INTRODUCTION TO GSM
INTRODUCTION TO GSM
INTRODUCTION
•
The Global System for Mobile Communications (GSM) is a set of
recommendations and specifications for a digital cellular telephone
network (known as a Public Land Mobile Network, or PLMN).
•
These recommendations ensure the compatibility of equipment from
different GSM manufacturers, and interconnectivity between different
administrations, including operation across international boundaries.
GSM networks are digital and can cater for high system capacities.
They are consistent with the world-wide digitization of the telephone
network, and are an extension of the Integrated Services Digital
Network (ISDN), using a digital radio interface between the cellular
network and the mobile subscriber equipment.
•
•
INTRODUCTION TO GSM
CELLULAR TELEPHONY
•
•
•
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A cellular telephone system links mobile subscribers into the public
telephone system or to another cellular subscriber.
Information between the mobile unit and the cellular network uses
radio communication. Hence the subscriber is able to move around
and become fully mobile.
The service area in which mobile communication is to be provided
is divided into regions called cells.
Each cell has the equipment to transmit and receive calls from any
subscriber located within the borders of its radio coverage area.
Radio
Cell
Mobile subscriber
INTRODUCTION TO GSM
GSM FREQUENCIES
•
GSM systems use radio frequencies between 890-915 MHz for
receive and between 935-960 MHz for transmit.
•
RF carriers are spaced every 200 kHz, allowing a total of 124
carriers for use.
•
An RF carrier is a pair of radio frequencies, one used in each
direction.
•
Transmit and receive frequencies are always separated by 45 MHz.
UPLINK FREQUENCIES
890
DOWNLINK FREQUENCIES
915
935
UPLINK AND DOWNLINK FREQUENCY SEPARATED BY 45MHZ
960
INTRODUCTION TO GSM
Extended GSM (EGSM)
•
EGSM has 10MHz of bandwidth on both transmit and receive.
•
Receive bandwidth is from 880 MHz to 890 MHz.
•
Transmit bandwidth is from 925 MHz to 935 MHz.
•
Total RF carriers in EGSM is 50.
UPLINK FREQUENCIES
880
890
DOWNLINK FREQUENCIES
915
925
935
UPLINK AND DOWNLINK FREQUENCY SEPARATED BY 45MHZ
960
INTRODUCTION TO GSM
DCS1800 FREQUENCIES
•
DCS1800 systems use radio frequencies between 1710-1785 MHz
for receive and between 1805-1880 MHz for transmit.
•
RF carriers are spaced every 200 kHz, allowing a total of 373
carriers.
•
There is a 100 kHz guard band between 1710.0 MHz and 1710.1
MHz and between 1784.9 MHz and 1785.0 MHz for receive, and
between 1805.0 MHz and 1805.1 MHz and between 1879.9 MHz
and 1880.0 MHz for transmit.
•
Transmit and receive frequencies are always separated by 95 MHz.
UPLINK FREQUENCIES
1710 MHz
1785 MHz
DOWNLINK FREQUENCIES
1805 MHz
UPLINK AND DOWNLINK FREQUENCY SEPARATED BY 95MHZ
1880 MHz
FEATURES OF GSM
FEATURES OF GSM
INCREASED CAPACITY
•
•
•
•
The GSM system provides a greater subscriber capacity than analogue
systems.
GSM allows 25 kHz per user, that is, eight conversations per 200 kHz
channel pair (a pair comprising one transmit channel and one receive
channel).
Digital channel coding and the modulation used makes the signal
resistant to interference from cells where the same frequencies are reused (co-channel interference); a Carrier to Interference Ratio (C/I) level
of 12 dB is achieved, as opposed to the 18 dB typical with analogue
cellular.
This allows increased geographic reuse by permitting a reduction in the
number of cells in the reuse pattern.
FEATURES OF GSM
AUDIO QUALITY
•
•
•
Digital transmission of speech and high performance digital signal
processors provide good quality speech transmission.
Since GSM is a digital technology, the signals passed over a digital air
interface can be protected against errors by using better error
detection and correction techniques.
In regions of interference or noise-limited operation the speech quality
is noticeably better than analogue.
USE OF STANDARDISED OPEN INTERFACES
• Standard interfaces such as C7 and X25 are used throughout the
system. Hence different manufacturers can be selected for different
parts of the PLMN.
• There is a high flexibilty in where the Network components are
situated.
FEATURES OF GSM
IMPROVED SECURITY AND CONFIDENTIALITY
•
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GSM offers high speech and data confidentiality.
Subscriber authentication can be performed by the system to check if
a subscriber is a valid subscriber or not.
The GSM system provides for high degree of confidentiality for the
subscriber. Calls are encoded and ciphered when sent over air.
The mobile equipment can be identified independently from the mobile
subscriber. The mobile has a identity number hard coded into it when it
is manufactured. This number is stored in a standard database and
whenever a call is made the equipment can be checked to see if it has
been reported stolen.
FEATURES OF GSM
CLEANER HANDOVERS
•
•
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GSM uses Mobile assisted handover techique.
The mobile itself carries out the signal strength and quality
measurement of its server and signal strength measurement of its
neighbors.
This data is passed on the Network which then uses sophisticated
algorithms to determine the need of handover.
SUBSCRIBER IDENTIFICATION
• In a GSM system the mobile station and the subscriber are
identified separately.
• The subscriber is identified by means of a smart card known as a
SIM.
• This enables the subscriber to use different mobile equipment while
retaining the same subscriber number.
FEATURES OF GSM
ENHANCED RANGE OF SERVICES
•
•
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•
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Speech services for normal telephony.
Short Message Service for point ot point transmission of text
message.
Cell broadcast for transmission of text message from the cell to all
MS in its coverage area. Message like traffic information or
advertising can be transmitted.
Fax and data services are provided. Data rates available are 2.4
Kb/s, 4.8 Kb/s and 9.6 Kb/s.
Supplementary services like number identification , call barring, call
forwarding, charging display etc can be provided.
FEATURES OF GSM
FREQUENCY REUSE
• There are total 124 carriers in GSM ( additional 50 carriers are
available if EGSM band is used).
• Each carrier has 8 timeslots and if 7 can be used for traffic then a
maximum of 868 ( 124 X 7 ) calls can be made. This is not enough
and hence frequencies have to be reused.
• The same RF carrier can be used for many conversations in several
different cells at the same time.
•
•
•
The radio carriers available are allocated
according to a regular pattern which repeats over
the whole coverage area.
The pattern to be used depends on traffic
requirement and spectrum availability.
Some typical repeat patterns are 4/12, 7/21 etc.
2
1
3
4
7
5
6
2
1
NETWORK COMPONENTS
NETWORK COMPONENTS
H
NMC
EIR
F
OMC-S
AUC
D
HLR
VLR
B
C
A
XCDR
IWF
MSC
UM
BSC
ABIS
OMC-R
EC
BTS
PSTN
UM
BTS
NETWORK COMPONENTS
Mobile Switching Centre (MSC)
• The Mobile services Switching Centre (MSC) co-ordinates the setting up
of calls to and from GSM users.
• It is the telephone switching office for MS originated or terminated traffic
and provides the appropriate bearer services, teleservices and
supplementary services.
• It controls a number of Base Station Sites (BSSs) within a specified
geographical coverage area and gives the radio subsystem access to
the subscriber and equipment databases.
• The MSC carries out several different functions depending on its position
in the network.
• When the MSC provides the interface between PSTN and the BSS in the
GSM network it is called the Gateway MSC.
• Some important functions carried out by MSC are Call processing
including control of data/voice call setup, inter BSS & inter MSC
handovers, control of mobility management, Operation & maintenance
support including database management, traffic metering and man
machine interface & managing the interface between GSM & PSTN N/W.
NETWORK COMPONENTS
Mobile Switching Centre (MSC) – Lucent MSC
NETWORK COMPONENTS
Mobile Station (MS)
• The Mobile Station consists of the Mobile Equipment (ME) and the
Subscriber Identity Module (SIM).
Mobile Equipment
• The Mobile Equipment is the hardware used by the subscriber to
access the network.
• The mobile equipment can be Vehicle mounted, with the antenna
physically mounted on the outside of the vehicle or portable mobile
unit, which can be handheld.
• Mobiles are classified into five classes according to their power
rating.
CLASS POWER OUTPUT
1
2
3
4
5
20W
8W
5W
2W
0.8W
NETWORK COMPONENTS
SIM
• The SIM is a removable card that plugs into the ME.
• It identifies the mobile subscriber and provides information about the
service that the subscriber should receive.
• The SIM contains several pieces of information
– International Mobile Subscribers Identity ( IMSI ) - This number
identifies the mobile subscriber. It is only transmitted over the air
during initialising.
– Temporary Mobile Subscriber Identity ( TMSI ) - This number also
identifies the subscriber. It can be alternatively used by the system.
It is periodically changed by the system to protect the subscriber
from being identified by someone attempting to monitor the radio
interface.
– Location Area Identity ( LAI ) - Identifies the current location of the
subscriber.
– Subscribers Authentication Key ( Ki ) - This is used to authenticate
the SIM card.
– Mobile Station International Standard Data Number ( MSISDN ) -
NETWORK COMPONENTS
SIM
• Most of the data contained within the SIM is protected against reading
(eg Ki ) or alterations after the SIM is issued.
• Some of the parameters ( eg. LAI ) will be continously updated to
reflect the current location of the subscriber.
• The SIM card can be protected by use of Personal Identity Number (
PIN ) password.
• The SIM is capable of storing additional information such as
accumulated call charges.
FULL SIZE SIM CARD
GSM
MINI SIM CARD
NETWORK COMPONENTS
Mobile Station International Subscribers Dialling Number ( MSISDN ) :
• Human identity used to call a MS
• The Mobile Subscriber ISDN (MSISDN) number is the telephone
number of the MS.
• This is the number a calling party dials to reach the subscriber.
• It is used by the land network to route calls toward the MSC.
CC
NDC
SN
98
XXX
12345
CC = Country code
NDC = National Destination Code
SN = Subscriber Number
NETWORK COMPONENTS
International Mobile Subscribers Identity ( IMSI ) :
• Network Identity Unique to a MS
• The International Mobile Subscriber Identity (IMSI) is the primary
identity of the subscriber within the mobile network and is
permanently assigned to that subscriber.
• The IMSI can be maximum of 15 digits.
MCC
MNC
MSIN
404
XX
12345..10
MCC = Mobile Country Code ( 3 Digits )
MNC = Mobile Network Code ( 2 Digits )
MSIN = Mobile Subscriber Identity Number
NETWORK COMPONENTS
Temporary Mobile Subscribers Identity ( TMSI ) :
• The GSM system can also assign a Temporary Mobile Subscriber
Identity (TMSI).
• After the subscriber's IMSI has been initialized on the system, the
TMSI can be used for sending messages backwards and forwards
across the network to identify the subscriber.
• The system automatically changes the TMSI at regular intervals, thus
protecting the subscriber from being identified by someone
attempting to monitor the radio channels.
• The TMSI is a local number and is always allocated by the VLR.
• The TMSI is maximum of 4 octets.
NETWORK COMPONENTS
Equipment Identity Register ( EIR )
• The Equipment Identity Register (EIR) contains a centralized
database for validating the international mobile station equipment
identity, the IMEI.
• The database contains three lists:
– The white list contains the number series of equipment identities
that have been allocated in the different participating countries.
This list does not contain individual numbers but but a range of
numbers by identifying the beginning and end of the series.
– The grey list contains IMEIs of equipment to be monitored and
observed for location and correct function.
– The black list contains IMEIs of MSs which have been reported
stolen or are to be denied service.
•
The EIR database is remotely accessed by the MSC’s in the
Network and can also be accessed by an MSC in a different PLMN.
.
NETWORK COMPONENTS
Equipment Identity Register ( EIR )
EIR
White List
Grey List
Black List
All Valid
assigned ID’s
Service allowed
but noted
Service denied
Range 1
Range 2
MS IMEI 1
MS IMEI 2
MS IMEI 1
MS IMEI 2
Range n
MS IMEI n
MS IMEI n
NETWORK COMPONENTS
International Mobile Equipment Identity ( IMEI ) :
• IMEI is a serial number unique to each mobile
• Each MS is identified by an International Mobile station Equipment
Identity (IMEI) number which is permanently stored in the Mobile
Equipment.
• On request, the MS sends this number over the signalling channel to the
MSC.
• The IMEI can be used to identify MSs that are reported stolen or
operating incorrectly.
TAC
FAC
SNR
SP
6
2
6
1
TAC
FAC
SNR
SP
=
=
=
=
Type Approval Code
Final Assembly Code
Serial Number
Spare
NETWORK COMPONENTS
HOME LOCATION REGISTER( HLR )
• The HLR contains the master database of all subscribers in the PLMN.
• This data is remotely accessed by the MSC´´s and VLRs in the network.
The data can also be accessed by an MSC or a VLR in a different
PLMN to allow inter-system and inter-country roaming.
• A PLMN may contain more than one HLR, in which case each HLR
contains a portion of the total subscriber database. There is only one
database record per subscriber.
• The subscribers data may be accessed by the IMSI or the MSISDN.
• The parameters stored in HLR are
– Subscribers ID (IMSI and MSISDN )
– Current subscriber VLR.
– Supplementary services subscribed to.
– Supplementary services information (eg. Current forwarding
address ).
– Authentication key and AUC functionality.
– TMSI and MSRN
NETWORK COMPONENTS
VISITOR LOCATION REGISTER ( VLR )
• The Visited Location Register (VLR) is a local subscriber database,
holding details on those subscribers who enter the area of the network
that it covers.
• The details are held in the VLR until the subscriber moves into the area
serviced by another VLR.
• The data includes most of the information stored at the HLR, as well as
more precise location and status information.
• The additional data stored in VLR are
– Mobile status ( Busy / Free / No answer etc. )
– Location Area Identity ( LAI )
– Temporary Mobile Subscribers Identity ( TMSI )
– Mobile Station Roaming Number ( MSRN )
• The VLR provides the system elements local to the subscriber, with
basic information on that subscriber, thus removing the need to access
the HLR every time subscriber information is required.
NETWORK COMPONENTS
Authentication Centre ( AUC )
•
•
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•
•
•
•
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The AUC is a processor system that perform authentication function.
It is normally co-located with the HLR.
The authentication process usually takes place each time the
subscriber initialises on the system.
Each subscriber is assigned an authentication key (Ki) which is
stored in the SIM and at the AUC.
A random number of 128 bits is generated by the AUC & sent to the
MS.
The authentication algorithm A3 uses this random number and
authentication key Ki to produce a signed response SRES( Signed
Response ).
At the same time the AUC uses the random number and
Authentication algoritm A3 along with the Ki key to produce a SRES.
If the SRES produced by AUC matches the one produced by MS is
the same, the subscriber is permitted to use the network.
NETWORK COMPONENTS
AUTHENTICATION PROCESS
HLR
VLR
AUC
Ki, A3, A8
A3 ( RAND, Ki ) = SRES
A8 ( RAND, Ki ) = Kc
Triples
Generated
MS
A3 , A8 , A5 , Ki
RAND
TRIPLES
RAND, Kc , SRES
SRES
RAND Kc SRES
SRES =
A3 (RAND , Ki )
SRES
SRES = SRES
BTS
A5 ,
HYPERFRAME NUM
Kc
AIR INTERFACE
ENCRYPTION
Kc =
A8 (RAND , Ki )
NETWORK COMPONENTS
Base Station Sub-System ( BSS ) :
• The BSS is the fixed end of the radio interface that provides control
and radio coverage functions for one or more cells and their
associated MSs.
• It is the interface between the MS and the MSC.
• The BSS comprises one or more Base Transceiver Stations (BTSs),
each containing the radio components that communicate with MSs in
a given area, and a Base Site Controller (BSC) which supports call
processing functions and the interfaces to the MSC.
• Digital radio techniques are used for the radio communications link,
known as the Air Interface, between the BSS and the MS.
• The BSS consists of three basic Network Elements (NEs).
– Transcoder (XCDR) or Remote transcoder (RXCDR) .
– Base Station Controller (BSC).
– Base Transceiver Stations (BTSs) assigned to the BSC. .
NETWORK COMPONENTS
Transcoder( XCDR )
• The speech transcoder is the interface between the 64 kbit/s PCM
channel in the land network and the 13 kbit/s vocoder (actually 22.8
kbit/s after channel coding) channel used on the Air Interface.
• This reduces the amount of information carried on the Air Interface and
hence, its bandwidth.
• If the 64 kbits/s PCM is transmitted on the air interface without
occupation, it would occupy an excessive amount of radio bandwidth.
This would use the available radio spectrum inefficiently.
• The required bandwidth is therefore reduced by processing the 64
kbits/s PCM data so that the amount of information required to transmit
digitized voice falls to 13kb/s.
• The XCDR can multiplex 4 traffic channels into a single 64 kbit/s
timeslot. Thus a E1/T1 serial link can carry 4 times as many channels.
• This can reduce the number of E1/T1 leased lines required to connect
remotely located equipment.
• When the transcoder is between the MSC and the BSC it is called a
remote transcoder (RXCDR).
NETWORK COMPONENTS
TRANSCODER(XCDR) - Siemens
NETWORK COMPONENTS
TRANSCODING
30 Timeslots
1 traffic channel / TS
64 Kbps / TS
4 E1 lines = 30 X 4
=120 Timeslots
MSC
Each Timeslot =16 X 4
= 64 Kb/s
30 timeslots = 30 x 4
=120 traffic channels
XCDR
BSC
Transcoded information from four calls
0
1
2
16
31
NETWORK COMPONENTS
Base Station Controller (BSC)
• The BSC network element provides the control for the BSS.
• It controls and manages the associated BTSs, and interfaces with
the Operations and Maintenance Centre (OMC).
• The purpose of the BSC is to perform a variety of functions. The
following comprise the functions provided by the BSC:
– Controls the BTS components.– Performs Call Processing.
– Performs Operations and Maintenance (O & M).
– Provides the O & M link (OML) between the BSS and the OMC.
– Provides the A Interface between the BSS and the MSC.
– Manages the radio channels.
– Transfers signalling information to and from MSs.
NETWORK COMPONENTS
Base Station Controller (BSC) – Siemens BSC
NETWORK COMPONENTS
Base Transceiver Station (BTS)
• The BTS network element consists of the hardware components,
such as radios, interface modules and antenna systems that
provide the Air Interface between the BSS and the MSs.
• The BTS provides radio channels (RF carriers) for a specific RF
coverage area.
• The radio channel is the communication link between the MSs
within an RF coverage area and the BSS.
• The BTS also has a limited amount of control functionality which
reduces the amount of traffic between the BTS and BSC.
NETWORK COMPONENTS
Base Transceiver Station (BTS)
NETWORK COMPONENTS
BTS Connectivity
Open ended Daisy Chain
MSC
BSC
BTS12
BTS13
BTS14
Star
BTS5
BTS11
BTS1
Daisy Chain with
a fork. Fork has a
return loop back
to the chain
BTS4
BTS2
BTS11
BTS6
BTS7
BTS3
BTS8
Daisy Chain with
a fork. Fork has a
return loop back
to the chain
BTS9
NETWORK COMPONENTS
Operation And Maintenance Centre For Radio (OMC-R)
•
•
•
The OMC controls and monitors the Network elements within a
region.
The OMC also monitors the quality of service being provided by the
Network.
The following are the main functions performed by the OMC-R
– The OMC allows network devices to be manually removed for or
restored to service. The status of network devices can be
checked from the OMC and tests and diagnostics invoked.
– The alarms generated by the Network elements are reported
and logged at the OMC. The OMC-R Engineer can monitor and
analyze these alarms and take appropriate action like informing
the maintenance personal.
– The OMC keeps on collecting and accumulating traffic statistics
from the network elements for analysis.
– Software loads can be downloaded to network elements or
uploaded to the OMC.
NETWORK COMPONENTS
Operation And Maintenance Centre For Radio (OMC-R)
NETWORK COMPONENTS
Base Station Identity Code
•
BSIC allows a mobile station to distinguish between neighboring base
stations.
•
It is made up of 8 bits.
7
6
0
0
5
4
NCC
3
2
1
0
BCC
BCC
NCC = National Colour Code( Differs from operator to operator )
BCC = Base Station Colour Code, identifies the base station to help
distinguish between Cell’s using the same BCCH frequencies
NETWORK COMPONENTS
MS Class Mark
• The MS is identified by it’s classmark which the mobile sends during
it’s initial message.
•
The classmark contains the following information
– Revision level - Identifies the phase of the GSM specifications the
mobiles complies with.
– RF Power Capabilities - The maximum power the mobile can
transmit. This information is held in the MS Power Class Number.
– Ciphering Algorithm - Indicates the ciphering algorithm
implemented in the mobile. There is only one algorithm (A5 ) in
GSM phase 1, however GSM phase 2 specifies different
algorithms (A5/0 to A5/7 )
– Frequency Capability - Indicates the frequency bands the MS can
receive and transmit on.
– Short Message Capability- Indicates whether the MS is able to
receive short messages or not.
MOBILE MAXIMUM RANGE
RANGE=
TIMIMG ADVANCE * BIT PERIOD* VELOCITY
2
TIMING ADVANCE = DELAY OF BITS (0-63)
BIT PERIOD= 577/156.25 = 3.693sec =3.693 * 10e-6 sec
VELOCITY= 3 * 10e5 Km/sec
RANGE= 34.9 Km
MULTIPLE ACCESS TECHNIQUES
•
In order for several links to be in progress simultaneously in the
same geographical area without mutual interference , multiple
access techniques are deployed.
•
The commonly used multiple access techniques are
– Frequency Division Multiple Access (FDMA )
– Time Division Multiple Access (TDMA )
– Code Division Multiple Access (CDMA )
TERRESTERIAL INTERFACE
•
The terrestrial interfaces comprises all the connections between
the GSM system entities ,apart from the Um or air interface.
•
The terrestrial interfaces transport the traffic across the system
and allows the passage of thousands of data messages to make
the system function.
•
The standard interfaces used are
– 2 Mb/s
– Signalling System (C7 or SS7
– Packet Switched Data
– A bis using the LAPD protocol (Link Access Procedure D )
•
INTERFACE NAMES
Each interface specified in GSM has a name associated with it.
NAME
INTERFACE
Um
MS ----- BTS
Abis
BTS ----- BSC
A
MSC ------ BSC
B
MSC ------ VLR
C
MSC ------ HLR
D
VLR ----- HLR
E
MSC ------ MSC
F
MSC ------ EIR
G
VLR ------ VLR
H
HLR ------ AUC
2 Mbits/s Trunk 30- channel PCM
This interface carries the traffic from the PSTN to the MSC,
between MSC’s, from the MSC to the BSC’s and from the BSC’s to
the BTS’s.
It represents the physical layer in the OSI model.
Each 2 Mb/s link provides 30 traffic channels available to carry
speech ,data or control information.
Typical Configuration
TS 0
TS 1-15
TS 16
TS 17 - 31
TS 0 - Frame allignment/ Error checking/ Signalling/ Alarms
TS 1-15 , 17-31 - Traffic
TS 16 - Siganlling
BSS CONNECTIONS
MSC
MTL
(C7 )
XCDR
OMC
OML (X.25)
BSC
CBC
CBL
RSL
( LAPD)
BTS
BTS
BTS
Cell Global Identity ( CGI ) :
LAI
MCC
MNC
LAC
CGI
MCC
MNC
LAC
CI
= Mobile Country Code
= Mobile Network Code
= Location Area Identity
= Cell Identity
CI
CHANNEL CONCEPT
CHANNELS
Downlink
Uplink
Physical channel - Each timeslot on a carrier is referred to as a physical
channel. Per carrier there are 8 physical channels.
Logical channel - Variety of information is transmitted between the MS and
BTS. There are different logical channels depending on the information
sent. The logical channels are of two types
• Traffic channel
• Control channel
CHANNEL CONCEPT
GSM Traffic Channels
Traffic Channels
TCH/F
Full rate 22.8kbits/s
TCH/H
Half rate 11.4 kbits/s
CHANNEL CONCEPT
GSM Control Channels
Control Channels
BCH ( Broadcast channels )
Downlink only
BCCH
Broadcast
control channel
SCH
Synchronisation
channel
Synch.
Channels
FCCH
Frequency
Correction channel
CCCH(Common Control Chan)
Downlink & Uplink
RACH
Random
Access Channel
CBCH
Cell Broadcast
Channel
PCH/
AGCH
Paging/Access grant
DCCH(Dedicated Channels)
Downlink & Uplink
SDCCH
Standalone
dedicated
control channel
FACCH
Fast Associated
Control Channel
ACCH
Associated
Control Channels
SACCH
Slow associated
Control Channel
CHANNEL CONCEPT
BCH Channels
BCCH( Broadcast Control Channel )
• Downlink only
• Broadcasts general information of the serving cell called System
Information
• BCCH is transmitted on timeslot zero of BCCH carrier
• Read only by idle mobile at least once every 30 secs.
SCH( Synchronisation Channel )
• Downlink only
• Carries information for frame synchronisation. Contains TDMA
frame number and BSIC.
FCCH( Frequency Correction Channel )
• Downlink only.
• Enables MS to synchronise to the frequency.
• Also helps mobiles of the ncells to locate TS 0 of BCCH carrier.
CHANNEL CONCEPT
CCCH Channels
RACH( Random Access Channel )
• Uplink only
• Used by the MS to access the Network.
AGCH( Access Grant Channel )
• Downlink only
• Used by the network to assign a signalling channel upon
successfull decoding of access bursts.
PCH( Paging Channel )
• Downlink only.
• Used by the Network to contact the MS.
CHANNEL CONCEPT
DCCH Channels
SDCCH( Standalone Dedicated Control Channel )
• Uplink and Downlink
• Used for call setup, location update and SMS.
SACCH( Slow Associated Control Channel )
• Used on Uplink and Downlink only in dedicated mode.
• Uplink SACCH messages - Measurement reports.
• Downlink SACCH messages - control info.
FACCH( Fast Associated Control Channel )
• Uplink and Downlink.
• Associated with TCH only.
• Is used to send fast messages like handover messages.
• Works by stealing traffic bursts.
CHANNEL CONCEPT
NORMAL BURST
FRAME1(4.615ms)
FRAME2
0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7
0.577ms
0.546ms
3
Guard Tail
Period Bits
57 bits
Data
26 bits
Flag Training
Bit sequence
57 bits
Flag
Bit
Data
3
Tail Guard
Bits Period
Carries traffic channel and control channels BCCH, PCH, AGCH, SDCCH,
SACCH and FACCH.
CHANNEL CONCEPT
NORMAL BURST
Data - Two blocks of 57 bits each. Carries speech, data or control info.
Tail bits - Used to indicate the start and end of each burst. Three bits always
000.
Guard period - 8.25 bits long. The receiver can only receive and decode if
the burst is received within the timeslot designated for it.Since the MS are
moving. Exact synchronization of burst is not possible practically. Hence
8.25bits corresponding to about 30us is available as guard period for a small
margin of error.
Flag bits - This bit is used to indicate if the 57 bits data block is used as
FACCH.
Training Sequence - This is a set sequence of bits known by both the
transmitter and the receiver( BCC of BSIC). When a burst of information is
received the equaliser searches for the training sequence code. The
receiver measures and then mimics the distortion which the signal has been
subjected to. The receiver then compares the received data with the
distorted possible transmitted sequence and chooses the most likely one.
CHANNEL CONCEPT
FREQUENCY CORRECTION BURST
FRAME1(4.615ms)
FRAME2
0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7
0.577ms
0.546ms
3
Guard Tail
Period Bits
•
•
•
142 bits
Fixed Data
3
Tail Guard
Bits Period
Carries FCCH channel.
Made up of 142 consecutive zeros.
Enables MS to correct its local oscillator locking it to that of the BTS.
CHANNEL CONCEPT
SYNCHRONISATION BURST
FRAME1(4.615ms)
0 1
2
3
4
5
6
FRAME2
7
0
1
2
3
4
5
6
7
0.577ms
0.546ms
3
39 bits
Guard Tail Encrypted
Bits
Period Bits
•
•
•
64 bits
Synchronisation
Sequence
39 bits
Encrypted
Bits
Carries SCH channel.
Enables MS to synchronise its timings with the BTS.
Contains BSIC and TDMA Frame number.
3
Tail Guard
Bits Period
CHANNEL CONCEPT
DUMMY BURST
FRAME1(4.615ms)
0 1
2
3
4
5
6
FRAME2
7
0
1
2
3
4
5
6
7
0.577ms
0.546ms
3
Guard Tail
Period Bits
•
57 bits
Data
26 bits
Flag Training
Bit sequence
57 bits
Flag
Bit
Data
3
Tail Guard
Bits Period
Transmitted on the unused timeslots of the BCCH carrier in the
downlink.
CHANNEL CONCEPT
ACCESS BURST
FRAME1(4.615ms)
FRAME2
0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7
0.577ms
8
Tail
Bits
•
•
41 bits
Synchronisation
Sequence
36 bits
3
Encrypted Tail
Bits
Bits
68.25 bits
Guard
Period
Carries RACH.
Has a bigger guard period since it is used during initial access and
the MS does not know how far it is actually from the BTS.
CHANNEL CONCEPT
NEED FOR TIMESLOT OFFSET
BSS Downlink
0 1
2
3
4
5
6
7
0
1
2
3
4
5
6
7
MS Uplink
0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7
•
If Uplink and Downlink are aligned exactly, then MS will have to
transmit and receive at the same time. To overcome this problem a
offset of 3 timeslots is provided between downlink and uplink
CHANNEL CONCEPT
NEED FOR TIMESLOT OFFSET
BSS Downlink
0 1
2
3
4
5
6
7
0
1
2
3
4
5
6
7
0
0
1
2
3
4
5
6
7
0
1
2
3
4
5
MS Uplink
5 6
7
3 timeslot
offset
•
As seen the MS does not have to transmit and receive at the same
time. This simplifies the MS design which can now use only one
synthesizer.
CHANNEL CONCEPT
26 FRAME MULTIFRAME STRUCTURE
4.615 msec
0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7
T T T T T T T T T T T T S T T T T T T T T T T T T I
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
120 msec
•
•
•
•
MS on dedicated mode on a TCH uses a 26-frame multiframe
structure.
Frame 0-11 and 13-24 used to carry traffic.
Frame 12 used as SACCH to carry control information from and to MS
to BTS.
Frame 25 is idle and is used by mobile to decode the BSIC of neighbor
cells.
BCCH/CCCH NON-COMBINED MULTIFRAME
Downlink
50
CCCH
BCCH
CCCH
40
Uplink
IDLE
CCCH BLOCK
BCCH BLOCK
SCH BLOCK
FCCH BLOCK
RACH BLOCK
50
40
CCCH
BCCH
CCCH
30
30
CCCH
BCCH
CCCH
20
20
CCCH
CCCH
10
10
CCCH
BCCH
0
0
BCCH/CCCH COMBINED MULTIFRAME
Uplink
Downlink
50
101
SACCH
CCCH
SACCH
CCCH
SACCH
BCCH
SACCH
BCCH
SDCCH
CCCH
SDCCH
CCCH
SDCCH
SDCCH
SDCCH
CCCH
SDCCH
CCCH
SDCCH
BCCH
CCCH
SDCCH
BCCH
CCCH
CCCH
CCCH
CCCH
CCCH
40
30
50
40
SDCCH
CCCH
101
SDCCH
CCCH
SDCCH
CCCH
SDCCH
CCCH
SDCCH
CCCH
SDCCH
CCCH
SACCH
CCCH
SACCH
CCCH
SACCH
CCCH
SACCH
CCCH
SDCCH
CCCH
BCCH
SDCCH
CCCH
BCCH
30
20
20
10
10
0
IDLE
CCCH BLOCK
BCCH BLOCK
SCH BLOCK
FCCH BLOCK
RACH BLOCK
SDCCH/4
SACCH/4
CCCH
CCCH
BCCH
BCCH
51
0
51
DCCH/8 MULTIFRAME
Uplink
Downlink
50
101
CCCH
A3
CCCH
A7
BCCH
A2
BCCH
A6
40
30
0
50
40
CCCH
A1
CCCH
A5
BCCH
A0
BCCH
A4
CCCH
D7
CCCH
D7
CCCH
D6
CCCH
D6
CCCH
D5
CCCH
D5
20
10
IDLE
SDCCH/8
SACCH/C8
30
20
BCCH
A0
101
BCCH
A4
CCCH
D7
CCCH
D7
CCCH
D6
CCCH
D6
CCCH
D5
CCCH
D5
CCCH
D4
CCCH
D4
CCCH
D3
CCCH
D3
CCCH
D2
CCCH
D2
CCCH
D1
CCCH
D1
CCCH
D0
CCCH
D0
CCCH
A7
CCCH
A3
CCCH
D4
CCCH
D4
CCCH
D3
CCCH
D3
CCCH
D2
CCCH
D2
CCCH
D1
CCCH
D1
BCCH
A6
BCCH
A2
CCCH
D0
CCCH
D0
CCCH
A5
CCCH
A1
51
10
0
51
CHANNEL CONCEPT
HYPERFRAME AND SUPERFRAME STRUCTURE
3h 28min 53s 760ms
0
1
6.12s
1
0
1 Hyperframe = 2048 superframes = 2,715,648 TDMA frames
2
2045
2
3
47
1
2
48
49
24
120ms
1
2047
1 Superframe = 1326 TDMAframes = 51(26 fr) 0r 26(51 fr) multiframes
0
0
2046
50
25
235.38ms
23
24
25
0
1
2
Traffic 26 - Frame Multiframe
48
49
50
Control 51 - Frame Multiframe
4.615ms
0
1
2
3
4
5
6
7
TDMA Frame
CODING, INTERLEAVING CIPHERING
SPEECH
CODING
SPEECH
DECODING
CHANNEL
CODING
CHANNEL
DECODING
INTERLEAVING
DEINTERLEAVING
BURST
ASSEMBLING
BURST
DISASSEMBLING
CIPHERING
DECIPHERING
MODULATION
Transmission
DEMODULATION
CODING
SPEECH CODING
•
The transmission of speech is one of the most important service of a
mobile cellular system.
•
The GSM speech codec, which will transform the analog signal(voice)
into a digital representation, has to meet the following criterias
• A good speech quality, at least as good as the one obtained with
previous cellular systems.
• To reduce the redundancy in the sounds of the voice. This reduction
is essential due to the limited capacity of transmission of a radio
channel.
• The speech codec must not be very complex because complexity is
equivalent to high costs.
•
The final choice for the GSM speech codec is a codec named RPELTP (Regular Pulse Excitation Long-Term Prediction).
CODING
SPEECH CODING
•
This codec uses the information from previous samples (this
information does not change very quickly) in order to predict the current
sample.
•
The speech signal is divided into blocks of 20 ms. These blocks are
then passed to the speech codec, which has a rate of 13 kbps, in order
to obtain blocks of 260 bits.
CODING
CHANNEL CODING
•
•
•
•
•
•
Channel coding adds redundancy bits to the original information in
order to detect and correct, if possible, errors ocurred during the
transmission.
The channel coding is performed using two codes: a block code and
a convolutional code.
The block code receives an input block of 240 bits and adds four
zero tail bits at the end of the input block. The output of the block
code is consequently a block of 244 bits.
A convolutional code adds redundancy bits in order to protect the
information. A convolutional encoder contains memory. This property
differentiates a convolutional code from a block code.
A convolutional code can be defined by three variables : n, k and K.
The value n corresponds to the number of bits at the output of the
encoder, k to the number of bits at the input of the block and K to the
memory of the encoder.
CODING
CHANNEL CODING ( Cont )
•
The ratio, R, of the code is defined as R = k/n.
Example - Let's consider a convolutional code with the following
values: k is equal to 1, n to 2 and K to 5. This convolutional code uses
then a rate of R = 1/2 and a delay of K = 5, which means that it will
add a redundant bit for each input bit. The convolutional code uses 5
consecutive bits in order to compute the redundancy bit. As the
convolutional code is a 1/2 rate convolutional code, a block of 488 bits
is generated. These 488 bits are punctured in order to produce a block
of 456 bits. Thirty two bits, obtained as follows, are not transmitted :
C (11 + 15 j) for j = 0, 1, ..., 31
k=1
1 bit input
•
Convolution code R = k/n = 1/2
n=2
2 bit input
The block of 456 bits produced by the convolutional code is then
passed to the interleaver
CODING
CHANNEL CODING FOR GSM SPEECH CHANNELS
• Before applying the channel coding, the 260 bits of a GSM speech
frame are divided in three different classes according to their function
and importance.
•
The most important class is the class 1a containing 50 bits.Next
important is the class 1b, which contains 132 bits.The least important
is the class 2, which contains the remaining 78 bits.
•
The different classes are coded differently.
•
First of all, the class 1a bits are block-coded. Three parity bits, used
for error detection, are added to the 50 class 1a bits.The resultant 53
bits are added to the class 1b bits.
•
Four zero bits are added to this block of 185 bits (50+3+132). A
convolutional code, with r = 1/2 and K = 5, is then applied, obtaining
an output block of 378 bits.
•
The class 2 bits are added, without any protection, to the output
block of the convolutional coder. An output block of 456 bits is finally
obtained.
CODING
Speech Channel Coding
260 bits
Class 1a
50 bits
Parity
check
Class 1b
132 bits
Class 1a 3
50 bits
Class 1b
132 bits
Convolution coding
378 bits
456 bits
Tail
bits
4
Class 2
78 bits
CODING
CHANNEL CODING FOR CONTROL CHANNELS
• In GSM the signalling information is just contained in 184 bits.
•
Forty parity bits, obtained using a fire code, and four zero bits are
added to the 184 bits before applying the convolutional code (r = 1/2
and K = 5). The output of the convolution code is then a block of 456
bits which does not need to be punctured.
Parity
bits
184 bits
Fire
code
184 bits
Convolution coding
456 bits
40 bits
4
Tail
bits
CODING
CHANNEL CODING FOR DATA CHANNELS
•
In data information is contained in 240 bits.
•
Four tails bits are added to the 240 bits before applying the
convolutional code (r = 1/2 and K = 5). The output of the
convolutional code is then a block of 488 bits which when punctuated
yields 456 bits.
240 bits
4
240 bits
Convolution coding
488 bits
Punctuate
456 bits
Tail
bits
INTERLEAVING
INTERLEAVING
•
An interleaving rearranges a group of bits in a particular way.
•
It is used in combination with FEC codes( Forward Error Correction
Codes ) in order to improve the performance of the error correction
mechanisms.
•
The interleaving decreases the possibility of losing whole bursts
during the transmission, by dispersing the errors.
•
As the errors are less concentrated, it is then easier to correct them.
INTERLEAVING
GSM SPEECH CHANNEL INTERLEAVING
•
A burst in GSM transmits two blocks of 57 data bits each.
•
Therefore the 456 bits corresponding to the output of the channel coder
fit into 8 ‘57 data’ bits (8 * 57 = 456). The 456 bits are divided into eight
blocks of 57 bits.
•
The first block of 57 bits contains the bit numbers (0, 8, 16, .....448), the
second one the bit numbers (1, 9, 17, .....449), etc.
•
The last block of 57 bits will then contain the bit numbers (7, 15, .....455).
•
The first four blocks of 57 bits are placed in the even-numbered bits of
four consecutive bursts.
•
The other four blocks of 57 bits are placed in the odd-numbered bits of
the next four bursts.
•
The interleaving depth of the GSM interleaving for speech channels is
eight.
•
A new data block also starts every four bursts. The interleaver for
speech channels is called a block interleaver.
INTERLEAVING
GSM SPEECH CHANNEL INTERLEAVING ( Diagram )
Full rate encoded speech blocks
from a single conversation
1
2
3
4
5
6
4
5
6
456 bits
456 bits
456 bits
Bursts
TDMA
Frames
0
1
Frame 1
2
3
4
5
Frame 3
Frame 2
6
7
0
1
2
3
4
5
6
7
0
1
2
3
4
5
Frame 4
6
7
0
1
2
3
4
5
6
7
INTERLEAVING
CONTROL CHANNEL INTERLEAVING
• A burst in GSM transmits two blocks of 57 data bits each.
•
Therefore the 456 bits corresponding to the output of the channel coder
fit into four bursts (4*114 = 456).
•
The 456 bits are divided into eight blocks of 57 bits. The first block of 57
bits contains the bit numbers (0, 8, 16, .....448), the second one the bit
numbers (1, 9, 17, .....449), etc. The last block of 57 bits will then contain
the bit numbers (7, 15, .....455).
•
The first four blocks of 57 bits are placed in the even-numbered bits of
four bursts.
•
The other four blocks of 57 bits are placed in the odd-numbered bits of
the same four bursts.
•
Therefore the interleaving depth of the GSM interleaving for control
channels is four and a new data block starts every four bursts.
•
The interleaver for control channels is called a block rectangular
interleaver.
INTERLEAVING
DATA INTERLEAVING
•
A particular interleaving scheme, with an interleaving depth equal to 22,
is applied to the block of 456 bits obtained after the channel coding.
•
The block is divided into 16 blocks of 24 bits each, 2 blocks of 18 bits
each, 2 blocks of 12 bits each and 2 blocks of 6 bits each.
•
It is spread over 22 bursts in the following way :
• the first and the twenty-second bursts carry one block of 6 bits
each
• the second and the twenty-first bursts carry one block of 12 bits
each
• the third and the twentieth bursts carry one block of 18 bits each
• from the fourth to the nineteenth burst, a block of 24 bits is placed
in each burst
•
A burst will then carry information from five or six consecutive data
blocks. The data blocks are said to be interleaved diagonally.
MODULATION
CIPHERING
•
Ciphering is used to protect signaling and user data.
•
A ciphering key is computed using the algorithm A8 stored on the
SIM card, the subscriber key and a random number delivered by the
network (this random number is the same as the one used for the
authentication procedure).
•
A 114 bit sequence is produced using the ciphering key, an algorithm
called A5 and the burst numbers.
•
This bit sequence is then XORed with the two 57 bit blocks of data
included in a normal burst.
•
In order to decipher correctly, the receiver has to use the same
algorithm A5 for the deciphering procedure.
MODULATION
•
Modulation is done using 0.3 GMSK
Other Networks
SIGNALLING
SIGNALLING SYSTEM
WHAT IS SIGNALLING ?
•
•
•
The term signaling is used in many contexts.
In technical systems, it very often refers to the control of different
procedures.
With reference to telephony, signaling means the transfer of
information and the instructions relevant to control and monitor
telephony connections.
SIGNALLING SYSTEM C7
GENERAL INTRODUCTION
•
•
•
•
•
Today’s global telecom networks are included in very complex
technical systems.
Naturally, a system of this type requires extensive signaling, both
internally in different nodes (for example, exchanges) and externally
between different types of network nodes.
During this training we will focus on external signaling.
Thus, the term signaling in the following slides always refers to
external signaling traffic.
The main purpose of using signaling in modern telecom networks –
where different network nodes must cooperate and communicate with
each other – is to enable transfer of control information between
nodes in connection with:
–Traffic control procedures as set-up, supervision, and release
of telecommunication connections and services
GENERAL INTRODUCTION
• Database communication, for example, database queries concerning
specific services, roaming in cellular networks, etc.
•
Network management
deblocking trunks.
•
Traditionally, external signaling has been divided into two basic types
procedures,
for
example,
blocking
or
– Access signaling (for example, Subscriber Loop Signaling) This
means signaling between a subscriber terminal (telephone) and
the local exchange.
– Trunk signaling (that is, Inter-Exchange Signaling) This is used
for signaling between exchanges.
SIGNALING IN TELECOMMUNICATION NETWORK
SIGNALLING
ACCESS SIG
TRUNK SIGNALLING
SUBSCRIBER LINE SIG.
CHANNEL ASSOCIATED SIG.
DIGITAL SUBSCRIBER SIG.
COMMON CHANNEL SIG.
Access Signaling
• There are many types of access signaling, for example, PSTN analogue
subscriber line signaling, ISDN Digital Subscriber Signaling System
(DSS1), and signaling between the MS and the network in the GSM
system.
• Signaling on the analogue subscriber line between a telephony
subscriber and the Local Exchange (LE) is performed by means of
on/off hook signals, dialed digits, information tones (dial tone, busy
tone, etc.), recorded announcements, and ringing signals.
• The dialed digits can be sent in two different ways: as decadic pulses
(used for old-type rotary-dial telephones), or as a combination of two
tones (used for modern pushbutton telephones). The latter system is
known as the Dual Tone Multi Frequency (DTMF).
• The information tones (dial tone, ringing tone, busy tone, etc.) are audio
signals used to keep the calling party (the A-subscriber) informed about
what is going on in the network during the set-up of a call.
Access Signaling
• Digital Subscriber Signaling System No. 1 (DSS1) is the standard
access signaling system used in ISDN. It is also called a D-channel
signaling system
• D-channel signaling is defined for digital access lines only.
• The signaling protocols are based on the OSI (Open System
Interconnection) reference model, layers 1 to 3.
• Consequently, the signaling messages are transferred as data
packets between the user terminal and the local exchange.
•
Due to the much more complex service environment at the ISDN
user’s site, the amount of signaling information and the number of
variations
Trunk Signaling
• The Inter-exchange Signaling information is usually transported on
one of the time slots in a PCM link, either in association with the
speech channel or independently.
• There are two commonly used methods for Inter Exchange Signaling.
Channel Associated Signaling (CAS)
– In CAS, the speech channel (in-band), or a channel closely
associated with a speech channel (out-band), is used for
signaling.
Common Channel Signaling (CCS)
– In this case a dedicated channel, completely separate from the
speech channel, is used for signaling. Due to the high capacity,
one signaling channel in CCS can serve a large number of
speech channels.
• In a GSM network, CCITT Signaling System No. 7 is used.
• Signaling System No. 7 is a Common Channel Signaling system.
CHANNEL ASSOCIATED SIGNALING (CAS)
• Channel Associated Signaling (CAS) means that the signaling is
always sent on the same connection (PCM link) as the traffic.
• The signaling is associated with the traffic channel.
• In a 2 Mb/s PCM link, 30 time slots are used for speech, whereas TS
0 is used for synchronization and TS 16 is used for the line signaling.
• All 30 traffic connections share TS 16 in a multiframe consisting of 16
consecutive frames.
• On TS 16, each traffic channel has a permanently allocated recurring
location for line signaling, where two traffic channels share TS 16 in
one frame.
COMMON CHANNEL SIGNALING (CCS)
• In CCS, signaling messages (or data packets) are transmitted over time
slots in a PCM link reserved for the purpose of signaling.
• The system is designed to use a common data channel (or signaling
link) as the carrier of all signals, required by a large number of traffic
channels.
• In 1968, CCITT specified a Common Channel Signaling system called
CCS System No. 6, which was designed especially for international
analogue telephony networks.
• However, very few installations of this system remain today. It has, as
already mentioned, been replaced by Signaling System No. 7.
• The first version of SS7 (1980) was designed for telephony and data.
• In the 80’s the demand for new services dramatically increased and the
SS7 was therefore developed to meet the signaling requirements,
specified for all these new services.
• Today SS7 is used in many different networks and related services
typically betn PSTN, ISDN, PLMN & IN services throughout the world.
OSI REFERENCE MODEL
• The Signaling System No. 7, which is a type of packet switched data
communication system, is structured in a modular and layered way.
• Such a design of SS7 is similar to the Open System Interconnection
model.
• Open Systems are systems that use standardized communication
procedures developed from the reference model.
• Thus, all such open systems are able to communicate with each
other.
• The word “system” can refer to computers, exchanges, data
networks, etc.
OSI MODEL REFERENCE DIAGRAM
APPLICATION
APPLICATION
PRESENTATION
PRESENTATION
SESSION
SESSION
TRANSPORT
TRANSPORT
NETWORK
NETWORK
LINK
LINK
PHYSICAL
PHYSICAL
COMMUNICATION PROCESS
• Each layer has its own specified functions and provides specific
services for the layers above.
• It is important to define the interfaces between different layers and the
functions within each layer.
• The way a function is realized within a layer is not predicted.
• Logically, the communication between functions always takes place
on the same level according to the protocols for that level.
• Only functions on the same level can “talk to each other”.
• In the transmitting system, the protocol for each layer adds
information to the data received from the layer above.
• The addition usually consists of a header and/or a trailer.
• In the receiving system, the additions are used, for example, to
identify bits or data fields carrying information for that specific layer
only.
• These fields are decoded by layer functionality and are removed
when delivering the message to the applications orlayers above.
•
•
•
When the data reaches the application layer on the receiving side,
it consists of only the data that originated in the application layer of
the sending system.
Logically, each layer communicates with the corresponding layer
in the other system.
This communication is called Peer-to-Peer communication and is
controlled by the layer’s protocol.
DESCRIPTION OF LAYERS
Application Layer
• This layer provides services for support of the user’s application
process and for control of all communication between applications.
• Examples of layer 7 functions are file transfer, message handling,
directory services, and operation and maintenance.
Presentation Layer
• This layer defines how data is to be represented, that is, the syntax.
• The presentation layer transforms the syntax used in the application
into the common syntax needed for the communication between
applications.
• Layer 6 contains data compression.
Session Layer
• This layer establishes connections between presentation layers in
different systems.
• It also controls the connection, the synchronization and the
disconnection of the dialogue.
• It allows the presentation layer to determine checkpoints, from which
the retransmission will start when the data transmission has been
interrupted.
Transport Layer
• This layer guarantees that the bearer service has the quality
required by the application in question.
• Examples of functions are error detection and correction (end-toend), and flow control.
• The transport layer optimizes the data communication, for example
by multiplexing or splitting data streams before they reach the
network.
Network Layer
• The basic network layer service is to provide a transparent channel.
• This means that the application requesting a channel ignores network
problems and the related signal exchange because that is the task of
the lower levels.
• It just requires an open channel, transparent for the transmission of
data, between transport layers in different systems.
• The Network Layer establishes, maintains, and releases connections
between the nodes in the network and handles addressing and
routing of circuits.
•
•
Data Link Layer
This layer provides an essentially error-free point-to-point circuit
between network layers.
The layer contains resources for error detection, error correction, flow
control, and retransmission.
Physical Layer
• This layer provides mechanical, electrical, functional, and
procedural resources for activating, maintaining, and blocking
physical circuits for the transmission of bits between data link
layers.
• The physical layer contains functions for converting data into
signals compatible with the transmission medium.
• For the communication between only two exchanges, layers 1 and
2 are sufficient.
• For the communication between all exchanges in the network, layer
3 must be added because it provides addressing and routing.
SIGNALING SYSTEM NO. 7 INTRODUCTION
• The Signaling System (SS)No. 7 is an elaborate set of
recommendations defining protocols for the internal management of
digital networks.
• These recommendations were introduced in 1980 and revised in 1984
and 1988 in different-colored books (yellow, red, and blue).
• CCITT SS No. 7 is intended primarily for digital networks, both
national and international, where the high transmission rates (64
kbps) can be exploited.
• It may also be used on analogue lines especially on international
trunks (CCITT SS No 6).
• CCS was initially meant for telephony only, but has now evolved into
non-telephony and non-connection related applications (for example,
location updating of a mobile subscriber).
• A dialogue with a database or between two databases is a typical
application for CS in GSM.
•
Thus, there is a need for a generic system that is able to support a
wide variety of applications in telecommunication.
•
The variety of applications is increasing as new types of telephony
systems and a wider use of databases in the network become
necessary (mobile telephony networks, ISDN, IN, etc.).
•
Even though the standardization of SS7 is now the responsibility of
ITU-T, for traditional and historical reasons, the system is often
called “CCITT No. 7 signaling system”.
•
The signaling system
recommendations.
•
The modular layer structure allows flexible usage of the
specifications.
used
in
GSM
follows
the
CCITT
USER PARTS
• The User Parts (UPs) contain functions dealing with the processing of
signal information before and after it is transmitted through the
signaling network.
• The MTP provides the means of reliable transport and delivery of UP
information across the SS7 network.
• It also has the ability to react to system and network failures that
affect the information from the UPs and take necessary action to
ensure that the information is safely conveyed.
• The User does not mean the subscriber involved in the call, but the
user of the MTP.
• The MTP is a common transport system developed to serve one or
more User Parts in the same node.
• Every Signaling Point(SP) consists of MTP & a number of its users.
• Only UPs of the same type can communicate with each other.
• To forward signaling messages between UPs, located in different
nodes, the MTP is used.
USERS OF SIGNALING SYSTEM CCITT NO 7
MAP
CAP
BSSAP
ISUP
TCAP
SCCP
MTP
CCITT SS NO. 7 PROTOCOLS IN GSM
TUP
MTP user parts
ISUP (ISDN User Part)
• It provides control-functions and signaling, needed in an ISDN, to deal
with ISDN subscriber calls and related functions.
TUP (Telephony User Part)
• It provides all necessary functions and signaling for dealing with a
telephony user.
• TUP is being replaced by ISUP in telecommunication networks.
DUP (Digital User Part)
• This UP is used for purposes such as file transfer and related
signaling.
SCCP
• The MTP was designed for the real-time applications of telephony.
• The connectionless nature of the MTP provides a low-overhead facility
suiting the requirements of telephony.
• Regarding GSM, other applications such as network management
need services such as expanded addressing capability and reliable
message transfer.
• The SCCP was developed to meet these requirements.
• The SCCP also sends its messages through the MTP.
• The SCCP provides functions for completely new services, for
example, non-circuit-related signaling.
• Some functions, not directly related to users, but necessary for
network control, are used.
• The main reason is that they are necessary for serving applications in
higher layers and for maintenance purposes.
SCCP
• These functions use SCCP services:
Transaction Capabilities (TC)
– First introduced in 1984, TC provides the mechanisms for
transaction-oriented applications and functions.
Operation and Maintenance Application Part (OMAP)
– Specifies network management functions and messages related
to operation and maintenance.
OSI Model
CCITT SS NO 7 Model
ASE
APPLICATION
USER PARTS
TCAP
PRESENTATION
SESSION
TRANSPORT
SCCP
NETWORK
SIGNALLING LINK
PHYSICAL
SIGNALLING DATA LINK
NSP
LINK
MTP
SIGNALLING NETWORK
CALL FLOW
Mobile originated call
BSS
MS
MSC
Channel Request (RACH)
Immediate Assignment [ Reject ] (AGCH)
SDCCH Seizure
CM Service Request
+ Connection Request < CMSREQ >
Connection [ Confirmed / Refused ]
Link Establishment
Authentication Request
Authentication Response
S
D
C
C
H
Ciphering Mode Command
Ciphering Mode Complete
DT1 <CICMD>
DT1 <CICMP>
Identity Request
Identity Response
Setup
Call Proceeding
Connection Management
Assignment Request
Assignment Request [ Failed ]
Assignment Command
Assignment [ Complete / Failure ]
Assignment [ Complete / Failure ]
T
C
H
TCH Seizure
Mobile terminated call
BSS
MS
Paging Request (PCH)
MSC
UDT < PAGIN >
Paging
Channel Request (RACH)
Immediate Assignment [ Reject ] (AGCH)
SDCCH Seizure
Paging Response
+ Connection Request < PAGRES >
Connection [ Confirmed / Refused ]
Link Establishment
Authentication Request
Authentication Response
Ciphering Mode Command
S
D
C
C
H
Ciphering Mode Complete
DT1 <CICMD>
DT1 <CICMP>
Identity Request
Identity Response
Setup
Call Confirmed
Connection Management
Assignment Request
Assignment Request [ Failed ]
Assignment Command
Assignment [ Complete / Failure ]
T
C
H
Assignment [ Complete / Failure ]
TCH Seizure
POWER CONTROL
RF POWER CONTROL
•
•
•
•
RF power control is employed to minimise the transmit power
required by MS or BS while maintaining the quality of the radio links.
By minimising the transmit power levels, interference to co-channel
users is reduced.
Power control is implemented in the MS as well as the BSS.
Power control on the Uplink also helps to increase the battery life.
POWER CONTROL IN THE MS
• The RF power level employed by the MS is indicated by means of the
5 bit TXPWR field sent either in the layer 1 header of each downlink
SACCH message block, or in a dedicated signalling block.
• The MS confirms the power level that it is currently employing by
setting the MS_TXPWR_CONF field in the uplink SACCH L1 header
to its current power setting. The value of this field is the power setting
actually used by the mobile for the last burst of the previous SACCH
period.
• The MS employs the most recently commanded RF power level
appropriate to the channel for all transmitted bursts on either a TCH
(including handover access burst), FACCH,SACCH or SDCCH.
• When accessing a cell on the RACH (random access) and before
receiving the first power command during a communication on a
DCCH or TCH (after an IMMEDIATE ASSIGNMENT), the MS uses
either the power level defined by the MS_TXPWR_MAX_CCH
parameter broadcast on the BCCH of the cell, or the maximum
TXPWR of the MS as defined by its power class, whichever is the
1111111 indicates this field does not have any TA value
8
7
Spare
Spare
6
5
4
3
2
Ordered MS Power Level
Ordered Timing Advance
1
Octet 1
Octet 2
POWER CONTROL MS
•
•
The range over which a MS is capable of varying its RF output
power is from its maximum output down to 20mW, in steps of
nominally 2dB.
0 - 43dBm…….15 - 13dBm.
TIMING OF POWER CHANGE BY MS
•
•
•
Upon receipt of a command on the SACCH to change its RF power
level (TXPWR field) the MS changes to the new level at a rate of
one nominal 2dB power step every 60ms (13 TDMA frames), i.e. a
full range change of 15 steps should take about 900ms .
The change commences at the first TDMA frame belonging to the
next reporting period . The MS changes the power one nominal 2
dB step at a time, at a rate of one step every 60 ms following the
initial change, irrespective of whether actual transmission takes
place or not.
In case of channel change the commanded power level is applied
on the new channel immediately.
BSS POWER CONTROL
•
•
Power control at BSS is optional.
The range over which the BS is capable of reducing its RF output
power from its maximum level is nominally 30dB, in 15 steps of
nominally 2dB.
RADIO LINK FAILURE
• The criterion for determining Radio Link Failure in the MS is based on
the success rate of decoding messages on the downlink SACCH.
• The radio link failure criterion is based on the radio link counter S.
• If the MS is unable to decode a SACCH message, S is decreased by 1.
• If a SACCH message is decoded successfully, S is increased by 2.
• If S reaches 0 a radio link failure is assumed & the MS aborts the conn.
• The RADIO_LINK_TIMEOUT parameter is transmitted by each BS in the
BCCH data.
4
3
2
1
0
Decoded
Not Decoded
RADIO LINK FAILURE
•
•
•
•
The MS continues transmitting as normal on the uplink until S
reaches 0.
The algorithm will start after the assignment of a dedicated channel
and S is initialized to RADIO_LINK_TIMEOUT.
The aim of determining radio link failure in the MS is to ensure that
calls with unacceptable voice/data quality, which cannot be improved
either by RF power control or handover, are either re-established or
released in a defined manner.
In general the parameters that control the forced release should be
set such that the forced release will not normally occur until the call
has degraded to a quality below that at which the majority of
subscribers would have manually released. This ensures that, for
example, a call on the edge of a radio coverage area, although of
bad quality, can usually be completed if the subscriber wishes.
CELL SELECTION AND RE-SELECTION
•
•
•
•
In Idle mode (i.e. not engaged in communicating with a BS), an MS will
do the cell selection and re-selection procedures .
The procedures ensure that the MS is camped on a cell from which it
can reliably decode downlink data and with which it has a high
probability of communications on the uplink. The choice of cell is
determined by the path loss criterion. Once the MS is camped on a
cell, access to the network is allowed.
An MS is said to be camped on a cell when it has determined that the
cell is suitable and stays tuned to a BCCH + CCCH of that cell. While
camped on a cell, an MS may receive paging messages or under
certain conditions make random access attempts on a RACH of that
cell, and read BCCH data from that cell.
The MS will not use the discontinuous reception (DRX) mode of
operation (i.e. powering itself down when it is not expecting paging
messages from the network) while performing the selection and
reselection algorithm. However use of powering down is permitted at
all other times in idle mode.
CELL SELECTION AND RE-SELECTION
•
•
•
•
•
For the purposes of cell selection and reselection, the MS is required
to maintain an average of received signal strengths for all monitored
frequencies. These quantities termed the "receive level averages” is
the averages of the received signal strengths measured in dBm.
The cell selection and reselection procedures make use of the "BCCH
Allocation" (BA) list. There are in two BA lists which may or may not be
identical, depending on choices made by the PLMN operator.
(i) BA (BCCH) - This is the BA sent in System Information Messages
on the BCCH. It is the list of BCCH carriers in use by a given PLMN in
a given geographical area. It is used by the MS in cell selection and
reselection.
(ii) BA (SACCH) - This is the BA sent in System Information Messages
on the SACCH and indicates to the MS which BCCH carriers are to be
monitored for handover purposes.
When the MS goes on to a TCH or SDCCH, it starts monitoring BCCH
carriers in BA (BCCH) until it gets its first BA (SACCH) message.
CELL SELECTION - NO BCCH DATA AVAILABLE
• The MS searches all 124 RF channels in the GSM system, takes
readings of RSS on each RF channel, and calculate the received
level average for each.
• The averaging is based on at least five measurement samples per
RF carrier spread over 3 to 5 secs.
• The MS tunes to the carrier with the highest average RSS &
determines whether or not this carrier is a BCCH carrier.
• If it is a BCCH carrier, the MS attempts to synchronise to this carrier
and read the BCCH data. The MS camps on the cell provided it can
successfully decode the BCCH data and this data indicates that it is
part of the selected PLMN, that the cell is not barred
(CELL_BAR_ACCESS = 0) & that the parameter C1 is greater than
0.
• If the cell is part of the selected PLMN but is barred or C1 is less than
zero, the MS uses the BCCH Allocation obtained from this cell and
subsequently only searches these BCCH carriers. Otherwise the MS
tune to the next highest carrier and so on.
CELL SELECTION - NO BCCH DATA AVAILABLE
• CELL_BAR_ACCESS may be employed to bar a cell that is only
intended to handle handover traffic etc. For example of this could be
an umbrella cell which encompasses a number of microcells.
• If at least the 30 strongest RF channels have been tried, but no
suitable cell has been found, provided the RF channels which have
been searched include at least one BCCH carrier, the available
PLMN's shall be presented to the user, otherwise more RF channels
shall be searched until at least one BCCH carrier is found.
• 30 RF channels are specified to give a high probability of finding all
suitable PLMN's, without making the process take too long.
CELL SELECTION - BCCH INFORMATION AVAIL.
• The MS stores the BCCH carriers in use by the PLMN selected when it
was last active in the GSM network. A MS may also store BCCH
carriers for more than one PLMN which it has selected previously (e.g.
at national borders or when more than one PLMN serves a country).
• If an MS includes a BCCH carrier storage option it searches only for
BCCH carriers in the list.
• If an MS decodes BCCH data from a cell of the selected PLMN but is
unable to camp on that cell, the BA of that cell is examined. Any BCCH
carriers in the BA which are not in the MS's list of BCCH carriers to be
searched is added to the list.
• If no suitable cell has been found after all the BCCH carriers in the list
have been searched, the MS acts as if there were no stored BCCH
carrier information. Since information concerning a number of channels
is already known to the MS, it may assign high priority to
measurements on those of the 30 strongest carriers from which it has
not previously made attempts to obtain BCCH information, and omit
repeated measurements on the known ones.
PATH LOSS CRITEREON( C1)
•
•
•
This parameter is used to ensure that the MS is camped on the cell
with which it has the highest probability of successful communication
on uplink and downlink.
The path loss criterion parameter C1 used for cell selection and
reselection is defined by:
C1 = (A - Max(B,0))
where A = Received Level Average - RXLEV_ACCESS_MIN
B = MS_TXPWR_MAX_CCH - P
RXLEV_ACCESS_MIN =Minimum received level at the MS
required for access to the system.
MS_TXPWR_MAX_CCH = Maximum TXPWR level an MS may
use when accessing the system.
P = Maximum RF output power of the MS.
All values are expressed in dBm.
PATH LOSS CRITEREON( C1)
A = + Good Downlink
- Poor Downlink
B = - Good Downlink
+ Poor Downlink
Monitoring of Received Level and BCCH data
• In Idle Mode an MS continues to monitor all BCCH carriers as
indicated by the BCCH Allocation .
• A running average of received level in the preceding 5 to 60 seconds
is be maintained for each carrier in the BCCH Allocation.
• For the serving cell receive level measurement samples is taken at
least for each paging block of the MS and the receive level average
is determined using samples collected over a period of 5 s or five
consecutive paging blocks of that MS, whichever is the greater
period.
Monitoring of Received Level and BCCH data
• At least 5 received level measurement samples are required per
receive level average value. New sets of receive level average
values is calculated as often as possible.
• The same number of measurement samples is taken for all non
serving cell BCCH carriers, and the samples allocated to each carrier
is as far as possible uniformly distributed over each evaluation
period.
• The list of the 6 strongest carriers is updated at least every minute
and may be updated more frequently.
• In order to minimise power consumption, MSs that employ DRX (i.e.
power down when paging blocks are not due) monitor the signal
strengths of non-serving cell BCCH carriers during the frames of the
Paging Block that they are required to listen to. Received level
measurement samples can thus be taken on several non-serving
BCCH carriers and on the serving carrier during each Paging Block.
• The MS includes the BCCH carrier of the current serving cell (i.e. the
cell the MS is camped on) in this measurement routine.
Monitoring of Received Level and BCCH data
• The MS has to decode the full BCCH data of the serving cell at least
every 30 seconds.
• The MS attempts to decode the BCCH data block that contains the
parameters affecting cell reselection for each of the 6 strongest nonserving cell BCCH carriers at least every 5 minutes.
• When the MS recognizes that a new BCCH carrier has become one of
the 6 strongest, the BCCH data shall be decoded for the new carrier
within 30 seconds.
• The MS attempts to check the BSIC for each of the 6 strongest non
serving cell BCCH carriers at least every 30 seconds, to confirm that it
is monitoring the same cell.
• If a change of BSIC is detected then the carrier is treated as a new
carrier and the BCCH data redetermined.
• When requested by the user, the MS monitors the 30 strongest GSM
carrier to determine, within 15 seconds, which PLMN's are available.
This monitoring is done so as to minimise interruptions to the
monitoring of the PCH.
CALL RE-ESTABLISHMENT
•
•
•
•
•
•
In the event of a radio link failure, call re-establishment may be
attempted if it is enabled in the database.
The received level measurement samples taken on surrounding cells
and on the serving cell BCCH carrier in the last 5 seconds is averaged,
and the carrier with the highest average received level which is part of
a permitted PLMN is taken.
A BCCH data block containing the parameters affecting cell selection
is read on this carrier.
If the parameter C1 is greater than zero, it is part of the selected
PLMN, the cell is not barred, and call re-establishment is allowed, call
re-establishment is attempted on this cell.
If the above conditions are not met, the carrier with the next highest
average received level is taken, and the MS repeats the above
procedure.
If the cells with the 6 strongest average received level values are tried
but cannot be used, the call re-establishment attempt is abandoned.
bs_ag_blk_res
• To ensure that some of the blocks are always left clear for access
grant messages the parameter bs_ag_blk_res is used to input the
number of blocks to be reserved for this purpose.
•
The reserved blocks is not be used for paging whatever the demand.
•
If more than one timeslot exists within a cell, this parameter will
reserve the indicated number of blocks on each timeslot.
•
This parameter is broadcast on the BCCH.
•
This parameter is used to calculate the number of paging groups
available.
COMBINED
No
No
No
No
No
No
No
No
Yes
Yes
Yes
CCCH BLOCKS
9
9
9
9
9
9
9
9
3
3
3
AGCH BLOCKS
0
1
2
3
4
5
6
7
0
1
2
PCH BLOCKS
9
8
7
6
5
4
3
2
3
2
1
Bs_pa_mfrms
• Used to indicate the number of 51 frame multiframes between
transmission of paging messages to MS of the same group.
•
Is transmitted on BCCH.
•
Used by the MS to calculate its paging group.
8
16
24
32
8
7
15
23
31
7
6
14
22
30
6
1 = 3 multiframes
5
13
21
29
5
2 = 4 multiframes
4
12
20
28
4
3
11
19
27
3
2
10
18
26
2
1
9
17
25
1
AGCH
AGCH
AGCH
AGCH
AGCH
BCCH
BCCH
BCCH
BCCH
BCCH
Value
0 = 2 multiframes
3 = 5 multiframes
4 = 6 multiframes
5 = 7 multiframes
6 = 8 multiframes
7 = 9 multiframes
PAGING
Example
cch_conf = 0
bs_ag_blk_res = 1
bs_pa_mfrms = 2
If cch_conf = 1
minimum = 2
If cch_conf = 6
Maximum = 81 * 4
Min time between pages = 2 * 235.5 = 471ms
Max time between pages = 9 * 235.5 =2.1195 sec
max_retran
•
An MS requests resources from the network by transmitting an
``access burst´´ containing the channel request message.
•
For a single request, channel request will be repeated upto M +
1 times where M = max_retran.
max_retrans
1
2
3
4
M
1
2
4
7
tx_integer
•
To reduce the chances of collision the wait period is
randomised for each MS.
•
After the first channel request is sent the next is repeated after
a random wait period in the set
(S, S+1,….., S+T-1)
•
Wait period from this set is chosen randomly from this set.
TX INTEGER
RACH SLOTS
S FOR NONCOMB CCCH
S FOR COMB
CCCH
3, 8, 14
4, 9, 16
5, 10, 20
6, 11, 25
7, 12, 32
55
76
109
163
127
41
52
58
86
115
AVAILABLE PAGING BLOCKS ON 1 CCCH_GROUP
Maximum AGCH reservation for non-combined multiframe = 7
Available paging blocks = 2
Maximum AGCH reservation for combined multiframe = 1
Available paging blocks = 2
Minimum AGCH reservation for non-combined multiframe = 0
Available paging blocks = 9
Minimum AGCH reservation for combined multiframe = 0
Available paging blocks = 3
No of paging blocks will have a range of 2 - 9
CALCULATION OF CCCH AND PAGING GROUP NO
CCCH_GROUP = [ ( IMSI mod 1000) mod (BS_CC_CHANS * N ) ]
div N
Paging group no = [ ( IMSI mod 1000) mod (BS_CC_CHANS * N )
] mod N
HANDOVER
HANDOVER
HANDOVER
• The GSM handover process uses a mobile assisted technique for
accurate and fast handovers, in order to:
•
•
•
•
•
–
Maintain the user connection link quality.
–
Manage traffic distribution
The overall handover process is implemented in the MS,BSS &
MSC.
Measurement of radio subsystem downlink performance and signal
strengths received from surrounding cells, is made in the MS.
These measurements are sent to the BSS for assessment.
The BSS measures the uplink performance for the MS being served
and also assesses the signal strength of interference on its idle
traffic channels.
Initial assessment of the measurements in conjunction with defined
thresholds and handover strategy may be performed in the BSS.
Assessment requiring measurement results from other BSS or other
information resident in the MSC, may be perform. in the MSC.
HANDOVER
HANDOVER (Cont)
•
The MS assists the handover decision process by performing
certain measurements.
•
When the MS is engaged in a speech conversation, a portion of the
TDMA frame is idle while the rest of the frame is used for uplink
(BTS receive) and downlink (BTS transmit) timeslots.
•
During the idle time period of the frame, the MS changes radio
channel frequency and monitors and measures the signal level of
the six best neighbor cells.
•
Measurements which feed the handover decision algorithm are
made at both ends of the radio link.
HANDOVER
MS END
• At the MS end, measurements are continuously signalled, via the
associated control channel, to the BSS where the decision for
handover is ultimately made.
•
MS measurements include:
–Serving cell downlink quality (bit error rate (BER) estimate).
–Serving cell downlink received signal level, and six best neighbor
cells downlink received signal level.
•
The MS also decodes the Base Station ID Code (BSIC) from the
six best neighbor cells, and reports the BSICs and the
measurement information to the BSS.
HANDOVER
BTS END
•
The BTS measures the uplink link quality, received signal level,
and MS to BTS site distance.
•
The MS RF transmit output power budget is also considered in
the handover decision.
•
If the MS can be served by a neighbor cell at a lower power, the
handover is recommended.
•
From a system perspective, handover may be considered due to
loading or congestion conditions. In this case, the MSC or BSC
tries to balance channel usage among cells.
HANDOVER
MS IDLE TIME REPORTING
• During the conversation, the MS only transmits and receives for one
eighth of the time, that is during one timeslot in each frame.
•
During its idle time (the remaining seven timeslots), the MS switches
to the BCCH of the surrounding cells and measures its signal
strength.
•
The signal strength measurements of the surrounding cells, and the
signal strength and quality measurements of the serving cell, are
reported back to the serving cell via the SACCH once in every
SACCH multiframe.
•
This information is evaluated by the BSS for use in deciding when
the MS should be handed over to another traffic channel.
•
This reporting is the basis for MS assisted handovers.
HANDOVER
MEASUREMENT IN ACTIVE MODE
Downlink
0
1
Frame 24
2
3
4
1
1.
2.
3.
4.
5
6
7
2
0
Uplink
Frame 25
1
2
0
1
3
3
4
Frame 24
2
3
4
1
5
6
7
Idle Frame
5
6
7
0
1
2
2
0
1
2
3
4
Frame 0
5
6
7
0
1
1
4
3
4
Frame 25
5
6
7
0
2
1
2
3
4
Idle Frame
5
6
7
2
0
1
2
Frame 0
MS receives and measures signal strength on serving cell(TS2).
MS transmits
MS measures signsl strength for at least one neighbor cell.
MS reads BSIC on SCH for one of the 6 strongest neighbor.
HANDOVER
NUMBER OF NEIGHBORS
• Maximum 32 averaging of RSS takes place.
•
Practically a cell neighbors can be equipped for a cell.
•
If high numbers of neighbors are equipped, then the accuracy of
RSS is decreased as should have 8 to 10 neighbors.
T T T T T T T T T T T T S T T T T T T T T T T T T I
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
T T T T T T T T T T T T S T T T T T T T T T T T T I
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
T T T T T T T T T T T T S T T T T T T T T T T T T I
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
T T T T T T T T T T T T S T T T T T T T T T T T T I
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
HANDOVER
NUMBER OF NEIGHBORS
• In one SACCH multiframe there are 104 TDMA frames.
•
Out of this 104 frames 4 frames are idle and are used to decode the
BSIC.
•
Remaining 100 TDMA frames are used to measure RSS( Received
Signal Strength) of the neighbor.
•
If 25 neigbors are equipped, then in one SACCH multiframe each
neigbor is measured 100/25 = 4 times and averaged out. This
produces a less accurate value.
•
If 10 neigbors are equipped, then in one SACCH multiframe each
neigbor is measured 100/10 = 10 times and averaged out. This
produces a more accurate value.
HANDOVER
INTERFERENCE ON IDLE CHANNEL
• GSM causes its own time interference.
•
The MS has a omni-directional antenna. Much of the MS power goes
to the server but a lot is interfering with surrounding cells using the
same channel.
•
The TDMA frames of adjacent cell are not aligned since they are not
synchronised. Hence the uplink in the surrounding cell suffers from
interference.
Channel 10
Cell 1
Channel 10
Cell 2
HANDOVER
INTERFERENCE ON IDLE CHANNEL
• The BSS keeps on measuring the interference on the idle timeslots.
•
Ambient noise is measured and recorded 104 times in one SACCH
multiframe.
•
These measurements are averaged out to produce one figure.
•
The BSS then distributes the idle timeslots into band 0 to band 5.
•
Since the BSS knows the interference level on idle timeslots, it uses
this data to allocate the best channel first and the worst last.
Inteference on idle channel measured on Idle Timeslot by BSS
0
1
2
3
4
5
6
7
HANDOVER
HANDOVER
The following measurements is be continuously processed in the BSS :
i) Measurements reported by MS on SACCH
- Down link RXLEV
- Down link RXQUAL
- Down link neighbor cell RXLEV
ii) Measurements performed in BSS
- Uplink RXLEV
- Uplink RXQUAL
- MS-BS distance
- Interference level in unallocated time slots
Every SACCH multiframe (480 ms) a new processed value for each of
the measurements is calculated..
HANDOVER
HANDOVER CONDITIONS
Handover is done on five conditions
– Interference
– RXQUAL
– RXLEV
– Distance or Timing Advance
– Power Budget
Interference - If signal level is high and still there is RXQUAL problem,
then the RXQUAL problem is because of interference.
RXQUAL - It is the receive quality. It ranges from 0 to 7 , 0 being the best
and 7 the worst
RXLEV - It is the receive level. It varies from -47dBm to -110dBm.
Timing Advance - Ranges from 0 to 63.
Power budget - It is used to save the power of the MS.
HANDOVER
HANDOVER TYPES
Intra-Cell Handover
BSC
0
BTS
•
•
•
1
2
3
4
5
6
7
Call is handed
from timeslot 3 to timeslot 5
Handover takes place in the same cell from one timeslot to another
timeslot of the same carrier or different carriers( but the same cell).
Intra-cell handover is triggered only if the cause is interference.
Intra-cell handover can be enabled or disabled in a cell.
HANDOVER
HANDOVER TYPES
Intra-BSC Handover
BSC1
0
1
2
3
4
5
6
7
BTS1
Call is handed from timeslot 3
of cell1 to timeslot 1 of cell2 .
Both the cells are controlled
by the same BSC.
0
•
1
2
3
4
5
6
7
Handover takes place between different cell which are controlled by
the same BSC.
HANDOVER
HANDOVER TYPES
Inter-BSC Handover
BSS1
0
1
2
3
4
5
6
7
BTS1
Call is handed from timeslot
of cell1 to timeslot 1 of cell2
Both the cells are controlled
by the different BSC.
MSC
BSS2
0
1
2
3
4
5
6
7
BTS2
•
Handover takes place between different cell which are controlled by
the different BSC.
HANDOVER
HANDOVER TYPES
Inter-MSC Handover
MSC1
BSS1
0
1
2
3
4
5
6
7
BTS1
Call is handed from timeslot 3
of cell1 to timeslot 1 of cell2 .
Both the cells are controlled
by the different BSC, each BSC
being controlled by different MSC
MSC2
BSS2
0
1
2
3
4
5
6
7
BTS2
•
Handover takes place between different cell which are controlled by
the different BSC and each BSC is controlled by different MSC.
LOCATION UPDATE
LOCATION UPDATE
• MSC should always know the location of the MS so that it can
contact it by sending pages whenever required.
•
The mobile keeps on informing the MSC about its current location
area or whenever it changes from one LA to another.
•
This process of informing the MSC is known as location updating.
•
The new LA is updated in the VLR.
•
LAI = MCC + MNC + LAC
3 digits
1-2 digits
Max 16 bits
MCC
MNC
LAC
MCC = Mobile country code.
MNC = Mobile Network Code.
LAC = Location area code. Identifies a location area within a GSM
PLMN network. The maximum length of LAC is 16 bits. Thus 65536
different LA can be defined in one GSM PLMN.
LOCATION UPDATE TYPES
•
Normal location update
•
Periodic location update
•
IMSI attach
Normal Location Update
•
Mobile powers on and is idle.
•
Reads the LAI broadcast on the BCCH.
•
Compares with the last stored LAI and if it is different does a
location update.
LOCATION UPDATE
MS
BSS
MSC
RACH
Imme. Assign
Location update request
Authentication request
Authentication response
DTI<CICMD>
Cipher mode command
Cipher mode complete
DTI<CICMP>
Location update accepted
IMSI ATTACH
•
Saves the network from paging a MS which is not active in the system.
•
When MS is turned off or SIM is removed the MS sends a detach signal
to the Network. It is marked as detached.
•
When the MS is powered again it reads the current LAI and if it is same
does a location update type IMSI attach.
•
Attach/detach flag is broadcast on the BCCH sys info.
PERIODIC LOCATION UPDATE
•
Many times the MS enters non-coverage zone.
•
The MS will keep on paging the MS thus wasting precious resources.
•
To avoid this the MS has to inform the MSC about its current LAI in a set
period of time.
•
This time ranges from 0 to 255 decihours.
•
Periodic location timer value is broadcast on BCCH sys info messages.
DISCONTINOUS TRANSMISSION
•
During conversation user talks alternatively.
•
In DTX mode of operation the transmitter are switched on only
for frames containing useful information.
•
Helps to increase battery life and reduce interference level.
T T T T T T T T T T T T S T T T T T T T T T T T T I
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
T T T T T T T T T T T T S T T T T T T T T T T T T I
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
SID T T T T T T T T T T T T S T T T T T T T T T T T T I
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
T T T T T T T T T T T T S T T T T T T T T T T T T I
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
IMPLEMENTATION OF DTX
Voice Activity Detector ( VAD )
20 ms speech
block
VAD
Speech / No speech
•
Determines which specific block of 20ms from the speech coder
contains speech.
•
Removes stationary noise.
•
Inserts comfort noise.
•
The frames containing this background noise are called SID frames
and are sent in blocks of 8 frames within every 104 frame block.
SYSTEM INFORMATION
MESSAGES
SYS INFORMATION MESSAGES
BROADCAST MESSAGES
•
System information is data about the network which the MS
needs to be able to communicate with the network in a
appropriate manner.
•
System information messages are sent on the BCCH and
SACCH.
•
There are six different types of system information messages.
•
System information messages 1 to 4 are broadcast on the BCCH
and are read by the MS in idle mode.
•
System information message 5 and 6 are sent on the SACCH to
the MS in dedicated mode.
•
System information messages 1 to 4 are broadcast on the BCCH
in a cyclic mode over 8 BCCH multiframes, i.e. 8 * 51 frames.
•
Every message is sent at least after every 1.8 sec.
SYS INFORMATION MESSAGES
BROADCAST MESSAGES
System
Information
1
2
3
4
BCCH
Multiframe
0
1
2 and 6
3 and 7
What is sent is optional on BCCH Multiframe 4 and 5
•
System information 5 and 6 are sent on the SACCH immediately
after HO or whenever nothing else is being sent.
•
Downlink SACCH is used for system information messages while
Uplink SACCH is used for measurement reports.
SYS INFORMATION MESSAGES
SYSTEM INFORMATION 1
When frequency hopping is used in cell MS needs to know which
frequency band to use and what frequency within the band it should
use in hopping algorithm.
Cell Channel Description
Cell allocation number :- Informs the band number of the
frequency channels used.
00 - Band 0 ( Current GSM band )
Cell allocation ARFCN :- ARFCN’s used for hopping. It is coded
in a bitmap of 124 bits.
124 123 122 121
016 015 014 013 012 011 010 009
008 007 006 005 004 003 002 001
SYS INFORMATION MESSAGES
SYSTEM INFORMATION 1
RACH Control Parameters
Access Control Class :- Bitmap with 16 bits. All MS spread out on
class 0 - 9. Priority groups use class 11-15. A bit set to 1 barres
access for that class. Bit 10 is used to tell the MS if emergency call
is allowed or not.
0 - All MS can make emergency call.
1 - MS with class 11-15 only can make emergency calls.
Cell barred for access :0 - Yes
1 - No
SYS INFORMATION MESSAGES
SYSTEM INFORMATION 1
RACH Control Parameters
Re-establishment allowed :0 – Yes
1 - No
max_retransmissions :- Number of times the MS attempts to
access the Network [ 1,2,4 or 7 ].
tx_integer :- Number of slots to spread access retransmissions
when a MS attempts to access the system.
Emergency Call Allowed :- Yes / No
SYS INFORMATION MESSAGES
SYSTEM INFORMATION 2
• Contains list of BCCH frequencies used in neighbor cells.
•
MS uses this list to measures the signal strength of the neighbors.
Neighbor Cell Description
BA Indicator :- Allows to differentiate measurement results related
to different list of BCCH frequencies sent to the MS.
BCCH Allocation number :- Band 0 is used.
BCCH ARFCN number :- Bitmap 1 -124
1 = Set
0 = Not set
PLMN permitted
RACH Control Parameters
SYS INFORMATION MESSAGES
SYSTEM INFORMATION 3
Location Area Identity
8
7
6
5
4
MCC DIG 2
1
1
1
1
MNC DIG 2
3
2
1
MCC DIG 1
Octet A
MCC DIG 3
Octet B
MNC DIG 1
Octet C
LAC
Octet D
LAC
Octet E
BCD
Binary
Cell Identity
8
7
6
5
4
CI
CI
3
2
1
Octet F
Binary
Octet G
SYS INFORMATION MESSAGES
SYSTEM INFORMATION 3
Control Channel Description
Attach / Detach
0 = Allowed
1 = Not allowed
cch_conf :- Defines multiframe struture
cch_conf Physical Channels Combined No of CCH
0
1 timeslot (0)
NO
9
1
1 timeslot (0)
YES
3
2
2 timeslots (0, 2)
NO
18
4
3 timeslots (0, 2, 4)
NO
27
6
4 timeslots (0, 2, 4, 6)
NO
36
bs_agblk :- Number of block reserved for AGCH [ 0-7 ].
Ba_pmfrms :- Number of 51 frame multiframes between
transmisiion of paging messages to MS of the same group.
T3212 :- Periodic location update timer [ 1-255 deci hours].
SYS INFORMATION MESSAGES
SYSTEM INFORMATION 3
Cell Options
dtx
pwrc :- Power control on the downlink.
0 = Not used
1 = Used
Radio link timeout :- Sets the timer T100 in the MS.
Cell Selection Parameters
Rxlev_access_min :- Minimum received signal level at the MS for
which it is permitted to access the system.
0-63 = -110 dBm to -47dBm
mx_txpwr_cch :- Maximum power the MS will use when accessing
the system.
Cell_reselect_hysteresis :- Used for cell reselection.
RACH Control Parameters
SYS INFORMATION MESSAGES
SYSTEM INFORMATION 4
Location Area Identification
Cell Selection Parameters
Rxlev_access_min
mx_txpwr_cch
Cell_reselect_hysteresis
RACH Control Parameters
max_retransmissions
tx_integer
Cell barred for access
Re-establishment allowed
Emergency Call Allowed
Access Control Class
SYS INFORMATION MESSAGES
SYSTEM INFORMATION 4
Channel Description
Channel type :- Indi. channel type SDCCH or CBCH( SDCCH/8).
Subchannel number :- Indicates the subchannel.
Timeslot number :- Indicates the timeslot for CBCH [0 - 7].
Training Sequence Code :- The BCC part of BSIC[0 - 7 ].
Hopping Channel(H) :- Informs if CBCH channel is hopping or
single. 0 - Single RF Channel 1 - RF hopping channel
ARFCN :- If H = 0
MAIO :- If H = 1 , informs the MS where to start hopping. Values [0
- 63].
HSN :- If H = 1 , informs the MS in what order in what order the
hopping should take place. Values [ 0 - 63]. HSN = 0 Cyclic
Hopping.
MA :- Indicates which RF Channels are used for hopping. ARFCN
numbers coded in bitmap.
SYS INFORMATION MESSAGES
SYSTEM INFORMATION 5
Sent on the SACCH on the downlink to the MS in dedicated mode.
Neighbour Cell Description
BA-IND :- Used by the Network to discriminate measurements
results related to different lists of BCCH carriers sent by the MS(
Type 2 or 5).
Values 0 or 1 ( different from type 2).
BCCH Allocation number :- 00 - Band 0 (Current GSM band).
BCCH ARFCN :- Neighboring cells ARFCN’s. Sent as a bitmap.
0 = ARFCN not used
1 = ARFCN used
124 123 122 121
016 015 014 013 012 011 010 009
008 007 006 005 004 003 002 001
SYS INFORMATION MESSAGES
SYSTEM INFORMATION 6
• MS in dedicated mode needs to know if the LA has changed.
•
MS may change between cells with different Radio link timeout
and DTX.
Cell Identity
Location Area Identification
Cell Options
dtx
pwrc
Radio link timeout
PLMN permitted
SYS INFORMATION MESSAGES
PAGING
•
Whenever the Network wants to contact the MS, it sends
messages on the paging channel.
•
Paging is sent on the PCH and it occupies 4 bursts.
•
MS has to monitor the paging channel to receive paging
messages.
•
MS does not monitor all paging channel but only specific paging
channels.
•
There are three types of paging messages
Paging
Type
1
2
3
No of MS
using IMSI
2
1
-
No of MS
using TMSI
2
4
Total no of
MS
2
3
4
SYS INFORMATION MESSAGES
CALCULATION OF PAGING GROUP
Following factors are used for calculation of paging group
•
CCCH_group
–
cch_conf in System Information 3 defines the number of
CCCH used in the cell.
–
CCCH can be allocated only TN 0, 2, 4, 6.
–
Each CCCH carries its own paging group of MS.
–
MS will listen to paging messages of its specific group.
•
bs_pa_mfrms
•
bs_ag_blk_res
SYS INFORMATION MESSAGES
CALCULATION OF PAGING GROUP
Total number of paging groups on 1 CCCH_GROUP(N)
No of paging groups N = Paging blocks * Repitition of paging blocks
= [ CCCH - bs_ag_blk_res ] * bs_pa_mfrms
Range of Paging Groups on 1 CCCH_Group
Minimum available Paging Groups = Min pag blocks * min bs_pa_mfrms
=2*2
=4
Maximum available Paging Groups = Max pag blocks * max bs_pa_mfrms
=9*9
= 81
SYS INFORMATION MESSAGES
AVAILABLE PAGING BLOCKS ON 1 CCCH_GROUP
Maximum AGCH reservation for non-combined multiframe = 7
Available paging blocks = 2
Maximum AGCH reservation for combined multiframe = 1
Available paging blocks = 2
Minimum AGCH reservation for non-combined multiframe = 0
Available paging blocks = 9
Minimum AGCH reservation for combined multiframe = 0
Available paging blocks = 3
No of paging blocks will have a range of 2 - 9
SYS INFORMATION MESSAGES
CALCULATION OF CCCH AND PAGING GROUP NO
CCCH_GROUP = [ ( IMSI mod 1000) mod (BS_CC_CHANS * N ) ]
div N
Paging group no = [ ( IMSI mod 1000) mod (BS_CC_CHANS * N )
] mod N
SOME KEY DATABASE
PARAMETERS
TIMER T3101
•
The MS requests for resources by sending channel request on
RACH.
•
The BSS allocates a SDCCH, if available, and sends a IMMEDIATE
ASSIGNMENT message on the downlink. This message contains the
details of allocated SDCCH, TSC , TA etc.
•
As soon as the BSS allocates and sends message on the AGCH, it
starts a timer T3101.
•
The MS logs on the SDCCH and sends a message on the UPLINK.
•
As soon as the BSS receives this message the timer T3101 is
stopped.
•
If no message is received by the BSS from the MS and timer T3101
expires, then the BSS releases the allocated SDCCH resource.
•
This timer is set in millisecs.
TIMER T3101
Successful SDCCH Access
MS
CELL
MS
CELL
RACH
RACH
IMMEDIATE ASSIGNMENT
(AGCH)
Unsuccessful SDCCH Access
SDCCH ALLOCATED
START TIMER T3101
IMMEDIATE ASSIGNMENT
(AGCH)
SDCCH ALLOCATED
START TIMER T3101
CL2I
IF MS SENDS CL2I ON
SDCCH STOP TIMER
T3101
IF T3101 EXPIRES AND
BSS DOES NOT RECEIVE
CL2I ON SDCCH RELEASE
ALLOCATED RESOURCES
Wait_indication parameter & Timer T3122
•
The MS requests for resources by sending channel request on RACH.
•
The BSS allocates a SDCCH, if available, and sends a IMMEDIATE
ASSIGNMENT message on the downlink. This message contains the
details of allocated SDCCH, TSC , TA etc.
•
If no SDCCH is available, the BSS sends a IMMEDIATE
ASSIGNMENT REJECT message to the MS.
•
As soon as the MS receives the IMMEDIATE ASSIGNMENT REJECT
message, it starts a timer T3122 and sets it equal to
wait_parameter_indication.
•
Till the timer T3122 is running, no channel request leaves the MS.
•
The next channel request is sent only after the expiry of T3122.
•
This wait_indication_parameter can be set from 0 to 255 secs.
Wait_indication parameter & Timer T3122
MS
CELL
RACH
NO SDCCH AVAILABLE
SET T3122 IN MS
EQUAL TO
WAIT_INDICATION
IMMEDIATE ASSIGNMENT
REJECT (AGCH)
INACTIVE MODE
IF T3122 EXPIRES,
MS CAN NOW SEND
A FRESH REQUEST
RACH
CALL QUEING
•
The MS requests for resources by sending a request to the BSS on
RACH.
•
The BSS assigns a SDCCH and the call setup takes place on the
SDCCH.
•
It is possible that after the call setup, their may not be a TCH available
for the MS due to congestion..
Assig. Req
The Network is able to place this
Queuing Indication
call in a queue along with other
Start T11
MS awaiting assignment of a
TCH.
Assign. Compl / Fail
•
•
The length of the queue can be
set in the database.
•
The time for which a MS can
stay in the queue can be set by
setting the value of T11 timer.
Stop T11
T11 expires. Clr Req.
TCH RESOURCE REPORTING
•
The number of dedicated traffic channel in use within a BSS may be
useful information for the MSC.
•
The number of TCH currently allocated can be indicated with a BSSMAP
REOURCE INDICATION MESSAGE.
•
The message can be sent to the MSC in different modes.
•
In one-off mode, the MSC asks for a report and the BSS generates one.
•
In the periodic mode, the MSC tells the BSS that it wants reports at
regular interval.
•
In spontaneous mode, the report is generated in response to TCH
demand.
TCH RESOURCE REPORTING
100% TCH Usage
No of TCH free
LWM
Resource Indication to MSC
No Handover request entertained
HWM
Traffic Usage
Resource Indication to MSC
Handover request entertained
T3109
•
The MS and BSS monitor the appearance of SACCH messages.
•
If an uplink failure occurs and the threshold of lost SACCH messages
is reached, the BSS will activate timer T3109.
•
The BSS will stop sending SACCH messages to the MS.
•
The MS will not receive any SACCH messages and hence T100 will
expire.
•
When T100 expires the MS will return to ideal mode.
•
The BSS will release the channel resources once T3109 expires.
•
For the downlink failure same pattern is followed except that T100
expires first.
•
T3109 should be set higher than T100 ensuring that the system
holds on to the radio link long enough for MS to release it first.
Otherwise it will be possible to have two MS on the same TCH.
T3109 LADDER DIAGRAM
MS
BSS
SACCH
SACCH NOT DECODED
LINK_FAIL EXCEEDED
DEACTIVATE D/L SACCH
START TIMER T3109
START TIMER T100
SACCH DEACTIVATED ON D/L
TIMER T3109 STILL RUNNING
T100 EXPIRES. RELEASE
RADIO RESOURCES
TIMER T3109 EXPIRES
BSS RELEASES RADIO RESOURCE
T3110 AND T3111 - Normal Release
•
The system initiates the release of a channel by sending a channel
release message to the MS and will start timer T3109.
•
SACCH messages on the downlink are disconnected.
•
On the receipt of the channel release, the MS starts internal timer
T3110 and disconnects the main signaling link.
•
When T3110 times out or when the signaling is disconnect confirmed,
the MS deactivates all RF links and returns to BCCH idle mode.
•
When the BSS receives disconnect message, it stops timer T3109
and starts T3111.
•
When T3111 has expired all RF links are terminated.
•
If T3109 times out all RF channels are deactivated and are then free
to be allocated.
•
T3109 should be greater than T3110.
T3110 AND T3111 - Normal Release
Channel Release
Start timer T3109
Start timer T3110
Disconnect
Stop timer T3109
Start timer T3111
Stop T3110 on the
receipt of UA
UA
T3111 expired
Release Radio
Resources
FREQUENCY HOPPING
INTRODUCTION TO FREQUENCY HOPPING
f1
f4
Modulated
RF signal
f3
f2
Voice
Information is
transmitted
by different frequencies
at
different timeslot
f1
Introduction to Frequency Hopping
•
Can be used to improve the quality of the network
•
Also can be used to increase the capacity of the Network thereby
reducing the number of sites required for CAPACITY.
The way it works
•
Each burst is transmitted on a different frequency
•
Both mobile and base station follow the same hopping sequence
Introduction to Frequency Hopping
•
Fading
– Causes quality deterioration
– Is frequency dependent
•
FH diversifies the impact of fading and improves quality.
•
The immunity to fading increases by exploiting its frequency
selectivity, because using different frequencies the probability of being
continuously affected by fading is reduced, so the transmission link
quality is improved.
• This improvement is much more noticeable for slow moving mobiles.
•
Increased Immunity to fading
• In a cellular urban environment in most cases multipath propagation
will be present and, as a consequence of that, important short term
variations in the received level are frequent . This is called Rayleigh
fading which results in quality degradation because some of the
information will be corrupted.
•
For a fast moving mobile, the fading situation can be avoided from
one burst to another because it also depends on the position of the
mobile so the problem is not so serious.
•
For a stationary one the reception may be permanently affected
resulting in a very bad quality, even a drop call.
•
Once the information is received by the mobile or the base station, the
only way to cope with the disturbance produced by the fading (errors
in the information bits) are the decoding and deinterleaving
processes, with an effectiveness limited by the number of errors they
have to deal with.
Increased Immunity to fading
• Interference
•
A result of frequency reuse & irregular terrain and sites
•
FH diversifies the impact of interference and improves quality
•
The situation of permanent interference coming from neighbour cells
transmitting the same or adjacent frequencies is avoided using
Frequency Hopping because the calls will spend the time moving
through different frequencies not equally affected by interfering
signals. This effect is called Interference Averaging.
Increased Immunity to fading
• Considering a non hopping system, the set of calls on the interferer
cells which can interfere with the wanted call is fixed for the duration of
those calls and some calls will be found with very good quality (no
interference problems) whereas some others with very bad quality
(permanent interference problems).
•
With hopping, that set of interfering calls will be continually changing
and the effect is that calls tend to experience an average quality rather
than extreme situations of either good or bad quality (all the calls will
suffer from a controlled interference but only for short and distant
periods of time, not for all the duration of the call).
•
This interference averaging means again spreading the raw bit errors
(BER caused by the interference) in order to have a random distribution
of them instead of bursts of errors, and therefore enhance the
effectiveness of decoding and deinterleaving processes to cope with the
BER and lead to a better value of FER.
TYPES OF HOPPING
Base Band Hopping (BBH)
•
The radio units transmit always the same frequency
•
Number of frequencies for hopping = Number of carriers
•
The radio units are always transmitting a fixed frequency and frequency
hopping is performed by moving the information for every call among
the available radio units in a cell on a per burst basis.
•
In reception the call is always processed by the same radio unit (the
one where the call started).
•
The number of frequencies to hop over is limited by the number of radio
units equipped in the cell.
•
The BCCH carrier can hop in timeslots 1 to 7 (without power
control/DTX).
TYPES OF HOPPING
Synthesiser Frequency Hopping (SFH)
•
The radio units change (retune) the frequency every burst.
•
The call always stays in the same radio unit.
•
Number of frequencies for hopping > Number of carriers.
•
The radio units can hop over a range of different frequencies( 64 in
case of Motorola).
•
Hybrid combiners are required in the base station (Cavity Combiners
can not be used with SFH).
•
The BCCH carrier can never hop.
Hopping Parameters
For frequency hopping operability, GSM defines the following set of
parameters:
•
Mobile Allocation (MA): Set of frequencies the mobile is allowed to hop
over. MA is a subset of all the frequencies allocated by the system
operator to the cell (cell allocation) although it can be the same. Eg:- If
the operator has frequencies from 1 -32, then he can use 1-15 for BCCH
and 17-32 for hopping ( MA).
•
Hopping Sequence Number (HSN): Determines the hopping order used
in the cell. 64 different HSNs can be assigned, where HSN = 0 provides a
cyclic hopping sequence and HSN = 1 to 63 provide various
pseudorandom hopping sequences.
•
Mobile Allocation Index Offset (MAIO): Determines inside the hopping
sequence which frequency the mobile starts to transmit on.
•
Frequency Hopping Indicator (FHI): Defines a hopping system made up
by an associated set of frequencies (MA) to hop over and a hopping
sequence (HSN).
INTRODUCTION
TO
RF PLANNING
INTRODUCTION TO RF PLANNING
• Designing a cellular system - particularly one that incorporates both
Macrocellular and Microcellular networks is a delicate balancing
exercise.
•
The goal is to achieve optimum use of resources and maximum
revenue potential whilst maintaining a high level of system quality.
•
Full consideration must also be given to cost and spectrum allocation
limitations.
•
A properly planned system should allow capacity to be added
economically when traffic demand increases.
•
As every urban environment is different, so is every macrocell and
microcell network. Hence informed and accurate planning is essential
in order to ensure that the system will provide both the increased
capacity and the improvement in network quality where required,
especially when deploying Microcellular systems.
•
RF planning plays a critical role in the Cellular design process.
•
By doing a proper RF Planning by keeping the future growth plan in
mind we can reduce a lot of problems that we may encounter in the
future and also reduce substantially the cost of optimization.
•
On the other hand a poorly planned network not only leads to many
Network problems , it also increases the optimization costs and still
may not ensure the desired quality.
INTRODUCTION TO RF
PLANNING
TOOLS USED FOR RF PLANNING
•
Network Planning Tool
•
CW Propagation Tool
•
Traffic Modeling Tool
•
Project Management Tool
INTRODUCTION TO RF
PLANNING
Network Planning Tool
•
Planning tool is used to assist engineers in designing and optimizing
wireless networks by providing an accurate and reliable prediction of
coverage, doing frequency planning automatically, creating neighbor
lists etc.
•
With a database that takes into account data such as terrain, clutter,
and antenna radiation patterns, as well as an intuitive graphical
interface, the Planning tool gives RF engineers a state-of-the-art tool to:
– Design wireless networks
– Plan network expansions
– Optimize network performance
– Diagnose system problems
•
The major tools available in the market are Planet, Pegasos, Cell Cad.
•
Also many vendors have developed Planning tools of their own like
Netplan by Motorola, TEMS by Ericsson and so on.
INTRODUCTION TO RF
PLANNING
Network Planning Tool (PLANET)
INTRODUCTION TO RF
PLANNING
Propagaton Test Kit
•
•
•
The propagation test kit consists of
– Test transmitter.
– Antenna ( generally Omni ).
– Receiver to scan the RSS (Received signal levels). The receiver
scanning rate should be settable so that it satisfies Lee’s law.
– A laptop to collect data.
– A GPS to get latitude and longitude.
– Cables and accessories.
– Wattmeter to check VSWR.
A single frequency is transmitted a predetermined power level from the
canditate site.
These transmitted power levels are then measured and collected by the
Drive test kit. This data is then loaded on the Planning tool and used for
tuning models.
INTRODUCTION TO RF
PLANNING
Propagaton Test Kit
INTRODUCTION TO RF
PLANNING
Traffic Modeling Tool
•
Traffic modelling tool is used by the planning engineer for Network
modelling and dimensioning.
•
It helps the planning engineer to calculate the number of network
elements needed to fulfil coverage, capacity and quality needs.
•
Netdim by Nokia is an example of a Traffic modelling tool.
INTRODUCTION TO RF
PLANNING
Project Management Tool
•
Though not directly linked to RF Design Planning, it helps in scheduling
the RF Design process and also to know the status of the project
•
Site database : This includes RF data, site acquisition,power, civil ,etc.
•
Inventory Control
•
Fault tracking
•
Finance Management
RF PLANNING
PROCEDURES
PRELIMINARY WORK
Propagation tool setup
 Set up the planning tool hardware. This includes the server
and or clients which may be UNIX based.
 Setup the plotter and printer to be used.
Terrain, Clutter, Vector data acquisition and setup
 Procure the terrain, clutter and vector data in the required
resolution.
 Setup these data on the planning tool.
 Test to see if they are displayed properly and printed correctly
on the plotter.
PRELIMINARY WORK
Setup site tracking database
 This is done using Project management or site management
databases.
 This is the central database which is used by all relevant
department, viz. RF, Site acquisition, Power, Civil engineering etc,
and avoids data mismatch.
Load master lease site locations in database
 If predetermined friendly sites that can be used are available,
then load this data into the site database.
PRELIMINARY WORK
Marketing Analysis and GOS determination
 Marketing analysis is mostly done by the customer.
 Growth plan is provided which lists the projected subscriber
growth in phases.
 GOS is determined in agreement with the customer (generally the
GOS is taken as 2%)
 Based on the marketing analysis, GOS and number of carriers as
inputs, the network design is carried out.
Zoning Analysis
 This involves studying the height restrictions for antenna heights
in the design area.
PRELIMINARY WORK
Set Initial Link Budget
 Link Budget Analysis is the process of analyzing all major gains
and losses in the forward and reverse link radio paths.
 Inputs
 Base station & mobile receiver sensitivity parameters
 Antenna gain at the base station & mobile station.
 Hardware losses(Cable, connector, combiners etc).
 Target coverage reliabilty.
 Fade margins.
 Output
 Maximum allowable path loss.
PRELIMINARY WORK
Initial cell radius calculation
 Using link budget calculation, the maximum allowable path loss is
calculated.
 Using Okumura hata emprical formula, the initial cell radius can
be calculated.
Initial cell count estimates
 Once the cell radius is known, the area covered by one site can
be easily calculated.
 By dividing the total area to be covered by the area of each cell, a
initial estimate of the number of cells can be made.
INITIAL SURVEY
Morphology Definition
 Morphology describes the density and height of man made or
natural obstructions.
 Morphology is used to more accurately predict the path loss.
 Some morphology area definitions are Urban, Suburban, rural,
open etc.
 Density also applies to morphology definitions like dense urban,
light suburban, commercial etc.
 This basically leads to a number of sub-area formation where the
link budget will differ and hence the cell radius and cell count will
differ.
Morphology Drive Test
 This drive test is done to prepare generic models for network
design.
 Drive test is done to characterize the propagation and fading
effects.
 The objective is to collect field data to optimize or adjust the
prediction model for preliminary simulations.
 A test transmitter and a receiver is used for this purpose.
 The received signals are typically sampled ( around 50 samples
in 40 ).
Propagation Tool Adjustment
 The data collected by drive testing is used to prepare generic
models.
 For a given network design there may be more than one model
like dense-urban, urban, suburban, rural, highway etc.
 The predicted and measured signal strengths are compared and
the model adjusted to produce minimum error.
 These models are then used for initial design of the network.
INITIAL DESIGN
Complete Initial Cell Placement
 Planning of cell sites sub-area depending on clutter type and
traffic required.
Run Propagation Analysis
 Using generic models prepared by drive testing & prop test, run
predictions for each cell depending on morphology type to predict
the coverage in the given sub-areas.
 Planning tool calculates the path loss and received signal
strength using Co-ordinates of the site location, Ground elevation
above mean sea level, Antenna height above ground, Antenna
radiation pattern (vertical & horizontal) & antenna orientation,
Power radiated from the antenna.
INITIAL DESIGN
Reset Cell Placement( Ideal Sites)
 According to the predictions change the cell placements to design
the network for contigious coverage and appropriate traffic.
System Coverage Maps
 Prepare presentations as follows
 Background on paper showing area MAP which include
highways, main roads etc.
 Phase 1 sites layout on transparency.
 Phase 1 sites composite coverage prediction.
 Phase 2 sites layout transparency.
 Phase 2 composite coverage prediction on transparency.
 If more phases follow the same procedure.
INITIAL DESIGN
Design Review With The Client
 Initial design review has to be carried out with the client so that he
agrees to the basic design of the network.
 During design review, first put only the background map which is
on paper. Then step by step put the site layout and coverage
prediction.
 Display may show some coverage holes in phase 1 which should
get solved in phase 2 .
SELECTION OF SITES
Prepare Initial Search Ring
 Note the latitude and longitude from planning tool.
 Get the address of the area from mapping software.
 Release the search ring with details like radius of search ring,
height of antenna etc.
Release search rings to project management
Visit friendly site locations
 If there are friendly sites available that can be used (infrastructure
sharing), then these sites are to be given preference.
 If these sites suite the design requirements, then visit these sites
first.
SELECTION OF SITES
Select Initial Anchor Sites
 Initial anchor sites are the sites which are very important for the
network buildup, Eg - Sites that will also work as a BSC.
Enter Data In Propagation Tool
 Enter the sites exact location in the planning tool.
Perform Propagation Analysis
 Now since the site has been selected and the lat/lon of the actual
site ( which will be different from the designed site) is known, put
this site in the planning tool and predict coverage.
 Check to see that the coverage objectives are met as per
prediction.
SELECTION OF SITES
Reset / Review Search Rings
 If the prediction shows a coverage hole ( as the actual site may
be shifted from the designed site), the surrounding search rings
can be resetted and reviewed.
Candidate site Visit( Average 3 per ring)
 For each proposed location, surveys should carried out and at
least 3 suitable site candidates identified.
 Details of each candidate should be recorded on a copy of the
Site Proposal Form for that site. Details must include:
» Site name and option letter Site location (Lat./Long)
» Building Height
» Site address and contact number
» Height of surrounding clutter
» Details of potential coverage effecting obstructions or
other comments(A, B, C,...)
SELECTION OF SITES
Drive Test And Review Best Candidate
 In order to verify that a candidate site, selected based on its
predicted coverage area, is actually covering all objective areas,
drive test has to be performed.
 Drive test also points to potential interference problems or
handover problems for the site.
 The test transmitter has to be placed at the selected location with
all parameters that have been determined based on simulations.
 Drive test all major roads and critical areas like convention
centers, major business areas, roads etc.
 Take a plot of the data and check for sufficient signal strength,
sufficient overlaps and splashes( least inteference to other cells).
SELECTION OF SITES
Drive Test Integration
 The data obtained from the drive test has to be loaded on the
planning tool and overlapped with the prediction. This gives a
idea of how close the prediction and actual drive test data match.
 If they do not match ( say 80 to 90 %) then for that site the model
may need tuning.
Visit Site With All Disciplines( SA, Power, Civil etc )
 A meeting at the selected site takes place in which all concerned
departments like RF Engineering, Site acquisition, Power, Civil
Engineer, Civil contractor and the site owner is present.
 Any objections are taken care off at this point itself.
Select Equipment Type For Site
 Select equipment for the cell depending on channel requirements
 Selection of antenna type and accessories.
Locate Equipment On Site For Construction Drawing
 Plan of the building ( if site located on the building) to be made
showing equipment placement, cable runs, battery backup
placement and antenna mounting positions.
 Antenna mounting positions to be shown separately and clearly.
 Drawings to be checked and signed by the Planner, site
acquisition, power planner and project manager.
Perform Link Balance Calculations
 Link balance calculation per cell to be done to balance the uplink
and the downlink path.
 Basically link balance calculation is the same as power budget
calculation. The only difference is that on a per cell basis the
transmit power of the BTS may be increased or decreased
depending on the pathloss on uplink and downlink.
EMI Studies
 Study of RF Radiation exposure to ensure that it is within limits
and control of hazardous areas.
 Data sheet to be prepared per cell signed by RF Planner and
project manager to be submitted to the appropriate authority.
Radio Frequency Plan/ PN Plan
 Frequency planning has to be carried out on the planning tool
based on required C/I and C/A and interference probabilities.
System Interference Plots
 C/I, C/A, Best server plots etc has to be plotted.
 These plots have to be reviewed with the customer to get the
frequency plan passed.
Final Coverage Plot
 This presentation should be the same as design review
presentation.
 This plot is with exact locations of the site in the network.
Identification of coverage holes
 Coverage holes can be identified from the plots and subsequent
action can be taken(like putting a new site) to solve the problem.
RADIO WAVE
PROPAGATION
BASIC DEFINITIONS
Isotropic RF Source
 A point source that radiates RF energy uniformly in all directions
(I.e.: in the shape of a sphere)
 Theoretical only: does not physically exist.
 Has a power gain of unity I.e. 0dBi.
Effective Radiated Power (ERP)
 Has a power gain of unity i.e. 0dBi
 The radiated power from a half-wave dipole.
 A lossless half-wave dipole antenna has a power gain of 0dBd or
2.15dBi.
Effective Isotropic Radiated Power (EIRP)
 The radiated power from an isotropic source
EIRP = ERP + 2.15 dB
BASIC DEFINITIONS
• Radio signals travel through space at the Speed of Light
C = 3 * 108 meters / second
• Frequency (F) is the number of waves per second (unit: Hertz)
• Wavelength () (length of one wave) = (distance traveled in one second)
(waves in one second)
= C / F
If frequency is 900MHZ then
wavelength  = 3 * 108
900 * 106
= 0.333 meters
BASIC DEFINITIONS
dB
• dB is a a relative unit of measurement used to describe power gain or
loss.
• The dB value is calculated by taking the log of the ratio of the measured
or calculated power (P2) with respect to a reference power (P1). This
result is then multiplied by 10 to obtain the value in dB.
dB = 10 * log10(P1/P2)
• The powers P1 ad P2 must be in the same units. If the units are not
compatible, then they should be transformed.
• Equal power corresponds to 0dB.
• A factor of 2 corresponds to 3dB
If P1 = 30W and P2 = 15 W then
10 * log10(P1/P2) = 10 * 10 * log10(30/15)
=2
BASIC DEFINITIONS
dBm
• The most common "defined reference" use of the decibel is the dBm, or
decibel relative to one milliwatt.
• It is different from the dB because it uses the same specific, measurable
power level as a reference in all cases, whereas the dB is relative to
either whatever reference a particular user chooses or to no reference at
all.
• A dB has no particular defined reference while a dBm is referenced to a
specific quantity: the milliwatt (1/1000 of a watt).
• The IEEE definition of dBm is "a unit for expression of power level in
decibels with reference to a power of 1 milliwatt."
• The dBm is merely an expression of power present in a circuit relative to
a known fixed amount (i.e., 1 milliwatt) and the circuit impedance is
irrelevant.}
BASIC DEFINITIONS
dBm
•
dBm = 10 log (P) (1000 mW/watt)
where dBm = Power in dB referenced to 1 milliwatt
P = Power in watts
•
If power level is 1 milliwatt:
Power(dBm) = 10 log (0.001 watt) (1000 mW/watt)
= 10 log (1)
= 10 (0)
=0
•
Thus a power level of 1 milliwatt is 0 dBm.
•
If the power level is 1 watt
1 watt Power in dBm = 10 log (1 watt) (1000 mW/watt)
= 10 (3)
= 30
BASIC DEFINITIONS
dBm
•
dBm = 10 log (P) (1000 mW/watt)
where dBm = Power in dB referenced to 1 milliwatt
P = Power in watts
•
If power level is 1 milliwatt:
Power(dBm) = 10 log (0.001 watt) (1000 mW/watt)
= 10 log (1)
= 10 (0)
=0
•
Thus a power level of 1 milliwatt is 0 dBm.
•
If the power level is 1 watt
1 watt Power in dBm = 10 log (1 watt) (1000 mW/watt)
= 10 (3)
= 30
BASIC DEFINITIONS
dBm
•
dBm = 10 log (P) (1000 mW/watt)
•
The dBm can also be negative value.
•
If power level is 1 microwatt
Power in dBm = 10 log (1 x 10E-6 watt) (1000 mW/watt)
= -30 dBm
•
•
Since the dBm has a defined reference it can be converted back to
watts if desired.
Since it is in logarithmic form it may also be conveniently combined
with other dB terms.
BASIC DEFINITIONS
dBv/m
•
To convert field strength in dbv/m to received power in dBm with a
50 optimum terminal impedance and effective length of a half wave
dipole /
0dBu = 10 log[(10-6)2(1000)(/)2/(4*50)] dBm
At 850MHZ
0dBu = -132 dBm
39dBu = -93 dBm
FREE SPACE PROPAGATION
•
Friis Formula
Pr = Pt GtGr2
(4d)2
Pt
Lp
Pr
d
•
•
Gt
Gr
Propagation Loss
Lp = 10log [4d / ]2
The square term is the propagation exponent. It is greater than 2
when obstructions exist.
Propagation Loss in dB:
L p = 32.44 + 20Log(d) +20Log(f)
f = MHz
d = km
PROPAGATION MECHANISMS
Reflection
•
Occurs when a wave impinges upon a smooth surface.
•
Dimensions of the surface are large relative to .
•
Reflections occur from the surface of the earth and from buildings and
walls.
Diffraction (Shadowing)
•
Occurs when the path is blocked by an object with large dimensions
relative to  and sharp irregularities (edges).
•
Secondary “wavelets” propagate into the shadowed region.
•
Diffraction gives rise to bending of waves around the obstacle.
Scattering
•
Occurs when a wave impinges upon an object with dimensions on the
order of  or less, causing the reflected energy to spread out
or“scatter” in many directions.
•
Small objects such as street lights, signs, & leaves cause scattering
MULTIPATH
• Multiple Waves Create “Multipath”
• Due to propagation mechanisms, multiple waves arrive at the
receiver
• Sometimes this includes a direct Line-of-Sight (LOS) signal
MULTIPATH
Multipath Propagation
•
Multipath propagation causes large and rapid fluctuations in a signal
•
These fluctuations are not the same as the propagation path loss.
Multipath causes three major things
•
Rapid changes in signal strength over a short distance or time.
•
Random frequency modulation due to Doppler Shifts on different
multipath signals.
•
Time dispersion caused by multipath delays
•
These are called “fading effects
•
Multipath propagation results in small-scale fading.
WHAT IS FADING ?
•
The communication between the base station and mobile station in
mobile systems is mostly non-LOS.
•
The LOS path between the transmitter and the receiver is affected by
terrain and obstructed by buildings and other objects.
•
The mobile station is also moving in different directions at different
speeds.
•
The RF signal from the transmitter is scattered by reflection and
diffraction and reaches the receiver through many non-LOS paths.
•
This non-LOS path causes long-term and short term fluctuations in
the form of log-normal fading and rayleigh and rician fading, which
degrades the performance of the RF channel.
Signal Power (dBm)
WHAT IS FADING ?
Large scale fading component
Small scale fading
component
LONG TERM FADING
•
Terrain configuration & man made environment causes long-term
fading.
•
Due to various shadowing and terrain effects the signal level
measured on a circle around base station shows some random
fluctuations around the mean value of received signal strength.
•
The long-term fades in signal strength, r, caused by
configuration and man made environments form a
distribution, i.e the mean received signal strength, r,
normally in dB if the signal strength is measured over a
at least 40.
•
Experimentally it has been determined that the standard deviation, ,
of the mean received signal strength, r, lies between 8 to 12 dB with
the higher  generally found in large urban areas.
the terrain
log-normal
varies logdistance of
RAYLEIGH FADING
• This phenomenon is due to multipath propagation of the signal.
• The Rayleigh fading is applicable to obstructed propagation paths.
• All the signals are NLOS signals and there is no dominant direct path.
• Signals from all paths have comparable signal strengths.
• The instantaneous received power seen by a moving antenna becomes
a random variable depending on the location of the antenna.
RICEAN FADING
• This phenomenon is due to multipath propagation of the signal.
• In this case there is a partially scattered field.
• One dominant signal.
• Others are weaker.
DIVERSITY ANTENNA
SYSTEMS
Diversity Antenna Systems
NEED OF DIVERSITY
Building
Building
Building
Diversity Antenna Systems
NEED OF DIVERSITY
•
In a typical cellular radio environment, the communication between the
cell site and mobile is not by a direct radio path but via many paths.
•
The direct path between the transmitter and the receiver is obstructed
by buildings and other objects.
•
Hence the signal that arrives at the receiver is either by reflection from
the flat sides of buildings or by diffraction around man made or natural
obstructions.
•
When various incoming radiowaves arrive at the receiver antenna,
they combine constructively or destructively, which leads to a rapid
variation in signal strength.
•
The signal fluctuations are known as ‘multipath fading’.
Diversity Antenna Systems
Multipath Propagation
•
Multipath propagation causes large and rapid fluctuations in a signal
•
These fluctuations are not the same as the propagation path loss.
Multipath causes three major things
•
Rapid changes in signal strength over a short distance or time.
•
Random frequency modulation due to Doppler Shifts on different
multipath signals.
•
Time dispersion caused by multipath delays
•
These are called “fading effects
•
Multipath propagation results in small-scale fading.
Diversity Antenna Systems
DIVERSITY TECHNIQUE
•
Diversity techniques have been recognised as an effective means
which enhances the immunity of the communication system to the
multipath fading. GSM therefore extensively adopts diversity
techniques that include
Diversity techniques
Interleaving
In time domain
Frequency Hopping
In Frequency domain
Spatial diversity
In spatial domain
Polarisation diversity
In polarisation domain
Diversity Antenna Systems
CONCEPT OF DIVERSITY ANTENNA SYSTEMS
•
Spatial and polarisation diversity techniques are realised through
antenna systems.
•
A diversity antenna system provides a number of receiving branches
or ports from which the diversified signals are derived and fed to a
receiver. The receiver then combines the incoming signals from the
branches to produce a combined signal with improved quality in
terms of signal strength or signal-to-noise ratio (S/N).
•
The performance of a diversity antenna system primarily relies on
the branch correlation and signal level difference between branches.
Diversity Antenna Systems
CONCEPT OF DIVERSITY ANTENNA SYSTEMS
Fade
Transmission
media 1
Information
Receiver
Transmission
Tmedia 2
Peak
Diversity Antenna Systems
Combining
Combined signal
fed to receiver
Signal 2
Signal 1
Signal Strength
Combined signal
Signal 1
Signal 2
Time
Diversity Antenna Systems
SPATIAL DIVERSITY ANTENNA SYSTEMS
•
The spatial diversity antenna system is constructed by physically
separating two receiving base station antennas.
•
Once they are separated far enough, both antennas receive
independent fading signals. As a result, the signals captured by the
antennas are most likely uncorrelated.
•
The further apart are the antennas, the more likely that the signals
are uncorrelated.
•
The types of the configuration used in GSM networks are:
 horizontal separation
 vertical separation
 composite separation.
Diversity Antenna Systems
TYPICAL SPATIAL ANTENNA DIVERSITY CONFIGURATIONS
Horizontal Separation
Vertical Separation
Diversity Antenna Systems
THREE ANTENNA SPATIAL CONFIGURATION
10 Separation
Receive 1
Transmit
Receive 2
Diversity Antenna Systems
TWO ANTENNA SPATIAL CONFIGURATION
10 Separation
Tx Rx
Duplexer
Receive 2
Transmit
Receive 1
Diversity Antenna Systems
POLARISATION DIVERSITY ANTENNA SYSTEMS
•
A single (say vertical) polarised electromagnetic wave is converted to
a wave with two orthogonal polarised fields while it is propagating
through scattering environment.
•
It has also been found that the two fields exhibit some extent of
decorrelation.
Diversity Antenna Systems
DUAL POLARISED ANTENNAS
• A dual-polarisation antenna consists of two sets of radiating elements
which radiate or, in reciprocal, receive two orthogonal polarised
fields.
• The antenna has two input connectors which separately connects to
each set of the elements.
• The antenna has therefore the ability to simultaneously transmit and
receive two orthogonally polarised fields.
H/V
Slant 45
Diversity Antenna Systems
ADVANTAGES OF DUAL POLARISED ANTENNAS
•
The best advantage of using the dual polarisation antenna is the
reduction in the number of antennas per sector.
•
Reduced size of the headframe of the supporting structure
•
Reduced windload and weight.
•
Reduced difficulty in site acquisition and installation.
•
Cost saving
– Requiring slim tower
– Requiring less installation time.
– Cost of one dual polarisation antenna is generally lower than that
of two
– Single polarised antennas
Diversity Antenna Systems
T R
TX RX
RX
RX
SINGLE POLE ANTENNA
DUAL POLE ANTENNA
DUAL POLE ANTENNA
DUAL POLE ANTENNA
DUAL POLARISED ANTENNA CONFIGURATIONS
RX
TX
T R
T R
TX RX
TX RX
INTERFERENCE
WHAT IS INTERFERNCE ?
•
Interference is the sum of all signal contributions that are neither noise
not the wanted signal.
EFFECTS OF INTERFERENCE
• Interference is a major limiting factor in the performance of cellular
systems.
•
It causes degradation of signal quality.
•
It introduces bit errors in the received signal.
•
Bit errors are partly recoverable by means of channel coding and error
correction mechanisms.
•
The interference situation is not reciprocal in the uplink and downlink
direction.
•
Mobile stations and base stations are exposed to different interference
situation.
SOURCES OF INTERFERENCE
•
Another mobile in the same cell.
•
A call in progress in the neighboring cell.
•
Other base stations operating on the same frequency.
•
Any non-cellular system which leaks energy into the cellular frequency
band.
TYPES OF INTERFERENCE
• There are two types of system generated interference
– Co-channel interference
– Adjacent channel interference
Co-Channel Interference
•
This type of interference is the due to frequency reuse , i.e. several
cells use the same set of frequency.
•
These cells are called co-channel cells.
•
Co-channel interference cannot be combated by increasing the power
of the transmitter. This is because an increase in carrier transmit power
increases the interference to neighboring co-channel cells.
•
To reduce co-channel interference, co-channel cells must be physically
separated by a minimum distance to provide sufficient isolation due to
propagation or reduce the footprint of the cell.
Co-Channel Interference
•
Some factors other then reuse distance that influence co-channel
interference are antenna type, directionality, height, site position etc,
•
GSM specifies C/I > 9dB.
Carrier f1
dB
Interferer f1
C
I
Distance
Co-Channel Interference
D
C1
C3
•
C1
C2
C3
C2
In a cellular system, when the size of each cell is approximately the
same, co-channel interference is independent of the transmitted power
and becomes a function of cell radius(R) and the distance to the centre
of the nearest co-channel cell (D).
Co-Channel Interference
•
Q = D / R = 3N
•
By increasing the ratio of D/R, the spatial seperation between the cochannel cells relative to the coverage distance of a cell is increased. In
this way interference is reduced from improved isolation of RF energy
from the co-channel cell.
•
The parameter Q , called the co-channel reuse ratio, is related to the
cluster size.
•
A small value of Q provides larger capacity since the cluster size N is
small whereas a large value of Q improves the transmission quality.
Adjacent-Channel Interference
•
Interference resulting from signals which are adjacent in frequency to
the desired signal is called adjacent channel interference.
•
Adjacent channel interference results from imperfect receiver filters
which allow nearby frequencies to leak into the passband.
•
Adjacent channel interference can be minimized through careful
filtering and channel assignments.
•
By keeping the frequency separation between each channel in a given
cell as large as possible , the adjacent interference may be reduced
considerably.
Adjacent-Channel Interference
Carrier f1
dB
Interferer f2
A
C
Distance
COUNTERING INTERFERENCE
POWER CONTROL
•
•
•
•
•
•
•
RF power control is employed to minimise the transmit power required
by MS or BS while maintaining the quality of the radio links.
By minimising the transmit power levels, interference to co-channel
users is reduced.
Power control is implemented in the MS as well as the BSS.
Power control on the Uplink also helps to increase the battery life.
Power received by the MS is continously sent in the measurement
report.
Similarly uplink power received from the MS by the BTS is measured
by the BTS.
Complex algorithm evaluate this measurements and take a decision
subsequently reducing or increasing the power in the Uplink or the
downlink.
COUNTERING INTERFERENCE
SECTORIZATION
•
For 120 degrees sectored site as compared to an omni site almost
1/3rd interference is received in the uplink.
•
The more selective and directional is the antenna, the smaller is the
interference.
Reduction in interference results in higher capacity in both links.
•
REPEATERS
REPEATERS
INTRODUCTION
Donor side antenna
Repeater receives
Donor signal at
~ -90dBm
Mobile side antenna
Repeater amplifies
the signal and
rebroadcasts the
signal
Donor Cell
Poor Coverage area
• Repeater units are designed to receive signals from a donor site,
amplify and rebroadcast the donor sites signals into poor coverage
areas or to extend the coverage range of the donor site.
• These repeater are bi-directional and do not translate frequency and
subsequently are limited in output power and gain.
• Repeaters provide between 50 to 80 dB of gain.
REPEATERS
INTRODUCTION
• There are two types of repeater band selective and channel
selective.
• Band selective repeater amplifies a band of frequency. Hence it
amplifies any frequency that falls within its band.
• Channel selective repeater allows selection of a number of
individual channels to amplify and rebroadcast.
• Typically a channel selective repeater allows selection of 2 to 4
channels.
• If the GSM900 or DCS1800 network incorporates frequency
hopping, then only band selective repeaters should be used.
Other Networks
TRANSMISSION
SYSTEMS
Introduction To Transmission Systems
•
Transmission systems form the backbone of any networks.
•
Normally transmission systems
Microwaves, leased lines.
•
In GSM normally the core network is located in the same premises
and are mostly interconnected by fixed wireline. In huge network
consisting of many MSC located at different places the
interconnection may be through any of the transmission systems
mentioned above.
•
The Access network consists of BSC’s with many BTS’s connected
to them in various transmission topologies. Normal practice is to
connect various BSC’c to the MSC via fiber and different BTS’s
connected to BSC via microwave in Daisy chain, star or any other
topology. However there can be many different ways of
implementation.
include
SDH,
PDH,
ATM,
E1
•
2.048 Mbps circuit provides high speed, digital transmission for
voice, data, and video signals at 2.048 Mbps.
•
2.048 Mbps transmission systems are based on the ITU-T
specifications G.703, G.732 and G.704, and are predominant in
Europe, Australia, Africa, South America, and regions of Asia.
•
The primary use of the 2.048 Mbps is in conjunction with
multiplexers for the transmission of multiple low speed voice and
data signals over one communication path rather then over multiple
paths.
•
The most common line code used to transmit the 2.048 Mbps
signal is known as HDB3 (High Density Bipolar 3) which is a
bipolar code with a specific zero suppression scheme where no
more then three consecutive zeros are allowed to occur.
Typical implementation of a E1
The 2.048 Mbps Framing Format
•
The 2.048 Mbps signal typically consists of multiplexed data and/or
voice which requires a framing structure for receiving equipment to
properly associate the appropriate bits in the incoming signal with
their corresponding channels.
•
The 2.048 Mbps frame is broken up into 32 timeslots numbered 031.
•
Each timeslot contains 8 bits in a frame, and since there are 8000
frames per second, each time slot corresponds to a bandwidth of 8
x 8000 = 64 kbps.
•
Time slot 0 is allocated entirely to the frame alignment signal (FAS)
pattern, a remote alarm (FAS Distant Alarm) indication bit, and
other spare bits for international and national use.
Framing Format
E1
•
The FAS pattern (0011011) takes up 7 bits (bits 2-8) in timeslot 0 of
every other frame.
•
In those frames not containing the FAS pattern, bit 3 is reserved for
remote alarm indication (FAS Distant Alarm) which indicates loss of
frame alignment when it is set to 1.
•
The remaining bits in timeslot 0 are allocated as shown in the
following Figure.
•
If the 2.048 Mbps signal carries no voice channels, there is no
need to allocate additional bandwidth to accommodate signaling.
•
Hence, time slot 1-31 are available to transmit data with an
aggregate bandwidth of 2.048 Mbps - 64 kbps (TSO) = 1.984
Mbps.
E1
TS 16 Multiframe Format
E1
•
If there are voice channels on the 2.048 Mbps signal, it is necessary
to take up additional bandwidth to transmit the signalling
information.
•
ITU-T Recommendation G.704 allocates time slot 16 for the
transmission of the channel-associated signalling information.
•
The 2.048 Mbps can carry up to thirty 64 kbps voice channels in
time slot 1-15 and 17-31.
•
Voice channels are numbered 1-30; voice channels 16-30 are
carried in time slot 17-31.
•
However, the 8 bits in time slot 16 are not sufficient for all 30
channels to signal in one frame. Therefore, a multiframe structure is
required where channels can take turns using time slot 16.
E1
•
Since two channels can send their ABCD signalling bits in each
frame, a total of 15 frames are required to cycle through all of the 30
voice channels.
•
One additional frame is required to transmit the multiframe
alignment signal (MFAS) pattern, which allows receiving equipment
to align the appropriate ABCD signalling bits with their
corresponding voice channels.
•
This results in the TS-16 multiframe structure where each
multiframe contains a total of 16 2.048 Mbps, numbered 0-15.
•
Figure on the previous slide shows the TS-16 multiframe format for
the 2.048 Mbps signal as defined by the ITU-T Recommendation
G.704.
E1
•
As can be seen in Figure , time slot 16 of frame 0 contains the 4-bit
long multiframe alignment signal (MFAS) pattern (0000) in bits 1-4.
The “Y” bit is reserved for the remote alarm (MFAS Distant Alarm)
which indicates loss of multiframe alignment when it is set to 1.
•
Time slot 16 of frames 1-15 contains the ABCD signalling bits of the
voice channels.
•
Time slot 16 of the nth frame carries the signalling bits of the nth and
(n+15)th voice channels. For example, frame 1 carries the signalling
bits of voice channels 1 and 16, frame 2 carries the signalling bits of
channels 2 and 17 etc.
•
It is also important to note that the frame alignment signal (FAS) is
transmitted in time slot 0 of the even numbered frames.
T1 Introduction
•
T1 is a digital communications link that enables the transmission
of voice, data, and video signals at the rate of 1.544 million bit per
second (Mb/s).
•
Introduced in the 1960s, it was initially used by telephone
companies who wished to reduce the number of telephone cables
in large metropolitan areas.
•
T1 simplifies the task of networking different types of
communications equipment since it can carrz both voice and data
on the same link.
T1 Introduction
•
To illustrate, Figure 1 on the next page shows what a company’s
communications network might look like without T1
•
Figure 1 shows that telephone, facsimile,
applications would all require separate lines.
•
Typically, voice and low-speed data applications are serviced by
analog lines, while high-speed data applications are serviced by
digital facilities.
•
Figure 2 on the next page depicts the same network with a T1 link
installed.
and
computer
T1
FIGURE 1
T1
FIGURE 2
PDH Overview
•
Long-established analog transmission systems that proved
inadequate were gradually replaced by digital communications
networks.
•
In many countries, digital transmission networks were developed
based upon standards collectively known today as the
Plesiochronous Digital Hierarchy (PDH).
•
Although it has numerous advantages over analog, PDH has
some shortcomings: provisioning circuits can be labor-intensive
and time-consuming, automation and centralized control
capabilities of telecommunication networks are limited, and
upgrading to emerging services can be cumbersome.
•
A major disadvantage is that standards exist for electrical line
interfaces at PDH rates, but there is no standard for optical line
equipment at any PDH rate, which is specific to each
manufacturer.
PDH Overview
•
This means that fiber optic transmission equipment from one
manufacturer may not be able to interface with other
manufacturers’ equipment.
•
As a result, service providers are often required to select a
single vendor for deployment in areas of the network, and are
locked into using the network control and monitoring
capabilities of that vendor.
•
Reconfiguring PDH networks can be difficult and labor-intensive
- resulting in costly, time-consuming modifications to the
network whenever new services are introduced or when more
bandwidth is required.
SDH Overview
•
Bellcore (the research affiliate of the Bell operating companies in
the United States) proposed a new transmission hierarchy in 1985.
•
Bellcore’s major goal was to create a synchronous system with an
optical interface compatible with multiple vendors, but the
standardization also included a flexible frame structure capable of
handling either existing or new signals and also numerous facilities
built into the signal overhead for embedded operations,
administration, maintenance and provisioning (OAM&P) purposes.
•
The new transmission hierarchy was named Synchronous Optical
Network (SONET).
•
The International Telecommunication Union (ITU) established an
international standard based on the SONET specifications, known
as the Synchronous Digital Hierarchy (SDH), in 1988.
SDH Overview
•
The SDH specifications define optical interfaces that allow
transmission of lower-rate (e.g., PDH) signals at a common
synchronous rate.
•
A benefit of SDH is that it allows multiple vendors’ optical
transmission equipment to be compatible in the same span.
•
SDH also enables dynamic drop-and-insert capabilities on the
payload; PDH operators would have to demultiplex and
remultiplex the higher-rate signal, causing delays and requiring
additional hardware.
•
Since the overhead is relatively independent of the payload,
SDH easily integrates new services, such as Asynchronous
Transfer Mode (ATM) and Fiber Distributed Data Interface
(FDDI), along with existing European 2, 34, and 140 Mbit/s
PDH signals, and North American 1.5, 6.3, and 45 Mbit/s
signals.
SDH Overview STM1 data rate calculation
•
SDH multiplexing combines low-speed digital signals such as 2,
34, and 140 Mbit/s signals with required overhead to form a frame
called Synchronous Transport Module at level one (STM-1).
•
Figure 1 shows the STM-1 frame, which is created by 9 segments
of 270 bytes each.
•
The first 9 bytes of each segment carry overhead information; the
remaining 261 bytes carry payload.
•
When visualized as a block, the STM-1 frame appears as 9 rows
by 270 columns of bytes.
•
The STM-1 frame is transmitted row #1 first, with the most
significant bit (MSB) of each byte transmitted first.
SDH Overview STM1 data rate calculation
•
This formula calculates the bit rate of a framed digital signal:
•
bit rate = frame rate x frame capacity
•
In order for SDH to easily integrate existing digital services into its
hierarchy, it operates at the basic rate of 8 kHz, or 125
microseconds per frame, so the frame rate is 8,000 frames per
second.
•
The frame capacity of a signal is the number of bits contained within
a single frame.
•
Figure 2 shows: frame capacity = 270 bytes/row x 9 rows/frame x 8
bits/byte = 19,440 bits/frame
•
The bit rate of the STM-1 signal is calculated as follows:
bit rate = 8,000 frames/second x 19,440 bits/frame = 155.52 Mbit/s
SDH Overview
FRAME PERIOD 125 MICRO SEC.
1
2
3
OVERHEAD
PAYLOAD
9 BITS
261 BITS
4
5
FIGURE 1
6
7
8
9
SDH Overview
1
2
OVERHEAD
3
4
5
PAYLOAD
1
2
3
4
5
6
7
8
9
270 BITS
FIGURE 2
6
7
8
9
SDH Overview Multiplexing of STM frames
•
As the Figure coming on the next slide shows, the ITU has
specified that an STM-4 signal should be created by byte
interleaving four STM-1 signals.
•
The basic frame rate remains 8,000 frames per second, but the
capacity is quadrupled, resulting in a bit rate of 4 x 155.52
Mbit/s, or 622.08 Mbit/s.
•
The STM-4 signal can then be further multiplexed with three
additional STM-4s to form an STM-16 signal.
•
Table 1 lists the defined SDH frame formats, their bit rates, and
the maximum number of 64 kbit/s telephony channels that can
be carried at each rate.
SDH Overview Multiplexing of STM frames
STM1 A
STM1 B
4:1
STM1 C
STM1 D
STM4
SDH Overview Multiplexing of STM frames
FRAME
FORMAT
STM - 1
STM - 4
STM - 16
BIT
RATE
155.52 Mbits/S
622.08 M bits/S
2.488 Gbits/S
MAX NUMBER OF
TELEPHONY CHANNELS
1920
7680
30720
Microwave Overview
•
Normally used for point to point transmission
•
Used mainly in the GHz range.
•
Normally distance between radios is less than 50Kms.