GSM Channels & Air Interface
1
The GSM radio interface
 The radio interface is the interface between
the mobile stations and the fixed
infrastructure. It is one of the most
important interfaces of the GSM system.
 One of the main objectives of GSM is
roaming. Therefore, in order to obtain a
complete compatibility between mobile
stations and networks of different
manufacturers and operators, the radio
interface must be completely defined.
--- The spectrum efficiency depends on the
radio interface and the transmission, more
particularly in aspects such as the capacity
of the system and the techniques used in
order to decrease the interference and to
improve the frequency reuse scheme.
 The specification of the radio interface has
then an important influence on the
spectrum efficiency.
Frequency allocation
 Two frequency bands, of 25 MHz each one,
have been allocated for the GSM system:
 The band 890-915 MHz has been allocated
for the uplink direction (transmitting from
the mobile station to the base station).
 The band 935-960 MHz has been allocated
for the downlink direction (transmitting
from the base station to the mobile
station).
GSM Band
GSM Bands in Pakistan (From Frequency Allocation Board,
Pakistan)
Definition of Channels
Logical Channel
Type of information to be transmitted e.g., traffic or control logical channels.
Transport Channel
How and with what format data is transmitted through physical links.
Physical Channel
Unit of radio resource of a radio system e.g., frequency band, time slot, code, etc.
RF Channel
Fixed frequency band of a radio system.


The MAC (media access control) sub-layer is responsible for mapping logical
channels onto transport channels.
The physical layer is responsible for mapping transport channels onto physical
Channels.
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What is MAC?
 Short for Media Access Control address,
a hardware address that uniquely identifies
each node of a network. In IEEE 802 networks,
the Data Link Control (DLC) layer of the OSI
Reference Model is divided into two sub-layers:
theLogical Link Control (LLC) layer and the Media
Access Control (MAC) layer.The MAC layer interfaces
directly with the network medium. Consequently, each
different type of network medium requires a different
MAC layer.
 On networks that do not conform to the IEEE 802
standards but do conform to the OSI Reference
Model, the node address is called the Data Link
Control (DLC) address.
GSM Physical Channel
 GSM physical channel relates to the recurrence of
one burst in every frame.
 This channel is characterized by both its
frequency and its position within the TDMA
frame.
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GSM Physical Channel Cntd....
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GSM Frame Structure Summary
.
GSM Logical Channel
 The logical channels consist of the information
carried over the physical channel.
 There are two major categories of GSM Logical
channels
1. Traffic channels
2. Control channels.
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GSM Physical Channel (in detail)
 A single GSM Carrier is divided into eight
timeslots.
 Therefore can support up to eight MS subscribers
simultaneously.
 The timeslots are arranged in sequence and are
conventionally numbered 0 to 7.
 Each repetition of this sequence is called a
“TDMA frame”.
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GSM Physical Channel Cntd....
BURST:







The information carried in one timeslot is called a “burst”.
The timing of the burst transmissions to and from the
mobiles is critical.
Each time slot of a TDMA frame lasts for duration of
156.25 bit periods or
576.9 µsec or 0.576 ms
so a frame takes 4.615 ms
GSM’s data transmission rate is 270.83 kbps per carrier
frequency.
Therefore one bit duration is 3.692 µsec
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Burst structure
 As it has been stated before, the burst is the unit in
time of a TDMA system. Four different types of bursts
can be distinguished in GSM:
 The frequency-correction burst is used on the FCCH.
It has the same length as the normal burst but a
different structure.
 The synchronization burst is used on the SCH. It has
the same length as the normal burst but a different
structure.
 The random access burst is used on the RACH and is
shorter than the normal burst.
 The normal burst is used to carry speech or data
information. It lasts approximately 0.577 ms and has
a length of 156.25 bits.
TB
Normál burst (NB)
000
Data
Training seq.
58 bit
26 bit
TB
Frequency Correction Burst (FB)
Synchronization burst (SB)
Access burst (AB)or RACH
Data
Sync. seq.
39 bit
TB
Sync. seq.
000
000
Data
64 bit
41 bit
TB
GP
8.25 bit time
TB
142 bit
000
000
58 bit
000...0
000
TB
Data
39 bit
Data
36 bit
GP
8.25 bit time
TB
000
GP
8.25 bit time
TB
GP
000
68.25 bit time
GSM Logical Channels
 There are two main groups of logical channels,
traffic channels and control channels.
Run over a physical channel, but not necessarily in all
its time slots
Have to be managed: set up, maintenance, tear down.
Traffic & Control channels are further classified into
groups.
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GSM Traffic Channels
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Traffic Channels
 A traffic channel (TCH) is used to carry speech and
data traffic.
 TCHs for the uplink and downlink are separated in
time by 3 burst periods.
 Therefore MS does not have to transmit & receive
simultaneously, thus simplifying the electronics.
 In addition to these full-rate TCHs, there are also
half-rate TCHs defined.
 Half-rate TCHs will effectively double the capacity of
a system at the cost of voice quality.
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Traffic Channels Cntd....
 Traffic channels can carry either
1. Speech or
2. Data
 Speech channels are supported by two different
methods of coding known as
1. Full Rate (FR)
2. Enhanced Full Rate (EFR)
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Traffic Channels Cntd....
 Enhanced Full Rate coding provides a speech
service that has improved voice quality from the
original Full Rate speech coding.
 EFR employs a new speech coding algorithm
and additions to the full rate channel coding
algorithm to accomplish this improved speech
service.
 it is only supported by Phase 2+ mobiles
onwards.
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Traffic Channels Cntd....
Full Rate TCH
Half Rate
TCH
TCH/HR
TRAFFIC
CHANNELS
TCH/FR
TCH/F 9.6
TCH/F 4.8
TCH/F 2.4
TCH/H4.8
TCH/H2.4
Full Rate Traffic Channels
Half Rate Traffic Channels
Name
Type
Data Rate
NAME
Type
Data Rate
TCH/FR
Speech
22.8 kbps
TCH/HR
Speech
11.4 kbps
TCH/F9.6
Data
22.8 kbps
TCH/H9.6
Data
11.4 kbps
TCH/F4.8
Data
22.8 kbps
TCH/H4.8
Data
11.4 kbps
TCH/F2.4
Data
22.8 kbps
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GSM Control Channels
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GSM CONTROL CHANNELS (CCH)
 There are three main control channels in the
GSM system
1. Broadcast Channel (BCH)
2. Common Control Channel (CCCH)
3. Dedicated Control Channel (DCCH)
 Each control channel consists of several logical
channels having different Control Functions.
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GSM CONTROL CHANNELS (CCH)
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Broadcast Channels (BCH)
 The broadcast channel operates on the forward
link of a specific ARFCNs
 It transmits data only in (TS 0) of certain
ARFCNs. Other TSs are available for TCHs.
 The BCH provides synchronization for all
mobiles within the cell.
 It is also monitored by mobiles in neighboring
cells so that the received power and MAHO
decisions can be made by out-of-cell users.
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i) BROADCAST CONTROL CHANNEL (BCCH)




BCCH is a forward control channel.
information such as cell and network identity.
The BCCH is transmitted by the BTS at all times.
The RF carrier used to transmit the BCCH is
referred to as the BCCH carrier.
 Information on BCCH is monitored by the MS
periodically (at least every 30 secs), when switched
on & not in a call.
 The BCCH is transmitted at constant power at all
times, and its signal strength is measured by all MS
which may seek to use it.
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BROADCAST CONTROL CHANNEL Cntd….
 BCCH Carries the following information (this is only a partial
list):
1.
2.
3.
4.
5.
6.
7.
8.
9.
Location Area Identity (LAI).
List of neighboring cells which should be monitored by the
MS.
List of frequencies used in the cell.
Cell identity.
Power control indicator.
DTX permitted.
Access control (for example, emergency calls, call barring).
CBCH description.
List of Channels currently in use within a cell.
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ii) FREQUENCY CORRECTION CHANNEL
(FCCH)
 FCCH is a forward control channel.
 It is transmitted on same ARFCN i.e. of BCCH
 The FCCH allows each subscriber unit to
synchronize its internal frequency standard
(local oscillator) to exact frequency of the base
station
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iii) SYNCHRONIZATION CHANNEL (SCH)
 It’s also a Downlink Channel.
 SCH allows each mobile to frame synchronize with the base
station.
 It transmits two Important Information
1.
2.
Frame number.
Base Site Identity Code (BSIC).
 The frame number (FN) ranges from 0 to 2715647.
 The BSIC is uniquely assigned to each BTS in a GSM
system.
 The BS issues course timing advancement command to the
mobile station over the SCH
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COMMON CONTROL CHANNEL (CCCH)
 The common control channels occupy TS 0 of
every GSM frame that is not otherwise used by
the BCH.
 CCCH consist of three different channels.
These channels are described in following
slides
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i) PAGING CHANNEL (PCH)
 It exists only on downlink (Forward channel).
 The paging channel (PCH) provides paging signals to
all mobiles.
 notifies a specific mobile of an incoming call which
originates from the PSTN.
 The PCH transmit the IMSI of the target subscriber,
along with a request for acknowledgment from the
mobile unit.
 the PCH is also used to provide cell broadcast ASCII
text messages to all subscribers, as part of the SMS
feature of GSM
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ii) RANDOM ACCESS CHANNEL (RACH)





The RACH is the only reverse link (uplink) channel.
MS acknowledges a page from the PCH on RACH.
RACH is also used by mobiles to originate a call.
The RACH uses slotted ALOHA access scheme.
At the BTS, every frame (even the idle frame) will
accept RACH transmissions from mobiles during
TS 0
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iii) ACCESS GRANT CHANNEL (AGCH).
 AGCH is used by the BS to provide forward link
communication.
 The AGCH is used by the BS to respond to a
RACH sent by a mobile station.
 It carries data for MS to operate in a particular
physical channel (time slot and ARFCN).
 The AGCH is the final CCCH message sent by
the base station before a subscriber is moved off
the control channel.
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iv) CELL BROADCAST CHANNEL (CBCH)
 CBCH is used to transmit messages to be
broadcasted to all MSs within a cell.
 it is considered a common channel because the
messages can be received by all mobiles in the
cell.
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DEDICATED CONTROL CHANNELS
(DCCH)
 There are three (03) types of Dedicated Control
Channels in GSM.
 like Traffic Channels they are bi-directional.
 Same format and function in both the forward
and reverse links.
 DCCHs may exist in any time slot and any
ARFCN except TS 0 of the BCH ARFCN.
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i) STAND-ALONE DEDICATED
COTROL CHANNEL (SDCCH)
 The SDCCH carries signaling data following the
connection of the MS with the BTS just before a
TCH assignment.
 The SDCCH ensures the Connection b/w MS
and BS during the verification of subscriber unit
& allocation of resources for the MS.
 It is a dedicated point-to-point signaling channel
which is not tied to the existence of a TCH
(stand-alone),
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i) STAND-ALONE DEDICATED
COTROL CHANNEL (SDCCH)……
 The SDCCH is requested from the MS via the
RACH and assigned via the AGCH.
 The SDCCH can be thought of as an
intermediate and temporary channel.
 A SDCCH may also be used for
1.
2.
3.
4.
5.
call setup,
Authentication
location updating
SMS point to point
e-Fax
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ii) SLOW-ASSOCITED CONTROL CHANNEL
(SACCH)
 The SACCH is always associated with a TCH or a
SDCCH and maps onto the same physical channel.
 Each ARFCN systematically carries SACCH data
for all of its current users.
 On the downlink, SACCH sends slow but regularly
changing control information to the mobile station.
1.
transmit power level instruction.
2.
and specific timing advance instruction.
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ii) SLOW-ASSOCITED CONTROL CHANNEL
(SACCH)…..
 On reverse channel SACCH sends
1. received signal strength (form Serving BTS).
2. quality of the TCH.
3. BCH measurement results (from neighboring
cells)
 The SACCH is transmitted during the 13th frame
of every speech dedicated control channel
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iii) FAST-ASSOCIATED CONTROL CHANNEL
(FACCH)
 FACCH carries urgent messages, and contains
essentially the same type of information as the
SDCCH.
 A FACCH is assigned whenever a SDCCH has
not been dedicated for a particular user and there
is an urgent message (e.g. handoff request).
 The FACCH gains access to a time slot by
“stealing” frames from the traffic channel to
which it is assigned
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GSM Basic Call Sequence
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Call Setup Procedure
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Speech coding
The transmission of speech is, at the moment, 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 criteria:



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 RPE-LTP
(Regular Pulse Excitation Long-Term Prediction). 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.
Discontinuous transmission (DTX)
 This is another aspect of GSM that could have been
included as one of the requirements of the GSM
speech codec. The function of the DTX is to suspend
the radio transmission during the silence periods. This
can become quite interesting if we take into
consideration the fact that a person speaks less than
40 or 50 percent during a conversation. The DTX
helps then to reduce interference between different
cells and to increase the capacity of the system. It
also extends the life of a mobile's battery.
The DTX function is performed
thanks to two main features:

The Voice Activity Detection (VAD), which has to determine
whether the sound represents speech or noise, even if the
background noise is very important. If the voice signal is
considered as noise, the transmitter is turned off producing
then, an unpleasant effect called clipping.

The comfort noise. An inconvenient of the DTX function is
that when the signal is considered as noise, the transmitter
is turned off and therefore, a total silence is heard at the
receiver. This can be very annoying to the user at the
reception because it seems that the connection is dead. In
order to overcome this problem, the receiver creates a
minimum of background noise called comfort noise. The
comfort noise eliminates the impression that the connection
is dead.
Timing advance
 The timing of the bursts transmissions is very
important. Mobiles are at different distances from the
base stations. Their delay depends, consequently, on
their distance.
 The aim of the timing advance is that the signals
coming from the different mobile stations arrive to the
base station at the right time. The base station
measures the timing delay of the mobile stations.
 If the bursts corresponding to a mobile station arrive
too late and overlap with other bursts, the base
station tells, this mobile, to advance the transmission
of its bursts.
Power control
 At the same time the base stations perform the timing
measurements, they also perform measurements on
the power level of the different mobile stations. These
power levels are adjusted so that the power is nearly
the same for each burst.
 A base station also controls its power level. The
mobile station measures the strength and the quality
of the signal between itself and the base station. If
the mobile station does not receive correctly the
signal, the base station changes its power level.
Discontinuous reception
 It is a method used to conserve the
mobile station's power.
 The paging channel is divided into
sub-channels corresponding to single
mobile stations.
 Each mobile station will then only
'listen' to its sub-channel and will stay
in the sleep mode during the other
sub-channels of the paging channel.
Multipath and Equalization
 At the GSM frequency bands, radio waves reflect from
buildings, cars, hills, etc. So not only the 'right' signal
(the output signal of the emitter) is received by an
antenna, but also many reflected signals, which corrupt
the information, with different phases.
 An equalizer is in charge of extracting the 'right' signal
from the received signal. It estimates the channel
impulse response of the GSM system and then
constructs an inverse filter. The receiver knows which
training sequence it must wait for. The equalizer will
then comparing the received training sequence with the
training sequence it was expecting, compute the
coefficients of the channel impulse response. In order
to extract the 'right' signal, the received signal is
passed through the inverse filter.