Channels on 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 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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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 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 halfrate 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 used a 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
broadcast 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 types of Dedicated Control
Channels in GSM.
• like Traffic Channels they are bidirectional.
• 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 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 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 subchannels corresponding to single
mobile stations. Each mobile station will then
only 'listen' to its subchannel and will stay in
the sleep mode during the other subchannels
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.