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.

7

What is MAC?

  Short for Media Access Control address, a hardware address that uniquely identifies each node of a network . In the Reference Model IEEE Data Link Control (DLC) layer 802 networks, of the OSI is divided into two sub-layers: the

Logical 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.

9

GSM Physical Channel Cntd....

10

 .

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.

2.

Traffic channels Control channels.

12

GSM Physical Channel (in detail)

  Therefore can support up to eight MS

subscribers

simultaneously.

 A single GSM Carrier is

divided

into

eight

timeslots.

The timeslots are arranged

in sequence

conventionally numbered 0 to 7.

and are  Each repetition of this sequence is called a “

TDMA frame

”.

13

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

frequency.

Therefore one

bit

duration is

3.692 µsec

per carrier 14

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 It has the same length as the normal burst but a different structure. The synchronization burst is used on the FCCH. the same length as the normal burst but a different structure. The random access burst is used on the SCH. It has shorter than the normal burst. The normal burst is used on the RACH and is is used to carry speech or data information. It lasts approximately 0.577 ms and has a length of 156.25 bits.

Normál burst (NB) TB 000 Data 58 bit Training seq.

26 bit Data 58 bit TB 000 GP 8.25 bit time Frequency Correction Burst (FB) Synchronization burst (SB) TB 000 TB 000 Data 39 bit 000...0

142 bit Sync. seq.

64 bit TB 000 GP 8.25 bit time Data 39 bit TB 000 GP 8.25 bit time Access burst (AB)or RACH TB 000 Sync. seq.

41 bit Data 36 bit TB 000 GP 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.

18

GSM Traffic Channels

19

Traffic Channels

   Therefore MS does not have to transmit & receive

simultaneously

, thus simplifying the

electronics

.  A traffic channel (TCH) is used to carry

speech data

traffic. and TCHs for the uplink and downlink are

separated

time by

3 burst periods

.

in In addition to these

half-rate full-rate

TCHs defined.

TCHs, there are also  Half-rate TCHs will effectively

double

a system at the cost of

voice quality

. the capacity of 20

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.

2.

Full Rate (FR) Enhanced Full Rate (EFR)

21

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 onwards.

Phase 2+

mobiles 22

Traffic Channels Cntd....

Full Rate TCH TCH/FR TCH/F 9.6

TCH/F 4.8

TCH/F 2.4

Full Rate Traffic Channels TRAFFIC CHANNELS Half Rate TCH/H4.8

TCH/H2.4

Half Rate Traffic Channels Name Type Data Rate NAME Type Data Rate TCH/FR TCH/F9.6

TCH/F4.8

TCH/F2.4

Speech Data Data Data

22.8 kbps 22.8 kbps 22.8 kbps 22.8 kbps

TCH/HR TCH/H9.6

TCH/H4.8

Speech Data Data

11.4 kbps 11.4 kbps 11.4 kbps 23

24

GSM Control Channels

25

GSM CONTROL CHANNELS (CCH)

 There are three main control channels in the GSM system

1.

2.

3.

Broadcast Channel (BCH) Common Control Channel (CCCH) Dedicated Control Channel (DCCH)

 Each control channel consists of

several

logical channels having different Control Functions.

26

GSM CONTROL CHANNELS (CCH)

27

28

Broadcast Channels (BCH)

  It transmits data only in

(TS 0)

of certain ARFCNs. Other TSs are available for

TCHs

.

 The broadcast channel operates on the

forward link

of a

specific

ARFCNs The BCH provides

synchronization

mobiles within the cell.

for all  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.

29

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 periodically

by the MS (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.

30

BROADCAST CONTROL CHANNEL Cntd….



3.

4.

5.

6.

7.

8.

9.

BCCH Carries the following information (this is only a partial list):

1.

2.

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.

31

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

32

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

system.

assigned to each BTS in a GSM The BS issues course

timing advancement

command to the mobile station over the SCH 33

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 34

i) PAGING CHANNEL (PCH)

     It exists only on

downlink

(Forward channel).

The paging channel (PCH) provides

paging

all mobiles.

signals to notifies a specific mobile of an

incoming originates

from the

PSTN

.

call which The PCH transmit the

IMSI

along with a mobile unit.

request

of the target subscriber, for

acknowledgment

from the the PCH is also used to provide cell

broadcast

ASCII text messages

to all subscribers, as part of the

SMS

feature of GSM 35

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

36

iii) ACCESS GRANT CHANNEL (AGCH).

 AGCH is used by the BS to provide

forward link

communication.

 The AGCH is used by the BS to

RACH

sent by a mobile station.

respond

to a  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.

37

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.

38

DEDICATED CONTROL CHANNELS (DCCH)

  like Traffic Channels they are

bi-directional

.

 There are

three (03)

types of Dedicated Control Channels in GSM.

Same

format

and and reverse links.

function

in both the forward  DCCHs may exist in

any

time slot and any

ARFCN

except

TS 0

of the BCH ARFCN.

39

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), 40

i) STAND-ALONE DEDICATED COTROL CHANNEL (SDCCH)……

 The SDCCH is requested from the

MS RACH

and

assigned

via the

AGCH

.

via the  The SDCCH can be thought of as an

intermediate

and

temporary

channel.

 A SDCCH may also be used for

1.

call setup, 2.

3.

4.

5.

Authentication location updating SMS point to point e-Fax

41

ii) SLOW-ASSOCITED CONTROL CHANNEL (SACCH)

  The SACCH is always

associated

SDCCH

and

maps

onto the

same

with a

TCH

or a physical channel.

Each ARFCN

systematically

for all of its current users.

carries SACCH data  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

.

42

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

13 th

frame of

every

speech

dedicated

control channel 43

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 44

GSM Basic Call Sequence

45

46

Call Setup Procedure

47

48

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.