GSM Channels & Air Interface

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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 GSM radio interface

---- 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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10

GSM Physical Channel Cntd....

.

GSM Frame Structure Summary

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 Logical Channel

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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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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GSM Physical Channel Cntd....

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.

Normál burst (NB)

Frequency Correction Burst (FB)

Synchronization burst (SB)

Access burst (AB)or RACH

TB

000

Data

58 bit

Training seq.

26 bit

Data

58 bit

TB

000

GP

8.25 bit time

TB

000

000...0

142 bit

TB

000

GP

8.25 bit time

TB

000

Data

39 bit

Sync. seq.

64 bit

Data

39 bit

TB

000

GP

8.25 bit time

TB

000

Sync. seq.

41 bit

Data

36 bit

TB

000

GP

68.25 bit time

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 Logical Channels

GSM Traffic Channels

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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

Traffic channels can carry either1. Speech or

2. Data Speech channels are supported by two different

methods of coding known as1. 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....

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TRAFFIC CHANNELS

Full Rate TCH Half Rate TCHTCH/FR

TCH/F 9.6

TCH/F 4.8

TCH/HR

TCH/H4.8

TCH/H2.4TCH/F 2.4

Full 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

TCH/HR

TCH/H9.6

TCH/H4.8

Speech

Data

Data

11.4 kbps

11.4 kbps

11.4 kbps

22.8 kbps

Traffic Channels Cntd....

Half Rate Traffic Channels

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GSM Control Channels

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

1. Location Area Identity (LAI).2. List of neighboring cells which should be monitored by the

MS.3. List of frequencies used in the cell.4. Cell identity.5. Power control indicator.6. DTX permitted.7. Access control (for example, emergency calls, call barring).8. CBCH description.9. List of Channels currently in use within a cell.

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BROADCAST CONTROL CHANNEL Cntd….

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. Frame number.2. 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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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. call setup,2. Authentication3. location updating 4. SMS point to point 5. e-Fax

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

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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On reverse channel SACCH sends1. 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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ii) SLOW-ASSOCITED CONTROL CHANNEL (SACCH)…..

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.