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were 1.3 million subscribers worldwide which had grown to more than 55 million by October 1997. With North America making a delayed entry into the GSM field with derivative of GSM called PCS1900 , GSM systems exist on every continent, and the acronym GSM now aptly stands for Global System for Mobilecommunications.
The developers of GSM chose an unproven (at the time)digital system,as opposed to the then-standard analog cellular systems like AMPS in the United States TACS in the United Kingdom. They had faith that advancements in compressionalgorithm and digital signal processors would allow the fulfillment of the original criteria and the continual improvement of the system interms of quality and cost . The over 8000 pages of GSM recommendations try to allow flexibility and cometitive innovation among suppliers but provide enough standardization to guarantee proper interwork between the components of the system.This is done by providing functionaandinter-face descriptions for each of the functional entities defined in the system. Services provided by GSM
From the beginning, the planners of GSM wanted ISDN compatibility in terms of the services offered and the control signalling used . However, radio transmission limitations, in terms of bandwidth and cost, do not allow the standard ISDN B-channel bit rate of 64 kbps to be practically achieved.
Using the ITU-T definitions , telecommunication services be divided into bearer services, teleservices , and supplementary services . The basic teleservice supported by GSM is telephony . As with anothercommunications , speech is digitally encoded and transmitted through the GSM network as a digital stream . There is emergency service , where the narest emergency- service provider is notified by dialing three digits (similar to 911).
A variety of data services is offered.GSM users can send and receive data,at rates up to 9600 bps , tousers on POTS ISDN, Packet Switched Public Data Networks , and Circuit Switched Public Data Networks using a variety of access methods and
protocols, such as X.25 or X.32.Since GSM is a digital network,a modem is not required between the user and GSM network,although an audio modem is required inside theGSMnetwork to interwork with POTS.
Other data services include Group 3 facsimile , as described in ITU-T recommend-
dation T.30, which is supported by use of an appropriate fax adaptorA unique feature of GSM, not found in older analog systems, is the Short Message Service (SMS). SMS is a bidirectional service for short alphanumeric messages . Messages are transported in a store-and-forward fashion.For point -to-point SMS, a message can be sent to another sub scriber to the serviceand anacknowledgement of receipt is provided to the sender . SMS can also be used in a cell-broadcast mode , for sending messages such as traffic up-dates or news updates. Messages can also be stored in the SIM card for later retrieval .
Supple mentary services are provided on top of teleservices or bearer services.Inthe current (Phase I) specifications, they include several forms of call forward ( such as call forwarding when the mobile subscriber is unreachable by the network ) , and call barring of outgoing or incoming calls , for example , when roaming in another country . Many additional supplementary services will be provided in the Phase 2 specifications , such as caller identification, call waiting, multi-party conversations . Control channels
Common channels can be accessed both by idle mode and dedicated mode mobiles. The common channels are used by idle mode mobiles to exchange the signallinginforma
tion required to change to dedicated mode . Mobiles already in dedicated mode monitor the surrounding base stations for handover and other information .The common channels are defined within a 51frame multiframe , so that
dedicatedmobile using the 26-frame multiframe TCH structure canstill monitor control channels .The common channels include:
? Broadcast Control Channel (BCCH)
Continually broadcasts, on the downlink, information including base station identity, frequency allocations, and frequency-hopping sequences.
? Frequency Correction Channel (FCCH) and Synchronisation Channel ( SCH )
Used to synchronise the mobile to the time slot structure of a cell by defining the boundaries of burst periods , and the time slot numbering . Every cell in a GSM network broadcasts exactly one FCCH andone SCH , which are by definition on time slot number 0 (within a TDMA frame).
? Random Access Channel (RACH)
Slotted Aloha channel used by the mobile to request access to the network. ? Paging Channel (PCH)
Used to alert the mobile station of an incoming call. ? Access Grant Channel (AGCH)
Used to allocate an SDCCH to a mobile for signalling (in order to obtain adedicated channel), following a request on the RACH. Channel coding and modulation
Because of natural and man-made electromagnetic interference,the encoded speech or data signal transmitted over the radio interface must be protected from errors . GSM uses convolutional encoding and block interleaving to achieve this protection . The exact algorithms used differ for speech and for different data rates .The method used forspeech blocks will be described below.
Recall that the speech codec produces a 260 bit block for every 20ms speechsample. From subjective testing , it was found that some bits of this block were more important for perceived speech quality than others . The bits are divided into three classes:
?
Class Ia 50 bits - most sensitive to bit errors
? ?
Class Ib 132 bits - moderately sensitive to bit errors Class II 78 bits - least sensitive to bit errors
Class Ia bits have a 3 bit Cyclic Redundancy Code added for error detection. If an error is detected,the frame is judged too damaged to be comprehensible and itisdiscarded. It is replaced by a slightly attenuated version of the previous correctly received frame . These 53 bits, together with the 132 ClassIb bits and a 4 bit tail sequence (a total of 189 bits), are input into a 1/2 rate convolutional encoder of constraint length 4 . Each input bit is encoded as two output bits, based on a combination of the previous 4 input bits.The convolutional encoder thus outputs 378 bits,to which are added the 78remaining Class II bits, which are unprotected.
Thus every 20 ms speech sample is encoded as 456 bits.
To further protect against burst errors commontothe radio interface each sample is interleaved . The 456 bits output by the convolutional encoder are divided into 8 blocks of 57 bits, and these blocks are transmitted in eight consecutive time-slot bursts . Since each time-slot burst can carry two 57 bit blocks , each burst carries traffic from two different speech samples. Speech coding
GSM is a digital system, so speech which is inherently analog, has to be digitized.
The method employed by ISDN, and by current telephone systems for multiplex- ing voice lines over high speed trunks and optical fiber lines, is PulseCoded Modulation (PCM) . The output stream from PCM is 64 kbps , too high a rate tobe feasible over a radio link. The 64 kbps signal,although simple to implement contains much redundancy. The GSM group studied several speech coding algorithms on the basis of subjective speech quality and complexity ( which is related to cost, processing delay, and power consumption once implemented)before arriving at the choice of a Regular Pulse Excited -- Linear Predictive Coder (RPE—LPC ) with a Long Term Predictor loop.Basically, information from previous samples , which does not change very quickly , is used to predict the current sample. The coefficients of
the linear combination of the previous samples , plus an encoded form of the residual, the difference between the predicted and actual sample, represent the signal. Speech is divided into 20 millisecond samples, each of which is encoded as 260 bits, giving a total bit rate of 13 kbps . This is the so- called Full-Rate speech coding. Recently , an Enhanced Full-Rate (EFR) speech coding algorithm has been implemented by some North American GSM1900 operators. This is said to provide improved speech quality using the existing 13 kbps bit rate. Power control
There are fiveclasses of mobile stations defined,accordin to their peak transmitter power, rated at 20 , 8, 5, 2, and 0.8 watts . To minimize cochannel interference and to conserve power , both the mobiles and the Base Transceiver Stations operate at the lowest power level that will maintain an acceptable signal quality .Power levels can
be stepped up or down in steps of 2 dB from the peak power for the class down to a minimum of 13 dBm 。
The mobile station measures the signal strength or signal quality(basedon the Bit Error Ratio), and passes the information to the BaseStationController ,which ultimately decides if and when the power level should be changed. Power control should be handled carefully, since there is the possibility of instability .This arises from having mobiles in co-channel cells alternatingly increase their powerin response toincreased cochannel interference caused by the other mobile increasing its power.This in unlikely to occur in practice but it is under study. . Frequency hopping
The mobile station already has to be frequency agile,meaning it can move between a transmit, receive, and monitor time slot within one TDMA frame,which normally are on different frequencies. GSM makes use of this inherent frequency agility to implement slow frequency hopping , where the mobile and BTS transmit each TDMA frame on a different carrier frequency. The frequency hopping