【文章內(nèi)容簡(jiǎn)介】 e Service (SMS). SMS is a bidirectional service for short alphanumeric messages . Messages are transported in a storeandforward point topoint 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 cellbroadcast mode , for sending messages such as traffic updates 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 current (Phase I) specifications, they include several forms of call forward ( such as call forwarding when the mobile subscriber is unreachable by the work ) , and call barring of outgoing or ining 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, multiparty conversations . Control channels Common channels can be accessed both by idle mode and dedicated mode mobiles. The mon 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 mon channels are defined within a 51frame multiframe , so that dedicatedmobile using the 26frame multiframe TCH structure canstill monitor control channels .The mon channels include: ? Broadcast Control Channel (BCCH) Continually broadcasts, on the downlink, information including base station identity, frequency allocations, and frequencyhopping 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 work 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 work. ? Paging Channel (PCH) Used to alert the mobile station of an ining 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 manmade electromagic 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 prehensible 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 bination of the previous 4 input 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 montothe 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 timeslot bursts . Since each timeslot 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 plexity ( 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 , information from previous samples , which does not change very quickly , is used to predict the current sample. The coefficients of the linear bination 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 FullRate speech coding. Recently , an Enhanced FullRate (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