The Global System for Mobile communications (GSM)OverviewOverviewDigital Cellular Systems WorldDigital Cellular Systems World--widewideGSMD-AMPSJapan DigitalPCS 1900DCS 1800CDMAMultiple Access TechniquesMultiple Access TechniquesIn the GSM/DCS mobile system each free physical channel can be used by every subscriber and there are not channels permanently dedicated to single userThis policy requires the introduction of 2 different techniques for the multiple accessTime Division Multiple Access (TDMA)Frequency Division Multiple Access (FDMA)TDMA principleTDMA principleT = Allocated timets(t)Slot for user 1Slot for user 2Slot for user 3Slot for user 5Slot for user 7Slot for user 8Slot for user 4Slot for user 6FDMAFDMABesides the TDMA in the GSM/DCS we have also the FDMA techniqueGSM/DCS is characterised by a hybrid access to the channelEach frame of 8 physical channels are multiplexed in the frequency domaineach frame is transmitted in a sub-band of 200 kHz124 carriers are available (the last one is not used for limiting the aliasing with other transmission systems)ffp1fp2fp3fp123fp124B=25 MHz1252BB<100 kHz100 kHzCarrier Frequency RangeCarrier Frequency RangeGSMUplink: 890 - 915 MHzDownlink: 935 - 960 MHzCarrier Pairs (in MHz)890.0 935.0890.1 935.1890.3 935.3.... ....914.9 959.9915.0 960.0Duplex Frequency = 45 MHzDCSUplink: 1710 - 1785 MHzDownlink: 1805 - 1880 MHzCarrier Pairs (in MHz)1710.0 1805.01710.1 1805.11710.3 1805.3.... ....1784.9 1879.91785.0 1880.0Duplex Frequency = 95 MHz124 Carriers374 CarriersGSM QuantisationGSM QuantisationIt is a logarithmic quantiserIt uses 13 bits : 213quantisation levelsSpeech EncoderSpeech EncoderIn the traditional telephone network the voice signal has a bandwidth ranging between 300 Hz and 3,4 kHz and it is quantised with a bit sequence at 64 kb/s (8 bits/Sampler • 8 kSampler/s)GSM/DCS adopts a speech encoder able to transmit voice with a data rate of 13 kb/s, ensuring at the same timea voice quality similar to the ETACS standardhigh robustness against transmission errorslimited transmission delaylow power consumptionlow cost implementationRPERPE--LTP LTP (1)(1)Regular Pulse Excitation/Long Term Prediction is the algorithm used by the GSM/DCS speech encoderIt performs an analysis of the voice for 20 ms consecutivelythe RPE technique tries to reproduce the signal with an equispaced impulse sequence filtered by a specific digital filter whose transfer function in the frequency domain estimates the voice spectrum envelop The speech is digitalised sampling at 8 kHz and quantising with 13 bitsbit rate of 104 kb/sRPERPE--LTP LTP (2)(2)This signal is then split up in sequences of 160 samples each 20 msSamples are analysed to evaluate the coefficients of the Linear Predictive Coding (LPC) filter whose transfer function estimates the voice spectrum envelop With the Long Term Prediction algorithm the coding of the samples is accomplishedAs result we get a burst of 260 bits each 20 ms bit rate of 13 kb/s It is foreseen the introduction in a next future of an encoder able to operate at 6,5 kb/sChannel CodingChannel CodingNoise, distortion and attenuation through the transmission channel determines a degradation of the signalUsing a coding of the transmitted information with the insertion of some redundancy symbols we manage to ensure a higher protection against errorsOf course this advantage is paid in terms of a higher number of transmitted bits and a reduction of the bit rateChannel encoders in GSM/DCSChannel encoders in GSM/DCSA cascade of 3 different types of coding are adopted in the GSM/DCS systemparity codecyclic code (Linear Block Code)convolutional codeEach information sequence of 260 bits is represented with a coded word of 456 bits (260 information bits + 196 coded bits)The required bit rate after the channel encoder is 22.8 kb/sDiagonal Diagonal InterleaverInterleaverIt is a technique usually used in the radio transmission systems in order to reduce the burst errors in single coded wordIt is performed permuting in a deterministic way the transmission order of bitsIt allows scattering an eventual burst error determined by the channel over more coded words this ensures the possibility of a proper correction even of long error sequencesGMSKGMSKThis is the modulation adopted in the GSM/DCS systemIts main features as all the CPM consists in ensuring a continual phase at each bit period T in the transition from a symbol to the next oneIt is performed with a FSK modulator with a gaussian filter useful to increase the frequency efficiencythe Power Spectral Density (PSD) of the modulated signal with this filter is characterised by a narrower bandwidththe aliasing with the adjacent channel is limitedThe modulated signal has a constant envelopeno problems with the non linear distortion introduced by the HPABurst and Frame FeaturesBurst and Frame FeaturesThe length of each burst (time slot) is of 577 µsIt includes 156.25 bitsEach bit has a length of 3,69 µsThe length of a frame is577 µs • 8 = 4.615 msThe bit rate required to transmit a frame through the Air Interface is156.25 / 0.577 µs = 270.8 kb/sIn each PCM time slot (125/32 In each PCM time slot (125/32 µµs) 8 bits are transmitteds) 8 bits are transmittedClassification of the Bursts Classification of the Bursts (1)(1)q Frequency Correction Burstused just to transmit Frequency Correction Channel (FCCH)142 bits are set to “1”q Synchronisation Burstused to transmit synchronisation informationthe training sequence includes a well known sequence of bitsq Dummy Burstit contains no information but only filling bitsClassification of the Bursts Classification of the Bursts (2)(2)q Access Burstused to send the Random Access CHannel (RACH) informationRACH contains the first message from MS to BTSit has a long guard period to allow BTS to calculate the MS distance from the BTS and to provide timing advance information to MSq Normal BurstNormal BurstNormal BurstIt is used to transmit both information and control bitsIt involves 156.25 bits2 x 3 tailing bitsfixed to 0 and used to inizialise the Viterbi’s equiliser memory2 x 57 sequences of information coded bits (payload)2 x 1 service bit26 bits as training sequenceused at the receiver for the eqaulisation8.25 bits as guard period for protection between 2 adjacent TSsThe 456 information coded bits to be transmitted each 20 ms are split in 8 sub-blocks of 57 bits573 357268.251 1577 µsLogical ChannelsLogical ChannelsThe physical channels (one
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