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94Fig. 3Ericsson built an experimental system in order todemonstrate the advantages of the TDMA method.A TDMA transceiver was connected to a basestation already in operation, belonging to the LosAngeles Cellular Telephone Company. TDMAunits and analog units were installed in a van thattoured urban and less densely populated areasalso be retained. This would not be possibleif the analog channel was to bereplaced by three 10 kHz FDMA channels.FDMA would necessitate a reassignmentof frequencies and filters inthe system or the introduction of complexpower amplifiers for both base stationsand mobile stations.Mobile assisted handoffThe anticipated rapid growth in subscribernumbers in conjunction withsmaller cell sizes makes it increasinglyimportant to be able to locate mobilestations faster and more accurately thanin present systems. The solution proposedby Ericsson is that a mobile unitshould measure the signal strength onchannels from neighbouring base stationsand report them to its current basestation. The land system - base stationsand MSCs (Mobile Services SwitchingCentres) - evaluates these measurementsand indicates the base station towhich the mobile unit will be handed offwhen it is about to leave a cell or for anyother reason would gain in radio linkquality by a handoff. The number ofhandoffs increases when the traffic percell increases and the cell size is reduced.If the analog system methodwere used - where neighbouring basestations measure the signal transmittedfrom a mobile unit - the signalling loadon the links between the base stationsand the MSCs would be very heavy, andthis would also require very high dataprocessing capacity in the MSCs. A decentralizedlocation procedure, whereeach mobile unit is a measurementpoint, will thus reduce the load on thenetwork.Flexible user data rateFuture increases in network capacity requirethat the standard permits exploitationof developments in the field ofspeech coding - developments whichcontinually reduce the bit rate requiredby the codec in order to maintain a givenspeech quality. If the bit rate from thespeech codec is reduced by half, thecapacity increases approximately to thesame degree. With FDMA, the bandwidthof the radio channel must bechanged in step with the bandwidth requiredby the user. This requires verynarrow receive filters (5 kHz) and stringentreceiver specification if it is to bepossible to introduce half-rate speechchannels in an FDMA system. If differentbit rates have to be used, for exampledifferent data rates and speech, the mobileunits must be equipped with aswitchable receiver, which is not a feasiblesolution for a small, cheap, handheldtelephone.With TDMA, different users may use differentdata rates; they are simply giventhe time required, i.e. half the number oftime slots for half-rate channels.This does not affect the radio part in thetransceiver and, hence, thus does notincrease the complexity of the telephones.The upper limit of the data rateoffered by the system - if its complexityis not to increase drastically - is determinedby the nominal channel bandwidth.In the North American system thedifference between TDMA and FDMA isat least a factor of three - in favour ofTDMA.Ericsson's experimentalsystemMany people considered that a changefrom the FDMA method used in presentdayanalog systems to the TDMA methodwould constitute natural progress.The question was whether the time wasright or whether TDMA needed moretime to mature. Ericsson built an experimentalsystem to demonstrate the feasibilityof TDMA. A TDMA transceiver wasconnected to an operating base stationbelonging to the Los Angeles CellularTelephone Company, near LAX airport.Mobile stations - TDMA and analog referencesets - were installed in a van,which toured urban and semi-rural areas,fig. 3. Most of the observers takingpart in the demonstration judged thespeech quality of the digital solution tobe as good as or better than that of theanalog method.The TDMA decisionTIA decisions are normally based onconsensus, i.e. different technical solutionsare discussed until the memberscan agree on one. However, in this caseneither the advocates of FDMA northose of TDMA would give in. The matterwas therefore settled by ballot. The resultwas that TDMA was chosen by alarge majority.
95Fig. 4The frame structure of the traffic channelsThe length of the TDMA frame is 20 ms. Thelength of the time slot is 20/3 ms for both full andhalf-rate channelsA Time slot, user 1B Time slot, user 2C Time slot, user 3Other decisionsSpeech codecOne of the most important factors in determiningthe capacity of a cellular systemis the bit rate of the speech codec.With a low bit rate the amount of spectrum-timeconsumed for one connectionis low, permitting more simultaneousconnections within the systembandwidth.The method used to choose the algorithmfor the speech coder was to testhardware from nine suppliers. The testincluded sensitivity to bit errors. Thecandidates could allocate a part of thebit stream to error correction. The totalbit rate should not exceed 13 kbit/s.The candidates were tested under variousconditions and the results wereevaluated through a subjective listeningtest in which 100 persons participated.The winning speech and channel codecalgorithm used 8 kbit/s for speech codingand 5 kbit/s for error detection andcorrection.This was the only case where hardwaretesting was used. All other decisionswere made by the committee. Thespeech codec chosen uses a variant ofthe CELP (Code Excited Linear Predictive)algorithm, and protection againstbit errors is provided by a CRC (CyclicRedundancy Check) error detectioncode followed by a convolutional codewith a constraint length equal to 6. Thetotal complexity for a duplex implementationis about 15-20 MIPS. The subjectivequality of the speech codec isabout the same as for the codec in theGSM system. The latter operates at13 kbit/s, whereas the CELP codec onlyrequires 8 kbit/s. This is partly due to theprogress in the speech coding field duringthe last two years and partly to thegreater complexity of the CELP codeccompared with the GSM codec.ModulationIt was decided from the beginning thatthree users would share one carrier.With a total bit rate of approximately16 kbit/s per user - including signallingand overhead - the bit rate for one radiochannel would be 48 kbit/s. Proposalsvaried between 42 and 54 kbit/s. As aworking hypothesis the designers soondecided on 13 kbit/s for the combinedspeech and channel coding process.Hardware development and implementationof the proposed speech and channelcoder could then start.The transmission of 48 kbit/s in a 30 kHzsignal requires a modulation schemethat accommodates 1.6 bits/Hz. Thiswould not have been possible with thepreviously most commonly used methodswith constant envelopes - e.g.GMSK (Gaussian Minimum Shift Keying)which was chosen for GSM. In theUS a modulation method with a nonconstantenvelope was chosen: n/4-shifted differentially encoded QPSK(Quadrature Phase Shift Keying). Thismodulation method gives four possiblesymbols, each corresponding to twobits.The pulse shaping was determined halfa year later when the total bit rate hadbeen selected (48.6 kbit/s). The pulseshaping determines the width of the radiospectrum. The US choice was RootRaised Cosine with a roll-off factor of0.35.In order to be able to transmit three digitalspeech channels in a bandwidth of30 kHz it was necessary to use a modulationmethod with a non-constant envelope- the first to be used in a cellularsystem. This method requires a somewhatmore complex power amplifier andalso higher power, i.e. the time betweenbattery recharges for hand-held stationsis reduced. The solution is consideredto provide a good balance between systemcapacity and demands on the mobileunits. In Europe the channel bandwidthwas chosen so that a constantenvelope scheme could be used. InNorth America this was not feasible,partly because of the requirement fordual mode (both analog and digitalspeech channels).Frame and time slot structure fortraffic channelsThe frame structure is shown in fig. 4.The structure is prepared for half-ratecoders so that future advances inspeech coding technology can be exploited.At the initial stage, when fullratechannels are used, the TDMA framehas a length of 20 ms. The time slotlength is 20/3 ms for both full and half-
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