Transport Network Development - ericssonhistory.com

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.