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.typically on the order of several milliseconds.A6-millisecondframe, for example, is divided into six 1-millisecond timeslots, with each time slot assigned to a specific user.Eachtime slot consists of a header and a packet of user data forthe call assigned to it (Figure T-5).The header generally con-tains synchronization and addressing information for theuser data.If the data in the header become corrupted as a result of atransmission problem signal fade, for example the entireslot can be wasted, in which case no more data will be trans-mitted for that call until the next frame.The loss of an entiredata packet is called frame erasure. If the transmission prob-lem is prolonged (i.e., deep fade), several frames in sequencecan be lost, causing clipped speech or forcing the retransmis- TIME DIVISION MULTIPLE ACCESS 3636 ms(30 KHz Channel)1 ms 1 ms 1 ms 1 ms 1 ms 1 msslot slot slot slot slot slotHeader Packet of User DataFigure T-5 Six TDMAtime slots.sion of data.Most transmission problems, however, will not besevere enough to cause frame erasure.Instead, only a few bitsin the header and user data will become corrupted, a conditionreferred to as single-bit errors. Network FunctionsThe IS-54 standard defines the TDMA radio interfacebetween the mobile station and the cell site radio.The radiodownlink from the cell site to the mobile phone and the radiouplink from the mobile phone to the cell site are functionallysimilar.The TDMAcell site radio is responsible for speechcoding, channel coding, signaling, modulation/demodula-tion, channel equalization, signal strength measurement,and communication with the cell site controller.The TDMAsystem s speech encoder uses a linear predictivecoding technique and transfers Pulse Code Modulated (PCM)speech at 64 kbps to and from the network.The channel coderperforms channel encoding and decoding, error correction, 364 TIME DIVISION MULTIPLE ACCESSand bit interleaving and deinterleaving.It processes speechand signaling information, builds the time slots for the chan-nels, and communicates with the main controller, modulator,and demodulator.The modulator receives the coded information and signal-ing bits for each time slot from the channel coders.It performsDQPSK modulation to produce the necessary digital compo-nents of the transmitter waveform.These waveform samplesare converted from digital to analog signals.The analog sig-nals are then sent to the transceiver, which transmits andreceives digitally modulated radiofrequency (RF) control andinformation signals to and from the cellular phones.The modulator/equalizer receives signals from the trans-ceiver.It performs filtering, automatic gain control, receivesignal strength estimation, adaptive equalization, anddemodulation.The demodulated data for each of the timeslots is then sent to the channel coder for decoding.Newer voice coding technology is available that producesnear-landline speech quality in wireless networks based onthe IS-136 TDMAstandard.One technology uses an alge-braic code excited linear predictive (ACELP) algorithm, anenhanced internationally accepted code for dividing waves ofsound into binary bits of data.The ACELP coders can beintegrated easily into current wireless base station radios aswell as new telephones.ACELP-capable phones enable usersto take advantage of the improved digital clarity over bothNorth American frequency bands 850 MHz for cellular and1.9 GHz for PCS.Call HandoffReceive signal strength estimation is used in the call hand-off process.The traditional handoff process involves the cellsite currently serving the call, the switch, and the neighbor-ing cells that potentially can continue the call.The neigh-boring cells measure the signal strength of the potential callto be handed off and report that measurement to the serving TIME DIVISION MULTIPLE ACCESS 365cell, which uses it to determine which neighboring cell canbest handle the call.TDMA systems, on the other hand,reduce the time needed and the overhead required to com-plete the handoff by assigning some of this signal strengthdata gathering to the cellular phone, relieving the neighbor-ing cells of this task and reducing the handoff interval [ Pobierz całość w formacie PDF ]
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.typically on the order of several milliseconds.A6-millisecondframe, for example, is divided into six 1-millisecond timeslots, with each time slot assigned to a specific user.Eachtime slot consists of a header and a packet of user data forthe call assigned to it (Figure T-5).The header generally con-tains synchronization and addressing information for theuser data.If the data in the header become corrupted as a result of atransmission problem signal fade, for example the entireslot can be wasted, in which case no more data will be trans-mitted for that call until the next frame.The loss of an entiredata packet is called frame erasure. If the transmission prob-lem is prolonged (i.e., deep fade), several frames in sequencecan be lost, causing clipped speech or forcing the retransmis- TIME DIVISION MULTIPLE ACCESS 3636 ms(30 KHz Channel)1 ms 1 ms 1 ms 1 ms 1 ms 1 msslot slot slot slot slot slotHeader Packet of User DataFigure T-5 Six TDMAtime slots.sion of data.Most transmission problems, however, will not besevere enough to cause frame erasure.Instead, only a few bitsin the header and user data will become corrupted, a conditionreferred to as single-bit errors. Network FunctionsThe IS-54 standard defines the TDMA radio interfacebetween the mobile station and the cell site radio.The radiodownlink from the cell site to the mobile phone and the radiouplink from the mobile phone to the cell site are functionallysimilar.The TDMAcell site radio is responsible for speechcoding, channel coding, signaling, modulation/demodula-tion, channel equalization, signal strength measurement,and communication with the cell site controller.The TDMAsystem s speech encoder uses a linear predictivecoding technique and transfers Pulse Code Modulated (PCM)speech at 64 kbps to and from the network.The channel coderperforms channel encoding and decoding, error correction, 364 TIME DIVISION MULTIPLE ACCESSand bit interleaving and deinterleaving.It processes speechand signaling information, builds the time slots for the chan-nels, and communicates with the main controller, modulator,and demodulator.The modulator receives the coded information and signal-ing bits for each time slot from the channel coders.It performsDQPSK modulation to produce the necessary digital compo-nents of the transmitter waveform.These waveform samplesare converted from digital to analog signals.The analog sig-nals are then sent to the transceiver, which transmits andreceives digitally modulated radiofrequency (RF) control andinformation signals to and from the cellular phones.The modulator/equalizer receives signals from the trans-ceiver.It performs filtering, automatic gain control, receivesignal strength estimation, adaptive equalization, anddemodulation.The demodulated data for each of the timeslots is then sent to the channel coder for decoding.Newer voice coding technology is available that producesnear-landline speech quality in wireless networks based onthe IS-136 TDMAstandard.One technology uses an alge-braic code excited linear predictive (ACELP) algorithm, anenhanced internationally accepted code for dividing waves ofsound into binary bits of data.The ACELP coders can beintegrated easily into current wireless base station radios aswell as new telephones.ACELP-capable phones enable usersto take advantage of the improved digital clarity over bothNorth American frequency bands 850 MHz for cellular and1.9 GHz for PCS.Call HandoffReceive signal strength estimation is used in the call hand-off process.The traditional handoff process involves the cellsite currently serving the call, the switch, and the neighbor-ing cells that potentially can continue the call.The neigh-boring cells measure the signal strength of the potential callto be handed off and report that measurement to the serving TIME DIVISION MULTIPLE ACCESS 365cell, which uses it to determine which neighboring cell canbest handle the call.TDMA systems, on the other hand,reduce the time needed and the overhead required to com-plete the handoff by assigning some of this signal strengthdata gathering to the cellular phone, relieving the neighbor-ing cells of this task and reducing the handoff interval [ Pobierz całość w formacie PDF ]