tion access to the network. Of course the real objective is to increase the ratio of users per unit bandwidth when compared to the analog AMPS access method. 18.5.2 Bit Rate Reduction of the Digital Voice Channel It became obvious to system designer that conversion to digital cellular required some different technique for coding speech other than conventional PCM found in the PSTN and described in Chapter 6. The following lists some techniques that have been considered or that have been incorporated in the various systems in North America, Europe, and Japan (Ref. 10). 1. ADPCM (adaptive differential PCM). Good intelligibility and good quality; 32-kbps data transmission over the channel may be questionable. 2. Linear predictive vocoders (voice coders); 2400 bps. Adopted by U.S. Department of Defense. Good intelligibility, poor quality, especially speaker recognition. 3. Subband coding (SBC). Good intelligibility, even down to 4800 bps. Quality suffers below 9600 bps. 4. RELP (residual excited linear predictive) type coder. Good intelligibility down to 4800 bps and fair to good quality. Quality improves as bit rate increases. Good quality at 16 kbps. 5. CELP (codebook-excited linear predictive). Good intelligibility and surprisingly good quality, even down to 4800 bps. At 8 kbps, near-toll quality speech. 18.6 NETWORK ACCESS TECHNIQUES 18.6.1 Introduction The objective of a cellular radio operation is to provide a service where mobile subscribers can communicate with any subscriber in the PSTN, where any subscriber in the PSTN can communicate with any mobile subscriber, and where mobile subscribers can communicate among themselves via the cellular radio system. In all cases the service is full duplex. 18.6 NETWORK ACCESS TECHNIQUES A cellular service company is allotted a radio bandwidth segment to provide this ser- vice. Ideally, for full-duplex service, a portion of the bandwidth is assigned for transmis- sion from a cell site to mobile subscriber, and another portion is assigned for transmission from a mobile us