Quality of service and bandwidth efficiency of cellular mobile radio with variable-bit-rate speech transmission

We have investigated the performance of an adjustable source/channel codec in a cellular mobile-radio environment. The speech transmission rate and the amount of forward error correction change in response to changing channel conditions. The channel rate is constant at 32 kb/s and when the channel is good all of these bits are used for speech transmission. In intermediate and poor channels the speech rate is 24 or 16 kb/s and the remaining channel symbols are used for forward error correction. Relative to conventional transmission this approach offers an improved grade of service. For example, the outage rate (the proportion of "poor or worse" communications) goes from 9% with fixed-rate to 3% with variable-rate transmission. Alternatively, this improved grade of service can be exchanged for higher bandwidth efficiency. The fixed-rate system (with 9% outage) has 23 users per cell. With 52 users per cell the outage of the variable-rate system is only 6%.