An Adaptive-rate Anti-jam System for Optimal Voice Communication

Systems for the digital transmission of voice have traditionally been xed rate in anti-jam situations. This design severely limits the potential of the communication system in the case of voice since compression of speech is a lossy process. Bit errors in the transmission of voice coded speech cause degradation which can be compensated for in several ways. The standard method is to trade o channel coding and source coding bit rates to achieve an accepted level of quality in a xed rate system. We present a method that provides optimal voice quality and intelligibility for any given transmission channel condition. The approach is performed via a fully adaptive-rate system using an adaptive-rate modem, channel coding, and a multimode voice coder. In general, the receiver utilizes channel state information to not only optimally demodulate and decode the currently corrupted symbols from the channel, but also to inform the transmitter, via a feedback channel, of the optimal strategy for voice/channel coding and modulation format. We compare several joint source-channel coding schemes at multiple transmission symbol rates and compare the performance to xed aggregate-rate channel-controlled variable rate voice coding systems. We show through simulation that the reduction in speech distortion is greater for a modulation change than a channel coding change in many realistic situations because of the variability of coding gain.