Error protection for tactical voice modems

The coding techniques included convolutional codes and Reed-Solomon block codes. The speech input to the modem was compressed by using a 1200 bps low rate voice (LRV) algorithm. The modem operated on a high frequency (HF) fading channel using M-ary phase shift keying (PSK) modulation. The coding techniques were treated independently from the LRV algorithm. The code rate was varied to determine the coding gains obtainable in a fading channel. The value of M was also varied in conjunction with the code rate such that the transmission bandwidth remained fixed. Using 4-ary PSK, an optimal code rate of 1/2 for convolutional codes was found such that the Eb/No was minimized. Applying the optimal code rate, an optimal allocation of bits could be defined between the voice and error protection algorithms to make best use of the channel capacity