CONVOLUTIONAL CODING/VITERBI DECODING OF PCM/FM

Abstract

Tests were run with a binary PCM/FM transmission channel using limiter/discriminator detection and (7, 1/2) convolutional coding with Viterbi decoding. In all cases, soft decision decoding showed little or no improvement relative to hard decision, evidently because of the pops in the FM demodulator. Under optimum PCM/FM conditions, namely p-p deviation = 0.7 fs and IFBW = fs, the coding improvement at BER = 10^-6 was about 2.2 dB. The binary symbol rate in the transmission channel is fs. In the course of the test, it was discovered that with PCM/FM under optimum conditions, channel errors frequently occurred in pairs. This severely reduces the value of parity bit when used with uncoded PCM/FM and also degrades the Viterbi decoder performance by about 1 dB.