Distance measures for viterbi decoding of coded and bandlimited 8PSK signals

Abstract

AbstractThe paper studies a digital modem that transmits a convolutionally encoded eight‐phase‐shift keyed (CE8PSK) signal over a satellite channel and employs a near‐maximum‐likelihood decoder at the receiver. Several different earth‐stations are assumed to have simultaneous access to a given transponder in the satellite as an arrangement of frequency‐division multiple access. Adjacent channel interference (ACI) is neglected here. The high‐power amplifier (HPA) at the earth‐station transmitter and the satellite transponder are assumed to be linear. The channel distorts the data signal, through the bandlimiting introduced by the equipment filters, and it adds stationary white Gaussian noise to the data signal at the receiver input. The resulting noisy and distorted data signal is coherently demodulated. Four different bandwidths of the transmitted CE8PSK signal are considered, together with three different demodulation processes. The decoder uses the Viterbi algorithm, together with one of six different distance measures to determine the costs of the stored vectors (survivors).The paper describes the various techniques involved in the modem, and presents the results of an extensive series of computer‐simulation tests to measure the performances of the system with all appropriate combinations of the signal bandwidth, demodulation process and distance measure. The results of the tests are then used to determine the preferred modem design.