THE TAR DECODER - A BANDPASS VITERBI FFT DECODER FOR CONVOLUTIONAL ENCODED SPREAD-SPECTRUM SIGNALS/

A new form of bandpass convolutional decoder termed the TAR decoder is presented. The decoder has as its basis the classic Viterbi algorithm , but uses a fast Fourier transform (FFT) in order to cope with a freq uency offset, phase rotating, received signal. The TAR decoder also us es a two-dimensional (2-D) despread signal history array in addition t o the usual path history registers. The advantage of the TAR decoder i s that the decoder does not need to be preceded by a modem and is not subject to the problems of cycle slips. It can operate at negative inp ut signal-to-noise ratio (SNR) values and is well suited to the decodi ng of low rate convolutional codes as may be used in code-division mul tiple-access (CDMA) systems. Symbol timing recovery is still required but this is straightforward in practical cases down to congruent to-15 dB SNR.