16-state nonlinear equalizer for IS-54 digital cellular channels

A performance evaluation for a number of equalizers for frequency selective fading channels has been carried out. Linear and decision feedback equalizers have been considered. IS-54 digital cellular channels based on TDM concepts have delay spreads that result in at most one data symbol of overlap. Using a standard fading model, we find that a 16-state sequence estimator, following a receive filter matched to the transmitter filter, provides excellent performance for delay spreads from zero to one symbol interval. It is a low-complexity detector, and for this situation it is superior to both linear and decision feedback equalizers in this application. We assume perfect channel state information to establish ultimate performance. In practical applications, at most three complex samples of the overall channel impulse that includes the receiver filter must be estimated. The frequency selective channel is a two-path model with time variation following standard Doppler variations for IS-54 channels and co-channel interference is included. We present results for both root-raised-cosine filtered π/4-DQPSK and QPSK modulation formats. In the appendix, we provide an analysis to support our best result. It is shown that if the interbeam delay is one symbol interval on a slowly varying, two-beam channel, and maximum likelihood sequence estimation has a performance that attains Mazo's (1991) matched filter lower bound, even when the root-Nyquist receiver filter is only matched to its transmitter filter counterpart and not to the complete channel response