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

An evaluation of a number of equalizers for frequency selective fading channels has appeared recently. 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, the authors 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 superior to both linear and decision feedback equalizers in this application. Herein is assumed perfect channel state information to establish ultimate performance. In practical application, at most three complex samples of the overall 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. Cochannel interference is included and results are presented for both root-raised-cosine filtered π/4-DQPSK and QPSK modulation formats