Decorrelating diversity equalizer for Rayleigh fading multipath channels

We explore both coherent and noncoherent diversity combining techniques for rapidly time-varying mobile radio frequency selective fading channels. In particular, we propose to use the decorrelating filter for mitigation of intersymbol interference (ISI) at the input to the combiner. This detector is suitable for a multi-ray channel propagation model. The decorrelating filter does not depend on rapidly time-varying channel coefficients and needs to be updated only if the relative delays between paths change. When it is possible to perform channel estimation, the decorrelating detector with maximal ratio combining provides reliable performance and requires lower complexity than linear equalizers. It is demonstrated that noncoherent detection for differentially encoded data (DPSK) is also feasible when the decorrelator is used, whereas the DPSK RAKE receiver is limited due to significant ISI. Signal design issues related to the implementation of the decorrelating detector and reliable linear equalizers for fading frequency-selective channels are also discussed.