Diversity receivers in a fast fading multipath channel: Dissertation

Some radio channels are fast fading in the sense that they are randomly time-variant and the Doppler spread is in the order of or larger than the symbol rate of the digital signal. Such channels can be found in land-mobile, aircraft, and satellite communications, especially when the symbol rate is small, the velocity of the mobile station is large, and the carrier frequency is large. With small symbol rates, a Doppler barrier exists, below which the complexity of the optimal receiver is increased. The Doppler barrier is the time-frequency dual of the ISI barrier with large symbol rates. In the thesis, the history of digital communications in fading multipath channels is first summarized. The ML sequence detector, consisting of a correlator and an estimator, is derived in a WSSUS fast Rayleigh fading channel. In the system model, each symbol is divided into several short segments and ideal symbol synchronization is assumed. Finally, suboptimal receivers based on linear prediction and smoothing are analyzed in a fast fading channel. The authors main contribution is the extension of Kams ideas to the segmented waveform concept, so that the channel may be considered time-invariant over a segment duration. The system model is very useful, since it allows for a very tidy theoretical development concerning receiver structures; this bridges the gap between the classical one-shot receivers and the modern sequence estimation approach. Of almost equal importance is that the results of the thesis also bridge the gap between receivers for slow and for fast fading channel models. In a fast fading frequency-nonselective channel, an unexpected diversity effect was observed. The performance of the receiver is improved since the fading in different parts of a symbol is partially uncorrelated. In addition, the RAKE principle is extended to a fast fading channel, and this gives an additional improvement in performance. To simplify the receiver, data modulation must somehow be avoided or removed before the estimator, and either a predictor or a smoother can be used in the estimator. Performance analysis includes MMSE and bit error probability analysis. Many numerical results are given, and their validity is demonstrated with Monte Carlo simulations. The results can be applied in both conventional narrow-band and spread-spectrum (CDMA) systems. An extensive bibliography consisting of some 250 references is included in the thesis.