Finite-length decision-feedback equalisation for optimum combining diversity in digital cellular mobile radio

The paper analyses the performance of optimum space combining diversity with finite-length decision-feedback equalisation operating in a mobile radio environment with frequency-selective fading and quadrature amplitude modulation transmission. Selection diversity, another very common diversity method, is also involved in the analysis. By analytic techniques and simulation, it is shown that for the same number of taps, generally, the improvement is greater with combining diversity structure. However, numerical results also indicate that combining diversity may be inferior to selection diversity in some cases where the channels are severely distorted and combining diversity may lead to noticeable amplitude dispersion which cannot be compensated for easily by using a finitely long postcombining equaliser. Estimates of average errors and outage probabilities as functions of length of equalisation are provided for different diversity structures. The required length of equalisation to approach the optimum performance (achievable with an infinitely long equaliser) for different values of relative delay spread is examined carefully. Admitting perfect elimination of intersymbol interference postcursors, considerably superior performance of decision feedback equalisation can be obtained in terms of a fairly small number of taps to achieve this best performance, in addition to a smaller minimum mean square error than linear equalisation if the channel suffers severe time-dispersive multipath fading, particularly in the combining diversity case