Spectral Efficient Doppler Diversity Transmissions in High Mobility Systems with Channel Estimation Errors

This paper studies spectral efficient Doppler diversity transmissions in the presence of imperfect channel state information (CSI). Fast time-varying fading in high mobility communication systems introduces Doppler diversity that can benefit system performance. On the other hand, it is more difficult to estimate and track fast time-varying channel, thus channel estimation errors might seriously degrade system performance in high mobility systems. We propose a practical pilot- assisted system design that can balance the tradeoff between Doppler diversity and channel estimation errors. At the transmitter, the data symbols are precoded with a rate-1 Doppler domain multiplexing scheme to achieve maximum Doppler diversity without sacrificing spectral efficiency. At the receiver, the information is detected by using imperfect CSI and a block decision feedback equalizer (BDFE), which is developed by exploiting the statistical properties of channel estimation errors. The analytical pairwise error probability (PEP) and a bit error rate (BER) lower bound are developed by considering a large number of design parameters, such as the percentage of pilot symbols in the transmitted symbols, the energy allocation between pilot and data symbols, and the maximum Doppler spread of the channel. Both simulation and analytical results show that maximum Doppler diversity can be achieved through optimizations of the various design parameters in the presence of imperfect CSI.