Maximum Doppler diversity transmissions for high mobility systems with imperfect channel state information

This paper studies the maximum Doppler diversity transmissions in the presence of imperfect channel state information (CSI) in high mobility systems. Due to fast time-varying fading in high mobility systems, channel estimation error is usually inevitable and they might have significant impacts on system performance. On the other hand, Doppler spread caused by fast time-varying fading introduces Doppler diversity that can benefit system performance. The fundamental tradeoff between channel estimation errors and Doppler diversity are studied by deriving the analytical symbol error rate (SER), which captures both effects in a single expression. It is shown that conventional maximal ratio combining (MRC) receiver is no longer optimum with channel estimation errors. A new optimum receiver that can collect the maximum Doppler diversity with imperfect CSI is developed by analyzing the statistical properties of the estimated channel coefficients. The SER of the new receiver is then derived and evaluated. The analytical and simulation results are used to identify the system parameters that can render the optimum tradeoff between Doppler diversity and channel estimation error.