Maximum Doppler Diversity Transmissions for High Mobility Systems with Imperfect Channel State Information using QAM Modulation Scheme

This paper uses QAM modulation scheme to studythe maximum Doppler diversity transmissions in high mobility systems when channel state information (CSI) is not known perfectly. A comparison between QAM and PSK modulation is also given for Doppler diversity transmissions. In fast time-varying fading, systems with high mobility causes channel estimation errors but it also introduces Doppler diversity because of the Doppler spread which benefits system performance. An optimum receiver, derived using the statistical properties of estimated channel coefficients, collects the maximum Doppler diversity when CSI is not known perfectly. Then, the final expression of SER is derived by carefully taking into consideration the constellation points for QAM modulation. This expression captures channel estimation errors and Doppler diversity in a single expression using QAM modulation scheme. The optimum parameters which give the best trade-off between Doppler diversity and channel estimation errors are identified.