Improved antenna-array multi-user OFDM in slow-fading frequency-selective channels based on oversampling

Multi-user orthogonal frequency division multiplexing (OFDM) with antenna arrays has been extensively investigated in the past with success. However, the performance of this type of OFDM in realistic multipath fading environments is limited when the number of carriers is small (i.e. less than 128 carriers are employed). In addition, the space correlation between different antenna elements was neglected in the past studies of this system. In this paper, a technique based on over-sampling at the receiver is employed to improve the performance of antenna array multi-user OFDM over slow-fading, frequency-selective (i.e. time-dispersive) channels. This oversampling technique improves the performance of both the channel estimation and data detection by several dB, under both independent antenna elements and dependent ones. Two multi-user receivers are employed: linear decorrelating receivers (LDR) and minimum mean-square error (MMSE) ones. The oversampling technique increases the complexity of the receiver linearly with the oversampling factor used. In addition, by using different oversampling factors in the pilot (channel estimation) and data detection modes respectively, the system complexity can be kept to a minimum value