Improved codes for differential space-time-frequency coded OFDM

With no channel state information (CSI) at the receiver, the highest possible diversity order for multiantenna transmissions over frequency selective channels, is given by the product of the number of transmit and receive antennas and the multipath channel length, the same as the case with perfect CSI at the receiver. In this paper, we consider improved nondiagonal codes for differential space-time-frequency (DSTF) coded orthogonal frequency division multiplexing (OFDM), enabling the maximum spatial and multipath diversity without CSI at the receiver. While full diversity gain can be achieved by using existing differential unitary diagonal codes, we propose a systematic approach to designing nondiagonal codes for DSTF systems, which are block diagonal (BD) and have the necessary unitary and full diversity properties. We investigate not only the diversity product, but also other aspects of the distance spectrum, namely the so-called effective multiplicity in designing BD codes. Based on permutation techniques, we have devised new excellent-performing BD DSTF codes, which improve the coding gain up to 3 dB as compared to their diagonal counterparts.