On the performance of super-orthogonal turbo-transmit diversity for CDMA cellular communication

To achieve maximum theoretical system capacity in a Rayleigh fading environment, it has been shown that a combination of multiple transmit-receive antennas (given ideal channel state information and independent fading between antenna pairs) transmit and receive diversity will lead to the maximum achievable system capacity. In a CDMA environment, where the advantages of low rate coding for spreading purposes can be exploited, a coding-diversity gain, also known as space-time gain, can be achieved. This paper presents the new concept of layered super-orthogonal turbo transmit diversity (SOTTD) for code division multiple access (CDMA) communications. The techniques of spreading and coding at low-rate are married with code-division transmit diversity (CDTD) and iterative “turbo” processing. In layered space-time TC CDTD, a turbo encoder (and its associated iterative decoder) is required for every transmit diversity branch available. In SOTTD a single turbo encoder-decoder pair is required, making it a practically feasible solution. The only requirement being that the number of constituent encoders Z, should be greater or equal to the transmit diversity order M_T. The principle of operation is to transmit the coded bits, stemming from the constituent encoders, via the spatial domain rather than via the time, code or frequency domain. The received data stream is then iteratively decoded using turbo decoding principles. SOTTD performance is compared with TC CDTD using both simulations and theoretical bounds