Space-time coded asynchronous DS-CDMA with decentralized MAI suppression: performance and spectral efficiency

Multiple-input, multiple-output (MIMO) techniques are considered in an asynchronous code-division multiple- access system with a flat fading channel. A unified maximum- likelihood receiver, based on chip-level oversampling, is proposed to jointly suppress the multiple-access interference and perform the space-time decoding. Analysis shows that the system performance is determined by the product measure of the space- time code and the effective signal-to-interference-plus-noise- ratio (SINR). Three expurgated union bounds are proposed to approximate the bit error rate for different MIMO schemes and SNRs. Next, spectral efficiency is analyzed by applying a Gaussian approximation to the multiple-access interference. Numerical results show that the chip-level oversampling, together with MIMO techniques, can effectively increase the spectral efficiency. The impact of channel estimation error on spectral efficiency is also investigated both analytically and numerically. Two tradeoffs are found in a lower bound of spectral efficiency. In the first tradeoff to optimize spectral efficiency, the increase of the number of users is shown to dominate the degradation of SINR. In the second tradeoff, increasing the number of training symbols in a fixed-length packet is shown to yield a higher SINR but a reduction in the amount of transmitted information. An optimal number of training symbols is found numerically as a function of the SNR and the packet length.