Multiuser detection of alamouti signals

In a MIMO multiple-access channel where users employ Space-Time Block Codes (STBC), interference cancellation can be used to suppress co-channel interference and recover the desired signal of each user at the receiver. Leveraging the special properties of Alamouti matrices, we first show that spatial multiplexing of Alamouti signals retains the space-time diversity gain of Alamouti signaling using our proposed low-complexity Alamouti BLAST-MMSE (A-BLAST) Algorithm. Next, in contrast to traditional transmit diversity that focuses on STBC construction at the transmitter, this paper looks at transmit diversity from the perspective of the receiver. In other words, the receiver gets to choose the STBCiquests, which are favourable to the channel assuming a fixed BLAST receive algorithm. In a multiuserMAC setting, we first present a systematic methodology to exploit different decomposition structure in Alamouti matrices, each with different tradeoff between performance and decoding complexity using possibly different MIMO receive algorithms. We then demonstrate that the notion of angles (the inner product of two quaternionic vectors) between multiuser channels determines the performance of MIMO receive algorithms. As an application of the general theory, we transform the decoding problem for several types of Quasi-Orthogonal STBC (QOSTBC) into multiuser detection of virtual Alamouti users. Building upon our A-BLAST Algorithm, we propose new algorithms for decoding single-user and multiuser QOSTBC. In particular, we show that bit error probability is a function of the quaternionic angle between virtual users (for a single user) or multiple users. This angle varies with the type of QOSTBC and leads to a new form of adaptive modulation called code diversity, where feedback instructs the transmitter how to choose from a plurality of codes.

I. Introduction

Multiple transmit and receive antennas can increase the transmission rate and improve the reliability of wireless systems by using Multiple-Input Multiple-Output (MIMO) techniques. One such technique is Space-Time Block Coding (STBC) that improves transmission reliability by correlating signals across different transmit antennas [1], [19]. At the receiver, space-time signal processing starts with a system of linear equations where signals are multiplied by channel gains. The value of algebraic structure to the construction of STBC is to transfer correlation from the transmitter to the receiver. For example, orthogonal signaling at the transmitter enables orthogonal separation of signals at the receiver with Maximum Likelihood (ML) decoding complexity that is constant, i.e., independent of the signal constellation size [19]. Diversity gain of STBC measures the link level advantage over a single path from the transmitter to the receiver. Of particular note is the Alamouti scheme that employs two transmit antennas to achieve the maximal diversity gain of two and a rate of one symbol per time slot [1]. The Alamouti code is the only orthogonal STBC that achieves full rate using complex modulation scheme, i.e., it is the only STBC that achieves the full diversity gain without needing to sacrifice data rate [19].