John Terry

Spherical space-time codes (SSTC)

We present the extension of the trellis space-time code (STC) concept to include signal mapping drawn from an N-dimensional sphere. The signal points were designed as such to increase the minimum squared distance between points in the constellation without increasing the average transmit energy. The mapping of the N-dimensional spherical constellations was performed in accordance to set partitioning rules for STC developed by AT&T but optimized for these codes. Performance evaluation for these spherical space-time codes (SSTC) are illustrated in an OFDM framework, which is a natural choice for multidimensional signaling.

On bandwidth efficient modulation for high-data-rate wireless LAN systems

We address the problem of high-data-rate orthogonal frequency division multiplexed (OFDM) systems under restrictive bandwidth constraints. Based on recent theoretic results, multiple-input multiple-output (MIMO) configurations are best suited for this problem. In this paper, we examine several MIMO configurations suitable for high rate transmission. In all scenarios considered, perfect channel state information (CSI) is assumed at the receiver. In constrast, availability of CSI at the transmitter is addressed separately. We show that powerful space-time codes can be developed by combining some simple well-known techniques. In fact, we show that for certain configurations, these space-time MIMO configurations are near optimum in terms of outage capacity as compared to previously published codes. Performance evaluation of these techniques is demonstrated within the IEEE 802.11a framework via Monte Carlo simulations.

Convergence analysis of finite alphabet beamformers for digital cochannel signals

In this paper, we examine the convergence behavior of finite alphabet (FA) beamformers. The two most popular implementations of FA beamformers for digital communication signals are the iterative least-squares with projection (ILSP) and the minimum mean-square error (MMSE) beamformer. To facilitate the analysis, for the binary communications case, we derive closed-form expressions for the mean weight vector, signal-to-noise ratio, and signal-to-interference ratio for both the ILSP and MMSE beamformers in terms of the bit-error rate (BER) performance at each iteration. Next, we generalize the analysis for the M-ary pulse-amplitude modulation and M-ary phase-shift keying cases. We show that both ILSP and MMSE beamformers have previously unreported bias terms in the array response vectors which are functions of the BER for each iteration. Furthermore, as the BER becomes arbitrarily small, we show that our solutions converge to the well-known asymptotic expressions widely published in the literature. Next, we provide a geometric interpretation of the effects of the noise bias vector in terms of angles between subspaces. Based on our analysis, we were able to develop necessary and sufficient conditions for convergence in the mean. We conclude with Monte-Carlo simulations to validate our analysis.