This study investigates two areas relating to practical multi-antenna ultra-wideband (UWB) indoor channel: the frequency dependency in line-of-sight (LOS) and non-LOS (NLOS) environments, and the differences in spatial characteristics when using virtual and physical arrays in multi-antenna channel characterisation. The authors quantify the frequency dependency of the channels in terms of their resilience to spatial fading and system capacity. The composite radiation pattern of the antenna and channel will impact the channel properties, the authors investigate these changes by measurement and analysis of channel data taken using both physical and virtual array configurations at the same locations. They find that at lower frequencies the resilience to spatial fading in LOS channels was less than half of the theoretic maximum. The difference in capacity and diversity gain between the LOS and NLOS channels was found to decrease with centre frequency. The different rates of increase result in a 4% average increase in capacity for a 500-MHz subband centred at 4.5 and 9.5 GHz for the LOS scenarios, whereas an increase of only 0.7% was experienced by the NLOS scenarios. In addition, this analysis indicates that the estimated diversity and capacity of multi-antenna channels decrease when using physical antenna arrays for measurements.

Co-channel interference is one of the major factors limiting the capacity in modern cellular networks. Interference rejection combining (IRC) is an effective technique to suppress co-channel interference, requiring receivers with multiple antennas. For terminals using IRC, the larger the number of available antennas, the more interfering signals that can be suppressed. However, production cost, size and energy consumption of the required radio-frequency units impose a limit in the number of practical antenna elements. In this paper, we investigate the combination of IRC with antenna selection (AS), as a cost-effective approach to improve the interference suppression performance of wireless receivers, and present different low-complexity AS algorithms. In addition, we propose algorithmic modifications in case the estimates of interfering channels are unknown at the receiver, and an estimate of the interference-plus-noise covariance matrix (CM) has to be used. It is found that, in the presence of channel estimation errors, AS techniques using the CM approach are able to provide better performance than techniques using estimates of interfering channels. These findings and the effectiveness of the proposed algorithms are validated by both computer simulations and experimental testing using a hardware prototype.

This work considers the placement of pilot symbols in orthogonal frequency division multiplexed transmissions, to minimise the mean squared error in the corresponding channel estimation. We show that further improvements in this widely investigated topic are possible, for example, in indoor wideband channels, which are subject to significantly correlated scattering processes. Noting that the naive approach incurs a prohibitive complexity, low complexity greedy solutions are developed. A measurement campaign is conducted to estimate real-world channel statistics in an indoor wideband environment. It is confirmed that practical channels are correlated, even to the extent of exhibiting rank deficiencies, and that the proposed placement algorithms result in improved channel estimation performance.

To facilitate coherent detection of orthogonal frequency division multiplexed transmissions, pilot symbols can be transmitted in some of the subcarriers. Their placement to minimise the mean squared error in channel estimation is considered. Such problems have been widely addressed previously for channels subject to wide sense stationary uncorrelated scattering processes. While the stationarity assumption is usually realistic, indoor wideband channels, for example, are subject to significantly correlated scattering processes. Furthermore, practical multiple antenna transmitters/receivers are bound to exhibit spatial correlations. This work considers pilot placement optimisation in a multiple antenna setting subject to arbitrary correlations in the delay and spatial domains. Noting that the naive approach incurs a prohibitive complexity, low complexity greedy solutions are developed. It is further proven that the conventional equi-spaced placement becomes optimal at high SNR even with correlations, when the correlation matrices are of full rank. A measurement campaign is conducted to estimate real-world channel statistics in an indoor wideband environment, and observe the applicability of developed algorithms. It is confirmed that practical channels are correlated to the extent of exhibiting rank deficiencies, and that the proposed placement algorithms result in improved channel estimation performance.

We propose a rate-lossless technique for reducing the peak-to-average power ratio (PAPR) in orthogonal frequency-division multiplexing systems employing antenna selection on a per-subcarrier basis. The proposed technique, which is from the family of the active constellation extension (ACE) approach to PAPR reduction, exploits the nulled subcarriers (for a given antenna) that arise through the antenna selection process to reduce the PAPR by introducing a controlled level of distortion to these subcarriers. We show that the new scheme is capable of achieving a much lower PAPR than traditional ACE methods applied to per-subcarrier antenna selection systems. Moreover, we demonstrate through simulation analyses that the distortion transmitted on the nulled subcarriers will not hinder practical system performance.

Adapting a modem's modulation scheme can significantly enhance the resulting performance over a wireless communications link. Non-adaptive modems are generally designed conservatively for worst-case channel conditions. Adaptive modems are able to exploit the higher channel capacity when good conditions are encountered. A number of different metrics can be used to drive the adaptation process (i.e. automatic repeat request (ARQ) or segment error rate (SER)). Not surprisingly, certain metrics are far more successful than others. This paper presents an investigation of adaptive modulation in the presence of interference. In particular, the case of non reciprocal interference is studied (i.e., cases where the level of interference is different on the forward and reverse links). Research results have shown that if adaptation is performed assuming a reciprocal channel, then poor performance will result for the link experiencing a higher level of interference. In this study, an algorithm is used to calculate a measure of the link quality. This value is then fed back to the other end of the link to adapt subsequent transmissions. Results indicate that the throughput of such a system can be considerably improved when transmitting adaptively using any one of DBPSK, DQPSK or 8DPSK modulation schemes. It is proposed that this simple approach be adopted for short range, ad hoc, wireless personal area networks (WPAN).