Deconstructing Space-Frequency Correlated Ultrawideband MIMO Channels (Invited Paper)

Conventional correlation models fail to accurately represent the correlation properties of ultrawideband (UWB) multiple-input multiple-output (MIMO) channels. In our pre- vious work, a framework for constructing correlated UWB MIMO channel models was proposed, where spatial correlation was introduced into both the multipath amplitude and time-of- arrival (ToA) in the channel impulse response. Based on this framework, in this paper we first present a simplified UWB MIMO channel model that represents a reasonable compromise between analytical tractability and model accuracy. We show that this model yields a structured space-frequency (SF) channel covariance matrix, making it suitable for theoretical system performance analysis. To illustrate the merit of this model, we apply it to evaluate the maximum diversity order in SF coded UWB MIMO systems. In contrast to previous work based on conventional wideband MIMO correlated channel models, we show that the maximum diversity order of a UWB MIMO system is not limited by the sum of the ranks of multipath amplitude correlation matrices. I. INTRODUCTION Ultrawideband (UWB) is a dynamic spectrum access (DSA) technique that aims to improve the radio spectrum utilization (1). Based on an underlay DSA approach (1), a UWB system seeks to coexist with the incumbents by spreading the transmit power over a very wide bandwidth. It thus holds great promise for enabling unlicensed short-range high-speed wireless access (2). A UWB MIMO system deploys antenna arrays at both ends of the communication link and can be used to increase the data rates and/or extend the coverage (3). Unfortunately, the performance of UWB MIMO systems degrades in the presence of channel correlation, which is often encountered in practical situations due to insufficient antenna spacing or sparse scattering. Consequently, an accurate and analytically tractable channel correlation model is critical for the design and performance analysis of realistic UWB MIMO systems. Conventional correlation models for wideband MIMO chan- nels with a tapped-delay-line structure assume that multipath components (MPCs) arrive at different antenna elements with varying (and possibly correlated) complex amplitudes but identical ToAs (4)-(6). Such models become inappropriate in the UWB regime due to the high delay resolution, which re- sults in distinguishable ToA differences even between closely- spaced antenna elements. In our previous work (7), we pro- posed a UWB MIMO channel modeling framework that con- siders both the amplitude and ToA correlations. The resulting channel model was shown to better fit the measurement results. Based on this framework, in this paper we aim to investigate the inherent structure of the UWB MIMO channels, understand the difference between UWB and wideband MIMO channels, and discuss the implications of the channel structure from the signal processing and information theoretic perspectives. II. UWB MIMO CHANNEL MODEL