Wideband space-time communication: from propagation-based models to information-theoretic design prescriptions

Abstract

While much of the activity on space-time, or multi-antenna, communication has focused on narrowband indoor systems, in this work, we consider the complementary setting of wideband outdoor channels typical of fourth generation cellular and fixed wireless systems, with a focus on orthogonal frequency division multiplexing (OFDM). Starting from propagation studies available in the literature, we obtain an analytical framework for information-theoretic design prescriptions that, compared to conventional space-time communication strategies, improve performance while reducing transceiver complexity. We introduce the notion of implicit feedback, applicable to both TDD and FDD systems, in which the base station learns the spatial covariance of the downlink spatial channel by averaging uplink measurements across frequency. We develop rules of thumb for optimizing antenna spacing for systems with such "free" feedback and show that the number of antennas at the base station can be scaled up, thus increasing the beamforming gain, without any increase in complexity at the mobile. Spatial covariance estimation also simplifies receiver processing on the uplink, enabling a novel method of noncoherent eigenbeamforming, which yields beamforming gains without explicit channel estimation. Key to these results is the observation that, for appropriately spaced antenna elements, a typical outdoor cellular channel has a small number of dominant spatial eigenmodes, even as the number of antenna elements at the base station is scaled up.