Overview of spatial channel models for antenna array communication systems

Throughout the history of wireless communications, spatial antenna diversity has been important in improving the radio link between wireless users. Historically, microscopic antenna diversity has been used to reduce the fading seen by a radio receiver, whereas macroscopic diversity provides multiple listening posts to ensure that mobile communication links remain intact over a wide geographic area. In recent years, the concepts of spatial diversity have been expanded to build foundations for emerging technologies, such as smart (adaptive) antennas and position location systems. Smart antennas hold great promise for increasing the capacity of wireless communications because they radiate and receive energy only in the intended directions, thereby greatly reducing interference. To properly design, analyze, and implement smart antennas and to exploit spatial processing in emerging wireless systems, accurate radio channel models that incorporate spatial characteristics are necessary. In this tutorial, we review the key concepts in spatial channel modeling and present emerging approaches. We also review the research issues in developing and using spatial channel models for adaptive antennas. Overview of Spatial Channel Models for Anfenna Array Communication Systems RICHARD B. ERTEL AND P A U L 0 CARDIERI, VIRGINIA POLYTECHNIC INSTITUTE KEVIN w. SOWERBY, UNIVERSITY OF AUCKLAND, N E W ZEALAND THEODORE s. RAPPAPORT AND JEFFREY H. REED, VIRGINIA POLYTECHNIC INSTITUTE ith the advent of antenna array systems for both interference cancellation and position lowtion applications comes the need to better understand the spatial properties of the wireless communications channel. These spatial properties of the channel will have an enormous impact on the performance of antenna array systems; hence, an understanding of these properties is paramount to effective system design and evaluation. The challenge facing communications engineers is to develop realistic channel models that can efficiently and accurately predict the performance of a wireless system. It is important to stress here that the level of detail about the environment a channel model must provide is highZy dependent on the type of system under consideration. To predict the performance of single-sensor narrowband receivers, it may be acceptable to consider only the received signal power and/or time-varying amplitude (fading) distribution of the channel. However, for emerging wideband multisensor arrays, in addition to signal power level, information regarding the signal multipath delay and angle of arrival (AOA) is needed. Classical models provide information about signal power level distributions and Doppler shifts of the received signals. These models have their origins in the early days of cellular radio when wideband digital modulation techniques were not readily available. As shown subsequently, many of the emerging spatial models in the literature utilize the fundamental principles of the classical channel models. However, modern spatial channel models build on the classical understanding of fading and Doppler spread, and incorporate additional conThis work was partially supported by the DARPA GloMo program, virgmia Tech’s Federal Highways Research Center of Excellence, Virginia Tech’s Bradlq Foundation, the Brazilian National Science Council CNPq, and NSF Presidential Faculty Fellowship. cepts such as time delay spread, AOA, and adaptive array antenna geometries. In this article, we review the fundamental channel models that have led to the present-day theories of spatial diversity from both mobile user and base station perspectives. The evolution of these models has paralleled that of cellular systems. Early models only accounted for amplitude and time-varying properties of the channel. These models were then enhanced by adding time delay spread information, which is important when dealing with digital transmission performance. Now, with the introduction of techniques and features that depend on the spatial distribution of the mobiles, spatial information is required in the channel models. As shown in the next sections, more accurate models for the distribution of the scatterers surrounding the mobile and base station are needed. The differentiation between the mobile and base station is important. Classical work has demonstrated that models must account for the physical geometry of scattering objects in the vicinity of the antenna of interest. The number and locations of these scattering objects are dependent on the heights of the antennas, particularly regarding the local environment. This article, then, explores some of the emerging models for spatial diversity and adaptive antennas, and includes the physical mechanisms and motivations behind the models. A literature survey of existing RF channel measurements with AOA information is also included. The article concludes with a summary and suggestions for future research. WireZess MuZtipath Channel Models This section describes the physical properties of the wireless communication channel that must be modeled. In a wireless system, a signal transmitted into the channel interacts with the environment in a very complex way. There are reflections from large objects, diffraction of the electromagnetic waves 1