Spherical-wave model for short-range MIMO

The plane-wave assumption has been used extensively in array signal processing, parameter estimation, and wireless channel modeling to simplify analysis. It is suitable for single-input single-output and single-input multiple-output systems, because the rank of the channel matrix is one. However, for short-range multiple-input multiple-output (MIMO) channels with a line-of-sight (LOS) component, the plane-wave assumption affects the rank and singular value distribution of the MIMO channel matrix, and results in the underestimation of the channel capacity, especially for element spacings exceeding half a wavelength. The short-range geometry could apply to many indoor wireless local area network applications. To avoid this underestimation problem, the received signal phases must depend precisely on the distances between transmit and receive antenna elements. With this correction, the capacity of short-range LOS MIMO channels grows steadily as the element spacing exceeds half a wavelength, as confirmed by measurements at 5.8 GHz. In contrast, the capacity growth with element spacing diminishes significantly under the plane-wave assumption. Using empirical fitting, we provide a threshold distance below which the spherical-wave model is required for accurate performance estimation in ray tracing.

I. Introduction

Multiple-input multiple-output (MIMO) transmission is an extremely spectrum-efficient technology that uses several antennas at both ends of the communication link [1], [2]. However, it has been revealed that some factors, such as the richness of the multipath, the correlation of the entries of the channel matrix, and the keyhole effect, might degrade MIMO system performance significantly in a real environment [3], [4]. Presence of a strong line-of-sight (LOS) component is sometimes viewed as a degradation in the context of MIMO, because a strong LOS is usually thought to result in a unity-rank channel, and a unity-rank channel is incapable of supporting multiple parallel data streams.