Distributed source model for short-range MIMO

The plane-wave, or point-source, 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 (SISO) and single-input/multiple-output (SIMO) systems, because the rank of the channel matrix is one. However, for short-range multiple-input- multiple-output (MIMO) systems with line-of-sight (LOS), the point-source assumption affects the rank and singular value distribution of the MIMO channel matrix, and results in the underestimation of channel capacity, especially for element spacings exceeding a half wavelength. The short-range geometry could apply to many indoor wireless local area network (WLAN) applications. To avoid this under-estimation 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 wavelength. We derive an empirically-based threshold distance below which the distributed source model is required for accurate performance estimation in ray tracing.