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The ray tracing approximation was used to compute propagation channel characteristics in an urban setting. The results were used to assess Multiple Input Multiple Output (MIMO) performance in this urban environment. The computation used mobile radio parameters and provided the maximum theoretical MIMO gain. The relative transmitter/receiver positions were moved within and around the urban area to evaluate how MIMO performance depended on position. A 2x2 MIMO system was used for simplicity and three propagation related parameters were varied to assess the behavior and capability of this method. They were MIMO antenna spacing, and ground and building electric properties.
The key finding was that consistent results required appropriate spatial averaging of the ray tracing solutions. This is due to the sensitivity of the ray tracing solutions to small changes in relative transmitter/receiver positions and computational parameters when structures are idealized as possessing flat surfaces only. It is argued that various physical phenomena will average out this sensitivity when surfaces are irregular. Without appropriate averaging, MIMO gains were overly sensitive to small controllable and un-controllable computational parameters.
Upon appropriate averaging, one finds that ray tracing solutions provided a realistic measure of MIMO performance that depended on antenna spacing and surface electrical properties in a consistent manner. For example, MIMO gains improved with increased antenna separation. Behavior such as “key-holing” where RF energy propagates in urban corridors was observed. In those cases, the multiple, reflecting waves remained correlated and MIMO gains were low.
The usefulness and ability of ray tracing for assessing urban setting MIMO performance gains was demonstrated. Results can be used in Force on Force models where communication effects are included, or to spatially map channel characteristics for use in mobile MIMO studies. Larger MIMO systems (N>2) can easily be considered.
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