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Summary form only given. Telecommunications techniques, MIMO, and cognitive radio are reviewed, and sensing the human user is presented as a part of radio-channel behavior. The effect of the user can be measured and optimized as a single element of the radio channel, in order to maximize the links performance. User-induced effects (both head and hand) were found to be as high as 5-15 dB, of which 5 dB could be compensated for by using antenna selection in the 1800 MHz band tested. Existing statistical measurements have proven that 40% of consumers commonly use lossy usage positions. A dual-antenna confi guration was designed at 1800 MHz and equipped with capacitive proximity sensors, providing a method for measuring and controlling the effects of users. The user proximity-sensor system is likely to improve the performance of four-antenna MIMO systems.