This letter considers spectrum sharing between a primary multiusermultiple-input multiple-output (MIMO) wireless energy transfer (WET) system anda coexisting secondary point-to-point MIMO wireless information transmission(WIT) system, where WET generates interference to WIT and degrades itsthroughput performance. We show that due to the interference, the WIT systemsuffers from a loss of the degrees of freedom (DoF) proportional to the numberof energy beams sent by the energy transmitter (ET), which, in general, needsto be larger than one in order to optimize the multiuser WET with user fairnessconsideration. To minimize the DoF loss in WIT, we further propose a newsingle-beam energy transmission scheme based on the principle of time sharing,where the ET transmits one of the optimal energy beams at each time. This newscheme achieves the same optimal performance for the WET system, and minimizesthe impact of its interference to the WIT system.
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