  This letter considers spectrum sharing between a primary multiuser
multiple-input multiple-output (MIMO) wireless energy transfer (WET) system and
a coexisting secondary point-to-point MIMO wireless information transmission
(WIT) system, where WET generates interference to WIT and degrades its
throughput performance. We show that due to the interference, the WIT system
suffers from a loss of the degrees of freedom (DoF) proportional to the number
of energy beams sent by the energy transmitter (ET), which, in general, needs
to be larger than one in order to optimize the multiuser WET with user fairness
consideration. To minimize the DoF loss in WIT, we further propose a new
single-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 new
scheme achieves the same optimal performance for the WET system, and minimizes
the impact of its interference to the WIT system.
