  RF energy harvesting is a promising potential solution to provide convenient
and perpetual energy supplies to low-power wireless sensor networks. In this
paper, we investigate the energy harvesting performance of a wireless sensor
node powered by harvesting RF energy from existing multiuser MIMO system.
Specifically, we propose a random unitary beamforming (RUB) based cooperative
beam selection scheme to enhance the energy harvesting performance at the
sensor. Under a constant total transmission power constraint, the multiuser
MIMO system tries to select a maximal number of active beams for data
transmission, while satisfying the energy harvesting requirement at the sensor.
We derive the exact closed-form expression for the distribution function of
harvested energy in a coherence time over Rayleigh fading channels. We further
investigate the performance tradeoff of the average harvested energy at the
sensor versus the sum-rate of the multiuser MIMO system.
