  In this paper, we study the average throughput performance of energy
beamforming in a multi-antenna wireless-powered communication network (WPCN).
The considered network consists of one hybrid access-point (AP) with multiple
antennas and a single antenna user. The user does not have constant power
supply and thus needs to harvest energy from the signals broadcast by the AP in
the downlink (DL), before sending its data back to the AP with the harvested
energy in the uplink (UL). We derive closed-form expressions of the average
throughput and their asymptotic expressions at high SNR for both delay-limited
and delay-tolerant transmission modes. The optimal DL energy harvesting time,
which maximizes the system throughput, is then obtained for high SNR. All
analytical expressions are validated by numerical simulations. The impact of
various parameters, such as the AP transmit power, the energy harvesting time,
and the number of antennas, on the system throughput is also investigated.
