  Simultaneous wireless information and power transfer (SWIPT) is anticipated
to have abundant applications in future machine or sensor based wireless
networks by providing wireless data and energy access at the same time. In this
paper, we study a multiuser multiple-input single-output (MISO) broadcast SWIPT
system, where a multi-antenna access point (AP) sends information and energy
simultaneously via spatial multiplexing to multiple single-antenna receivers
each of which implements information decoding (ID) or energy harvesting (EH).
Since EH receivers in practice operate with considerably higher received power
than ID receivers, we propose a receiver location based transmission
scheduling, where receivers that are close to the AP are scheduled for EH while
those more distant from the AP for ID. We aim to maximize the weighted
sum-power transferred to all EH receivers subject to a given set of minimum
signal-to-interference-and-noise ratio (SINR) constraints at different ID
receivers. In particular, we consider two types of ID receivers (referred to as
Type I and Type II, respectively) without or with the capability of cancelling
the interference from (a priori known) energy signals. For each type of ID
receivers, we formulate the joint information and energy transmit beamforming
design as a non-convex quadratically constrained quadratic program (QCQP).
First, we obtain the globally optimal solutions for our formulated QCQPs by
applying an optimization technique so-called semidefinite relaxation (SDR). It
is shown via SDR that no dedicated energy beam is needed for Type I ID
receivers to achieve the optimal solution; while for Type II ID receivers,
employing no more than one energy beam is optimal. Next, we establish a new
form of the celebrated uplink-downlink duality to develop alternative
algorithms to obtain the same optimal solutions as SDR.
