  In this paper, we study the optimal design for simultaneous wireless
information and power transfer (SWIPT) in downlink multiuser orthogonal
frequency division multiplexing (OFDM) systems. For information transmission,
we consider two types of multiple access schemes, namely, time division
multiple access (TDMA) and orthogonal frequency division multiple access
(OFDMA). At the receiver side, due to the practical limitation that circuits
for harvesting energy from radio signals are not yet able to decode the carried
information directly, each user applies either time switching (TS) or power
splitting (PS) to coordinate the energy harvesting (EH) and information
decoding (ID) processes. For the TDMA-based information transmission, we employ
TS at the receivers; for the OFDMA-based information transmission, we employ PS
at the receivers. Under the above two scenarios, we address the problem of
maximizing the weighted sum-rate over all users by varying the time/frequency
power allocation and either TS or PS ratio, subject to a minimum harvested
energy constraint on each user as well as a peak and/or total transmission
power constraint. For the TS scheme, by an appropriate variable transformation
the problem is reformulated as a convex problem, for which the optimal power
allocation and TS ratio are obtained by the Lagrange duality method. For the PS
scheme, we propose an iterative algorithm to optimize the power allocation,
subcarrier (SC) allocation and the PS ratio for each user. The performances of
the two schemes are compared numerically as well as analytically for the
special case of single-user setup. It is revealed that the peak power
constraint imposed on each OFDM SC as well as the number of users in the system
play a key role in the rate-energy performance comparison by the two proposed
schemes.
