Joint Precoding and Transmit Power Allocation for Multiuser Multiantenna SWIPT Employing OFDM With Optimal Time-Switching and α-Fairness

Time-switching is a common simultaneous wireless information and power transfer (SWIPT) receiver technique in which the receiver alternates between information decoding and power harvesting, with durations governed by the time-switching ratio. Existing methods for optimizing the time-switching ratio typically rely on convex problem formulations or are restricted to simplified problem structures solvable via the Lambert W function. In multiuser (MU) orthogonal frequency division multiplexing (OFDM) systems, where optimal resource allocation is non-convex due to MU interference, optimizing the time-switching ratio is particularly challenging, as it introduces coupling across subcarriers. Prior work that maximized data rates in MU multiple-input single-output (MISO) OFDM systems—without resorting to interference suppression — did not address time-switching ratio optimization for this reason. This paper presents a method for joint transmit power allocation, precoding, and time-switching ratio optimization in MU-MISO SWIPT systems employing OFDM over the broadcast channel (BC). Rather than treating the time-switching ratio as a fixed parameter, the proposed approach incorporates a recently established mapping between optimal time-switching ratios and weighted resource allocations, enabling efficient per-user optimization. The method is further generalized using α-Fairness utility functions to balance throughput and energy harvesting according to a fairness parameter. By leveraging Lagrange duality and the time-sharing property, the algorithm retains interference as harvestable energy while achieving near-optimal and fair resource allocation