Space-Time Signal Optimization for SWIPT: Linear Versus Nonlinear Energy Harvesting Model

In simultaneous wireless information and power transfer systems, optimization of transmit signals is critical to system performance. Although the optimization problem can be efficiently solved under a linear energy harvesting model, the obtained solution may not work well in practice, since the energy harvester contains nonlinear elements, such as diodes. On the other hand, while a nonlinear model can be used to capture the dynamics of energy harvesting circuits, it introduces additional complexity at the optimization stage. Specifically, under a nonlinear model, traditional convex optimization is not applicable, since the energy harvesting function is fractional. To address this challenge, this letter first derives an optimal solution for static channels by introducing pseudo-inverse of the nonlinear model. Then, an iterative algorithm that converges to a sub-optimal solution is proposed for time varying channels. With the developed methods, the performance-complexity tradeoff between linear and nonlinear models is illustrated.