Title: Magnetic MIMO Signal Processing and Optimization for Wireless Power Transfer,
Abstract: In magnetic resonant coupling (MRC) enabled multiple-input multiple-output
(MIMO) wireless power transfer (WPT) systems, multiple transmitters (TXs) each
with one single coil are used to enhance the efficiency of simultaneous power
transfer to multiple single-coil receivers (RXs) by constructively combining
their induced magnetic fields at the RXs, a technique termed "magnetic
beamforming". In this paper, we study the optimal magnetic beamforming design
in a multi-user MIMO MRC-WPT system. We introduce the multi-user power region
that constitutes all the achievable power tuples for all RXs, subject to the
given total power constraint over all TXs as well as their individual peak
voltage and current constraints. We characterize each boundary point of the
power region by maximizing the sum-power deliverable to all RXs subject to
their minimum harvested power constraints. For the special case without the TX
peak voltage and current constraints, we derive the optimal TX current
allocation for the single-RX setup in closed-form as well as that for the
multi-RX setup. In general, the problem is a non-convex quadratically
constrained quadratic programming (QCQP), which is difficult to solve. For the
case of one single RX, we show that the semidefinite relaxation (SDR) of the
problem is tight. For the general case with multiple RXs, based on SDR we
obtain two approximate solutions by applying time-sharing and randomization,
respectively. Moreover, for practical implementation of magnetic beamforming,
we propose a novel signal processing method to estimate the magnetic MIMO
channel due to the mutual inductances between TXs and RXs. Numerical results
show that our proposed magnetic channel estimation and adaptive beamforming
schemes are practically effective, and can significantly improve the power
transfer efficiency and multi-user performance trade-off in MIMO MRC-WPT
systems.