  This paper reviews emerging wireless information and power transfer (WIPT)
technique with an emphasis on its performance enhancement employing
multi-antenna techniques. Compared to traditional wireless information
transmission, WIPT faces numerous challenges. First, it is more susceptible to
channel fading and path loss, resulting in a much shorter power transfer
distance. Second, it gives rise to the issue on how to balance spectral
efficiency for information transmission and energy efficiency for power
transfer in order to obtain an optimal tradeoff. Third, there exists a security
issue for information transmission in order to improve power transfer
efficiency. In this context, multi-antenna techniques, e.g., energy
beamforming, are introduced to solve these problems by exploiting spatial
degree of freedom. This article provides a tutorial on various aspects of
multi-antenna based WIPT techniques, with a focus on tackling the challenges by
parameter optimization and protocol design. In particular, we investigate the
WIPT tradeoffs based on two typical multi-antenna techniques, namely limited
feedback multi-antenna technique for short-distance transfer and large-scale
multiple-input multiple-output (LS-MIMO, also known as massive MIMO) technique
for long-distance transfer. Finally, simulation results validate the
effectiveness of the proposed schemes.
