  The implementation of wireless power transfer in wireless communication
systems opens up a new research area, known as wirelessly powered
communications (WPC). In next-generation heterogeneous networks where
ultra-dense small-cell base stations are deployed,
simultaneous-wireless-information-and-power-transfer (SWIPT) is feasible over
short ranges. One challenge for designing a WPC system is the severe near-far
problem where a user attempts to decode an information-transfer (IT) signal in
the presence of extremely strong SWIPT signals. Jointly quantizing the mixed
signals causes the IT signal to be completely corrupted by quantization noise
and thus the SWIPT signals have to be suppressed in the analog domain. This
motivates the design of a framework in this paper for analog spatial
cancellation in a multi-antenna WPC system. In the framework, an analog circuit
consisting of simple phase shifters and adders, is adapted to cancel the SWIPT
signals by multiplying it with a cancellation matrix having unit-modulus
elements and full rank, where the full rank retains the spatial-multiplexing
gain of the IT channel. The unit-modulus constraints render the conventional
zero-forcing method unsuitable. Therefore, the paper presents a novel
systematic approach for constructing cancellation matrices. For the
single-SWIPT-interferer case, the matrices are obtained as truncated
Fourier/Hadamard matrices after compensating for propagation phase shifts over
the SWIPT channel. For the more challenging multiple-SWIPT-interferer case, it
is proposed that each row of the cancellation matrix is constructed as a
Kronecker-product of component vectors, with each component vectors designed to
null the signal from a corresponding SWIPT interferer similarly as in the
preceding case.
