  The new paradigm of wirelessly powered two-tier heterogeneous networks
(HetNets) is considered in this paper. Specifically, the femtocell base station
(FBS) is powered by a power beacon (PB) and transmits confidential information
to a legitimate femtocell user (FU) in the presence of a potential eavesdropper
(EVE) and a macro base station (MBS). In this scenario, we investigate the
secrecy beamforming design under three different levels of FBS-EVE channel
state information (CSI), namely, the perfect, imperfect and completely unknown
FBS-EVE CSI. Firstly, given the perfect global CSI at the FBS, the PB energy
covariance matrix, the FBS information covariance matrix and the time splitting
factor are jointly optimized aiming for perfect secrecy rate maximization. Upon
assuming the imperfect FBS-EVE CSI, the worst-case and outage-constrained SRM
problems corresponding to deterministic and statistical CSI errors are
investigated, respectively. Furthermore, considering the more realistic case of
unknown FBS-EVE CSI, the artificial noise (AN) aided secrecy beamforming design
is studied. Our analysis reveals that for all the above cases both the optimal
PB energy and FBS information secrecy beamformings are of rank-1. Moreover, for
all considered cases of FBS-EVE CSI, the closed-form PB energy beamforming
solutions are available when the cross-tier interference constraint is
inactive. Numerical simulation results demonstrate the secrecy performance
advantages of all proposed secrecy beamforming designs compared to the adopted
baseline algorithms.
