  In this paper, downlink secure transmission in simultaneous information and
power transfer (SWIPT) system enabled with massive multiple-input
multiple-output (MIMO) is studied. A base station (BS) with a large number of
antennas transmits energy and information signals to its intended users, but
these signals are also received by an active eavesdropper. The users and
eavesdropper employ a power splitting technique to simultaneously decode
information and harvest energy. Massive MIMO helps the BS to focus energy to
the users and prevent information leakage to the eavesdropper. The harvested
energy by each user is employed for decoding information and transmitting
uplink pilot signals for channel estimation. It is assumed that the active
eavesdropper also harvests energy in the downlink and then contributes during
the uplink training phase. Achievable secrecy rate is considered as the
performance criterion and a closed-form lower bound for it is derived. To
provide secure transmission, the achievable secrecy rate is then maximized
through an optimization problem with constraints on the minimum harvested
energy by the user and the maximum harvested energy by the eavesdropper.
Numerical results show the effectiveness of using massive MIMO in providing
physical layer security in SWIPT systems and also show that our closed-form
expressions for the secrecy rate are accurate.
