  In future practical deployments of massive multi-input multi-output (MIMO)
systems, the number of radio frequency (RF) chains at the base stations (BSs)
may be much smaller than the number of BS antennas to reduce the overall
expenditure. In this paper, we propose a novel design framework for joint data
and artificial noise (AN) precoding in a multiuser massive MIMO system with
limited number of RF chains, which improves the wireless security performance.
With imperfect channel state information (CSI), we analytically derive an
achievable lower bound on the ergodic secrecy rate of any mobile terminal (MT),
for both analog and hybrid precoding schemes. The closed-form lower bound is
used to determine optimal power splitting between data and AN that maximizes
the secrecy rate through simple one-dimensional search. Analytical and
numerical results together reveal that the proposed hybrid precoder, although
suffers from reduced secrecy rate compared with theoretical full-dimensional
precoder, is free of the high computational complexity of large-scale matrix
inversion and null-space calculations, and largely reduces the hardware cost.
