  In this paper, we study a massive multiple-input multiple-output (mMIMO)
relay network where multiple source-destination pairs exchange information
through a common relay equipped with a massive antenna array. The source users
perform simultaneous wireless information and power transfer (SWIPT) and the
power-splitting (PS) scheme is used at the relay to first harvest energy from
the received signals, and then, to transmit the decoded signals using the
harvested energy. Relay performs maximum ratio combining/maximum ratio
transmission (MRC/MRT) beam-forming on the received signal. Under the
three-dimensional (3D) directional channel model, we derive a closed-form lower
bound expression for the average signal-to-interference-plus-noise ratio using
results from random matrix theory, which leads to an asymptotic approximation
of the achievable sum-rate. Based on that, we study a joint optimization
problem over the tilt and PS ratio to maximize the achievable sum-rate. Grid
search algorithm is used to solve the non-convex problem. Simulation results
verify our theoretical analysis and the efficiency of our optimized design. In
particular, our optimized system outperforms a conventional system with $\pi/4$
tilt and PS of $0.5$, by at least $61\%$.
