  In this paper, we consider relay-assisted simultaneous wireless information
and power transfer (SWIPT) for two-hop cooperative transmission, where a
half-duplex multi-antenna relay adopts decode-and-forward (DF) relaying
strategy for information forwarding. The relay is assumed to be energy-free and
needs to harvest energy from the source node. By embedding power splitting (PS)
at each relay antenna to coordinate the received energy and information, joint
problem of determining PS ratios and power allocation at the multi-antenna
relay node is formulated to maximize the end-to-end achievable rate. We show
that the multi-antenna relay is equivalent to a virtual single-antenna relay in
such a SWIPT system, and the problem is optimally solved with closed-form. To
reduce the hardware cost of the PS scheme, we further propose the antenna
clustering scheme, where the multiple antennas at the relay are partitioned
into two disjoint groups which are exclusively used for information decoding
and energy harvesting, respectively. Optimal clustering algorithm is first
proposed but with exponential complexity. Then a greedy clustering algorithms
is introduced with linear complexity and approaching to the optimal
performance. Several valuable insights are provided via theoretical analysis
and simulation results.
