  Considering a three-node multiple antenna relay system, this paper proposes a
two-phase amplify-and-forward (AF) relaying protocol, which enables the
autonomous relay to simultaneously harvest wireless power from the source
information signal and from an energy signal conveyed by the destination. We
first study this energy-flow-assisted (EFA) relaying in a single-input
single-output (SISO) relay system and aim at maximizing the rate. By
transforming the optimization problem into an equivalent convex form, a global
optimum can be found. We then extend the protocol to a multiple antenna relay
system. The relay processing matrix is optimized to maximize the rate. The
optimization problem can be efficiently solved by eigenvalue decomposition,
after linear algebra manipulation. It is observed that the benefits of the
energy flow are interestingly shown only in the multiple antenna case, and it
is revealed that the received information signal and the energy leakage at the
relay can be nearly separated by making use of the signal space, such that the
desired signal can be amplified with a larger coefficient.
