We investigate a collaborative-relay beamforming design for simultaneous wireless information and power
transfer. A non-robust beamforming design that assumes availability of perfect channel state information (CSI) in the
relay nodes is addressed.  In practical scenarios, CSI errors are usually inevitable; therefore, a robust collaborative-
relay beamforming design is proposed.  By applying the bisection method and the semidefinite relaxation (SDR)
technique, the non-convex optimization problems of both non-robust and robust beamforming designs can be solved.
Moreover, the solution returned by the SDR technique may not always be rank-one; thus, an iterative sub-gradient
method is presented to acquire the rank-one solution. Simulation results show that under an imperfect CSI case, the
proposed robust beamforming design can obtain a better performance than the non-robust one.

关键词： Simultaneous wireless information and power transfer,  Channel state information,  Robust beamforming,  Semidefinite relaxation,  Iterative sub-gradient