Power Allocation for MIMO Systems with Multiple Non-Regenerative Single-Antenna Relays

We consider a multiple-input multiple-output (MIMO) system assisted by multiple non-regenerative single-antenna relays. This system where the antennas of all the relays are distributed geographically is fundamentally different from a single relay case with multiple antennas. The optimal power allocation for a relay with multiple antennas has already been known. In that solution, the power is distributed among the eigenmodes which are computed by the singular value decompositions of the first and the second hop channels. However, no closed form solution is known for the proposed system. In this paper, we propose a near optimal inter-relay power allocation method that combines the relay selection with the water-filling type closed form solution based on an asymptotic approach. In fact, the asymptotic approach itself is effective only for a specific antenna configuration, but we demonstrate through numerical simulations that if we combine this approach with the relay selection technique, the performance is comparable to the optimal case by exhaustive search even for a general antenna configuration.