Rate-Optimized Power Allocation for OFDM Transmission with Multiple DF/Regenerative Relays and an Improved Protocol

We consider an OFDM (orthogonal frequency division multiplexing) point to point transmission scheme improved by means of multiple relays. For each carrier, symbols sent by the source during a first time slot, may be retransmitted during a second time slot by the relays, which are assumed to be of the decode-and-forward (DF) type. For each relayed carrier the destination implements maximum ratio combining. Assuming perfect CSI (channel state information) knowledge the paper investigates the power allocation problem in order to maximize the rate offered by the scheme. Similarly to the protocol proposed in, the source is allowed to transmit a new symbol during the second time slot when none of the relay is assisting. The constraints of decodability at the relays are properly handled. The optimization is conducted for a constraint on the sum of powers at the source and at the relays. Next to the optimized solution, a suboptimum method based on relay selection is proposed and discussed. The theoretical analysis is illustrated by numerical results.