Rate-optimized power allocation for OFDM transmission with multiple DF relays and individual power constraints

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 DF (Decode-and-Forward) type. For each relayed carrier the destination implements maximum ratio combining between what is received over the direct link and what is received from the relay(s). Perfect CSI (channel state information) knowledge is assumed. The paper investigates the power allocation problem in order to maximize the rate offered by the scheme. The source is allowed to transmit a new symbol during the second time slot when none of the relays is assisting. The constraints of decodability at the relays are properly handled. The optimization is conducted for individual constraints on the powers at the source and at the relays. The theoretical analysis is illustrated by numerical results.