On the Optimal Power Allocation for Nonregenerative OFDM Relay Links

We consider a two-hop communication scheme using OFDM modulation. The relay is assumed to be nonregenerative (or amplify-and-forward). We examine the possibilities of power allocation (PA) over the frequency subchannels at source and relay, with respect to separate transmit power constraints, if channel state information at the transmitter (CSIT) is given. We provide the optimal PA at the relay (source) that maximizes the instantaneous rate for a given source (relay) PA. Furthermore, we show that alternate, separate optimization of source and relay PA converges to the solution of the joint optimization of source and relay PA. To further enhance the rate of the considered scheme, the OFDM subchannels of the source to relay and relay to destination channel can be paired according to their actual magnitude. In the case of a joint sum power constraint, we propose to allocate the transmit power between source and relay with respect to average channel attenuation of first and second hop. It is shown that this leads to a information rate that is near to the optimal rate achieved by a joint optimization of source and relay PA with joint transmit power constraint.