On the Optimal Power Allocation for Nonregenerative OFDM Relay Links


Ingmar Hammerström and Armin Wittneben


Proc. IEEE International Conf. on Communications (ICC), Istanbul, Turkey, June 2006.

DOI: 10.1109/ICC.2006.255341

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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 over the frequency subchannels at source and relay, with respect to separate transmit power constraints, if channel state information at the transmitter is given. We provide the optimal power allocation at the relay (source) that maximizes the instantaneous capacity for a given source (relay) power allocation. Furthermore, we show that alternate, separate optimization of source and relay power allocation converges to the solution of the joint power allocation optimization at source and relay. To further enhance the capacity 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. Furthermore, we propose to allocate the transmit power between source and relay in the case of a joint sum power constraint with respect to average channel attenuation of first and second hop. It is shown that this leads to a capacity that is near to the optimal capacity of joint power allocation optimization with joint transmit power constraint.

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