Power Allocation Schemes for Amplify-and-Forward MIMO-OFDM Relay Links


Ingmar Hammerström and Armin Wittneben


IEEE Transactions on Wireless Communications, VOL. 6, NO. 8, pp. 2798 - 2802, 2007.

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We consider a two-hop MIMO-OFDM communication scheme with a source, a relay, and a destination. The relay is assumed to be regenerative (i.e., decode-and-forward) or nonregenerative (i.e., amplify-and-forward). We examine the possibilities of power allocation (PA) over the subchannels in frequency and space domain to maximize the instantaneous rate of this link if channel state information at the transmitter (CSIT) is available. We consider two approaches: (i) separate optimization of source or relay PA with separate node transmit power constraints and (ii) joint optimization of source and relay PA with joint transmit power constraint. For nonregenerative relaying we provide the optimal PA at the relay (or source) with a node transmit power constraint that maximizes the instantaneous rate for a given source (or relay) PA. Furthermore, we show that repeating this separate optimization of source and relay PA with node power constraints alternately converges and improves the achievable rate of the considered link. Since the joint optimization of source and relay PA is analytically not tractable we present PAs which achieve a very tight lower bound on the rates achieved by the optimal PA. We show that regenerative relaying with and without CSIT achieves much higher average rates than nonregenerative relaying with CSIT in most of the cases. Moreover, we show that the performance gain due to CSIT compared to no CSIT is much higher in the case of nonregenerative relaying than for regenerative relaying.


power allocation, water-filling, MIMO, OFDM, relaying

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