A new approach to distributed cooperative beamforming in relay networks withfrequency selective fading is proposed. It is assumed that all the relay nodesare equipped with finite impulse response (FIR) filters and use afilter-and-forward (FF) strategy to compensate for the transmitter-to-relay andrelay-to-destination channels. Three relevant half-duplex distributed beamforming problems are considered.The first problem amounts to minimizing the total relay transmitted powersubject to the destination quality-of-service (QoS) constraint. In the secondand third problems, the destination QoS is maximized subject to the total andindividual relay transmitted power constraints, respectively. For the first andsecond problems, closed-form solutions are obtained, whereas the third problemis solved using convex optimization. The latter convex optimization techniquecan be also directly extended to the case when the individual and total powerconstraints should be jointly taken into account. Simulation resultsdemonstrate that in the frequency selective fading case, the proposed FFapproach provides substantial performance improvements as compared to thecommonly used amplify-and-forward (AF) relay beamforming strategy.
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