Distributed beamforming and subcarrier power allocation for OFDM-based asynchronous two-way relay networks

We consider an asynchronous two-way relay network where the signal paths, going through different relays, are subject to different propagation and/or processing delays. Such a relay channel can be viewed as an artificial multi-path channel which causes inter-symbol-interference (ISI). To combat such an ISI, orthogonal frequency division multiplexing (OFDM) is deployed at the two transceivers while the relays are to use simple amplify-and-forward (AF) relaying protocol, thereby realizing a beamformer in a distributed manner. For such a communication scheme, we present two max-min design approaches to obtain jointly optimal subcarrier power loading at the transceivers and distributed beamforming weights at the relays. Numerical examples are presented to compare the performance of the two algorithms.