Joint subcarrier power loading and distributed beamforming in OFDM-based asynchronous relay networks

In this paper, we consider an asynchronous relay network where different relaying paths are subject to different propagation delays thus causing inter-symbol-interference (ISI) at the destination. To eliminate such an ISI, orthogonal frequency division multiplexing (OFDM) is deployed at the source and destination but not at the relays. In fact each relay performs a simple amplify-and-forward operation: it multiply its received signal with a complex coefficient. We model such a network as a multipath channel where the equivalent channel impulse response can be carefully adjusted through optimal design of the complex relay weights. Based on this model, we use a max-min fair design approach where the smallest of the subcarrier SNRs is maximized subject to individual constraints on the source and relay transmit powers. Our numerical results show that the proposed asynchronous scheme achieves SNRs which are only 2 dB lower than those achieved by the synchronous scheme where the relays perfectly compensate their corresponding delays.