Filter-and-forward beamforming in asynchronous relay networks

We consider optimal filter-and-forward (FF) beamforming (BF) in multi-user peer-to-peer (MUP2P) relay networks with frequency selective fading in which the signals received at a destination are subject to severe inter-symbol interference (ISI) and multi-user interference (MUI). In the FF protocol, the amount of interference not only depends on the choice of the filter weights at the relays but also on the choice of a decoding delay at the destinations. State-of-the-art methods assume the network to be perfectly synchronized and select the same fixed decoding delay for all destinations. In a practical MUP2P scenario, however, users experience different path delays not only with respect to different relays but also with respect to other users. Hence, unlike in a conventional cellular network, perfect synchronization is impossible in practice and a common decoding delay for all users is not a good choice. Therefore, we propose to jointly optimize the filter weights at the relays and the individual decoding delays at the destinations to effectively mitigate ISI and MUI. The proposed method outperforms the existing approaches most significantly in asynchronous networks.