Filter-and-forward distributed beamforming in relay-assisted cognitive networks

In this paper, two novel approaches to filter-and-forward beamforming (FF-BF) in an underlay based relay-assisted cognitive network with frequency-selective channels are proposed. In our scheme, secondary users are allowed to transmit through multiple relays simultaneously with primary users over the same spectrum. Such a network suffers from both inter-symbol interference (ISI) and bidirectional interference between the networks. We design FF-BF coefficients of the relays to mitigate these two types of interference. Simulation results show that the proposed approaches improve system performance considerably in frequency-selective environments in comparison to the conventional amplify-and-forward beamforming strategy.