Interference cancellation and alignment techniques for multiple-input and multiple-output cognitive relay networks

We consider a multiple-input and multiple-output (MIMO) cognitive radio (CR) network with a MIMO relay that opportunistically accesses the same frequency band as that of a MIMO primary network. In particular, both interference cancellation and interference alignment techniques have been investigated to enhance the achievable degrees of freedom (DoF) for the MIMO CR network. Based on the number of antennas at the primary network and the secondary network, the authors analytically quantify the maximum achievable DoF of the secondary network by using the proposed techniques. It is shown that the DoF obtained by the CR network in the presence of a MIMO relay is higher than that could be obtained without a relay. The analyses consider both sufficient and insufficient number of antennas at the relay in terms of the ability to separate and decode both the primary and secondary transmitted signals. The simulation results support the analytically quantified achievable DoF results.