In this paper we propose an interference alignment (IA) scheme in which the performance of the secondary users (SUs) transmitting their information on the same frequency band of the pre-existing primary links is improved in terms of achievable sum rate without generating any interference on the primary network. In our scenario, in which the primary users (PUs) are cooperating through IA and a set of SUs join the network, we propose an efficient approach to find the values of the pre- and postprocessing filters of these SUs. Moreover, the SUs must not receive/generate any interference from/to the PUs.
In order to preserve the primary network from the interferences caused by the SUs, we also employ a method in which the distance between the subspace of the received signal from each secondary transmitter at the PUs and the set of interference subspaces at the primary links is minimized. In result, we would tackle with a computationally hard optimization problem with the sum rate of the secondary links as the objective function. We present the solutions in the cases where one or two SUs join the network. In situations where more SUs join the network simultaneously, we may apply other levels of IA between the secondary links after suppressing the mutual interferences between the primary and secondary networks. The analytical results are confirmed through simulations.