Interference Alignment with Cooperative Primary Receiver in Cognitive Networks

In cognitive radio systems, secondary users may transmit signals by aligning their signal directions to the primary user´s unused directions. However, their achievable transmission rates are very low in moderate to high SNR regime because the primary user takes most of available degrees of freedom (DoF). In order to increase the secondary transmission rate, we consider the case that the primary receiver performs interference suppression filtering. By counting the number of variables and the number of equations in the interference alignment (IA) condition, we analyze the secondary users´ achievable DoF, which is higher than the conventional scheme. Simulation results show that the primary receiver´s participation in IA significantly increases the secondary rate with negligible primary rate reduction.