Abstract:  Cognitive users can only access idle bandwidth in traditional cognitive networks, but they cannot share the same bandwidth with the primary user simultaneously. In order to make full use of the spectrum, we propose a capacity optimal opportunistic interference alignment algorithm. By ensuring the channel capacity of primary users, cognitive users and primary users can share the same bandwidth, and the cognitive users cause no interference to the primary user. In the proposed algorithm, the hybrid network base station estimates channel state information and calculates channel capacity. It then determines the pre-coding vectors and decoding vectors according to the capacity optimal rule. Cognitive users access the networks by capacity optimal opportunistic interference alignment when the primary user’s channel capacity almost does not decline and the whole network can still obtain good channel capacity. Simulation results show that the proposed algorithm can obtain greater channel capacity and increase the utilization ratio of spectrum than other existing algorithms under the same condition of channel state information.

Key words: cognitive networks, opportunistic interference alignment, channel capacity, pre-coding