Space-Time Opportunistic Interference Alignment In Cognitive Radio Networks

For a multiuser multiple-input-multiple-output (MIMO) overlay cognitive radio (CR) network, a space-time opportunistic interference alignment (ST-OIA) technique has been proposed that allows spectrum sharing between primary users (PU) and secondary users (SU) while ensuring zero interference to the PU. The CR system consists of one primary user (PU) and K secondary users (SU) where local channel state information is available at both the transmitters and receivers of SUs. The PU uses space-time water-filling (ST-WF) algorithm to optimize the PUs transmission and in the process, frees up unused eigenmodes that can be exploited by the SU. Because ST-WF achieves higher capacity per antenna than other methods, at low to moderate SNR regimes, it makes it ideal for implementation in CR networks. The SUs align their transmitted signals in such a way their interference impairs only the PUs unused eigenmodes. For this solution with multiple SUs exploiting the benefits of cooperative spectrum sensing to work, there are three separate conditions which must be met. For single user MIMO PU and SU link, the first condition requires there should be zero interference at the PU receiver and secondly, there should be zero interference to both the PU and SU receivers. The third condition caters for the multiple SU scenario which requires limited cooperation between the PU receiver and the multiple SUs to ensure interference from multiple SUs are aligned along unused eigenmodes. Finally, the SU system is assumed to be a time division duplex (TDD) system such that principle of Reciprocity is employed towards optimizing the SUs transmission rates.