Interference as a Source of Green Signal Power in Cognitive Relay Assisted Co-Existing MIMO Wireless Transmissions

This paper investigates the potential of exploiting interference as a source of green signal energy in parallel transmissions of co-existing radio systems assisted by a cognitive relay. Assuming a cognitive radio setup, the purpose of the relay is to allow the resources of a primary downlink to be efficiently utilised by a secondary system. While conventionally the relay aims to completely remove the interference, we investigate a strategy of making use of interference energy when the cross-interference between the two systems is mutually constructive. In this way, the interference that already exists in the communication medium provides a source of green signal energy that mutually enhances the received signal power of primary and secondary users without the need to raise the transmitted power. In this direction two adaptive linear precoding techniques are proposed for the cognitive relay and compared to conventional precoding. The effect of green interference on the received signal to noise ratio (SNR) of the primary and secondary users is studied through theoretical analysis and used to predict the resulting error probability and outage performance. The results show that by exploiting free interference power, the secondary downlink can access the primary resources without deteriorating the primary users´ performance.