Achievable rates for cognitive radios opportunistically permitting excessive secondary-to-primary interference

We propose a cognitive radio system in fading environments where the secondary sender opportunistically violates the secondary-to-primary (S-P) interference power limit. Assuming a slowly-varying fading interference channel with two-senders and two-receivers, the primary sender adjusts its rate and power depending on its own channel, while ignoring the secondary user. Through an optimization process, the secondary sender´s rate and power are determined and the decision is made on whether the secondary sender violates the S-P interference power limit. The primary receiver removes the effect of excessive S-P interference by multiuser decoding (MUD) which jointly decodes the primary and secondary senders´ data. Achievable rates of the proposed system are examined by computer simulation. It was observed that the proposed system tends to violate the SP interference power limit when the S-P channel gain is larger than the other channel gains. Remarkably, the proposed system can provide a considerably high data rate for the secondary user even without sacrificing the data rate for the primary user over a wide range of signal-to-noise ratios.