These

cooperations are facilitated by dynamically and opportunistically assigning one of the secondary users as a relay to assist the other

one, which results in more efficient spectrum hole detection. Also, we investigate the impact of erroneous detection of spectrum holes

and thereof missing communication opportunities on the capacity of the secondary channel. The capacity of the secondary users with

interference-avoiding spectrum access is affected by 1) how effectively the availability of vacant spectrum is sensed by the secondary

transmitter-receiver pair, and 2) how correlated are the perceptions of the secondary ransmitter-receiver pair about network spectral

activity. We show that both factors are improved by using the proposed cooperative protocols.