In 802.22 simulations, the secondary stops transmissions when the primary comes on. Since the transmissions are stopped the TCP RTO Expires and window drops to one. 

Subsequently, for each packet whose ack is not received, the RTO doubles. Thus over the period of operation of the primary the TCP RTO increases greatly. 

Hence when the incumbent goes off, the secondary doesn't start transmission since TCP continues to wait for RTO timer to expire.

Users are therefore advised to run cognitive radio simulation with UDP in the transport layer.