Fig. 3

Conventional CR system and proposed IA-based spectrum sensing. According to the busy/idle state of PU in a, this figure represents the process of conventional CR system in b, default mode in c, and dual transceiver mode in d. Since the conventional CR system can only sense the primary signal only during the short sensing time, the secondary system gives harmful interference to the primary system during the data transmission as shown in b. As shown in c, the sensor node can continuously sense the primary signal. At time (1), the sensor node broadcasts the primary non-detection report to SUs so that SUs can utilize the data channel using interference alignment. At time (2) and time (3), the sensor node detects the primary signal so that it sends the primary detection report at (4). As we can see in c, SUs can seize their transmission until the primary signal is not detected. d The case that dual transceivers are used. At time (5) when the sensor node detects the primary signal, it can immediately send the detection notification using the dedicated control channel