A combination of subscriber clustering and link interleaving provides a cognitive radio system (CR) 100 with opportunities to sense an incumbent system's spectrum on secondary basis. The CR system (100) uses clustering to identify out-of-band channels. The CR system (100) uses link interleaving during a second mode of operation to sense and detect any incumbent (120) on in-band channels. A list of out-band channels are sensed by clusters (0, 1, 2, 3) sequentially to generate a ranked list of potential channels for future use by the CR system. These out-of-band channels can be used opportunistically in case of in-band incumbent detection.