A New Signal Structure for Active Sensing in Cognitive Radio Systems

This paper presents a new signal structure for spectrum sensing under active cognitive radio (CR) transmission (i.e., active sensing). Specifically, the known pilots used for the base station of primary users (PU) are duplicated and reallocated in the CR transmission signal properly so that there is no overlap between the reallocated pilot subcarriers and the original subcarriers. Given the CR signal structure, the signal received by a spectrum sensor of the CR system becomes correlated on the subcarriers when PU reoccupation occurs during the data-transmission periods, which makes it easy to detect PU activities by computing the spectral correlation function of the received signal. Both theoretical analyses and simulation results are given to demonstrate the effectiveness of the proposed active-sensing approach. The proposed method attains a detection probability of 0.99 and a false-alarm probability of 0.01 within 170 orthogonal frequency-division multiplexing symbols under a signal-to-noise ratio of -25dB and a signal-to-interference ratio of -20dB. The performance would be degraded under practical conditions, but is acceptable in most cases if the carrier frequency offset is less than 0.3. Compared with traditional cyclostationary feature detection, the proposed approach requires only about half the sensing time to achieve the same sensing accuracy, at about a 3% loss of throughput.