To obtain the optimal performance of the cooperative spectrum sensing based on counting rule decision fusion

the relation between the information contained in local decision and received signal-to-noise ratio of secondary user（SU）is studied theoretically.The condition with which the optimal decision fusion can be achieved is given.An algorithm is proposed to sort the SUs and exclude those whose signal-to-noise ratio are lower than a threshold from cooperative scheme.The results of calculation verified the correctness of our theorectical analysis.The simulation results illustrated the good performance of the algorightm on sorting the SUs and screening them.