A challenge met in realizing spectrum sensing for cognitive radio is how to identify occupied channels quickly, thus reducing the time required to identify unoccupied channels that can be utilized by the cognitive radio. The spectrum sensor makes a series of measurements and then utilizes those measurements to determine if the channel is occupied or not An adaptive spectrum sensing scheme is proposed, which exploits the simple fact that for typical spectrum sensing systems the accumulation of channel statistics is a non-decreasing process. The adaptive sensing scheme is a variable-length decision procedure, having a hard limit on the maximum number of observations, and yielding the same performance as the optimal fixed-sample-size Neyman-Pearson detection. The behavior of the expected number of observations is investigated, and is compared with the sequential probability ratio tests.
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