An adaptive signal detector to detect a signal in a spectrum space includes a software defined receiver for calculating a cumulative decision statistic for determining the presence of the signal in the spectrum space. The cumulative decision statistic is determined by a sequential energy detector that includes a block energy detector that has a block termination rule, a block sampling instant and a block decision phase. A truncated sequential energy detector has a truncated sequential termination rule and a truncated sequential decision phase. The block energy detector and truncated sequential energy detector are interoperated to provide the cumulative decision statistic in which the block energy sampling instants provide samples that are independent and identically distributed. The cumulative decision statistic determines the presence of the signal in the spectrum space based upon a non-Bayesian dynamic mathematical modeled relationship of the cumulative decision statistic.