Signal instances are detected from within a long period of a received radiofrequency RF signal envelope, grouped according to signal level, and periodicities are determined among the grouped signal instances. Periodicity of one of the groups is matched to a known periodicity, and a frequency domain is estimated at a time instance derived from the matched periodicity. If an expected frequency pattern is found that occupies the derived time instance in the estimated frequency domain, then a receiver is synchronized to a candidate signal that lies within the frequency pattern. Then content of the candidate signal is decoded and that content is used to access a cognitive radio system (e.g., register to the system, start the normal operation in the network such as spectrum detection and/or transmission in a traffic channel, etc.).