Managing RF spectrum usage by secondary, or unlicensed, devices in portions of the RF spectrum where licensed, or primary, services operate is a requirement of cognitive radio systems. To perform the necessary spectrum scanning, a transition period, such as a TTG or RTG can be extended and the scanning performed during the extended period. The presence of a primary service signal can also detected by detecting an energy signature of the primary service signal in a known sequence. In both cases, there is no need to define quiet periods during which secondary devices scan the RF spectrum, thus resulting in saving of resources and improved QoS.