International fora and some national administrations define a cognitive radio (CR) as a pioneering radio communication system that would be capable of altering and adapting its transmitter and receiver parameters based on communication and the exchange of information with related detectable radio communication systems. It is crucial for the acceptance of CR that the CR radio emissions do not impact the performance of incumbent radiocommunication systems. Possible co-existence techniques can be divided into collaborative and non-collaborative methods where the former allow a limited amount of communication between the incumbent systems and the CR, and the latter require autonomous operation of the CR network. In the non-collaborative techniques, the CR has to detect and avoid the transmission channels of the incumbent systems. Since the signal levels can be rather low, in particular when spread spectrum techniques are used, and because errorneous conclusions can be drawn due to shadowing and multi-path fading (the hidden node problem), it is not recommended that CR techniques are deployed in bands where interference sensitive or licensed QoS operations are generally used.