Experimental approach

Spectrum sensing plays a critical role in improving spectrum utilization for cognitive radio networks. The majority of existing sensing approaches aim to detect the existence of a signal on a busy channel without differentiating whether a signal originates from a primary user or not. In this paper, we attempt to solve this challenge by an experimental approach using GNU radio and Universal Software Radio Peripheral (USRP) board. A two-stage sensing strategy is proposed to detect channel states. The first stage detects whether a channel is busy or idle by observing both the energy level and bandwidth. Once the channel is busy, we further determine the channel is occupied by a Primary User (PU) and an Secondary User (SU). From experiments, we observe that a receiver cannot successfully receive a signal if it implements different demodulation schemes with a transmitter. For example, a Binary Phase Shift Keying (BPSK) demodulator cannot detect a Quadrature Phase Shift Keying (QPSK) signal transmitted by a transmitter. Motivated by this observation, we propose that at the second stage, PUs and SUs are negotiated to conduct different modulations. Then SUs are equipped with two demodulators that will be able to demodulate both the PUs signal and the SUs signal. Given that SU and PU perform distinct modulations, once the PU comes to its licensed channel, the SU communicating on that channel will recognize it and withdraw from the channel immediately. Extensive experiments using GNU radio and USRP boards are carried out under various scenarios to evaluate the effectiveness of the proposed approach.