EVM Based Primary User Monitoring in Cognitive Radio Systems

The periodic sensing requirements in cognitive radios cause interruptions for secondary user (SU) communication. As a result, the throughput of the secondary system during sensing can be very low. In this paper, we propose a monitoring scheme for Orthogonal Frequency Division Multiplexing (OFDM) based cognitive radio which can monitor the spectrum during ongoing communication and detect the emergence of primary users (PUs). Our scheme permits secondary cognitive radios to perform spectrum monitoring while receiving packets. The proposed method exploits the pilot tones that are inherent to many OFDM based standards to measure the error vector magnitude (EVM) of the group of pilots (GOPs). A step change in the EVM curve signifies the emergence of the PU.