Reduction of out-of-band power and peak-to-average power ratio in OFDM-based cognitive radio using alternating projections

Non-contiguous OFDM technique can be used for the cognitive radio systems. The sidelobes of the OFDM-modulated tones cause the out-of-band power which can induce large interference to the incumbent communication systems. Another major drawback of OFDM-based systems is the high peak-to-average power ratio. In this paper, we propose a scheme to jointly reduce the out-of-band power and the peak-to-average power ratio of the OFDM transmission. A subset of the frequency tones do not carry any data but are weighted and the time-domain OFDM symbol is expanded at both edges with a few adjusting chips. The joint reduction scheme is developed based on the low-complexity method of alternating projections onto convex sets. In a few iterations, the transmitter removes the large peaks of the time-domain OFDM signal and reduces the out-of-band power. The receiver detects the data on the subcarriers with little performance degradation.