PAPR reduction in OFDM based cognitive radio with blockwise-subcarrier activation

In this paper, we consider the high peak-to-average power ratio (PAPR) problem of non-contiguous orthogonal frequency division multiplexing (NC-OFDM) signals in cognitive radio systems. A high PAPR can lead to saturation in the power amplifier (PA) of secondary users (SUs) and consequently increase spectral spreading, and cause interference to adjacent primary users (PUs). To overcome this problem, existing PAPR reduction techniques for OFDM systems can be applied to NC-OFDM, but they should provide a low PAPR with no side information and relatively low complexity. We consider NC-OFDM with blockwise-subcarrier activation and show that it can intrinsically employ tone reservation (TR) as a PAPR reduction technique. The proposed TR reserves subcarriers within inactive subblocks that are not used by the primary and secondary users. This eliminates data rate loss due to reserved peak reduction tones (PRTs). Further, dynamic PRT allocation in NC-OFDM typically requires side information about the PRT locations to be sent to the receiver. Since we choose PRTs from inactive subblocks, they are simply discarded at the receiver without any side information. The power spectral density (PSD) and bit error rate (BER) are evaluated at the output of the nonlinear PAs to provide a realistic performance comparison.