A sub-optimal sidelobe suppression technique for OFDM-based cognitive radios

Orthogonal frequency division multiplexing (OFDM) is a transceiver technology capable of achieving spectrally efficient, high data rate wireless transmissions. Being also capable of transmitting in a non-contiguous fashion by utilizing several seperate spectral whitespaces, it is an ideal candidate for dynamic spectrum access (DSA)-based transceiver systems. However, the large sidelobes emerging from OFDM signal transmissions need to be reduced in order to conform to the FCC spectrum masks, thus enabling the coexistence between licensed and unlicensed transceivers. In this paper, we propose a cancellation carrier-based sidelobe suppression technique for OFDM-based cognitive radios. A selected number of subcarrier locations on either side of the OFDM spectrum are reserved for inserting cancellation carriers whose amplitude and phase are determined by using a straight-forward algebraic technique. The algorithm performing this algebraic technique is based on the fact that the subcarriers near the edges of the OFDM signal spectrum possess a greater impact on the sidelobe power levels than those that are interior in the transmission spectrum. Simulation results show that with only one cancellation carrier on each side of a 64 subcarrier QPSK-OFDM spectrum, there is a reduction of as much as 10 dB on the peak-interference causing sidelobe by considering the impact of 25% of the total number of number of subcarriers on the interference power levels. With two cancellation carriers on each side, the reduction is around 13 dB.