Sidelobe Suppression for OFDM-Based Cognitive Radios Using Constellation Expansion

In this paper, we present a novel algorithm for reducing sidelobe interference power levels in OFDM-based cognitive radios. Existing techniques for sidelobe suppression can be computationally intensive when determining the complex-valued amplitude levels for the cancellation subcarriers. Exploiting the fact that different sequences have different sidelobe power levels, the proposed algorithm employs a constellation expansion-based iterative approach in order to suppress the sidelobe power levels. An important advantage of the proposed technique is that, no side information needs to be transmitted. Simulation results show that the proposed algorithm can be employed in a wide range of operating conditions at the cost of a slight increase in the bit error rate and the peak-to-average power ratio characteristics.

I. Introduction

Orthogonal frequency division multiplexing (OFDM) has been a prime candidate for spectrum pooling-based wireless transmission systems since subcarriers in the vicinity of the licensed users can be deactivated in order to minimize interference from the unlicensed users [1], [2]. Although secondary transmissions improve spectral efficiency by utilizing spectral white spaces unused by the primary users, OFDM-based secondary users may potentially interfere with existing transmissions due to their large out-of-band (OOB) sidelobe levels. Therefore, the interference from the secondary users needs to be suppressed sufficiently in order to enable spectrum sharing with primary users.