An Efficient Adaptive Technique with Low Complexity for Reducing PAPR in OFDM-Based Cognitive Radio

Cognitive radio (CR) is considered nowadays as a strong candidate solution for the spectrum scarcity problem. On standards level, many cognitive radio standards have chosen Non-Contiguous Orthogonal Frequency Division Multiplexing (NC-OFDM) as their modulation scheme. Similar to OFDM, NC-OFDM suffers from the problem of having a high Peak to Average Power Ratio (PAPR). If not solved, either the transmitted signal will be distorted, which will cause interference to primary (licensed) users, or the effeciency of the power amplifier will be seriously degraded. The effect of the PAPR problem in NC-OFDM based cognitive radio networks is worse than normal OFDM systems. In this paper, we propose enhanced techniques to reduce the PAPR in NC-OFDM systems. We start by showing that combining two standard PAPR reduction techniques (interleaver-based and selective mapping) results in a lower PAPR than using them individually. Then, an “adaptive number of interleavers” will be proposed that achieves the same performance of conventional interleaver-based PAPR reduction while reducing the CPU time by 41.3%. Finally, adaptive joint interleaver with selective mapping is presented, and we show that it gives the same performance as conventional interleaver-based technique, with reduction in CPU time by a factor of 50.1%. 1. Introduction There is an unparalleled increase in the usage of wireless devices in the last decade. However, most of the frequency spectrum has already been licensed exclusively to operators by government agencies, such as Federal Communications Commission (FCC). Therefore, there exists an apparent spectrum scarcity for new wireless applications and services. In recent studies, especially by the FCC, it is reported that there are vast temporal and spatial variations in the allocated spectrum utilization. The spectrum utilization efficiency can be as low as 15% [1]. Recently, cognitive radio [2, 3] has attracted much attention, as it can solve the spectrum scarcity problem [4] by allowing cognitive users (unlicensed users) to occupy the spectrum band when the primary users (licensed users) do not use their licensed spectrum. The main functions that a cognitive radio can perform are: spectrum sensing, spectrum management, spectrum mobility, and Spectrum Sharing [4]. From standards point of view, most of the cognitive radio-based standards have selected OFDM, or a variation of it, as their modulation scheme. The challenges of OFDM-based cognitive radio can be grouped into three categories, which are described in details in [5]. One of these categories is