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

Hefdhallah Sakran,Omar Nasr,Mona Shokair

doi:10.5402/2012/584941

Abstract

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%.

Highlights

There is an unparalleled increase in the usage of wireless devices in the last decade
The Complementary Cumulative Distribution Function (CCDF) shows the probability of having the Peak to Average Power Ratio (PAPR) of an Non-Contiguous Orthogonal Frequency Division Multiplexing (NC-OFDM) frame exceeding a given PAPR0
We have introduced three proposed algorithms for PAPR reduction of NC-OFDM signals

Summary

Introduction

There is an unparalleled increase in the usage of wireless devices in the last decade. 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 related to PAPR problem. The authors in [7] proposed a joint sidelobe and PAPR reduction method that can simultaneously reduce the inherent OFDM out-of-band (OOB) radiation and the OOB radiation caused by a nonlinear power amplifier. In this method, selective mapping (SLM) sequences, which are generated from sequences with a low average sidelobe power, are used to improve the PAPR performance. We present three proposed techniques for reducing PAPR in NC-OFDM-based systems.

PAPR for NC-OFDM System

Proposed Techniques

Simulation Results

Conclusions