Adaptive-Mode Peak-to-Average Power RatioReduction Algorithm for OFDM-based CognitiveRadioRakesh Rajbanshi Alexander M. Wyglinski Gary J. MindenInformation and Telecommunication Technology CenterThe University of KansasLawrence, Kansas 66045–7612Email: {rajbansh, alexw, gminden}@ittc.ku.eduAbstract— In this paper, we present a novel low complexityalgorithm for reducing the peak-to-average power ratio (PAPR)occurring in OFDM-based cognitive radios. Although severalPAPR reduction algorithms exist in the literature, they are oftenonly effective for specific scenarios. Our proposed algorithmexploits the agility of cognitive radio technology to rapidly chooseand employ the appropriate PAPR reduction approach from aset of approaches to achieve a large decrease in PAPR, given thecurrent operating conditions. The results show that for a widerange of operating conditions, the proposed algorithm achievesa large decrease in PAPR, unlikely the PAPR results when onlya single reduction approach is employed across the same widerange.I. INTRODUCTIONAs access to available spectrum is becoming increasinglydifficult, several researchers proposed the concepts of spectrumpooling and the use of non-contiguous portions of spectrum forhigh data rate transmission [1]. Non-contiguous OFDM (NC-OFDM) is a promising candidate for such a flexible spectrumpooling system, where the implementation achieves high datarates via collective usage of a large number of non-contiguoussubcarrier bands. Thus, the NC-OFDM can avoid interferingwith incumbent users respecting their rights to the spectrum,while still being able to transmit information.In general, OFDM offers high spectral efficiency, robustnessto channel fading, immunity to impulse interference, andthe capability to handle frequency-selective fading withoutresorting to complex channel equalization schemes [2, 3].However, one disadvantage of OFDM is that it could exhibita large envelope variation in the time domain, which is oftencharacterized by a large peak-to-average power ratio or PAPR(refer to Section III for details). When high PAPR occurs,the digital-to-analog (D/A) converter and power amplifier ofthe transmitter would require a large dynamic range to avoidamplitude clipping, thus increasing both power consumptionand component cost.To reduce PAPR in OFDM transceivers, there are severalsolutions proposed in the literature that could be employed bythe system. These solutions include the use of error controlcoding [4–8] and constellation shaping techniques [9–11].With respect to the latter, one constellation parameter thatcould be adjusted on a per-subcarrier basis is the subcarrierphase information. However, most of these algorithms possessa high computational complexity, especially for a large numberof subcarriers. Data randomization, such as interleaving, canbe employed to disrupt the long correlation patterns of aframe, hence reducing the PAPR of an OFDM signal [12].However, interleaving is effective only when the data frameshave moderate PAPR values. It was proposed that interleavingcan be combined with selective mapping (SLM) techniquesor partial transmit sequences (PTS) techniques for achievinggreater reduction in PAPR [12]. However, combining inter-leaving with these techniques will substantially increase theircomputational complexity.In this paper, we propose a novel low complexity algorithmthat adaptively chooses which PAPR reduction approach, orcombination thereof, to employ based on the current oper-ating conditions, such as instantaneous PAPR. The PAPRreduction approaches employed by the proposed algorithm inthis work are: (1) interleaving, and (2) per-subcarrier phaseadjustments. The proposed algorithm is then evaluated withinthe framework of cognitive radio transceiver employing NC-OFDM. The rest of the paper is organized as follows: Abrief overview of an NC-OFDM transceiver is presented inSection II. The definition of PAPR is presented in Section III.The proposed algorithm is presented in Section IV. Thesimulation results from this work are presented in Section Vand several conclusions are made in Section VI.II. NC-OFDM TRANSCEIVERA general schematic of an NC-OFDM transceiver employ-ing the proposed PAPR reduction algorithm is shown in Fig. 1.The basic principle of NC-OFDM is to split a high-speed datastream, x(n),intoN slower data streams that are transmittedsimultaneously over orthogonal subcarriers. For NC-OFDM,the active subcarriers need not be contiguous and are locatedin unoccupied spectrum bands1. Without loss of generality,these streams are modulated using M-ary phase shift keying1The usable spectrum bands are determined by dynamic spectrum sensingand channel sounding [13–15].1-4244-0063-5/06/$2000 (c) 2006 IEEES/P P/SMPSK ModulatorIFFT CPPhase-UpdatingAlgorithm()xn()sn0A1A1NA,0njeE,1nNjeEInterleaverAdaptive-ModePAPR AlgorithmFrom Dynamic Spectrum SensingToReceiverSubcarrierON/OFF(a)(b)(c)(d)(a) Transmitter with proposed adaptive mode PAPR reduction algo-rithm.S/P CP FFTMPSK DemodulatorP/SEqualization180o Phase Shifter()rn()xn,0njeE,1nNjeE,0njeE,1nNjeEDeInterleaver(a)(b)(c)(d)(b) Receiver with PAPR reduction information.Fig. 1. Schematic of an NC-OFDM transceiver employing adaptive-modePAPR reduction algorithm.(MPSK). As shown in Fig. 1(a), the proposed adaptive-modePAPR reduction algorithm decides upon computing phaseadjustments, interleaving the data sequence, or a combinationof both to reduce PAPR of an OFDM signal. The PAPRreduction information is also transmitted to the receiver. Theinverse fast Fourier transform (IFFT) is then used to transformthese modulated subcarrier signals into the time domain.Prior to transmission, a guard interval, with a length equalto or greater than the channel delay spread, is added toeach OFDM symbol using the cyclic prefix (CP) block inorder to mitigate the effects of intersymbol interference (ISI).Following the parallel-to-serial (P/S) conversion, the basebandNC-OFDM signal, s(n), is then passed through the transmitterradio frequency (RF) chain, which amplifies the signal andupconverts it to the desired center frequency.The complex envelope of a baseband NC-OFDM signal,consisting of all N contiguous subcarriers over a time interval[0,T], is given bys(t)=1NN−1k=0Akej2πkt/T(1)where Ak=1∠αk= ejαkis the symbol of the kthsubcarrier2, T is the OFDM symbol duration, and j =√−1.The symbol of the kthunavailable subcarrier is Ak=0.2For example,