Proposed Techniques for combined PAPR

reduction and Sidelobe Suppression in

OFDM based Cognitive Radioby

Shivani Darji

1 Abstract

Cognitive Radio emerges as a design paradigm which can potentially alle-viate the problem of spectrum shortage. Spectrum sensing is the main re-quirements for the establishment of cognitive radio. Orthogonal FrequencyDivision Multiplexing (OFDM) is a recognized transmission technique forCognitive Radio (CR) networks. In this work, OFDM technique is inves-tigated as a candidate for CR systems. However, the major drawbacks ofOFDM systems is that they exhibit high Out OF Band Radiation (OOBR)due to high spectral sidelobe. The high spectral sidelobe which are caused inOFDM systems based Cognitive Radio can cause interference with PrimaryUsers (PU) that indirectly affects the performance. Moreover, another majordrawback of using OFDM systems is that they exhibit large Peak to AveragePower Ratio (PAPR) values. For zero distortion of the OFDM signal, theRF High Power Amplifier (HPA) must not only operate in its linear regionbut also with sufficient back-off. Thus, HPA with a large dynamic range arerequired for OFDM systems. These amplifiers are very expensive and aremajor cost components. Thus, if we reduce the PAPR it not only meansthat we are reducing the cost of OFDM system and reducing the complexityof A/D and D/A converters, but also increasing the transmit power, thus, forsame range improving received SNR, or for the same SNR improving range.

2 Introduction

Cognitive radio (CR) is an emerging communication design paradigm [1],[2]in which the radios devices equipped with transceivers can sense the under-lying radio environment and adapt their transmission/reception parameters

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like operating frequency, power, modulation rate, etc. The specific instancein which the operating frequency is adjusted is known as Dynamic SpectrumAccess(DSA). DSA has the potential to alleviate the problem of spectrumshortage [3][4]. The problem of spectrum shortage can create problems inallocating spectrum for the numerous wireless applications that are beingconceived. On the other hand a recent survey by Federal CommunicationsCommission(FCC), USA, suggests that vast portions of allocated spectrumis vastly underutilized. A radio enabled with DSA can indeed access thespectrum opportunistically whenever the licensed users are inactive, therebyleading to efficient spectrum utilization. Henceforth, we refer to networksthat use DSA as CR networks. Since CR networks allow unlicensed usersto access opportunistically therefore it contains two types of users namelyprimary (licensed) users(PU) and secondary (unlicensed) users(SU).

3 OFDM based CR networks

OFDMs underlying sensing and spectrum shaping capabilities together withits flexibility and adaptivity make it probably the best transmission technol-ogy for CR systems. There are many requirements of CR which can full fillby OFDM such as spectrum sensing, spectrum shaping, adaptive to envi-ronment, interoperability etc. As we are considering OFDM based CognitiveRadio application related to OFDM using Cognitive Radio is being discussed.

3.1 Spectrum Sensing

The spectrum sensing is one of the most important part of Cognitive Radio.In OFDM systems, conversion from time domain to frequency domain isachieved by using FFT. The underlying FFT operation in OFDM systemhere makes it more better to sense a spectrum in frequency domain. Thusthere is no requirement of using any extra hardware for scanning all thepoings in time-frequency grid of OFDM system operating band because ofthe hardware reuse of FFT cores. Using the time-frequency grid, the selectionof bins that are available for exploitation (spectrum holes) can be carried outusing simple hypothesis testing. In [5] FFT is applied to the received signal.By using the output of the FFT, the receiver tries to detect the existence ofa primary user in the band [5].

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3.2 Spectrum Shaping

With efficient spectrum sensing by CR, then comes the next step of spectrumshaping. Ideally, it is desired to allow cognitive users to freely use availablebands in the spectrum. It is desired to have a flexible spectrum mask andcontrol over waveform parameters such as signal bandwidth, power level, andcenter frequency. OFDM systems can provide such flexibility, by disabling aset of subcarriers so that the spectrum of OFDM signals can be adaptivelyshaped to fit into the required spectrum mask [6].

“The spectrum mask is define as the allowable maximum in-band and out-of-band signal power spectrum” which is already known to the CR system.An example of spectrum shaping procedures in OFDM-based CR systems isillustrated in Figure. 1 when the two PUs are detected using the output ofthe FFT block, and subcarriers that can cause interference to these PUs areturned off. The transmitter then uses the unoccupied part of the spectrumfor the signal transmission.

Figure 1: Spectrum sensing and shaping using OFDM.

4 Major Drawbacks of OFDM based CR

As there are advantages of using OFDM based CR there are also few dis-advantages. The two major drawbacks that can degrade the performanceis Out-Of-Band-Radiation (OOBR) and very high Peak to Average PowerRatio (PAPR).

4.1 Out of Band Radiation (OOBR)

The out-of-band radiation in OFDM transmission is caused to due to spec-tral sidelobe which further causes interferes with wireless communications

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in adjacent channels and endangers the co-existence of the incumbent radiosystems of the spectrum. The reason for high power of sidelobe is the use ofthe IFFT block in the OFDM transmitter [7]. OFDM uses sinc-type pulsesin representing symbols transmitted over all the subcarriers per time con-stant. Large sidelobes resulting from this sinc-type pulses are a source ofinterference to the PU or other rental systems that might be present in thevicinity of the spectrum used by the unlicensed system. There are severalmethods for sidelobe suppression of the OFDM systems given as follows,

• In [7], OOBR of SUs can be reduced by inserting the guard band. But,it will consequently reduce the spectrum efficiency. Various schemeshave been proposed in both time domain and frequency domain toimprove performance of the primary users by reducing OOBR. In [7],the time domain signal is multiplied with a raised cosine windowingfunction to lower the sidelobes, but at a cost of expanding the symbolduration which results in lower throughput.

• To improve the spectral efficiency various advanced techniques havebeen proposed but the technique which have achieve more suppressionof OOBR with better spectral efficiency are Active interference can-cellation (AIC) [8] and and the introduction of cancellation carrier [9].Both of these techniques results in enough suppression of OOBR butsuffer from Signal to Noise Ratio (SNR) degradation and also extrapower is wasted in cancellation subcarrier.

• In [10], adaptive symbol transition technique is used in which theOFDM symbols extended adaptively at the cost of decrease in the use-ful symbol energy.

• Methods in frequency domain rely on spectrum shaping techniques andcan be classified as cancellation and precoding techniques. In [11], wediscuss spectral precoding scheme. In these approaches, the informa-tion symbols are mapped to a new set of precoded symbols under someconstraints. In this method large suppression of OOBR is achievedwithout losing BER performance but the notched frequencies selectionalgorithm is computationally more complex and expensive.

• In [12], a scheme based on optimization process is proposed. However,the method in [12] does not have any closed form solution and hencecomputationally expensive. Also, the OOBR rejection is achieved at acost of significant BER performance loss.

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4.2 Peak to Average Power Ratio

One of the main drawbacks Multi-Carrier (MC) systems is the high variationof the amplitude of the time-domain samples that can occur in the signalafter the IFFT, resulting in a high value of the Peak-to-Average Power Ratio(PAPR). Mathematically it can be defined as the,

PAPR =max |x(t)|2

E|x(t)|2(1)

The signal that contains samples with high amplitude, equivalent to highPAPR value, and the presence of nonlinear elements in the transmissionchain, e.q. high power amplifiers (HPA), result in in-band distortions, in-creased out-of-band emissions, and as a consequence result in Bit-Error Rate(BER) degradation at the receiver. Let us stress that the gain in OOB reduc-tion obtained due to the application of one of the abovementioned algorithmscan be reduced if the time-domain signal of high PAPR is processed in thenonlinear element. Thus efficient methods to minimize the PAPR value arerequired. Techniques for PAPR reduction has rich literature. Among themschemes like

• Among them, schemes like clipping and filtering [13], Partial TransmitSequence (PTS) [14], precoding based techniques [15] and Precodingbased Selected Mapping (SLM) [16] are popular. Clipping methodcauses distortion to the signal and BER degradation. The later bothPTS techniques and SLM need additional complexity.

5 Different Techniques for sidelobe suppres-

sion

5.1 Mask compliant precoder for OFDM spectrum shap-ing

Mask compliant precoder for shaping the spectrum of OFDM is presentedin [12]. Unlike existing methods which can focus on minimizing or forcingthe sidelobes to zero, the Mask Compliant precoder is designed for control-ling the out-of-band emissions to a particular power level without impactingthe bit error rate (BER) performance. A small distortion is added to theinformation symbols forcing the emitted spectrum under a prescribed radiofrequency mask. “The spectrum mask is define as the allowable maximum in-band and out-of-band signal power spectrum.” The block diagram of OFDM

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Figure 2: Mask compliant precoder block diagram of OFDM transmit-ter/receiver pair with sidelobe suppression.

transmitter and receiver for mask complaint precoder is shown in figure. 2. Aprecoder that forces the power spectrum of OFDM signals under a given RFmask is proposed in the given work. Using the proposed precoder significantBER performance is improved compared to other precoder. Moreover, withthe use of adaptive modulation, the BER performance under strict maskscan be further improved.

5.2 Multiuser spectral precoding for OFDM based CRsystems

A spectral precoding approach for multiple OFDM-based CR users to re-duce OOBR leakage and enhance spectrum compactness in shown [11]. WeCan construct individual precoders to render selected spectrum nulls, our ap-proach suppresses the overall OOB radiation without sacri?cing bit error rateperformance of CR users. It also ensures user independence thus with lowencoding and decoding complexities. This approach can improve bandwidthefficiency by carefully selecting notched frequencies. The block diagram ofmultiple spectrum precoding for OFDM based CR systems is shown in figure.3.

While OOB leakage cannot be completely eliminated so that guard bandsare usually require. As a result, the relative throughput loss brought by guardbands can be significant if CR users operate over narrow subbands. To reducethe guard bands and enhance bandwidth efficiency spectrum shaping can beincluded which suppresses the OOBR leakage to the adjacent frequency bands.Here for the shaping of the spectrum Existing approaches are performed intime or frequency domain.

Spectral precoding approach can be used in frequency domain which canreduce the OOBR signi?cantly and are applicable to multiple users. The pre-

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Figure 3: The block diagram of multiple spectrum precoding for OFDMbased CR systems

coding matrix for these approaches is constructed from delicately designedbasis sets or determined to render time continuity of adjacent OFDM sym-bols or spectrum nulls at notched frequencies. With a block diagonal precod-ing matrix, it is possible to ensure user independence. Here the correlativeprecoder , the projection precoder , and the 1-continuous OFDM precoderare non-orthogonal and will cause signi?cant BER performance degradationwhen only a few subcarriers are available and utilized by CR users. Spectralprecoding dealing with this challenges.

5.3 Adaptive Symbol transmission (AST)

Another method that can be investigated for Suppression of the OFDM side-lobes is referred to as adaptive symbol transition (AST) [10]. Similar to thewindowing technique, the OFDM symbols are extended in time to reducethe effect of symbol transition. However, instead of using a predefined fil-ter shape, the transition signal is optimized adaptively based on transmitteddata and detected PU bands for reducing the interference to PU. The blockdiagram of AST for OFDM based CR systems is shown in figure. 4. The

Figure 4: The block diagram of AST for OFDM based CR systems

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proposed AST technique can studied which extends OFDM symbols and usesthe extension to reduce ACI to other users operating in the same band. TheAST can achieve a significant gain of 28 dB over conventional sidelobe sup-pression techniques (RC windowing and conventional). Moreover, AST doesnot increase the signal PAPR and keeps a low SNR.

6 Proposed Method

• Many solutions were proposed for solving either of these problems whilein this paper, an efficient technique for reducing both PAPR and side-lobe power is proposed. We proposed a technique in which we involvecombined suppression of spectral sidelobes and reduction of PAPR.

• A novel PAPR reduction techniques, namely, Zadoff-Chu matrix Trans-form (ZCT) precoding based PAPR reduction technique and ZCT post-coding based PAPR reduction technique for OFDM-WLAN systems.In the proposed schemes, the reshaping of the ZCT is carried out oneway for precoding and another way for postcoding. For precoding wereshape the ZCT row wise and precode the constellation symbols beforethe IFFT with ZCT and for postcoding, we reshape the ZCT columnwise and implement ZCT after the IFFT.

• For sidelobe suppression we will use a method of Cancelation Carri-ers (CC) technique that depends on adding additive carriers, whichreduce the sidelobe power and do not cause intersymbol interference,but increases PAPR. Hence we will use ZCT precoding and postcodingto reduce PAPR. Therefore, we use combined suppression of sidelobeswith reducing PAPR, because spectral sidelobes can regrow after thehigh peak power signal passes through power amplifier. A techniquethat can deal with both problem together will give better performance.

References

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[15] I. Baig and V. Jeoti, “PAPR Analysis of DHT-Precoded OFDM Sys-tem for M-QAM,” the 3rd International Conference on Intelligent andAdvanced Systems (ICIAS2010), June 2010.

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