PAPR Reduction Techniques in OFDM: A Brief Review

PAPR Reduction Techniques in OFDM: A Brief Review

Mr. Suraj Shete. Prof. R. N. Patil. Prof. P. M. Jadhav.

Electronics engineering Dept.

Electronics engineering Dept.

Electronics engineering Dept.

DKTEs TEI, Ichalkaranji. DKTEs TEI, Ichalkaranji. DKTEs TEI, Ichalkaranji.

Abstract: OFDM is multi-carriercommunication technique which is used in bothwired and wireless communication. OFDM is thetime domain signal which is a sum of severalsinusoids and these sinusoids makes Peak to AveragePower Ratio (PAPR). This paper represents anoverview of reduction of PAPR using all proposedtechniques. According with performance oraccording with application we can use appropriatetechnique.

KEYWORDS: OFDM, PAPR, SLM, PTS, WHT.

INTRODUCTION:

Communication is one of theimportant aspects of life. With the advancement in ageand its growing demands, there has been rapid growthin the field of communications. Signals, which wereinitially sent in the analog domain, are being sent more and more in the digital domain these days. For bettertransmission, even single-carrier waves are beingreplaced by multi-carriers. Multi-carrier systems likeCDMA and OFDM are now-a-days beingimplemented commonly. OFDM is multi-carriercommunication technique which is used in both wiredand wireless communication.In the OFDM system,orthogonally placed sub-carriers are used to carry thedata from the transmitter end to the receiver end.Presence of guard band in this system deals with theproblem of ISI and noise is minimized by largernumber of sub-carriers. OFDM sends many low speedtransmissions simultaneously and hence it avoids theproblem of Inter Symbol Interference (ISI). OFDM isthe time domain signal which is a sum of severalsinusoids and this sinusoids makes Peak to AveragePower Ratio (PAPR).The high peaks usually drive the power amplifier into saturation, clipping thetransmitted waveform and introducing in-band andinterference affects the systems that operates atsurrounding frequencies.out- band interference. The in-band interferencereduces the system performance while out-band.

1. Papr:

   OFDM is generated by first choosing the spectrumrequired, based on the input data and modulationscheme used. Each carrier to be produced is assignedsome data to transmit. OFDM consists of lots ofindependent modulated sub- carriers. This leads toproblem of peak to average power ratio(PAPR). ThenPAPR is defined as- the ratio period between themaximum instantaneous power and its average powerduring an OFDM symbol.

   PAPR= max [| x(t)|2] / [{E(A)}2] [1]

   Where max [|x(t)|2] is maximum or peak power and[{E (A)}2] is average power of transmitted symbol. Toget proper values of PAPR oversampling is necessary.Oversampling can be performed by padding IFFTsource data with zeros. So PAPR is major drawback ofOFDM system so in this paper we represent somePAPR reduction techniques.

2. PAPR REDUCTION TECHNIQUES: FOR

PAPR reduction individual as well as combinedtechniques are used. The effect of each technique isdifferent.

1] INDIVIDUAL TECHNIQUES: To reduce thePAPR,

several individual techniques have beenproposed, which can be divided into three categories.

1. SINGAL SCRAMBLING TECHNIQUES:

   1. Block coding techniques

      Different code words are used in block codingtechnique.In this code words are transmitted along with original OFDM symbol, but for better result selection of proper code word areessential.In blockcoding technique no need to transmit side informationand received signal can be easily decoded.Errorcorrection can be done in this technique.But thismethod results in distortion.

   2. Sub block coding

      In sub Block coding large input frames are dividedinto small sub blocks.The ¾ code rate of total code isadded to last odd parity checking bit .For large inputsteam this method is more efficient than Block codingtechnique.

   3. Selective mapping (SLM)

      In this method selection of optimal combination ofphase factors with lower PAPR is carried out.Inselective mapping side information is passed only withselected signal.That means in SLM only chosen signalis mapped.

   4. Partial Transmit Sequence (PTS)

      In PTS the whole sequence is divided into non- overlappingsub blocks.This sub blocks are assignedby independent rotation factor.This factor generatestime domain data with lowest amplitude.This selectionof lowest

      amplitude gives results in reduction ofPAPR.PTS gives better performance than SLM.

   5. Adaptive Interleaving

      In adaptive interleaving early threshold termination isestablished So searching process is terminated whenthe value of PAPR reaches below the predefinedthreshold value.We can adapt the threshold valuedepending upon the application,so adaptation is done.

   6. Tone Reservation

   In tone reservation,reserved tones can be used tominimize the PAPR.When there are number of tonesare small then there will be small PAPR reduction andvice-versa.In this method no need of decoding at thereceiver end.

2. SIGNAL DISTORTION TECHNIQUES

   1. Clipping and filtering

      This is effective method for PAPR reduction in whichinitially clipping is done and then filtering is carriedout.Since clipping is non-linear process which increase the band noise distortion,Bit Error Rate(BER) anddecrease spectral efficiency.Filtering after clippingreduces out of band distortion.For PSK modulationthis method is more effective.

   2. Peak Windowing

   In Peak Windowing technique different windowshaving large signal peaks like Cosine window, Kaiser window, Hamming window,Hanningwindow,Gaussian window etc. are multiply withoriginal OFDM signal.The resultant spectrum isnothing but convolution of applied window andoriginal OFDM signal.Depending upon the applicationwe can use the standard window.

3. PRE-CODING TECHNIQUES

1. Modifying FFT and IFFT algorithms

   This is pre-processing method for PAPR reduction.Inthis method slight modification is done in matrixcoefficient of IFFT matrix at transmitter and FFTmatrix at receiver.In this method two columns of IFFTmatrix are replaced with their linear combination inorder to reduce PAPR. These columns correspond tomaximum and minimum value of OFDM signal.Alsowith the slight change in FFT matrix we get betterPAPR reduction.

2. Golay complementary sequence

   In Golay complementary sequence two sequenceswhose aperiodic autocorrelations sum becomes zero inall out-of- phase positions are used for PAPRreduction. These sequences are called Golaycomplementary sequences.This sequences are oftendefined over an alphabet of size 2(binary),4(quanternary) or 8(octary).In this, PAPR reduction isvalid only for OFDM signals without guardfrequencies.With guard frequencies the PAPRreduction is not so good.

3. Reed Muller code

   Reed Muller code is efficient way for PAPRreduction. In this Reed Muller code is applied to givenOFDM signal but bandwidth expansion and power lossoccurs.

4. Walsh Hadamard Transform (WHT)

The Walsh Hadamard Transform (WHT) is a nonsinusoidal,orthogonal linear transform. WHTdecomposes a signal into set of basic functions. Thesefunctions are Walsh functions, which are square waveswith values of +1 or -1. The proposed hadamardtransform scheme may reduce the occurrence of thehigh peaks comparing the original OFDM system. Theidea to use the WHT is to reduce the autocorrelation ofthe input sequence to reduce the peak to averagepower problem and it requires no side information tobe transmited to the receiver. In Walsh HadamardTransform (WHT) is more efficient way because inWHT no bandwidth expansion, no power increase, nodata rate loss occurs. Also no Bit Error Rate (BER)degradation happens and it is distortion-less. Thusimplementation of Walsh Hadamard Transform(WHT) pre-coding to reduce Peak to Average PowerRatio (PAPR) in conventional Orthogonal FrequencyDivision Multiplexing (OFDM) system is moreeffective than other techniques.WHT is becomes moreeffective when it is combined with other techniques.

2] COMBINED TECHNIQUES

A combined technique shows much better performancethan individual techniques.Combined techniques arelike as:

1. Joint companding transform with Hadamardtransform

   In this method Joint companding transform andHadamard Transform both are used to reduce PAPR ofOFDM signal.In this Hadamard Transform is appliedbefore the IFFT and companding transform is appliedafter IFFT at the transmitting end while InverseCompanding Transform is applied before FFT andInverse Hadamard Transform is applied after FFT atthe receiver end.This method maintain goodperformance in BER as compared to ordinarycompanding techniques,also PAPR reductionperformance is also improved.

2. Double WHT

   In this method two times WHT applied to achievebetter performance than WHT PAPR reductiontechnique.This method is more robust than WHTagainst nonlinearity.It have lower clipping probabilitythan conventional OFDM.

3. DCT combined with SLM

   In this method,DCT is combined with SLM usingRiemen sequence.This is more effective than bothindividual PAPR reduction methods.If DCT is carriedout before SLM then we can achieve 1.8 db PAPRreduction while if SLM is carried out before DCT thenwe get 1.2 db PAPR.

4. SLM combined with WHT and dummy sequenceinsertion

In this method, SLM is combined with WHT anddummy sequence is inserted into resultant signal.Better performance is achieved by this method.

CONCLUSION

Thus for PAPR reduction in OFDM differentindividual and combined methods are used. The effectof each method is different.Depending uponapplication or depending upon performance we canuse appropriate method.

REFERENCES

1. MalharChauhan, Saurabh Patel, Hardik Patel,Different techniques to reduce the PAPR in OFDMsystem International Journal of Engineering Researchand Applications (IJERA), year may-jun2012, vol-2,issue-3, pp.1292-1294.

2. Zhongpeng Wang School of Information andElectronic Engineering, Zhejiang University ofScience and Technology, Hangzhou, China ReductionPAPR of OFDM Signals by Combining SLMwith DCT Transform

3. Keyvan Kasiri1, Iraj Hosseini1, Omid Taheri1,Mohammad Javad Omidi1, P. Glenn gulak2 1ecedepartment, is Fahan University of technology 2ecedepartment, university of to ronto a preprocessingmethod for PAPR reduction in ofdm systems bymodifying FFT and IFFT matrices

4. PepinMagnanganaZokoGoyoro, Ibrahim JamesMoumouni, SroyAbouty SLM Using RiemannSequence Combined with DCT Transform for PAPRReduction in OFDM Communication Systems

5. A. Zolghadrasli and M. H. Ghamat, An Overviewof PAPR Reduction Techniques for MulticarrierTransmission and Propose of New Techniques forPAPR Reduction, Iranian Journal of Electrical andComputer Engineering, Vol. 7, No. 2, Summer-Fall2008, pp. 115-120.