A Review Paper on: The PAPR Analysis of MIMO-OFDM Systems

Int. Journal of Electrical & Electronics Engg. Vol. 2, Spl. Issue 1 (2015) e-ISSN: 1694-2310 | p-ISSN: 1694-2426

123 NITTTR, Chandigarh EDIT-2015

A Review Paper on: The PAPR Analysis ofMIMO-OFDM Systems

1Raj Lakshmi Shukla , 2Garima Saini1 2 ME Scholar, Assistant Professor, ECE Department

1,2 NITTTR Chandigarh1 [email protected],[email protected]

ABSTRACT:Orthogonal frequency division Multiplexing(OFDM) is one of the most popular multicarrier ormultiplexing modulation techniques which transmits manysignals over a single path in high speed wirelesscommunication. OFDM convert high data rate stream in tosmaller data rate stream. Due to this high data rate andability to combat frequency selective fading, OFDM has astrong candidate for 4G wireless networks. Because ofOFDM-MIMO advantages in multipath fading channel e.g.stout against ISI, ICI and some other advantages like bestQoS for multiple users, efficient convention of bandwidth it issuggested to be the modulation technique for next generation4G networks e.g. LTE. OFDM combined with multiple-inputmultiple-output (MIMO) to increase system capacity over thetime variant frequency-selective channels and the diversitygain. The radio transmitter stations for covering and gettingenough transmitted power in their desired area has to useHigh Power Amplifier (HPA) operable near to the saturationregion or else, an aspect memory-less nonlinear distortion willaffect the communication path. But along with all itsadvantages there are some disadvantages also. e.g. HighPAPR (Peak to Average Power Ratio) at the transmitter endand BER (Bit Error Rate) at the receiving end. As OFDM isonly used in the downlink of 4G networks. To reduce theproblems of OFDM-MIMO systems some procedures asSLM, PTS, Clipping, Coding, & Pre-coding etc are suggestedbut none of them is proficient enough to reduce the PAPRand BER up to standard value. This Paper will discuss sometechniques of PAPR reduction, and both their advantages anddisadvantages.

Keywords: Orthogonal Frequency Division Multiplexing multiplein multiple out (OFDM-MIMO), Peak–to-Average Power Ratio(PAPR), High Power Amplifier (HPA), selective mapping (SLM)Transmit sequence (PTS), Probability Distribution Function(PDF).

I. INTRODUCTIONMIMO-OFDM is an attractive technique for high data ratewireless communication systems, but it exhibits a largepeak-to-average power ratio (PAPR), due to thesuperposition of the individual nonlinear region of HighPower Amplifier (HPA) [1-3], and the signal distortioninduces the degradation of Bit Error Rate (BER). HighPAPR causes signal distortions such as the in-banddeformation and out-of-band emission and induces thedegradation of bit error rate performance due to thenonlinearity effects [1]. Therefore, RF power amplifiersshould operate in a very large linear region to avoid thesignal peaks from getting into the non-linear region of thepower amplifier causing in-band distortion. i.e., intermodulation among the subcarriers and

out of band radiation. To surmount this, the poweramplifiers should be operated with a large power back-offs

and this indirectly leads to very inefficient amplificationand increases in transmitter power. There are severalreduction technique that have been proposed to reduce thePAPR problem in MIMO-OFDM systems such as ClippingAnd Filtering[2], Partial Transmit Sequences (PTS)[3],Selected Mapping Technique[4], tone reservation (TR) andtone injection (TI)[5]. And some coding techniques. SLMand PTS are scrambling techniques which usesprobabilistic methods [4]. The principle of probabilisticmethod depends on reducing the probability of high PAPRby generating several OFDM symbols (multiplecandidates) carrying the same information and selectingthe one having the lowest PAPR [5]. The absence of thePAPR reduction techniques will cause the increase in thetransmit power, the in Bit error rate at the receiver, the lossin data rate, and the computational complexity. [6]

II SYSTEM MODEL

As in Fig 1 in a MIMO-OFDM system with N subcarriers(or tones) the individual data streams are first passedthrough OFDM modulators which perform an IFFT onblocks of length N followed by a parallel-to-serialconversion. A cyclic prefix (CP) of length Lcp _ Lcontaining a copy of the last Lcp samples of the parallel-to-serial converted output of the N-point IFFT is thenprepended. The resulting OFDM symbols of length N +Lcp are launched simultaneously from the individualtransmit antennas. The Cyclic Prefix is essentially asecurity and a guard interval which serves to eliminateinterference between OFDM symbols and turns linearconvolution into circular convolution such that the channelis diagonalized by the FFT. In the receiver the individualsignals are passed through OFDM demodulators which











II SYSTEM MODEL












II SYSTEM MODEL



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first discard the CP and then perform an N-point FFT. Theoutputs of the OFDM demodulators are finally separatedand decoded.

III: PEAK TO AVERAGE POWER RATIOPeak-to-Average Power Ratio (PAPR) is defined as therelation between the maximum power divided by theaverage power of the OFDM system [6]. The PAPR isdefined as the ratio of the peak power of the signal to itsaverage power, which is in other words a measure of thefluctuations or amplitude ebb and flow of the signal. Thetransmit signals in an orthogonal frequency-divisionmultiplexing (OFDM) system can have high peak values inthe time domain since many subcarrier components areadded via an inverse fast Fourier transformation (IFFT)operation. High PAPR results from the nature of themodulation itself where multiple subcarriers /sinusoids areadded together to form the signal to be transmitted. Whentotal of N sinusoids add, the peak magnitude would have avalue of N, where the (rms) average might be quite lowdue to disparaging interference between the sinusoids. Infact, the high PAPR is one of the most detrimental aspectsin an OFDM system as it decreases the signal-to-quantization noise ratio (SQNR) of the analog-digitalconvertor (ADC) and digital-analog convertor (DAC)while deteriorating the efficiency of the power amplifier inthe transmitter.

(1)

IV. DECISIVE FACTOR FOR PAPR REDUCTIONIn this section a decisive factor is characterised for thetechniques used for both PAPR and BER reduction. Thereare many different techniques and some hybrid techniquesused for PAPR and BER reduction. But still none gaveacceptable results. The technique must reduce the PAPRand BER largely plus the following performance factorsmust be considered for OFDM based system in order to beacceptable.

a) Wherewithal of PAPR ReductionThe primary factor of selecting PAPR reduction techniqueis the wherewithal and capability of PAPR reduction. Atechnique is believed to be finest if it reduces PAPRlargely. OOB radiation and IB distortion are two of themany considerable factors for selecting a technique.

b) Low Average PowerA procedure must not only reduce PAPR but also theaverage power of the signal which should not be increasedfrom an acceptable region or else it will require a largelinear region for operation in HPA, which will enlarge theBER rate of the OFDM system.

c) Low ComplexityThe technique should also not increase the complexity ofthe overall system. Complexity and intricacy includes bothtime and hardware requirements for implementation andthereby the functioning of the system.

d) Less Bandwidth Expansion

Some techniques like scrambling techniques needs sideinformation, which increase the use of the bandwidth.Some coding techniques also enlarge the bandwidth due tocode rate generation. A technique must not be such whichenhances and increases the bandwidth to value whichultimately causes degradation and filth in the throughputand hence efficiency is reduced..

e) Less BER Performance DegradationThe main objective of the PAPR reduction technique is isto achieve better performance including BER as comparedto usual and conventional OFDM system.

f) Less Additional Power NeedThe technique must require no need of additional powerfor PAPR reduction, as it will degrade BER performanceof the system plus power efficiency is the main goal ofwireless based systems.

h) Other FactorsSeveral previous factors like nonlinear devices such asADC/ DAC convertors and cost should be considered.

V. PEAK TO AVERAGE POWER REDUCTIONTECHNIQUE

Peak to Average Power ratio (PAPR) is the major problemin the Orthogonal Frequency Division Multiplexing(OFDM) [7]. For reduction or minimization of PAPRvarious techniques are used [8]. Some of the techniquesused for the PAPR reduction like Clipping, ScramblingTechniques, Tone Reservation and Injection techniques,Active Constellation Extension, Adaptive Pre DistortionTechniques, coding techniques are the foundation of PAPRreduction. These techniques must change hardware oftransmitter and receiver according to the techniques whichis used for PAPR reduction. Also some techniques whichdoes not change hardware, are based on the softwareprogramming and are called the channel coding.

1. CLIPPING AND FILTERING

This is simplest technique used for PAPR reduction ofOFDM signal. A Clip is also called as non linear saturationwhich is employed around the peaks to reduce the peaksbefore HPA to reduce PAPR and so is called ClippingTechnique. This is simple technique but it yet introducesOut Of Band Radiation and In Band Distortion in OFDMSignal. It also destroys the Orthogonality of OFDMsubcarriers. The part of the signal which are above theallowed region are clipped in a simple clipping technique.Joint flitering and clipping technique reduce the OOBradiation but IB distortion are still there since this methoddegrades OFDM system performance e.g. spectralefficiency and BER. Peak Windowing improves spectralefficiency but it increase OOB radiation and BERModulation. Envelop scaling is used for PAPR reductiondue to equality envelop properties of all subcarriers input[9] Clipping means the amplitude of the signal is clipped atthe predefined values which limit the peak value of theinput signal to a predetermined or threshold value [10]. LetX[n] denote the input signal and Xc [n] denote the clippedsignal of X[n], which can be represented as,

[ ] = −Y [ ] ≤−Y



[ ] [ ] <YY [ ] ≥ Y (2)

Where Y is the threshold or predetermines value ofclipping level. Clipping is simple but yet it has somedrawback. Clipping cause signal distortion which increasebit-error-rate performance. After the filtering operationperformed on the clipped signal clipping level may exceedto the specified for the clipping operation.

2. CODING

Coding is the technique of reduction of PAPR by selectingsuch a codeword that reduced the PAPR. Coding does notcause any distortion and out of band radiation. But still ithas the drawback of bandwidth efficiency as the code rateis reduced. It also suffer from the complexity of thealgorithm to find the optimal code and store large tablesfor encoding and decoding for a multi-carrier system Pre-coding based techniques are combined with scramblingtechniques, clipping techniques, ACE etc to get bestperformance.Scrambling Techniques uses the concept of phase rotation.These techniques (Selective Mapping Technique) & PTS(Partial Transmit Technique) are very popular techniquefor PAPR reduction . As number of phase rotationincreases these techniques simply augment complexity.Also the side information is needed for receiver to deciphersignal in these techniques

3. SELECTED MAPPIMG (SLM)

Selected Mapping (SLM) technique reduces PAPR of thesystem by selected mapping so it is call Selected Mappingtechnique [11]. A set of generated signal has the sameinformation in selected mapping, and the most favourablesignal is selected for transmitted. In the SLM the input datastream is multiply by random weighted factor and thelowest PAPR is chosen for transmission as can be seenfrom Fig 2. To recover the original data at the receiver sidemultiplying sequence can be sent as side information. SLMmethod for reduction of the PAPR does not eliminate thepeak. The drawback of this technique is the sideinformation that requires to be transmitted to the receiverof the system in order to recover the original data stream.After IFFT operation select one with low PAPR fortransmits. For measurement the PAPR commutativedistributed function (CDF) and Complementarycommutative distributed function (CCDF) is used. CCDFis measure the probability that the PAPR of a data blockexceed the given threshold value.

4. PARTIAL TRANSMIT SEQUENCE (PTS)

In this technique first step is the phase shifting of the sub-block and then multiplication of data structure by randomvector [12]. The main idea behind this technique is thatinput data block is divided into non-overlapping sub-blockand each sub-block is phase shifted by a constant factor toreduce PAPR as can be seen from Fig 3. PTS technique ismodified version of the SLM technique. PTS method has acapability that minimizes the PAPR more then SLM andmain advantage of the PTS technique is no need to sendside information. This advantage reduces the complexity ofthe PTS technique .

5. TONE RESERVATION (TR)

Tone reservation technique is proposed for PAPRreduction. In this technique small set of tone is used forPAPR reduction. This can be originated as convexproblem. The amount of PAPR reduction is dependent onthe number of reserved tones, their location, quantity ofcomplexity and allowed power on them [13]. Tonereservation shows that reserving a small fraction of tonesgive larger minimization in PAPR even using simplealgorithm. The advantages of tone reservation method isthat it is less complex, no side information is sent, and alsono additional operation can be performed at the receiverside.

6. TONE INJECTION (TI)

This technique is based on the increased size of theconstellation. Each point of the original constellation is



[ ] [ ] <YY [ ] ≥ Y (2)


2. CODING












[ ] [ ] <YY [ ] ≥ Y (2)


2. CODING











NITTTR, Chandigarh EDIT -2015 126

mapped into several equivalent points in the extendedconstellation [13]. So that all the information can bemapped into several equidistant constellation point. InTone injection the new point with an extendedconstellation is replaced with a place of basic constellationpoint which injects a tone with a proper phase andfrequency in multi carrier system. The drawback of thetone reservation is need to decode side information at thereceiver side and so that needed extra IFFT operationwhich increases the complexity of the system.

7. ACTIVE CONSTELLATION EXTENSION (ACE)

Here some of the outer signal constellation points in thedata blocks are extended towards the outside of the originalconstellation such that PAPR of system is reduced.[14].8. ADAPTIVE PRE-DISTORTION TECHNIQUE

These techniques can recompense for the non linearconsequences of HPA (High Power Amplifier). The nonlinear deviation of the amplifier can be handled by theusual adjustment of the input constellation with the help ofminimum usage of hardware. The convergence time ofpredistorter and the MSE (Mean Square Error) can belessened via screening out and broadcasting techniques bythe training design assistance.All these techniques are comparatively poles apart andentail different restriction like multifaceted optimization,(out-of-band) emission, IB (in-band) deformation, sideefficiency diminishing, computational complexity,bandwidth expansion, BER (bit error rate) degradation anddata rate loss.Relative

VI CONCLUSIONThis paper investigates one of the bottleneck problems thatexist in OFDM wireless communication systems. HighPeak average power ratio (PAPR of OFDM signal), anddiscuss how to reduce it by different effective algorithms.Both scrambling and non scrambling techniques have beendiscussed. Each technique of PAPR reduction in OFDMbased system are different from each other and impactdifferent constraints as in bandwidth expansion, OOBradiation, IB distortion, reduction of spectral efficiencyradiation, BER reduction, high peak and average power,and taken as a whole system’s intricacy and complexity.Precoding based techniques results good with no need ofside information and works with less complexity. Clippingcause signal distortion which increase bit-error-rateperformance. Coding increases bandwidth efficiency as thecode rate is reduced. It also suffer from the complexity ofthe algorithm. SLM method for reduction of the PAPRdoes not eliminate the peak. The drawback of thistechnique is the side information that requires to betransmitted to the receiver of the system in order to recoverthe original data stream. Though PTS technique does notneed side information but still the implementationcomplexity is increased due to number of IFFT operationsand due to complex multiplication and summation. Thedrawback of the tone reservation is the need to decode sideinformation at the receiver side and so that extra IFFToperation are needed which increases the complexity of the

system. Adaptive Predistortion technique can be dependentupon the layout circuitry of hardware. Less is the hardwarethe deviation can be handled of HPA. Precoding basedtechniques results good with no need of side informationand works with less complexity Pre-coding plus othertechniques gives best result for PAPR reduction. Alsohybrid SLM and PTS allows further PAPR reduction eventhough it requires too many calculations But it reduces theefficiency of the use of the side bits.

VII FUTURE WORKThough all the techniques in individual and hybrid effectsgives good results but still every method is having certaindrawbacks. So the future work will be discussing somehybrid techniques amongst scrambling and non scramblingand checking for The Cumulative Distribution Function(CDF) which is the most important parameters of theOFDM-MIMO systems to measure the PAPR.

VIII ACKNOWLEDGEMENTAuthors would like to thank to Dr.M.P.Puniya, DirectorNITTTR,Chandigarh, Dr.S.B.L.Sachan,HOD-ECE,NITTTR Dr.Prabal Chakravorty, Director VGI, Dadri,without whose support and valuable guidance thingswould not have been practically implemented.Last wewould thank our colleagues for giving their endlesssupport.

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A Review Paper on: The PAPR Analysis of MIMO-OFDM Systems