report on papr ofdm

8/10/2019 Report on Papr Ofdm

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Peak-to-Average Power Ratio ReductionTechniques For OFDM Signals

Abstract: In this paper, different techniques for reducing Peak To Average Power Ratio (PAPR) is discussed. The performance of anorthogonal frequency division multiplexing (OFDM) system is degraded if the peak-to average power ratio (PAPR) is high. An OFDM systemdynamic range is typically two or four times larger than a single carrier

system increasing value of the dynamic range will lead to an increased

cost, power consumption of transmitter amplifier and also lead to high peak to average power ratio (PAPR). This is one of the major drawbacksof OFDM system. A serious problem of large PAPR presents whenenergy-inefficient nonlinear power amplifiers are used. Severaltechniques have been proposed to reduce PAPR in OFDM system, out ofthat few are selective mapping (SLM), tone reservation and partialtransmit sequence. SLM method could be used to reduce PAPR,however, it causes the BER degradation and increases computationalcomplexity. The good improvement in PAPR given by the presenttechniques permits the reduction of the complexity and cost of thetransmitter.One of the challenging issues for Orthogonal Frequency DivisionMultiplexing (OFDM) system is its high Peak-to-Average Power Ratio(PAPR). In this paper, we review and analysis different OFDM PAPRreduction techniques, based on computational complexity, bandwidthexpansion, spectral spillage and performance. We also discuss somemethods of PAPR reduction for multiuser OFDM broadbandcommunication systems


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IntroductionDue to the high data rate transmission, orthogonal frequency division multiplexing(OFDM) is a promising technique in the current broadband wirelesscommunication system. The basic principle of OFDM is to split a high-rate datastream into a number of lower rate streams that are transmitted simultaneouslyover a number of subcarriers.The carriers can be made orthogonal by appropriately choosing the frequencyspacing between them. The main concept of OFDM is orthogonality of thesubcarriers. Since the carriers are all sine and cosine waves, we know that areaunder one period of these are zero. Therefore these are all orthogonal to each other.The orthogonality allows simultaneous transmission of a lot of sub carriers in atight frequency space without interference from each other. The orthogonality also

allows high spectral efficiency.


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Orthogonal frequency division multiplexing (OFDM) technique has been acceptedfor many applications such as mobile and indoor wireless communications.It is well known that OFDM is spectrally efficient but power inefficient due to thelarge peak-to average power ratio (PAPR) inherent in the OFDM signals, a largePAPR value also makes the signal vulnerable to nonlinearities in the transmission.

A number of approaches have been proposed for reducing the PAPR of OFDMsignals. For example, selective mapping or transformation [1], [2] may statisticallyreduce the PAPR with a relatively simple implementation cost.

Although these schemes may offer considerable reduction of the PAPR, a rigorousoptimization of reduction process becomes computationally challenging for the

system with a large number of subcarriers. Systematic coding techniques may beattractive since they can deterministically bound the PAPR with littlecomputational cost at the transmitter, but designing the low PAPR codes whilemaintaining a reasonable coding rate becomes quite difficult as the number ofsubcarriers increases.

Probably simplest for the PAPR reduction is digital clipping and filtering of theOFDM signal . The problem arises, however, that low pass filtering the clippedOFDM signal samples results in considerable regrowth of peak power in additionto a certain amount of degradation in bit-error performance. In clipping technique ,OFDM signal peaks larger than some threshold are deliberately clipped off. Eventhough this is a simple technique, it introduces in-band distortion and out of bandnoise.

In Selective Mapping (SLM) technique the sequence with the lowest PAPR aftermaking the few different phase changes on the identical input data sequence isselected and transmitted. To recover data, the receiver must have knowledge aboutthe generation process of OFDM signal and phase information. This information isknown as Side information (SI), which results in some loss of efficiency.

To mitigate the performance degradation in the propagation channel, channelcoding is usually used in communication systems.


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OFDM

The principle of orthogonal frequency division multiplexing (OFDM)

modulation has been in existence for several decades. However, in recent years

these techniques have quickly moved out of textbooks and research laboratories

and into practice in modern communications systems. The techniques are

employed in data delivery systems over the phone line, digital radio andtelevision, and wireless networking systems. What is OFDM? And why has it

recently become so popular?

OFDM is symbol based, and can be thought of as a large number of low bit

rate carriers transmitting in parallel. All these carriers transmitted using

synchronized time and frequency, forming a single block of spectrum. This is to

ensure that the orthogonal nature of the structure is maintained.

Since these multiple carriers form a single OFDM transmission, they are

commonly referred to as subcarriers, with the term of carrier reserved for

describing the RF carrier mixing the signal from base band. There are several ways

of looking at what make the subcarriers in an OFDM signal orthogonal and why

this prevents interference between them.

OFDM can be simply defined as a form of multicarrier modulation where its

carrier spacing is carefully selected so that each subcarrier is orthogonal to the

other subcarriers


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As is well known, orthogonal signals can be separated at the receiver bycorrelation techniques; hence, intersymbol interference among channels can be

eliminated. Orthogonality can be achieved by carefully selecting carrier spacing,

such as letting the carrier spacing be equal to the reciprocal of the useful symbol

period.

DEVELOPMENT OF OFDM SYSTEMS

The development of OFDM systems can be divided into three parts. Thiscomprises of Frequency Division Multiplexing, MulticarrierCommunication and Orthogonal Frequency Division Multiplexing.

3.2.1 Frequency Division Multiplexing

Frequency Division Multiplexing is a form of signal multiplexing whichinvolves assigning non overlapping frequency ranges or channels todifferent signals or to each user of a medium.

3.2.2 Multicarrier Communication

As it is ineffective to transfer a high rate data stream through a channel,

the signal is split to give a number of signals over that frequency range.Each of these signals are individually modulated and transmitted overthe channel. At the receiver end, these signals are fed to a de multiplexer where it is demodulated and re combined to obtain theoriginal signal.


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3.3 OFDM THEORYOrthogonal Frequency Division Multiplexing is a special form of

multicarrier modulation which is particularly suited for transmission

over a dispersive channel. Here the different carriers are orthogonal to

each other, that is, they are totally independent of one another. This is

achieved by placing the carrier exactly at the nulls in the modulation

spectra of each other.

2.3 FREQUENCY DIVISION MULTIPLEXING MODULATION SYSTEM

FDM extends the concept of single carrier modulation by using multiple

sub carriers within the same single channel. The total data rate to be

sent in the channel is divided between the various sub carriers.


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Inter Symbol Interference

Inter symbol interference (ISI) is a form of distortion of a signal inwhich one symbol interferes with subsequent symbols.


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This is an unwanted phenomenon as the previous symbols have similareffect as noise, thus making the communication less reliable. ISI isusually caused by multipath propagation or the inherent non linearfrequency response of a channel causing successive symbols to blurtogether.The presence of ISI in the system introduces error in the decision device

at the receiver output. Therefore, in the design of the transmitting andreceiving filters, the objective is to minimize the effects of ISI andthereby deliver the digital data to its destination with the smallest errorrate possible.

Inter Carrier Interference

Presence of Doppler shifts and frequency and phase offsets in an OFDMsystem causes loss in orthogonality of the sub carriers. As a result,interference is observed between sub carriers. This phenomenon isknown as inter carrier interference (ICI) .

Cyclic Prefix

The Cyclic Prefix or Guard Interval is a periodic extension of the last part of an OFDM symbol that is added to the front of the symbol in thetransmitter, and is removed at the receiver before demodulation [1].The cyclic prefix has to two important benefits The cyclic prefix acts as a guard interval. It eliminates the inter symbolinterference from the previous symbol.


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convolution which in turn maybe transformed to the frequency domainusing a discrete fourier transform. This approach allows for simplefrequency domain processing such as channel estimation andequalization.

Inverse Discrete Fourier Transform

By working with OFDM in frequency domain the modulated QPSK datasymbols are fed onto the orthogonal sub-carriers. But transfer of signalover a channel is only possible in its time-domain. For which weimplement IDFT which converts the OFDM signal in from frequencydomain to time domain.

IDFT being a linear transformation can be easily applied to the systemand DFT can be applied at the receiver end to regain the original data infrequency domain at the receiver end. Since the basis of Fouriertransform is orthogonal in nature we can implement to get the timedomain equivalent of the OFDM signal from its frequency components.


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MODULATION & DEMODULATION IN OFDM SYSTEMS

Usually, in practice instead of DFT and IDFT we implement Fast Fourier

Transformation for an N-input signal system because of the lower

hardware complexity of the system.


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Modulation

Modulation is the technique by which the signal wave is transformed inorder to send it over the communication channel in order to minimizethe effect of noise. This is done in order to ensure that the received datacan be demodulated to give back the original data. In an OFDM system,the high data rate information is divided into small packets of data whichare placed orthogonal to each other. This is achieved by modulating thedata by a desirable modulation technique (QPSK). After this, IFFT is

performed on the modulated signal which is further processed by passing through a parallel to serial converter. In order to avoid ISI

we provide a cyclic prefix to the signal.

3.4.2 Communication Channel

This is the channel through which the data is transferred. Presence ofnoise in this medium affects the signal and causes distortion in its datacontent.

Demodulation is the technique by which the original data (or a part of it)is recovered from the modulated signal which is received at the receiverend. In this case, the received data is first made to pass through a low

pass filter and the cyclic prefix is removed. FFT of the signal is doneafter it is made to pass through a serial to parallel converter. Ademodulator is used, to get back the original signal.The bit error rate and the signal to noise ratio is calculated by takinginto consideration the un modulated signal data and the data at thereceiving end.


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3.5 ADVANTAGES & DISADVANTAGES OF AN OFDMSYSTEM

AdvantagesDue to increase in symbol duration, there is a reduction in delay spread.Addition of guard band almost removes the ISI and ICI in the system.

Conversion of the channel into many narrowly spaced orthogonal sub carriers render it immune to frequency selective fading.

As it is evident from the spectral pattern of an OFDM system,

orthogonally placing the sub carriers lead to high spectral efficiency.Can be efficiently implemented using IFFT.

DisadvantagesThese systems are highly sensitive to Doppler shifts which affect thecarrier frequency offsets, resulting in ICI.

Presence of a large number of sub carriers with varying amplituderesults in a high Peak to Average Power Ratio (PAPR) of the system,which in turn hampers the efficiency of the RF amplifier.


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A typical Frequency division multiplexing signal spectrum is shown infig.2.2. The data do not have to be divided evenly nor do they have to originate

from the same information source. Advantages include using separate modulation

demodulation customized to a particular type of data, or sending out banks of

dissimilar data that can be best sent using multiple, and possibly different,

modulation schemes. Current national television systems committee (NTSC)

television and FM stereo multiplex are good examples of FDM. FDM offers an

advantage over single-carrier modulation in terms of narrowband frequency

interference since this interference will only affect one of the frequency sub

bands. The other sub carriers will not be affected by the interference. Since each

sub carrier has a lower information rate, the data symbol periods in a digital

system will be longer, adding some additional immunity to impulse noise and

reflections [2].

FDM systems usually require a guard band between modulated sub

carriers to prevent the spectrum of one sub carrier from interfering with another.

These guard bands lower the systems effective information rate when compared

to a single carrier system with similar modulation.


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2.4 ORTHOGONALITY AND OFDM

If the FDM system above had been able to use a set of sub carriers that were

orthogonal to each other, a higher level of spectral efficiency could have been

achieved. The guard bands that were necessary to allow individual demodulation

of sub carriers in an FDM system would no longer be necessary. The use of

orthogonal sub carriers would allow the sub carriers spectra to overlap, thus

increasing the spectral efficiency. As long as orthogonality is maintained, it is still

possible to recover the individual sub carriers signals despite their overlapping

spectrums. If the dot product of two deterministic signals is equal to zero, these

signals are said to be orthogonal to each other. Orthogonality can also be viewed

from the standpoint of stochastic processes. If two random processes are

uncorrelated, then they are orthogonal. Given the random nature of signals in a

communications system, this probabilistic view of orthogonality provides an

intuitive understanding of the implications of orthogonality in OFDM.

OFDM is implemented in practice using the discrete Fourier transform (DFT).

Recall from signals and systems theory that the sinusoids of the DFT form an

orthogonal basis set, and a signal in the vector space of the DFT can be

represented as a linear combination of the orthogonal sinusoids. One view of the

DFT is that the transform essentially correlates its input signal with each of the

sinusoidal basis functions.


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If the input signal has some energy at a certain frequency, there will be a peak inthe correlation of the input signal and the basis sinusoid that is at that

corresponding frequency. This transform is used at the OFDM transmitter to map

an input signal onto a set of orthogonal sub carriers, i.e., the orthogonal basis

functions of the DFT.

Similarly, the transform is used again at the OFDM receiver to process the

received sub carriers.

The signals from the sub carriers are then combined to form an estimate of the

source signal from the transmitter. The orthogonal and uncorrelated nature of

the sub carriers is exploited in OFDM with powerful results. Since the basic

functions of the DFT are uncorrelated, the correlation performed in the DFT for a

given sub carrier only sees energy for that corresponding sub carrier. The energyfrom other sub carriers does not contribute because it is uncorrelated.

This separation of signal energy is the reason that the OFDM sub carriers

spectrums can overlap without causing interference.

2.5 MATHEMATICAL ANALYSIS

With an overview of the OFDM system, it is valuable to discuss the

mathematical definition of the modulation system.


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It is important to understand that the carriers generated by the IFFT chip

are mutually orthogonal. This is true from the very basic definition of an IFFT

signal. This will allow understanding how the signal is generated and how receiver

must operate.

Mathematically, each carrier can be described as a complex wave:

The real signal is the real part of s c (t). Ac (t) and c (t), the amplitude and phase of

the carrier, can vary on a symbol by symbol basis. The values of the parameters

are constant over the symbol duration period t. OFDM consists of many carriers.

Thus the complex signal S s(t) is represented by:

This is of course a continuous signal. If we consider the waveforms of each

component of the signal over one symbol period, then the variables A c (t) and c

(t) take on fixed values, which depend on the frequency of that particular carrier,

and so can be rewritten:


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If the signal is sampled using a sampling frequency of 1/T, then the resulting signal

is represented by:

At this point, we have restricted the time over which we analyze the signal to N

samples. It is convenient to sample over the period of one data symbol. Thus we

have a relationship: t=NT If we now simplify above equation, without a loss of

generality by letting 0=0, then the signal becomes:

Now above equation can be compared with the general form of the inverseFourier transform:


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In Ss(KT) the function A ne jn is no more than a definition of the signal in the

sampled frequency domain, and S s(kT) is the time domain representation. last 2

expression are equivalent if:

This is the same condition that was required for orthogonality. Thus, one

consequence of maintaining orthogonality is that the OFDM signal can be defined

by using Fourier transform procedures.

2.6 OFDM GENERATION AND RECEPTION

OFDM signals are typically generated digitally due to the difficulty in

creating large banks of phase locks oscillators and receivers in the analog domain.

Fig 2.3 shows the block diagram of a typical OFDM transceiver. The transmitter

section converts digital data to be transmitted, into a mapping of subcarrier

amplitude and phase.


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It then transforms this spectral representation of the data into the timedomain using an Inverse Discrete Fourier Transform (IDFT). The Inverse Fast

Fourier Transform (IFFT) performs the same operations as an IDFT, except that it

is much more computationally efficiency, and so is used in all practical system

Fig.2.3 Block diagram of a basic OFDM transceiver.


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The receiver performs the reverse operation of the transmitter, mixing theRF signal to base band for processing, then using a Fast Fourier Transform (FFT) to

analyze the signal in the frequency domain. The amplitude and phase of the sub

carriers is then picked out and converted back to digital data. The IFFT and the

FFT are complementary function and the most appropriate term depends on

whether the signal is being received or generated. In cases where the signal is

independent of this distinction then the term FFT and IFFT is used

interchangeably.

Peak To Average Power Ratio (PAPR)

An OFDM signal is the sum of Ci complex random variables, each of it can beconsidered as a complex modulated signal at a different frequency. In some cases,all signal components can add up in phase and producea large output and in some cases, they may cancel each other, producing zerooutput. Thus, the peak-to-average ratio (PAPR) of an OFDM system is very large.

In the transmitter front-end, a power amplifier is required with a wide linear rangethat can include the peaks in the transmitted waveform. DAC's and ADC's mustalso have a wide signal range to avoid clipping. The symbols that have a largePAPR are vulnerable to errors. Peak to Average power Ratio is defined by Muller

and Huber (1997) [7]:


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where E denotes the expectation and Sx(t) is the OFDM transmitted signal. Thisexpectation value is the valuethat we expect to measure most often if repeated measurements were made on thesystem. The PAPR also can

be expressed in dB as in equation (2).

OFDM is one of the many multicarrier modulation techniques, whichprovides high spectral efficiency, low implementation complexity, lessvulnerability to echoes and non linear distortion. Due to theseadvantages of the OFDM system, it is vastly used in variouscommunication systems. But the major problem one faces whileimplementing this system is the high peak to average power ratio ofthis system. A large PAPR increases the complexity of the analog to digital and digital to analog converter and reduces the efficiency ofthe radio frequency (RF) power amplifier [3,6]. Regulatory andapplication constraints can be implemented to reduce the peaktransmitted power which in turn reduces the range of multi carriertransmission. This leads to the prevention of spectral growth and thetransmitter power amplifier is no longer confined to linear region inwhich it should operate. This has a harmful effect on the batterylifetime. Thus in communication system, it is observed that all thepotential benefits of multi carrier transmission can be out - weighed by

a high PAPR value .


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Presence of large number of independently modulated sub-carriers in anOFDM system the peak value of the system can be very high ascompared to the average of the whole system. This ratio of the peak toaverage power value is termed as Peak-to-Average Power Ratio.Coherent addition of N signals of same phase produces a peak which is

N times the average signal.The major disadvantages of a high PAPR are-1. Increased complexity in the analog to digital and digital to analogconverter.

2. Reduction is efficiency of RF amplifiers.

PAPR OF A MULTICARRIER SIGNALLet the data block of length N be represented by a vector . Duration ofany symbol in the set X is T and represents one of the sub carriers set.As the N sub carriers chosen to transmit the signal are orthogonal toeach other, so we can have where and NT is the duration of the OFDMdata block X . The complex data block for the OFDM signal to betransmitted is given by

The PAPR of the transmitted signal is defined as


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Reducing the max|x(t)| is the principle goal of PARP reductiontechniques. Since, discrete- time signals are dealt with in most systems,many PAPR techniques are implemented to deal with amplitudes ofvarious samples of x(t) . Due to symbol spaced 25output in the first equation we find some of the peaks missing which can

be compensated by oversampling the equation by some factor to give thetrue PAPR value.

Continuous-time PAPR

In general, the PAPR of OFDM signals is defined as the ratio between themaximum instantaneous power and its average power

where is the average power of and it can be computed in the frequencydomain because Inverse Fast Fourier Transform (IFFT) is a (scaled)unitary transformation.

2) Discrete-time PAPRThe PAPR of the discrete time sequences typically determines thecomplexity of the digital circuitry in terms of the number of bitsnecessary to achieve a desired signal to quantization noise for both the

digital operation and the DAC. However, we are often more concernedwith reducing the PAPR of the continuous-time signals in practice,since the cost and power dissipation of the analog components oftendominate.


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To better approximate the PAPR of continuous-time OFDM signals, theOFDM signals samples are obtained by times oversampling. -timesoversampled time-domain samples are -point IFFT of the data blockwith zero-padding. Therefore, the oversampled IFFT output can beexpressed as

Passband PAPR

Note that, if is large, an OFDM system usually does not employ pulseshaping, since the power spectral density of the band-limited OFDMsignal is approximately rectangular. Thus, the amplitude of OFDM RFsignals can be expressed as

where is the carrier frequency and . Therefore, the peak of RF signals is

equivalent to that of the complex baseband signals


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PAPR Reduction Techniques

PAPR reduction techniques vary according to the needs of the systemand are dependent on various factors. PAPR reduction capacity, increasein power in transmit signal, loss in data rate, complexity of computationand increase in the bit-error rate at the receiver end are various factorswhich are taken into account before adopting a PAPR reductiontechnique of the system. [3].

The PAPR reduction techniques on which we would work upon andcompare in our later stages are as follows:

AMPLITUDE CLIPPING AND FILTERING

A threshold value of the amplitude is set in this process and any sub-

carrier having amplitude more than that value is clipped or that sub-carrier is filtered to bring out a lower PAPR value.


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AMPLITUDE CLIPPING AND FILTERING

Amplitude clipping is considered as the simplest technique which may be under taken for PAPR reduction in an OFDM system. A thresholdvalue of the amplitude is set in this case to limit the peak envelope of theinput signal.

The main objective of this technique is to generate a set of data blocks atthe transmitter end which represent the original information and then to

choose the most favorable block among them for transmission. Let usconsider an OFDM system with N orthogonal sub carriers. A data block is a vector composed of N complex symbols , each of themrepresenting modulation symbol transmitted over a sub carrier. X ismultiplied element by element with U vector composed of N complexnumbers , , defined so that , where |.| denotes the modulus operator. Eachresulting vector , where , produces after IDFT, a corresponding OFDMsignal given by

where T is the OFDM signal duration and is the sub carrier spacingAmong the modified data blocks, the one with the lowest PAPR isselected for transmission. The amount of PAPR reduction for SLMdepends on the number of phase sequences U and the design of the

phase sequences. 30


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SELECTED MAPPING

In this a set of sufficiently different data blocks representing theinformation same as the original data blocks are selected. Selection ofdata blocks with low PAPR value makes it suitable for transmission.

PARTIAL TRANSMIT SEQUENCE

Transmitting only part of data of varying sub-carrier which covers allthe information to be sent in the signal as a whole is called Partial

Transmit Sequence Technique.

Tone Reservation

In this scheme, some OFDM subcarriers are reserved. These reserved subcarriersdon t carry any data information, are only used for reducing PAPR. This method iscalled Tone Reservation .

This technique includes number of set of reservation of tones. By using this

technique reserved tones can be used to minimize the PAPR.This method is used for multicarrier transmission and also shows the reservingtones to reduce the PAPR. This technique depends on amount of complexity.

When the number of tones is small reduction in PAPR may represent nonnegligible samples of available bandwidth. Advantage of this tone reservation isvery positive that no process is needed at receiver end. And also do not need totransmit the side information along with the transmitted signal. In this techniquenumber of loops are used and the signal will pass from each loop and dependingnumber of iterate the output PAPR value will be displayed.


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As per the technique above figure shows the previous simulation by keeping CR=1, that why reduction in the PAPR can be done up to 1.5dB only but the timesalways wants some changes so by this technique due to some changes better resultcan be display. In table the data shows reduction in CCDF of PAPR.


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The signal generated in BPSK modulation signal then signal passed from 1000

loops. OFDM signal s peaks can be remove at some level by using this technique.In this technique by using previous reservation terms further can be evaluate. Nowas per shown the fig.3 this is the simulation result by using the tone reservationtechnique to reduce the PAPR.

This proposed technique includes signal power and mean power. Now in thistechnique OFDM signal is transmitted after performing IFFT.

Here as per the signal power and combination of mean power and CR. Reduction

in PAPR has been possible.


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Clipping Ratio(CR)=2

Here particular in this technique loop is 1000. And the reducing of data we can see

here it is continuous decreasing. The value of CCDF is checked for 32 Times because here M=32.

3.2 Partial Transmit sequenceFor PAPR reduction using partial transmit sequence a typical OFDM system withinput data block in Xhas been partitioned into M disjoint sub-blocks of clusters, which are represented

by the


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However, the ESA requires an exhaustive search over all combinations of theallowed phase factors and has exponential search complexity with the number ofsub blocks. To reduce the computational complexity, some simplified searchtechniques have been proposed such as the Iterative Flipping Algorithm (IFA) .

Although the IFA significantly reduces the search complexity, there has been somegap between its PAPR reduction performance and that of the ESA. A Cross-Entropy (CE) based method has been proposed by Jung - Cheih Chen for obtainingthe optimal phase factors for the PTS technique to reduce the PAPR. Jung- CheihChen has proposed a Quantum-Inspired Evolutionary Algorithm (QEA) basedmethod to obtain the optimal phase factor for the PTS technique. Abolfazl has

proposed an Auto- Correlation Function (ACF) to develop a new PTS sub blocking

technique using Error-Correcting Codes (ECCs). This technique minimizes thenumber of repeated subcarrier with a sub-block and provides better PAPRreduction than pseudorandom or m-sequence sub blocking.

3.3 Selective Mapping Technique

In SLM, the input data sequences have been multiplied by each of the phasesequences to generate alternative input symbol sequences. Each of the alternativeinput data sequences is made the IFFT operation, and then the one with the lowestPAPR is selected for transmission . A block diagram of the SLM technique has

been depicted in Fig.7. Each data block is multiplied by V different phase factors,each of length N , v v,0 bv,1 bu,N-1T (v=0,1,..V-1), resulting in V different data

blocks. Thus , the vth phase sequence


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Among the data block Xv(v= 0,1,.V -1), only one with the lowest PAPR has beenselected for transmission and the corresponding selected phase factors bv,n alsoshould be transmitted to receiver as side information. For implementation of SLMOFDM systems the SLM technique needs V IFFT operation and the number ofrequired bits as side information is 2 for each data block. Therefore, the ability ofPAPR reduction in SLM depends on the number of phase factors V and the designof the phase factors.

Some extension of SLM also has been proposed to reduce the computationalcomplexity and number of the bits for side information transmission. SelectedMapping (SLM) has been one of the most popular signal scrambling techniques

used to reduce the PAPR of OFDM signals. In symbol scrambling techniques theinput data sequence has been scrambled using a number of specializedscrambling sequences. The sequence which produces the lowest PAPR is the oneused for transmission.

The SLM technique proposed takes the OFDM subcarrier data block to betransmitted and multiples it element-wise by a number of phase adjustment vectorsets. The new statistically independent phase adjusted OFDM frames represent thesame transmitted information, but have different PAPR values.The OFDM frame that has the lowest PAPR is then selected to be transmitted. As

reported in the SLM technique can provide a 0.1% probability PAPR reduction of2.8 dB (from 10.4 dB to 7.6 dB) when applied to 128 subcarrier QPSK-OFDMsymbols. However, one drawback of this technique is that some additionalinformation relating to the phase vector set that produces the lowest PAPR alsorequires to be transmitted along with the OFDM signal. This extra informationincreases the overhead.

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