9. ECE - IJECIERD - final

International Journal of Electronics, Communication & Instrumentation Engineering Research and Development (IJECIERD) ISSN 2249-684X Vol. 2 Issue 4 Dec - 2012 73-82 Š TJPRC Pvt. Ltd.,

PAPR REDUCTION IN OFDM BY CLIPPING TECHNIQUE 1

PRASHANT MARUTI JADHAV, 2L.S.ADMUTHE & 3A.P.BHADVANKAR 1

Department of Electronics TEI, Rajwada Ichalkaranji.

Department of Electronics TEI, Rajwada Ichalkaranji 3 Department of Electronics & TC Sharad Institute of technology, Yadrav 2

ABSTRACT Orthogonal frequency division multiplexing (OFDM) is an attractive technique for wireless communication applications. . Long term evolution (LTE) is the last step toward the 4th generation (4G) of radio technologies designed to increase the capacity and speed of mobile telephone networks. LTE has adopted orthogonal frequency division multiplexing (OFDM) for the downlink transmission. OFDM meets the LTE requirement for spectrum flexibility and enables cost-efficient solutions for very wide carriers with high peak rates. Potentially large peak-to-average power ratio (PAPR) of the transmitting signals has limited its application. This high PAPR causes interference when the OFDM signals are passed through an amplifier which does not have enough linear range. OFDM signal with a large peak-to-mean envelope power ratio, result in significant distortion when passed through a nonlinear device such as a transmitter power amplifier. We investigate, through extensive computer simulations, the effects of clipping and filtering on the performance of OFDM, including the power spectral density, the crest factor, and the bit-error rate. Our results show that clipping and filtering is a promising technique for the transmission of OFDM signals using realistic linear amplifiers. One such algorithm is the iterative clipping and filtering (ICF). Also technique of interleaving is discussed and simulated.

Simulation results confirm that the using QPSK modulation we get maximum PAPR reduction while IFFT size is 32 for 512 data bits. It’s up to: 17.384791 dB. Simulation results show that the proposed scheme may obtain significant PAPR reduction while the BER of performance of system is improved at the same clipping parameter. ICF is a widely used technique to reduce the PAPR of OFDM signals. In this paper different modulation techniques like 16-QAM, QPSK and BPSK are simulated for clipping techniques in two ways.

KEYWORDS: - Orthogonal Frequency Division Multiplexing (OFDM), Peak-to-Average power ratio (PAPR), Peak Envelope Power (PEP), Selective Level Mapping (SLM)

INTRODUCTION ORTHOGONAL frequency division multiplexing (OFDM) is a very attractive technique for the transmission of high-bit-rate data in a radio environment [4]. The further increasing demand on high data rates in wireless communications systems has arisen in order to support broadband services. Long term evolution (LTE) is standardized by the third generation partnership project (3GPP) and is an evolution to existing 3G technologies in order to meet projected customer needs over the next decades. Current working assumptions in 3GPP LTE are to use orthogonal frequency division multiplexing access (OFDMA) for downlink and single carrier-frequency division multiple access (SC-FDMA) for uplink.

[1], [2], [3]. However, any multicarrier signal with a large number of sub channels is burdened with a large crest factor (CF peak voltage/rout mean square (rms) voltage). When passed through a nonlinear device, such as a transmitter power amplifier, the signal may suffer significant spectral spreading and in-band distortion. The conventional solutions to this problem are to use a linear amplifier or to back off the operating point of a nonlinear amplifier; both approaches resulting in a significant power efficiency penalty. Moreover, the low data rate on the subcarriers mitigates the effect of the multipath problem in wireless environment. Advantage of OFDM system is that it can be implemented via (inverse) fast

74

Prashant Maruti Jadhav, L.S.Admuthe & A.P.Bhadvankar

Fourier transform ((I)FFT) [10] which makes it fast and efficient. Unfortunately, due to the superposition of a large number of individual sub channels, the amplitude of the transmitted OFDM signal generally suffers from high peak-to-average power ratio (PAPR). This fact complicates implementation of the analog radio frequency (RF) frontend. When the PAPR is high, the digital-to-analog converter (DAC) and power amplifier (PA) of the transmitter require high dynamic ranges to avoid amplitude clipping. Such high dynamic range increases complexity, reduces efficiency, and increases cost of the components. On the other hand, if the dynamic range is too low, there would be substantial amount of signal distortion which in turn will raise the amount of bit error rate (BER). Furthermore, the distortion would cause unwanted out-of-band radiation. Various kinds of methods to combat PAPR problem have been proposed [11]. To reduce the PAPR, many techniques have been proposed. Such as clipping, coding, partial transmit sequence (PTS), selected mapping (SLM), interleaving [20][21], nonlinear companding transforms[22] [23], hadamard transforms[24] and other techniques etc. these schemes can mainly be categorized into signal scrambling techniques, such as PTS, and signal distortion techniques such as clipping, companding techniques. Among those PAPR reduction methods, the simplest scheme is to use the clipping process. However, using clipping processing causes both in-band distortion and out-of-band distortion and further causes an increasing of error bit rate of system. In this paper focus is given on the clipping technique to reduce PAPR of OFDM system. Using simulation results the effect of clipping technique is compared for the various modulation systems as BPSK, QPSK and QAM. The remainder of this paper is organized as follows: in section II, we describe the wireless communication systems model.

In section III, amplitude clipping PAPR reduction techniques is analyzed. In section IV, we simulated and compare the clipping method with the different level of clip and filtering level for various modulation techniques. Finally, conclusions are made in section V according to simulation results.

OFDM Basics ORTHOGONAL frequency-division multiplexing (OFDM) is a technique widely used for wireless applications [12].Due to its multicarrier feature, OFDM systems are more sensitive than single-carrier systems to frequency synchronization errors [13].

Serial to paralle l

IFFT

Parallel to serial

DAC and LPF

Serial data in

Parallel to serial

Channel FFT

Serial to Parallel

ADC

Serial data out Fig. I: Block Diagram of OFDM system

OFDM is a special case of FDM. As we know in single carrier modulation channels are frequency selective so they add ISI at the receiver side because channels not having flat response in frequency domain. Even though if we do amplification at the receiver side noise also get amplified so multicarrier modulation is so much popular. OFDM is a multicarrier modulation in which orthogonality allows lot of subcarriers in tight frequency space without interference from each other. Due to limitation of bandwidth in communication we need to divide data stream in to many small bands and also carriers. Then we multiply carriers with data stream then we modulate each carrier at lower data rate and adding them together for transmission. OFDM is a form of multicarrier transmission that sends information simultaneously over N

75

Papr Reduction In OFDM By Clipping Technique

orthogonal carriers. It introduces frequency diversity by making the bandwidth of each carrier smaller than the coherence bandwidth of the channel. Each carrier may still suffer from flat-fading, however. OFDM is considered a good candidate for high data rate wireless systems and is currently used for the Hyper LAN II standard [14]. The transmitted signal over a symbol duration T is: [15]

 N −1  s (c , t ) = Re ∑ ci exp( j 2π ( f 0 + if s )t   i =0  c = (c0 c1 ...c N −1 )

0≤t ≤T

The code word c consists of N symbols chosen from a many modulation method. All of the code words form the set C. For M PSK.

ci = e

j

2π ( ai ) M

a i εZ M

The duration of an OFDM symbol T is N times the duration of the symbols ci plus the duration of the cyclic prefix or guard band. The complex envelope of the transmitted signal, sampled at 1/T, is: N −1

~ s (c , n ) = ∑ ci exp( j 2πni / N ) i =0

This equation can be recognized as the IDFT of the sequence co …cN-1. FFT is the efficient algorithm to find the DFT, so in block diagram of OFDM at the transmitter side we are using IFFT block first and at the receiver side we are using FFT block. FFT converts time domain signal in to frequency domain and IFFT is vice versa. In transmitter taking FFT means only multiplying by D-1 block after multiplication we serially convert the parallel signal and transmit it. This whole process is nothing but linear convolution of two signals so there is found overlapping of last L-1 bits to avoid this we can pad zeros or we can cyclic prefix to the original data blocks. An important limitation of OFDM is that it suffers from a high Peak-to-Average Power Ratio (PAPR). OFDM is the time domain signal which is a sum of several sinusoids leads to High PAPR; resulting from the coherent sum of several carriers. This forces the power amplifier to have a large input back off and operate inefficiently in its linear region to avoid inter modulation products. High PAPR also affects D/A converters negatively and may lower the range of transmission.

Basics of PAPR PAPR is defined as:

PAPR =

max s(t )

[

E s (t )

2

2

]

Theoretically, the PAPR can be as high as N, but the occurrence of such peaks is rare. The summation of a large number of carriers assumes a Gaussian distribution. The numerator, max|s(t)|2, is also known as the PEP (Peak Envelope Power).

It is also equal to:

PEP = ~ s (t ) ~ s * (t ) .

Therefore, it is desirable to reduce the PAPR.

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Prashant Maruti Jadhav, L.S.Admuthe & A.P.Bhadvankar

CLIPPING TECHNIQUE Clipping and Filtering A high PAPR brings disadvantages like increased complexity of the ADC and DAC and also reduced efficiency of radio frequency (RF) power amplifier. One of the simple and effective PAPR reduction techniques is clipping, which cancels the signal components that exceed some unchanging amplitude called clip level. In Clipping, the amplitudes of the input signal are clipped to a predetermined value. However, clipping yields distortion power, which called clipping noise, and expands the transmitted signal spectrum, which causes interfering [16].

Clipping and filtering technique is effective in removing components of the expanded spectrum. Although filtering can decrease the spectrum growth, filtering after clipping can reduce the out-of-band radiation, but may also cause some peak re-growth, which the peak signal exceeds in the clip level [17].

input OFDM

Clipping

Clipping

and

and

frequency

frequency

domain

domain

filtering

filtering

Output OFDM

Fig II: Block Diagram of OFDM System in Clipping and Filtering Approach The technique of iterative clipping and filtering reduces the PAPR without spectrum expansion. However, the iterative signal takes long time and it will increase the computational complexity of an OFDM transmitter [16]. But without performing interpolation before clipping causes it out-of-band. To avoid out-of-band, signal should be clipped after interpolation. However, this causes significant peak re-growth. So, it can use iterative clipping and frequency domain filtering to avoid peak re-growth. In the system used, serial to parallel converter converts serial input data having different frequency component which are base band modulated symbols and apply interpolation to these symbols by zero padding in the middle of input data. Then clipping operation is performed to cut high peak amplitudes and frequency domain filtering is used to reduce the out of band signal, but caused peak re-growth [17]. This consists of two FFT operations. Forward FFT transforms the clipped signal back to discrete frequency domain. The in-band discrete components are passed unchanged to inputs of second IFFT while out of band components are null. But heavy clipping causes about 1 dB lower average EVM. Clipping introduces in band distortion and out-of-band signals, which can be controlled by proper filtering.

REPEATED CLIPPING AND FREQUENCY DOMAIN FILTERING A clipping method in its basic form is based on simple time domain signal limitation. Clipped signal

c(t) can be

expressed by following relationship: (t)=

.

, | ,|

| |

Where A is the clipping level and

is the phase of original signal

. By this limitation, the peak values of

signal are removed that results in PAPR reduction. However, the clipping introduces signal distortion resulting in adjacent channel emissions. This undesirable effect can be suppressed by low pass filtering of clipped signal that unfortunately further increases the PAPR.

77

Papr Reduction In OFDM By Clipping Technique

Armstrong [18] developed a method based on K-times repetition of the clipping and filtering process. Therefore both PAPR and adjacent spectral emissions are reduced, although the PAPR reduction is far from simple clipping case. In this paper results for repeated clipping are discussed .

COMBINATION OF INTERLEAVING WITH REPEATED CLIPPING AND FILTERING In paper [20], authors used a combination of interleaving (adaptive symbol selection) with simple clipping followed by a filter increasing the PAPR. We have chosen a concatenation of interleaving and repeated clipping and frequency domain filtering or its simplified non iterative alternative. First, the interleaving approach is used and the signal with lowest PAPR is then passed through clipping and filtering method. The intention to combine these two methods is to obtain signal with lower PAPR than in the case of interleaving method and with lower distortion (and thus lower bit error rate) than in the case of standalone Repeated clipping and filtering.

Sn

Sk

QAM/PS K mapping

S/ P

IFFT

Sk ’

QAM/PSK DE mapping

D/A and HPA

P/S and PAPR Reduction

Sn ’ P/ S

FFT

Wn

S/P and Inverse PAPR Reduction

A/D

Fig I: Actual Simulation Approach of OFDM System As both methods used in the combination suffer from high complexity, the main disadvantage of the combined method is above all the complexity. Moreover, side information (SI) to identify the interleaver with lowest PAPR has to be sent to receiver for each OFDM symbol. Without this side information, it is not possible to decode the data. As the correct decoding of side information is fundamental for the performance of OFDM modem, the SI can thus be either mapped using modulation with lower number of states or encoded by FEC. The complexity of the presented combined method can be dramatically reduced using the recently proposed method Simplified clipping and filtering instead of the repeated clipping and frequency domain filtering method. This case has been also considered in our paper and this method is recommended for practical use.

CONCLUSIONS In this paper some PAPR reduction carried out by clipping technique in two ways for modulation techniques like BPSK, QPSK, 16-QAM. Results are compared as per the tabular data shown over here.

Here we can conclude that in case of BPSK modulation we get maximum PAPR reduction for IFFT size of 32 while data points are 512. It’s up to : 13.525361 dB. Here also we can conclude that In case of QPSK modulation we get maximum PAPR reduction while IFFT size is 32 for 512 data bits. It’s up to: 17.384791 dB Table I: Simulation Results with HPA Effect for BPSK and QPSK

PSK type BPSK

IFFT Data size points 8

32

PAPR without clipping after HPA 14.060386

PAPR with clipping and HPA 5.856270

78

Prashant Maruti Jadhav, L.S.Admuthe & A.P.Bhadvankar

BPSK BPSK BPSK QPSK QPSK QPSK QPSK QPSK

16 16 32 8 8 16 16 32

64 128 512 256 512 256 512 512

15.511691 17.990750 21.304113 14.792368 16.733045 19.738100 18.211730 25.225245

6.557389 4.897381 7.778752 7.081363 7.685571 7.694189 6.630577 7.840454

Simulation results with phase rotation and without phase rotation:

BER curve for 8 frames for 16-QAM:

BER curve for 16 frames for 16-QAM:

79

Papr Reduction In OFDM By Clipping Technique

BER curve for 32 frames for 16-QAM:

BER curve for 100 frames for 16-QAM:

Table II: Simulation Results with Filtering for QAM No of frames

PSD

8 16 32 100

0.025020 0.023034 0.023787 0.027488

PAPR before Clipping 6.719467 5.641196 8.496442 4.490738

PAPR after Clipping 1.790023 1.838601 2.094995 1.689178

BER

0.171250 0.167656 0.168177 0.165967

Results for repeated clipping and filtering: Table III: Simulation Results with Repeated Clipping and Filtering for QAM PAPR 1 CLIP

ST

PAPR 2 CLIP

ND

PPAR 3 CLIP

RD

PAPR CLIP

4 TH

NO OF DATA POINTS

ORIGINAL PAPR

1024

9.074727

7.533764

6.842138

6.459559

6.253227

2048

9.417887

7.698206

6.932880

6.507286

6.277734

4096

9.737573

7.861174

7.023105

6.554821

6.302306

8192

10.08860

8.013226

7.106953

6.599251

6.325240

80

Prashant Maruti Jadhav, L.S.Admuthe & A.P.Bhadvankar

PAPR for 1024 bits:

PAPR for 2048 bits:

PAPR for 4096 bits:

PAPR for 8192 bits:

For different variations in number of frame for QAM modulation we get the almost maximum reduction in PAPR of up to 6dB. Where BER remains almost constant for varying frames. Also PSD remains constant and within range of up to 0.02 to 0.03. For 40 frames I get the maximum PAPR reduction but PSD is also MAX. No specific PAPR reduction technique has been the best solution for all multicarrier transmission system. It has been suggested that the PAPR reduction technique should be carefully chosen according to various system requirements.

Clipping and Partial Transmit Sequence are more practical than other techniques.

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Papr Reduction In OFDM By Clipping Technique

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