USE OF CLIPPING AND LINEAR PHASE FIR FILTERING TO REDUCE PAPR IN OFDM SYSTEM Pradeep K G M 1 and K.Dasharath 2 1,2 Department of ECE, Sphoorthy Engineering College Abstract- Orthogonal frequency division multiplexing (OFDM) is becoming the chosen modulation technique for wireless communications. The growth of wireless communication technologies has been producing the intense demand for high-speed, efficient, reliable voice & data communication. The next generation wireless communication technology long term evolution (LTE) which is designed to increase the capacity and speed of existing mobile telephone & data networks has adopted a multicarrier transmission technique known as orthogonal frequency division multiplexing. OFDM and other multi-carrier system together deal with a problem of Peakto-Average Power Ratio (PAPR). The reason for the occurrence of this problem in OFDM is the occurrence of significant distortion due to fact that it has high PAPR when allowed to pass through a non-linear device. OFDM signal transmission can be improved significantly by the use of clipping technique. In this paper, a clipping based filtering method has been implemented & also analyzed their modulation effects on reducing PAPR. However, clipping the peak envelope of the input signal to a predetermined value causes in-band distortion. A trade-off is necessary for reducing PAPR with increasing bit error rate (BER), computational complexity or data rate loss etc. using proposed technique along with reducing PAPR, BER performance also can be improved. Keywords - OFDM, Bit error rate ( BER), Peak-to-Average Power Ratio (PAPR), Multicarrier modulation (MCM), clipping and filtering (C&F) I. INTRODUCTION Recently, multimedia applications for wireless communications are widespread. The requirements of high data rate transmission for wireless communications become necessary. Orthogonal frequency division multiplexing (OFDM) is a multicarrier modulation (MCM) technique which seems to be an attractive candidate for fourth generation (4G) wireless communication systems. The additional increasing demand on high data rates in wireless communications systems has arisen in order to carry broadband services. OFDM is a promising technique owing to the high spectral efficiency, good immunity against multipath effects and narrow band interferences, and easy implementation. However, OFDM faces the Peak-to-Average Power Ratio (PAPR) problem. The high PAPR signal, when transmitted through a nonlinear power amplifier, creates spectral broadening and also will increase the dynamic range of the digital to analog converter (DAC). The result will be an increase in the cost of the system and efficiency degradation. A. MOTIVATION FOR THE PROJECT Orthogonal Frequency Division Multiplexing (OFDM) is considered to be a promising technique against the multipath fading channel for wireless communications. However, due to the inherent multicarrier nature of OFDM, the summation of subcarriers with the same phase will usually lead to a high peak-to-average power ratio (PAPR) of the time-domain signal which is a major drawback of multicarrier transmission system that leads to power inefficiency in RF section of the transmitter. The PAPR problem of OFDM has received much interest from the research community and a number of techniques have been developed to reduce it. The clipping and filtering (C&F) procedure may be the simplest to approach a specified PAPR of OFDM signals. DOI:10.21884/IJMTER.2017.4184.0O0UY 55
The proposed scheme uses optimal filtering technique which can achieve nearly same PAPR reduction effect, same out-of-band radiation and better BER performance than that of the existing Clipping and Filtering technique. B.OBJECTIVE OF THE PROJECT The main objective of the project is to develop a technique -Clipping and Optimal Filteringthat can achieve PAPR reduction effect, better BER performance considering the complexity of the system and to prove the same in the MATLAB platform. C.PROBLEM FORMULATION OFDM is limited mainly by its high Peak-to-Average Power Ratio (PAPR). The clipping and filtering technique efficiently reduces PAPR. However, clipping the peak envelope of the input signal to a predetermined value causes in-band distortion. Thus, the C&F method reduces PAPR at the cost of increase in bit error rate (BER). The proposed scheme can achieve nearly same PAPR reduction effect and better BER performance as that of the existing technique, at the cost of slight loss of spectral efficiency. D.PROBLEM STATEMENT In the proposed Clipping and optimal filtering scheme, the clipped signal passes through an FFT block, Composed filtering (FIR based BPF) block and an IFFT block after filtering. This technique results in effective reduction of PAPR and an improved BER performance. II. METHODOLOGY Figure II.1 Block diagram for the proposed project. It has the basic OFDM system blocks with the proposed clipping and linear filtering to effectively reduce the PAPR of the OFDM system. @IJMTER-2017, All rights Reserved 56
A. Sequence of Operations Performed (Algorithm) Step 1: Initialization: Here Initialization of Number of bits per QAM/QPSK symbol: Alphabet size: (QPSK) FFT size: N=128 CP Size: 32 Sampling period Frequency: Fs= BW*L=8MHZ Band width (BW): 1MHz Oversampling Factor (L):8 OFDM Symbol Period: Carrier Frequency (Fc): 2MHz Baseband Sampling Frequency of Filter: 8 Order of filter: 104 Density factor of filter: 20 Sampling period: Step 2: Define Binary Sequences (Message) In this step a matrix of order (b N) is generated using a function RANDI in MATLAB. It contains random binary numbers. "0" and "1" occur with equal probability. Step 3: Apply QAM/QPSK In this step modulation is performed using psk mod function. The constellation diagram is shown below Figure II.A.1 QPSK constellation Step 4: Apply N-Point IFFT & Interpolation with Oversampling Factor L The input of the IFFT block is the interpolated signal introducing N(L 1) zeros (also known as zero padding) in the middle of the original signal. In this system, the L- times oversampled discrete-time signal is generated as, Step 5: Add Cyclic Prefix A cyclic prefix is a repetition of the first section of a symbol that is appended to the end of the symbol. In addition, it is important because it enables multi-path representations of the original signal to fade so that they do not interfere with the subsequent symbol. @IJMTER-2017, All rights Reserved 57
Step 6: Apply Clipping International Journal of Modern Trends in Engineering and Research (IJMTER) Where represents the phase of x, x is the unclipped signal,x clip is clipped signal and Tm the clipping level. Step 7: If ncf=0 go to step 11 This step is to generate an unclipped signal so as to use it to compare with clipped and C&F (clipped and filtered) signal. Step 8: Define and Calculate clipping Ratio (New CR) Clipping ratio is defined as the ratio of the clipping level to the root mean square value of the unclipped signal and it is related to the desired PAPR (denoted ) as follows: Where Is the root mean square value of the unclipped signal. Step 9: If new CR>CR Update ( ) and Go to step 9 After each clipping process new CR is calculated and compared with the values taken to be the CR (0.8, 1.0, 1.2, 1.4 and 1.6) values. Step 10: Apply Filtering This step includes FFT, Filtering and IFFT blocks. Syntax to filter the signal is Y = filter (B, A, X) which filters the data in vector X with the filter described by vectors A and B to create the filtered data Y. Where, B is h (filter co-efficient), h = firpm(n,f,a,w,{dens}) n(order of the filter)= 104 f(vector of pairs of normalized frequency points)=ff/(fs/2) Where, FF= [0 1.4 1.5 2.5 2.6Fs/2] and Fs= 8 a(desired amplitudes at the points specified in f)= [0 0 1 1 0 0] w (vector of weights)=[10 1 10] dens(density factor of filter) =20 Figure II.B.2 Magnitude response of the filter The above result is the magnitude response of the filter A is taken to be X clipped signal. Over all process of 3 blocks that is, at the output of IFFT block is Step 11: Remove cyclic prefix, Perform FFT and Demodulation This step includes the operations done at the receiver side: remove CP from each OFDM symbol, pass it through the FFT block (fft function is used) and finally demodulate. @IJMTER-2017, All rights Reserved 58
Step 12: Go To Step 2 To Process The Next OFDM Symbol. This step is to process the next incoming signal and this continues as long as there is a signal to be processed. III.RESULTS AND DISCUSSIONS Figure III.A.1 shows random 128 data points generated using RANDI function to be transmitted. Vertical axis represents data phase and horizontal axis represents data points Figure III.B.2 constellation.for QPSK modulation the constellation is 4 and we can see the distribution of constellation points across in-phase and quadrature phase (horizontal and vertical axis) FigureIII.C.3 shows OFDM modulated signal without clipping. Vertical axis represents amplitude of the signal and horizontal axis represents time. @IJMTER-2017, All rights Reserved 59
Figure III.D.4 Clipped OFDM modulated signal. It is the OFDM modulated signal which has been clipped. Threshold value taken is. We can observe that the signal above and below the threshold (above +0.4 and below-0.4) has been clipped. Figure III.E.5 OFDM signal after HPA. When the OFDM (unclipped) modulated signal passes through the power amplifier. This is to show the effect of high power amplifier on the signal that passes through it. @IJMTER-2017, All rights Reserved 60
Figure III.F.6 clipped signal after HPA.When the OFDM (clipped) modulated signal passes through the power amplifier. This is to show the effect of high power amplifier on the signal that passes through it. Figure III.F.7Received unclipped data points. It shows the data points that have been recovered at the receiver side after the demodulation process. This is without clipping. @IJMTER-2017, All rights Reserved 61
FigureIII.G.8 Received unclipped data points. It shows the data points that have been recovered at the receiver side after the demodulation process. This is with clipping. Figure III.H.9 PAPR Statistics of OFDM Signals and BER Performance for CR= (0.8, 1.0, 1.2, 1.4, 1.6). It shows the proof for the objective of the project. @IJMTER-2017, All rights Reserved 62
IV. PAPR STATISTICS OF OFDM SIGNALS The plain line (blue) corresponds to OFDM signal without clipping, the group of lines (green) (for different CR values ranging from 0.8 to 1.6) to the left of plain line corresponds to clipped OFDM signal and the remaining group of lines (blue lines with symbols on it) to the left in the plot corresponds to clipped and optimal filtered OFDM signal. We can observe that PAPR has reduced comparatively. V. BER PERFORMANCE Here the line(blue) below all lines in the plot corresponds to OFDM signal without clipping, and the lines(green) above it corresponds to clipped OFDM signal and lastly the lines (blue lines with symbols on it) at the top corresponds to clipped and optimal filtered OFDM signal. We can observe the BER performance though not better than that for the clipped signals it is than the existing method considering its complexity. Figure 7.10PAPR Statistics of OFDM Signals and BER Performance for CR= (0.8, 1.0, 1.2,1.4,1.6). It is same as figure 7.9 but to be more specific only the plot corresponding to Clipping Ratio (CR) = 0.8 is shown. Now we can clearly observe that PAPR is reduced (nearly 5dB) and BER= (these values keeps slightly changing every time the code is run). It is better than the method in which only a low pass filter is used and not so better than the method proposed in [7] but the complexity is comparatively reduced. VI. CONCLUSION AND SCOPE FOR FUTURE WORK A.CONCLUSION The main objective of the project was to develop a technique -Clipping and Optimal Filteringthat can achieve PAPR reduction effect, better BER performance considering the complexity of the system and to prove the same in the MATLAB platform. Accordingly PAPR reduction is achieved at the acceptable cost of BER performance. We have another advantage of reduced complexity of the system. @IJMTER-2017, All rights Reserved 63
B.SCOPE FOR FUTURE WORK 1. The BER performance degradation in this project though acceptable, can be further reduced to avail more benefit of the PAPR reduction system. 2. Future work can involve the design of a system that doesn t allow the loss of spectral efficiency as there is slight loss of it in this system. 3. Filtering the clipped signal can reduce out-of-band radiation at the cost of peak re growth. The signal after filtering operation may exceed the clipping level specified for the clipping operation. So a better filtering technique which doesn t allow for this disadvantage can be adopted. REFERENCES [1] Houshou Chen, Hsinying Liang PAPR Reduction of OFDM Signals Using Partial Transmit Sequences and Reed- Muller Codes Communications Letters, IEEE Volume: 11, Issue: 6. 2007. [2] Nikookar, H.; Lidsheim, K.S., PAPR reduction of OFDM by random phase, Personal, Indoor and Mobile Radio Communications, 2002. [3] XiaowenGu; SeungminBaek; Suwon Park, PAPR reduction of OFDM signal using an efficient SLM technique, Advanced Communication Technology (ICACT), 2010. [4] Jung-Chieh Chen, Chao-Kai Wen, PAPR Reduction of OFDM Signals Using Cross-Entropy-Based Tone Injection Schemes Signal Processing Letters, IEEE Volume: 17, Issue: 8,2010. [5] Yang, L., Chen, R.S., Siu, Y.M., Soo, K.K., PAPR Reduction of OFDM Signals Using Partial Transmit Sequences with Low Computational Complexity, Broadcasting, IEEE Transactions on Volume: 52, Issue: 1, 2006. [6] Y.C. Wang and Z.Q. Luo, Optimized Iterative Clipping and Filtering for PAPR Reduction of OFDM Signals, IEEE Transactions on Communication, vol. 59, no.1, pp 33-37, Jan. 2011. [7] Shripad P. Mohani, Mrunmayee S. Bhandarkar & Mukul S. Sutaone, Distortion Compensated Optimized ICF Technique for PAPR Reduction in OFDM, ISSN (Print): 2278-5140, Volume-2, Issue 1, 2013. [8] Begared Salih Hassen, Peak to Average Power Ratio Reduction in OFDM Systems Using Clipping and Filtering Technique, (IJSCE) ISSN: 2231-2307, Volume-4, Issue-1, March 2014. [9] T. Jiang and Y. Wu, An overview: peak-to-average power ratio reduction techniques for OFDM signals, IEEE Transactions on Broadcasting, vol. 54, no. 2, pp. 257 268, Jun. 2008. [10] J. Armstrong, Peak-to-average power reduction for OFDM by repeated clipping and frequency domain filtering," Electronics Letters, vol. 38, no. 5, pp. 246-247, Feb. 2002. @IJMTER-2017, All rights Reserved 64