A Generalized Performance Analysis of Papr Reduction for A Given Transmitted Symbols in OFDM System Authors Syed Roshan Ali Project Guide Dr.Veena Desai M.Tech Student,Department of ECE, Gogte Institute Of Technology, Belgaum, Karnataka, India ABSTRACT - E-mail:syedroshanalis@gmail.com Communication is one of the important aspects of life. With the advancement in age and its growing demands, there has been rapid growth in the field of communications. Signals, which were initially sent in the analog domain, are being sent more and more in the digital domain these days. For better transmission, even single carrier waves are being replaced by multicarriers. Multi carrier systems like CDMA and OFDM are now a days being implemented commonly. In the OFDM system, orthogonally placed sub carriers are used to carry the data from the transmitter end to the receiver end. Presence of guard band in this system deals with the problem of ISI and noise is minimized by larger number of sub carriers. But the large Peak to Average Power Ratio of these signal have some undesirable effects on the system. In this thesis we have focused on learning the basics of an OFDM System and have undertaken various methods to reduce the PAPR in the system so that this system can be used more commonly and effectively. Key Words : CDMA, OFDM, ISI, PAPR,CCDF. I. INTRODUCTION Since the very genesis of man, communication has been one of the main aspects in human life.previously various methods like sign languages were implemented for this purpose. As various civilizations started coming into existence, many innovative ideas came to the minds of the people special birds and human messengers were employed to meet these challenges. As ages rolled by, post system developed and transportation vehicles like trains and ships were used to maintain link between people miles apart. But by the turn of the nineteenth century, a great leap in communication system was observed when wireless communication was introduced. After the advent of wireless communication huge change has been observed in the lifestyle of people. Wireless communication which was initially implemented analog domain for transfer has is now-a-days mostly done in digital domain. Instead of a single carrier in the system multiple sub-carriers are implemented to make the process easier. Syed Roshan Ali IJMEIT Volume 2 Issue 6 June 214 Page 386
1.1 Electronic Communication System Electronics communication system has revolutionized the face of the world. Communication with someone a mere century back was only possible by physical mode. But now that can be done just by clicking a switch on the telephone pad or by just a click of the mouse. Even live television report, live games telecast could not be possible without wireless communication. A simple communication system consists of a transmitter end which send the data and a receiver end at which the data is received. Usually there received data is not the same as the data sent. Because of the noise present in the medium the signal gets affected and distortion is observed in the signal. Various modulation techniques are under taken in order to ensure that the signal sent is safely available at the receiver end. Therefore, power amplifiers with a large dynamic range are required for OFDM systems. Reducing the PAPR is pivotal to reducing the cost of OFDM systems. II. OFDM THEORY Orthogonal 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. Fig 2.1 OFDM Spectrum 1.2 Motivation Fig 1.1: A block diagram representation of electronic communication system 2.1 Orthogonality Two periodic signals are orthogonal when the integral of their product over one period is equal to zero. For the case of continuous time: Multi-carrier modulation (MCM) has recently gained fair degree of prominence among modulation schemes due to its intrinsic robustness in frequency selective fading channels. This is one of the main reason to select MCM a candidate for systems such as Digital Audio and Video Broadcasting (DAB and DVB), Digital Subscriber Lines (DSL), and Wireless local area networks (WLAN), metropolitan area networks (MAN), personal area networks(pan), home networking, and even beyond 3G wide area networks (WAN). Orthogonal Frequency Division Multiplexing (OFDM), a multi-carrier transmission technique that is widely adopted in different communication applications. OFDM systems support high data rate transmission. However, OFDM systems have the undesirable feature of a large peak to average power ratio (PAPR) of the transmitted signals. The transmitted signal has a non-constant envelope and exhibits peaks whose power strongly exceeds the mean power. Consequently, to prevent distortion of the OFDM signal, the transmit amplifier must operate in its linear regions. For the case of discrete time: Where m n in both ca. 2.2 Sub Carriers Each sub carrier in an OFDM system is a sinusoid with a frequency that is an integer multiple of a fundamental frequency. Each sub carrier is like a Fourier series component of the composite signal, an OFDM symbol. Syed Roshan Ali IJMEIT Volume 2 Issue 6 June 214 Page 387
The sub carrier s waveform can be expressed as: Where, The sum of the sub carriers is then the baseband OFDM signal: Reducing the max x (t) is the principle goal of PARP reduction techniques. Since, discrete- time signals are dealt with in most systems, many PAPR techniques are implemented to deal with amplitudes of various samples of x(t). Due to symbol spaced output in the first equation we find some of the peaks missing which can be compensated by oversampling the equation by some factor to give the true PAPR value. For analysis of PAPR reduction techniques the most widely used performance measure is Complementary Cumulative Distribution function (CCDF), which gives the probability that the PAPR of a data block exceeds a given threshold z. The CCDF of the PAPR of a data block of N symbols with Nyquist rate sampling is derived as The CDF of the amplitude of a signal sample is given by III. THE PAPR OF OFDM SYSTEM Presence of large number of independently modulated subcarriers in an OFDM system the peak value of the system can be very high as compared to the average of the whole system. This ratio of the peak to average 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 analog converter. 2. Reduction in efficiency of RF amplifiers. 3.1 PAPR OF A MULTICARRIER SIGNAL Let the data block of length N be represented by a vector X = [X, X 1,X N-1 ]. Duration of any symbol X k in the set X is T and represents one of the sub carriers set {f n, n =,1, N-1}. As the N sub carriers chosen to transmit the signal are orthogonal to each other, so we can have f n = nδf, where nδf = 1/NT and NT is the duration of the OFDM data block X. The complex data block for the OFDM signal to be transmitted is given by The PAPR of the transmitted signal is defined as The CCDF of the PAPR of the data block is desired is our case to compare outputs of various reduction techniques. This is given by IV. PAPR REDUCTION TECHNIQUES PAPR reduction techniques vary according to the needs of the system and are dependent on various factors. PAPR reduction capacity, increase in power in transmit signal, loss in data rate, complexity of computation and increase in the bit-error rate at the receiver end are various factors which are taken into account before adopting a PAPR reduction technique of the system. a) Signal Scrambling Techniques Block Coding Techniques Block Coding Scheme with Error Correction Selected Mapping (SLM) Partial Transmit Sequence (PTS) Interleaving Technique Tone Reservation (TR) Tone Injection (TI) b) Signal Distortion Techniques Peak Windowing Envelope Scaling Peak Reduction Carrier Syed Roshan Ali IJMEIT Volume 2 Issue 6 June 214 Page 388
Clipping and Filtering The PAPR reduction techniques on which we would work upon and compare in our later stages are as follows: SELECTED MAPPING (SLM) In this a set of sufficiently different data blocks representing the information same as the original data blocks are selected. Selection of data blocks with low PAPR value makes it suitable for transmission. 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. Among the modified data blocks, the one with the lowest PAPR is selected for transmission. The amount of PAPR reduction for SLM depends on the number of phase sequences U and the design of the phase sequences. 4.2 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 threshold value of the amplitude is set in this case to limit the peak envelope of the input signal. Signal having values higher than this pre-determined value are clipped and the rest are allowed to pass through un-disturbed as shown in fig 4.2. 4.1 SELECTED MAPPING (SLM) The main objective of this technique is to generate a set of data blocks at the transmitter end which represent the original information and then to choose the most favorable block among them for transmission as shown in the fig 4.1. Let us consider an OFDM system with N orthogonal sub carriers. A data block is a vector x =(x n ) N composed of N complex symbols x n, each of them representing modulation symbol transmitted over a sub carrier. X is multiplied element by element with U vector B u = (b u, n ) N composed of N complex numbers b u,n, u ε{,1,,u-1} defined so that b u,n = 1, where. denotes the modulus operator. Each resulting vector Xu = (x u,n ) N, where x u,n = b u,n.x n, produces after IDFT, a corresponding OFDM signal S u (t) given by Where, T is the OFDM signal duration and Δf = 1/T is the sub carrier spacing. Fig. 4.2: Block diagram of clipping and filtering where, B(x) = the amplitude value after clipping. x = the initial signal value. A = the threshold set by the user for clipping the signal. The problem in this case is that due to amplitude clipping distortion is observed in the system which can be viewed as another source of noise. This distortion falls in both in band and out of band. Filtering cannot be implemented to reduce the in band distortion and an error performance degradation is observed here. On the other hand spectral efficiency is hampered by out of band radiation. Out of band radiation can be reduced by filtering after clipping but this may result in some peak re growth. A repeated filtering and clipping operation can be implemented to solve this problem. The desired amplitude level is only achieved after several iteration of this process. Fig4.1: Block diagram of SLM Technique 5. OVERALL ANALYSIS OF DIFFERENT TECHNIQUES The PAPR reduction technique should be chosen with awareness according to various system requirements. Syed Roshan Ali IJMEIT Volume 2 Issue 6 June 214 Page 389
Table -1: Comparison of PAPR Reduction Techniques PAPR Reduction Technique Selected Mapping (SLM) Clipping and Filtering Partial Transmit Sequence (PTS) Block Coding Interleaving Tone Reservation (TR) Tone Injection (TI) Distort ion t decreas e Parameters Raise in Powe r Yes Yes Data rate a t decrea a a a a t decrea Required Operation at Transmitter(TX) / Receiver(RX) TX : N times IDFTs Operation RX : Side Information Extraction, Inverse SLM TX : Clipping RX : ne TX : N times IDFTs Operation RX : Side Information Extraction, Inverse PTS TX : Coding RX : Decoding TX : N times IDFTs Operation,(N-1) times Interleaving RX : Side Information Extraction,deinter leaving TX : Tone reservation RX : ne TX injection RX : :Tone V. SIMULATION RESULTS In this section the PAPR Reduction and performances of rmal OFDM, Modified SLM technique and Clipping and Filtering technique are evaluated by Simulation. In our simulation the number of sub-carriers are N=64,with M=8 alphabet size and quadrature phase shift keying(qpsk) symbols are input to the OFDM system. Simulation Results shows that Clipping and Filtering Technique is more efficient than the other two techniques. The Fig 5.1 shows the simulation result for rmal OFDM signal and SLM Modified signal for N=64 transmitted symbols, M=8 Alphabet Size with QPSK symbols as input to OFDM system. 1.8.6.4.2.4.3.2.1 rmal OFDM signal 1 2 3 4 5 6 1 2 3 4 5 6 Fig 5.1: rmal OFDM Signal,SLM Modified OFDM Signal for N=64 no. of transmitted symbols, M=8 Alphabet Size and Length factor of L=1.1. The below Fig 5.2 shows the simulation result for rmal OFDM signal with zero padding, Clipped OFDM signal and Clipped and Filtered OFDM signal for N=64 number of transmitted sub carriers and alphabet size of M=8 with QPSK symbols as input to OFDM system. 1.5 SLM modified OFDM signal 1 2 3 4 5 6 7 1.5 rmal OFDM signal with zero padding 1 2 3 4 5 6 7 2 1 Clipped OFDM signal Clipped and Filtered OFDM signal 2 4 6 8 1 12 14 Fig 5.2: rmal OFDM signal with zero padding, Clipped OFDM Signal, Clipped and Filtered OFDM signal for N=64 Syed Roshan Ali IJMEIT Volume 2 Issue 6 June 214 Page 39
no. of transmitted symbols, M=8 Alphabet Size and length factor L=1.1. The Simulation results shows that Selected Mapping (SLM) technique is more efficient when we transmit up to N=16 number of symbols with alphabet size M=8.As further when we increase the number of transmitted symbols from N=16,the Clipping and Filtering technique is more efficient Simulation results for N=64 with M=8 PAPR of the rmal OFDM = 19.44 db. PAPR of the SLM Modified technique = 13.98 db. PAPR of the Clipped and Filtering technique = 1.7 db. Hence the Clipping and Filtering technique is more efficient with Efficiency = 44.93 %, where as SLM techniques efficiency = 18.1 % as compared to rmal OFDM technique. VI. CONCLUSION In this paper, techniques for reducing PAPR has been proposed for different number of transmitted symbols and their respective performance analyzed. The PAPR reduction performances were evaluated using MATLAB simulation tool. A modified selective mapping technique is proposed in this paper to improve the performance of the OFDM system with respective PAPR. Results of simulation of modified SLM technique show that the PAPR reduction of OFDM system is more than clipping and filtering up to 16 number of transmitted symbols but the Experimental results clearly proves that there is a significant reduction in PAPR using Clipping and Filtering technique than Modified SLM technique above 16 number of transmitted symbols. The proposed scheme reduced the PAPR by about 3.28 db and increa the efficiency by about 16.83 % for N=64 and M=8. It is also worth noting that the proposed Clipping and filtering technique can be implemented with low complexity compared with coding, partial transmit and selective mapping. In short, the proposed clipping and Filtering technique is feasible for incorporation with OFDM systems above 16 number of transmitted symbols and more suitable for real-time applications. ACKNOWLEDGEMENT The completion of any project report brings with it a sense of satisfaction, but it is never complete without thanking those people who made it possible and whose constant support has crowned our efforts with success. I would like to express my heartfelt thanks to my guide Dr. Veena Desai,Department of ECE, Gogte Institute of Technology, Belgaum, Karnataka, India for her kind support, valuable guidance and encouragement. REFERENCES [1].HimanshuBhushan Mishra,MadhusmitaMishra,Sarat Kumar Patra, Selected Mapping Based PAPR Reduction in Wimax without sending the side information, IEEE,211. [2] SeungHee Han, Jae Hong Lee, An overview of Peak-totransmission, IEEE Wireless commun.,apr. 25 [4] N.V. Irukulapati, V.K. Chakka and A. Jain, SLM based PAPR reduction of OFDM signal using new phase sequence, Electronics Letters 19th vember 29 vol. 45 no. 24.. [3] Stephane Y. Le Goff, Samer S. Al-Samahi, Boon KeinKhoo, Charalampos C. Tsimenidis, and Bayan S. Sharif, Selected Mapping without Side Information for PAPR Reduction in OFDM, IEEE Trans. Wireless Commun., vol. 8, no. 7, pp. 332-3325, Jul. 29. [4] Tao Jiang, " An Overview: Peak to-average Power Ratio Reduction Techniques for OFDM signals" IEEE Transactions on broadcasting,vol.54,.2,june 28. [5] A. Ghassemi and T. A.. Gulliver, Fractional Selective mapping using Decimation in time IFFT/FFT., Publication in WCNC 28 Proceedings IEEE, pp. 543-547.[6] Yang Jie, Chen Lei, Liu Quan and Chan De,.A Modified selected mapping technique to reduce the Peak to Average Power Ratio of OFDMsignal., IEEE transaction on consumer Electronics, Vol53,.3, pp. 846-851, August 27. [7] Ahmed, E., W. Aziz, S. Saleem and Q. Islam, 212. Performance Analysis of OFDM System fordifferent Channel Lengths and Multipath Channel Taps. Advances in Electrical Engineering Systems, 1 (2): 124-128. [8] Ochiai, H. and K. Imai, 1997. Block Coding Scheme Based on Complementary Sequences for Multicarrier Signals. IEICE Transactions Fundamentals, E8-A (11): 2136-2143. [9] Ochiai, H. and K. Imai, 21. On the Distribution of the Peak-to-Average Power Ratio in OFDM Signals. IEEE Transactions on Communications, 49 (2): 282-289. [1] Shang-Kang Deng and Mao-Chao Lin, 25. OFDM PAPR reduction using clipping with distortion control. IEEE International Conference of Communications, Taipei, Taiwan, 4: 2563-2567. Syed Roshan Ali IJMEIT Volume 2 Issue 6 June 214 Page 391
[11] Seung Hee Han, Student Member, IEEE, and Jae Hong Lee, Senior Member, IEEE. Modified Selected Mapping Technique for PAPR Reduction of Coded OFDM Signal.,IEEE Transactions on broadcasting, vol. 5, NO. 3, September 24. [12] R. E. Blahut, Algebraic Codes for Data Transmission. Cambridge, U.K.: Cambridge University Press, 23. [13] V. Tarokh and H. Jafarkhani, On the computation and reduction of the peak-to-average power ratio in multicarrier communications, IEEE Trans. Commun., vol. 48, pp. 37 44, Jan. 2. [14] X.Li and L. J. Cimini, Effects of clipping and filtering on the Performance of OFDM, IEEE Commun. Letters, Vol.2.5,pp.131-133, May 1998. [15] R. W. Bauml, R. F. H. Fischer, and J. B. Huber, Reducing the peak-to-average power ratio of multicarrier modulation by selected mapping, Electron. Lett.vol.32, no. 22, pp. 256-257, Oct. 1996. [16] E. Lawrey and C. J. Kikkert, Peak to average power ratio reduction of OFDM signals using peak reduction carriers, in Proc. ISSPA 99, Brisbane, Australia, Aug. 1999, pp. 737 74. [17] Y.Wu and W. Y. Zou, Orthogonal frequency division multiplexing: A multi-carrier modulation scheme, IEEE Trans. Consumer Electronics, vol. 41, no. 3, pp. 392 399, Aug. 1995. BIOGRAPHY Mr. Syed Roshan Ali was born in INDIA in 1991.He obtained his BE degree from Government Engineering College, Haveri, India in 212 under VTU, Belgaum. Currently he is doing M.Tech in Digital Communication And Networking, in Gogte Institute Of Technology, Belgaum, India. His research intrest are Digital Signal Processing, Wireless Communication system and Multimedia Communication. Mr. Syed Roshan Ali can be reached at syedroshanalis@gmail.com Syed Roshan Ali IJMEIT Volume 2 Issue 6 June 214 Page 392