PAPR reduction techniques in OFDM.pptx

Communication

1. “ Comparision of papr reduction techniques in ofdm system

2. CONTENTS : Introduction OFDM Overview Orthogonality Principle OFDM Transceiver Introduction to PAPR PAPR reduction techniques SLM technique for PAPR reduction PTS technique for PAPR reduction Hybrid techniques for PAPR reduction Results and conclusion References

3. Introduction : Single Carrier System Transmitted using only one carrier at a time Frequency Division Multiplexing involves assigning non – overlapping frequency ranges or channels to different signals or to each user of a medium.

4. Multicarrier Communication the signal is modulated separately and transmitted Fig : Multicarrier Communication

5. Orthogonality principle   related to dot product of signals Mathematically represented as Time T Base frequency = 1/T , T= symbol period

6. OFDM Subcarrier Spectra Carriers allows their overlapping while disabling the occurrence of crosstalk. Fig. 1 Fig. 2

7. Orthogonality test Fig.: Different Sinusoidal signals Fig 1.5: Orthogonality test result. Fig 1.5: Orthogonality test result. Fig.: Orthogonality test result.

8. OVERVIEW ABOUT OFDM OFDM was invented more than 40 years ago . OFDM has been adopted for several technologies : 1. Asymmetric Digital Subscriber Line (ADSL) services. 2. IEEE 802.11a/g, IEEE 802.16a. 3. Digital Audio Broadcast (DAB). 4. Digital Terrestrial Television Broadcast: DVD in Europe, ISDB in Japan 5. 4G , IEEE 802.16, and IEEE 802.20.

9. OFDM can be viewed modulation cum multiplex technique. Modulation technique Viewed by the relation between input and output signals Multiplex technique Viewed by the output signal which is the linear sum of the modulated signals How Signals are transmitted without interference???

10. Spectrum of normal time domain sequence b) Spectrum of classical frequency division multiplexing (FDM) c) Spectrum of OFDM Fig :OFDM can be thought as

11. How OFDM signal is generated ? Fig :- OFDM Transceiver

12. Mathematical Equation The baseband OFDM signals can be written as W here is the central frequency of the mth sub channel and is the corresponding transmitted symbol .

13. Sub – Carriers subcarriers are sinusoids the sum of the sub – carriers is then the baseband OFDM Fig : Subcarriers

14. Fig:-Time domain OFDM signal

15. Cyclic prefix helps in proper recovery of signal Disadvantage - some loss in efficiency results as cyclic prefix has no new information Cyclic Prefix Fig : Cyclic Prefix

16. Advantages and Drawbacks of OFDM 1 . Advantages : OFDM converts frequency selective fading channels to non-selective fading subchannels (flat fading). Bandwidth efficient flexible to the channel conditions. Robust to ISI and ICI  

17. Ch.1 Ch.2 Ch.3 Ch.4 Ch.5 Ch.6 Ch.7 Ch.8 Ch.9 Ch.10 Saving of bandwidth Ch.3 Ch.5 Ch.7 Ch.9 Ch.2 Ch.4 Ch.6 Ch.8 Ch.10 Ch.1 50% bandwidth saving frequency frequency Fig: Bandwidth saving using OFDM

18. 2. Drawbacks : sensitive to the carrier frequency offset and Doppler shift.   more complicated than single-carrier modulation . High peak to average power ratio problem

19. Peak-to-Average ratio : . PAPR causes 1.Intermodulation Distortion 2.Out-of-band radiation = 10   Fig :- PAPR

20. Why papr is large? OFDM uses N-point IFFT. IFFT uses the principle of linear combination. The central limit theorem states that a linear combination of a large number of independent random signals is approximated by Gaussian. (* N is usually large in OFDM.) This implies that some samples have a high probability of high peaks.

21. 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. High PAPR=16 Low PAPR=2.25 PAPR example

22. THE CCDF OF THE PAPR F ( z ) = 1 – exp( z ) PAPR is random Variable Level crossing rate theorem is used for measurement PAPR is calculated by using CCDF as CCDF(PAPR o ) = Pr (PAPR > PAPR o ) Complementary Commulative Distributive Function  

23. CRITERIA FOR SELECTION OF PAPR REDUCTION TECHNIQUE PAPR reduction capability Power increase in transmit signal BER increase at the receiver Loss in data rate Computational complexity

24. Signal Scrambling Techniques • Selected Mapping (SLM) • Partial Transmit Sequence (PTS ) • Tone Reservation (TR) • Tone Injection (TI ) b) Signal Distortion Techniques • Peak Windowing • Peak Reduction Carrier • Clipping and Filtering PAPR Reduction Techniques

25. Simple approach requiring multiple sequential FFTs . Live with in-band clipping noise and effect on performance.   Clipping and Filtering method Fig :-Clipping and filtering

26. Tone Reservation • This method describes an additive method for reducing PAPR • some subcarriers are reserved • This methods may reduce the system bit rate if many tones are reserved for PAPR reduction purposes Drawbacks: Increase in the average energy per bit which might reduce the BER performance improvement . Loss of spectral efficiency due to tone reservation

27. Selective Level Mapping (SLM) Basic scrambling technique. Symbols are divided and multiplied with phase sequences. It requires transmitting bits of side information per OFDM symbol Fig :- Block diagram of SLM technique

28. Flowchart for Implementation of SLM/PTS

29. Fig : CCDF of PAPR with Different set of phase vectors( Elapsed time is 0.865826 seconds.) Results :

30. Fig : CCDF of PAPR with Different set of phase vectors( Elapsed time is 0.576960 seconds .) Fig : CCDF of PAPR with Different set of phase vectors( Elapsed time is 1.268421 seconds.)

31. Fig:- CCDF of PAPR of SLM technique using QAM modulation

32. Input OFDM signal is divided into sub-blocks. Phase optimization method is used. 6) Partial Transmit Sequence (PTS ) Fig:- PTS block diagram

33. Fig :- CCDF of PAPR for PTS technique

34. Conclusions : By using SLM Method PAPR is reduced but computational complexity and side information is increased. PTS method gives better performance as compared to SLM, but has more computational complexity.

35. References [ 1] R. van Nee and R. Prasad, OFDM for Wireless Multimedia Communications, Boston: Artech House, 2000.   [2] Hong-Jie Chou, Ping-You Lin and Jung-Shan Lin, “PAPR Reduction Techniques with Hybrid SLM-PTS Schemes for OFDM Systems.”IEEE Transactions, 2012.   [3] Hermann Rohling, “OFDM Concepts for Future Communication Systems”. Springer,ISSN: 1860-4862, ISBN: 978-3-642-17495-7, e-ISBN: 978-3-642-17496-4, DOI: 10.1007/978-3-642-17496-4   [4] John G. Proakis, “Digital Communications”, 4 th edition. McGraw Hill.   [5] E. Alsusa and L. Yang, “A Low-Complexity Time-Domain Linear Symbol Combining Technique for PAPR Reduction in OFDM Systems,” IEEE Transactions on Signal Processing, Vol. 56, No. 10, Oct. 2008.  

36. References [ 6] S. H. M¨uller and J. B. Huber, “A Novel Peak Power Reduction Scheme for OFDM,” Proceedings of International Symposium on Personal, Indoor and Mobile Radio Communications (PIMRC’97), Vol. 3, Helsinki, Finland, pp. 1090-1094, Sep. 1997.   [7] S. H. M¨uller and J. B. Huber, “OFDM with Reduced Peak-to-Average Power Ratio by Optimum Combination of Partial Transmit Sequences,” IEE Electronics Letters, Vol. 33, No. 5, pp. 368-369, Feb. 1997. [ 8] P. A. Pushkarev, K.-W. Ryu, K.-Y. Yoo and Y.-W. Park, “A Stud yon the PAR Reduction by Hybrid Algorithm Based on the PTS and SLM Techniques,” Proceedings of the 57th IEEE Vehicular Technology Conference, Vol. 2, pp. 1263-1267, 2003.   [9] R. B¨auml, R. F. H. Fischer and J. B. Huber, “Reducing the Peak-to-Average Power Ratio of Multicarrier Modulation by Selected Mapping ,”IEE Electronics Letters, Vol. 32, No. 22, pp. 2056-2057, Oct. 1996.   [10] Zakee Ahmed, Shete A.K,”A Comparative Study of PAPR Reduction Techniques in OFDM ”, International Journal of Engineering and Innovative Technology (IJEIT) Volume 2, Issue 1, July 2012

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