1 PAPR Reduction through Lossy Coding in BPSK OFDM M.Vasantha Lakshmi, B.Kanmani M.Vasantha Lakshmi, assistant Professor, BMS College of Engineering, Bangalore -560019 phone: 9741557906; e-mail: ([email protected]). B.Kanmani, Professor and Dean Academic, BMS College of Engineering, Bangalore-560019. (e-mail: [email protected]). ABSTRACT OFDM is a multicarrier modulation technique efficiently used for high speed data communication. OFDM provides high spectral efficiency and high tolerance to multipath fading hence it is considered as one of the popular technique for digital communication. In OFDM output is the superposition of multiple subcarriers hence, some instantaneous power output may become very high greater than the mean power of the system resulting in the high Peak to Average Power of the system. But the major disadvantage of OFDM system is the high Peak to Average Power Ratio (PAPR) which results in the degradation of the signal quality. Such high PAPR cannot be used for any practical applications. To reduce the PAPR in the OFDM signal, a Lossy Coding method was proposed and implemented in Lab VIEW. By generating the OFDM symbols by considering different carrier frequencies from 10 to 60 KHz for symbol length M=6, PAPR has been calculated for all possible combinations of the inputs. It was observed that out of the 2M different combinations of the inputs only four combinations has very high PAPR compared to the other possible combinations of the inputs with the introduced Bit Error Rate (BER). Though we introduce the BER, typical OFDM is of larger symbol length and by increasing the value of M the Bit error rate will reduce. The experiment has been repeated for symbol length ranging from 6 to 16 and for different sets of carrier frequencies. It was observed that for all the symbol length from 6 to 16, only four combinations have very high PAPR. By toggling the LSB of the four combinations by using Combinational Logic, the PAPR can be reduced to 40%.. Keywords: Bit Error rate, OFDM, Peak to Average Power, Symbol Length 1. INTRODUCTION In a 4G Wireless communication systems one of the popular multicarrier techniques used is the Orthogonal Frequency Division Multiplexing (OFDM). In OFDM bandwidth is divided into data streams and then modulated using different carrier frequencies. This increases the spectral efficiency and hence finds the wide application in the field of wireless communication. OFDM is the efficient method for high speed data transmission because of the spectral efficiency. With the growth of telecommunication and the increasing technology, there is a greater demand for the higher bandwidth. OFDM efficiently utilizes the available bandwidth of the spectrum but still there is a greater scarcity of the bandwidth allocation with the increasing demand. The limitation in the modulation schemes of the existing communication systems does not support the higher data rate. OFDM output is the superposition of the multiple sub carriers. While increasing the bandwidth some power may become very high instantaneously greater than the mean power of the system which results in the high peak power compared with the average power of the system resulting in high PAPR. This is the major disadvantage of the OFDM systems. High PAPR will result in the degradation of the quality of the signal and hence reduces the efficiency of the system to a greater extent. Hence it becomes a major goal to reduce this high peak power in any of the communication systems. Several methods like Signal Scrambling Techniques and signal Distortion Techniques has been proposed to reduce the PAPR in OFDM systems and has been implemented and a significant reduction in PAPR has been achieved. In the Section –II, we propose a new method to reduce the PAPR in BPSK OFDM through Lossy Coding. This has been implemented in LabVIEW and the result of the implementation is presented in Section-III. Finally we conclude in Section-IV. 2. PROPOSED METHOD To reduce the PAPR in OFDM system, we propose a new Lossy Coding method with the introduced Bit Error Rate. The methodology used is a follows: Let M be the length of the OFDM symbol to be transmitted where M = 1, 2,…..16. The possible combinations are 2M. OFDM signals have been generated for all the possible combinations of M bits by considering different carrier frequencies for each bit. The peak power has been calculated for all the possible combinations and it has been observed that 2 out of the 2M combinations, only 4 combinations have very high peak value compared with the other possible combinations of M bits. This high peak value results in the increase in the PAPR. The proposed method is to avoid these four combinations with the high PAPR and represent these four combinations in terms of other possible combinations with the reduced PAPR. This can be done by toggling the LSB of the bit pattern based on the Combinational Logic as shown in Figure 1. The proposed method reduces the PAPR with the introduced BER. Though we introduce the BER, Typical OFDM is of larger symbol length and by increasing the value of M, the BER will be reduced. For M=6, the possible combinations are 000000 to 111111. For all the 2M combinations, OFDM symbols are generated with the carrier frequencies as 10, 20, 30, 40, 50 and 60Hz and the peak values are calculated. The peak values lie between 9.5 and 36. For the four combinations 000000, 010101, 101010 and 111111, the peak values are 36 and the remaining combinations has a peak value ranging from 16.3 to 9.5. Based on the observations, if we toggle the LSB of these four bit patterns say instead of 000000 if we transmit 000001 the BER will be reduced from 36 to 16 with the 50% reduction in the PAPR with the introduced BER of 0.01 which is not acceptable. By increasing the value of M, BER will be reduced. The experiment has been performed using LabVIEW 2014 as hardware implementation to simulate the Lossy encoding method to compute PAPR. Any circuit simulation tool like Multisim/ Pspice or others can be used to perform the same. The results can be reproduced with the suitable hardware. Figure 2 shows the OFDM Signal generation for the bit pattern 000000 for M=6 along with the peak value of 36 generated and Figure 3 shows the OFDM signal generated for the toggled bit pattern of 000000 to 000001 with the reduced peak value of 16. Figure 4 shows the OFDM signal generated for the Bit pattern 111111 for M = 6 with the peak value of 36 and Figure 5 shows the OFDM signal generated for the toggled bit pattern 111111 to 111110 with the reduced value of 16. Figure1 Combinational Logic of toggling the LSB 3 a) Figure 2 OFDM signal for 000000 bit pattern for M=6 Figure 3 OFDM signal with the toggled bit pattern 000001for M=6 Figure 4 OFDM signal for 111111 bit pattern for M =6 Figure 5 OFDM signal with the toggled bit pattern 111110 for M =6 4 Harish Kumar Pal and Anand Kumar Singh “PAPR reduction technique using Advanced Peak windowing mehod of OFDM system “International Journal of Soft computing and engineering, Volume 3, Issue -2, May 2013. ISSN: 2231-2307. 9. Gu. Chen, R. Ansari and Y. Yao, “Improved Peak Windowing for PAPR Reduction in OFDM”, Proceedings of IEEE 69th Vehicular Technology Conference, (2009) April 26-29. 10. Foomooliareon and W.A.C. Fernando, “PAPR reduction in OFDM systems”, Thammast Int,. J..Sc..Tech, volume 7, Number 3, Sept-December. 2002 11. E.Lawry and C.J.Kikkart, “Peak to average power reduction OFDM signals using Peak reduction Carriers” Proceedings of Fifth international Symposium on Signal Processing and its applications, ISSPA, Brisbane, Australia, pp. 737-740, August 1999. 12. C.E.Tan and I.J.Wassel, “Data Bearing Peak reduction carriers for OFDM systems”, ICICS-PCM 2003, Laboratory for Communication Engineering, Singapore, 15-18 December. 8. 3. CONCLUSION The proposed Lossy encoding method has been implemented in LABVIEW for the value of M ranging from 6 to 16. It has been observed that only four combinations out of 2M have very high peak value compared with the other combinations. If we change the bit patterns of the four combinations by toggling the LSB of the bit patterns, significant reduction in the peak value has been achieved with the introduced BER. By increasing the value of M the BER will be reduced. ACKNOWLEDGMENT The research work presented in this paper has been supported by the College through TEQIP-II of MHRD, Government of India and the authors acknowledge the technical support by the National Instruments. REFERENCES 1. Dae Woo Lim, seok joong Heo and Jong Seon No “ An Overview of Peak to average power reduction Schemes for OFDM signals “, Journal of Communications and Networks, Volume 11, Number 3, June 2009. 2. Md. Ibrahim Abdullah, Md. Zulfiker Mahmood, Md. Shamin Hussain, Md. Nural Islam, “Comparitive study of PAPR Reduction techniques in OFDM” APRN Journal of Systems and Software, Volume 1, No. 8, November 2011, ISSN: 2222-9833. 3. Suverna Sengar, Partha Prathim Bhattacharya” Performance Improvemnet in OFDM system by PAPR reduction ”, Signal and Image Processing: An international Journal, Volume 3, Number 2 , April 2012. 4. 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BIOGRAPHY M.Vasantha Lakshmi, obtained her Bachelors in Electronics and Communication Engineering form Visveswaraiah Technological University in 2003, M.Tech Degree in Digital communication and Networking from Visveswaraiah Technological University in 2008 and pursuing PhD in the domain of Wireless Communication. She has been with BMS College of Engineering, Bangalore, since 2005, and has to her credit over 8 International publications. As Assistant Professor in the department, she teaches under-graduate courses related to Communication Domain. Her prior employment as a teaching faculty was with Kuppam College of Engineering (AP). She is Member IEEE, and ARSI. 5 B Kanmani, obtained her Bachelors in Electronics and Communication Engineering form Nagarujuna University in 1987, M.Tech. degree in Digital communication from Indian Institute of Technology, Kanpur in 1990, and PhD from the Indian Institute of Science Bangalore (IISc) in the year 2006. She has been with BMS College of Engineering, Bangalore, since 1995, and has to her credit over 45 International publications. She has delivered lectures on ‘Signals & Systems’, during the VTU-EDUSAT Program -10, and contributed modules on CNX.ORG. Having held the position of Head of the department of Telecommunication Engineering, for more than a decade, she has steered the department to effective implementation of Outcomes Based Education. As Professor in the department, she teaches under-graduate courses related to Signal Processing and Communication. Presently, she holds the position of Dean Academics. Her prior employment as a teaching faculty was with Thadomal Shahani College of Engineering (Mumbai) and K L College of Engineering (Guntur). She is Senior Member IEEE, and Life Member ISTE.