Keywords
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| High power amplifier (HPA), NCT, OFDM, PAPR, ETU (extended typical urban channel), Additive white Gaussian noise (AWGN). |
I .INTRODUCTION
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| Orthogonal frequency division multiplexing (OFDM) is used in many wireless broadband communication systems because it is a simple and scalable solution to inter symbol interference caused by a multipath channel. It is used in wireless local area networks (WLAN) and wireless metropolitan area networks (WMAN) including IEEE802.11a/g and worldwide interoperability for microwave access (Wi-MAX). As a promising technique, OFDM has been widely applied in modern wireless communications due to its high spectral efficiency and low susceptibility to the multipath propagation. Very recently the use of OFDM in optical systems has attracted increasing interest. However, a major drawback of OFDM-based transmission systems is its high instantaneous peak-to-average power ratio (PAPR), which leads to undesired in-band distortion and out-of-band radiation if the linear range of the high power amplifier (HPA) is not sufficient. In an OFDM system with subcarriers, the complex baseband representation of OFDM signal is given by [1]. |
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| Wherej = √−1 and the vector xk denotes the frequency-domain OFDM symbols and T is the symbol duration. To reduce the PAPR we proposed algorithm is Non-Linear Companding transform (NCT) technique is used .It uses strict monotonically increasing function to transform the given signal to companded signal and hence can be recovered [3]. It compresses the high level signals and expands the low level signals. Many PAPR reduction schemes causes spectrum side-lobes generation, but the nonlinear companding transforms (NCT) can offer better PAPR reduction, less spectrum side-lobes, less BER and phase error performance than the u-law and exponential companding techniques [2]. |
| The analytical expression regarding the achievable reduction in BER and PAPR, attenuation factor of a signal and the selection criteria of transform parameters are derived and executed through computer simulations. The proposed algorithm is compared with exponential companding technique [4] for investigating the proposed algorithm performance. |
II .LITERATURE REVIEW
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| This literature review attempts to provide brief overview of the Peak-to-Average power ratio reduction in OFDM by using nonlinear companding transform technique. It discusses the NCT technique, to reduce PAPR and NCT is compared with exponential companding technique. NCT uses threshold points like cut off and inflexion points to reduce Peak Power in OFDM. In order to investigate the system performance, the companded signal is passed through different noise channels like AWGN, ETU and Rician fading channels. In addition, by choosing proper transform parameters, the impact caused by companding distortion can be significantly reduced [4] |
III .PAPR IN OFDM MODEL
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| Generally, an OFDM signal is the sum of N independent data symbols modulated by phase shift keying (PSK) or QAM modulation. Let X (0), X (1) ,….., X (N-1) represents the data sequence to be transmitted in an OFDM symbol with N subcarriers [5]. The base band representation of OFDM is given by |
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| Where n=0, 1 ,…….., JN-1is time index and J is the oversampling ratio. According to the central limit theorem, when N is large, both the real and imaginary parts of x (t) become’s Gaussian process. Thus it is possible that the maximum amplitude of OFDM signal may well exceed its average amplitude. Practical hardware i.e. A/D and D/A converters, a power amplifier has finite dynamic range; therefore the peak amplitude of OFDM signal must be limited [6]. |
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| It is easy to see from (2) that PAPR reduction may be achieved by decreasing the numerator value and increasing the denominator or both. |
| Complementary Cumulative Distribution Function (CCDF) curves present vital role in the OFDM signal to be transmitted. It uses to measure the effectiveness of PAPR reduction with different transforms |
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IV. NCT ALGORITHM DESCRIPTION
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| A.NCT ALGORITHM MODELING |
| Step 1.To obtain the OFDM symbols |
| Step 2.Divide the entire symbols into two disjoint sub-blocks and append zeros of ’j’ oversampling Symbols between them |
| Step 3.To calculate CDF of these symbols |
| Step 4.To equate this CDF and inverse CDF for companding and de-companding of OFDM symbols. |
| Step 5 .To evaluates the performance of the proposed algorithm under different channel environment. |
V. RESULT AND DISCUSSION
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| PAPR OF THE COMPANDED SIGNAL |
| PAPR is pictured in fig.1. This algorithm offers a tolerable flexibility in the PAPR reduction by adopting the values of K2 and c .Accordingly, the ultimate PAPR of the companded signal can be in effect, imprisoned in the separation [4.8db, 7.8db]. |
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| TRANSFORM GAIN OF THE COMPANDED SIGNAL |
| Gain is depicted in fig 2.The achievable transform gain G in the PAPR is from 6 to 7.7 dB |
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| ATTENUTION FACTOR OF PROPOSED ALGORITHM |
| The theoretical results of attenuation is depicted in fig .3, from that we can see that attenuation gradually tends to 1 as k2 and c decrease. |
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| COMPLEMENTARY CUMULATIVE DISTRIBUTION FUNCTION |
| Fig.4, depicts the CCDF of the PAPR with different transforms. The new algorithm was roughly 0.3dB and 0.1Db inferior to exponential but surpasses the origin signal by 0.1 to 0.8 dB. |
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| BER PERFORMANCE UNDER DIFFERENT MULTIPATH FADING CHANNELS |
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| BER UNDER ETU CHANNEL |
| ETU channel is moreover used in mobile environment for communicating in high density areas like Japan, Tokyo and Europe countries. In ETU channel path gain is very much high and BER is less when compared to remaining noisy channels. |
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| SPECTRAL CHARACTERISTICS |
| The spectral regrowth comparision is shown in fig.8,we can find that the new algorithm with c=0.25,k2=-0.45 can produce about 2db to 4db lower out-of-band interference than other referred methods at normalized frequency 0.4. |
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VI .CONCLUSION
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| Due to its simplicity and effectiveness, NCT is an attractive solution to reduce the PAPR of OFDM signal .This algorithm can offer the transform gain in PAPR of 6 to 7.7db compared to the original signal .Both theoretical analysis and computer simulations show that the algorithm offers improved performance in terms of BER, in-band and out-ofband distortions while reducing PAPR effectively. |
VII .FUTURE SCOPE
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| In future, there are following possible topics to continue this paper work. |
| 1. This paper can be extended for different types of modulation technique (64QAM, BPSK), it can be used higher modulation order and high number of sub carrier to be utilized in other wireless standards. |
| 2. BER expressions for other fading channels like Nagakami-m channels can be evaluated and confirmed. |
References
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- T. Hwang, C. Yang, G. Wu, S. Li, and G. Y. Lee, “OFDM and its wireless application: A survey,” IEEE Trans. Veh. Technol., vol. 58, no.4, pp. 1673–1694, May 2009.
- T. Jiang and Y. Wu, “An overview: Peak-to-average power ratio reduction techniques for OFDM signals,” IEEE Trans. Broadcast., vol. 54,no. 2, pp. 257–268, Jun. 2008.
- X. B. Wang, T. T. Tjhung , and C. S. Ng, “Reduction of peak-to-average power ratio of OFDM system using a companding technique,”IEEE Trans. Broadcast., vol. 45, no. 3, pp. 303–307, Sep. 1999.
- T. Jiang, Y. Yang, and Y. Song, “Exponential companding transform for PAPR reduction in OFDM systems,” IEEE Trans. Broadcast.,vol. 51, no. 2, pp. 244–248, June 2005.
- Y. Jiang, “New companding transform for PAPR reduction in OFDM,” IEEE Commun. Lett., vol. 14, no. 4, pp. 282–284, Apr. 2010.
- D. Dardari, V. Tralli, and A. Vaccari, “A theoretical characterization ofnonlinear distortion effects in OFDMsystems,” IEEE Trans.Commun.,vol. 48, no. 10, pp. 1755–1764, Oct. 2000.
- P. Banelli, “Theoretical analysis and performance of OFDM signals in nonlinear fading channels,” IEEE Trans.Wireless Commun., vol. 2,no.2, pp. 284–293, Mar. 2003.
- Digital Video Broadcasting: Framing Structure, Channel Coding, and Modulation for Digital Terrestrial Television, ETSI EN 300 744v1.4.1,2001.
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