VOL. 13, NO. 24, DECEMBER 2018 ISSN 1819-6608 ARPN Journal of Engineering and Applied Sciences ©2006-2018 Asian Research Publishing Network (ARPN). All rights reserved. www.arpnjournals.com 9728 NONLINEARITY MITIGATION IN IM/DD OPTICAL OFDM USING NEW SLM SCHEME Chhavi Sharma 1 , Arvind Kumar 2 and S. K Tomar 1 1 Department of Electronics and Communication Engineering, The Institute of Engineering and Technology, Mahatma Jyotiba Phule Rohilkhand University, Bareilly, India 2 Department of Electronics and Communication Engineering, National Institute of Technology, Kurukshetra, India E-Mail: yash_naina@yahoo.co.in ABSTRACT Optical orthogonal frequency-division multiplexing (O-OFDM) systems have received a great deal of attention as a transmission technology for high-capacity long-haul optical transport networks. However, it is also very sensitive to nonlinear effects due to high peak to average power ratio (PAPR) problem as conventional OFDM system. This paper proposes Fast Hartley based selective mapping with Riemann Sequence for PAPR reduction in IM/DD Optical OFDM system. Results show that the proposed method performs better in terms of PAPR and bit error rate performance and it is less complex than other conventional methods. Keywords: O-OFDM, PAPR, SLM. INTRODUCTION Orthogonal Frequency Division Multiplexing (OFDM) has recently been introduced in optical communications because of its robustness against channel dispersion and its high spectral efficiency [1]. It is a promising technique for long haul and high speed optical transmission systems. It is also becoming popular because it can effectively reduce inter symbol interference caused by dispersive channel also it is immune to chromatic dispersion (CD) and polarization dispersion (PMD) [2]. The use of OFDM in optical communications mitigates transmission impairments and at the same time, provides high-data rate transmission across dispersive optical media. It is also used in wireless and wireline applications and in almost every major communication standards. Among the proposed OFDM systems, Intensity Modulation and Direct-Detection (IM/DD) technique inherently is a low cost and attractive for future cost- sensitive, short-range & high-speed transmission such as passive optical network (PON), indoor optical wireless communication and interconnections in data centre [3]. The two most popular IM/DD OFDM systems are DC- offset OFDM (DCO-OFDM) and asymmetrically clipped optical OFDM (ACO-OFDM) [4]. Fast Hartley transform (FHT) has been proposed as an alternative modulation/demodulation processing in IM/DD optical OFDM systems [16]. The complexity of FHT with real input constellation is half of that of FFT with complex input constellation [5].Thus; FHT is perfect for IM/DD systems due to its low complexity. However high peak to average power ratio (PAPR) and sensitivity to carrier frequency offset are two major drawback of optical OFDM (OOFDM) like conventional OFDM. Various techniques have been proposed to reduce the PAPR comprising clipping and filtering [7], coding schemes [8], companding transform [10], Tone Reservation and Tone Injection [6], Active constellation extension (ACE) [11], multiple signal representation techniques such as SLM and PTS [13-14] and interleaving [12] etc. All these techniques are selected according to various criteria such as bandwidth expansion, loss in data rate, system complexity, BER increase, power increase. Among all these basic techniques for PAPR reduction, SLM is an attractive technique as it can obtain better PAPR by modifying the OFDM signal without distortion. Generation and selection of phase sequence is one of the important aspects of the SLM technique for good PAPR reduction. In [15 ] rows of Riemann matrix are used as phase factors for SLM and it is shown that this scheme performs better than other previously proposed phase sequences such as Hadamard and Chaotic sequences but it has only discussed the PAPR performance without applying into optical OFDM transmission. This paper proposes a low complexity SLM technique for PAPR reduction in optical OFDM system. In this research work, FHT based SLM technique with Riemann sequence as phase factor sequence is proposed for PAPR reduction in OOFDM- IM/DD system. Here for comparison purpose, the signal is mapped into M- QAM format if FFT is used, whereas with the FHT scheme aM-PAM or BPSK format is considered as in [4].We adapt the FHT-based SLM for optical OFDM (OOFDM) using Riemann sequence as phase factor sequence. To the best of our knowledge, no demonstration of FHT-based SLM for IM/DD optical OFDM systems based on Riemann Sequence exists in the literature. The paper is organized as follows. Initially the transmitter and receiver models of conventional SLM, FHT based SLM, generation of Riemann matrix and its use as phase sequence is discussed then experimental set up for FHT based SLM-OOFDM with Riemann matrix as phase sequence and simulation results are presented and finally conclusion of the proposed work given. OFDM WITH FHT BASED SLM Conventional SLM and FHT based SLM SLM is a PAPR reduction technique suitable for a wide range of applications. In this method, the data symbols are copied into U sections, each multiplied by U