Improved nonlinear tolerance in ultra-dense WDM OFDM systems Abdelgader Abdalla a,n , Ricardo Ferreira a , Ali Shahpari a , Jacklyn D. Reis a,b , Mario Lima a , Antonio Teixeira a a Department of Electronics, Telecommunications and Informatics, University of Aveiro, Instituto de Telecomunicações, 3810-193 Aveiro, Portugal b CPqD, 13086-902, Campinas - SP, Brazil article info Article history: Received 20 January 2014 Received in revised form 4 April 2014 Accepted 7 April 2014 Available online 21 April 2014 Keywords: Optical communication UDWDM OFDM Precoding Nonlinear tolerance abstract This paper demonstrates and experimentally validates the Zadoff–Chu matrix transform (ZCT) precoding as a peak-to-average power ratio (PAPR) reduction technique for coherent optical orthogonal frequency division multiplexing (OFDM) over ultra-dense wavelength division multiplexing (UDWDM). The work provides performance comparison between conventional and the precoded UDWDM–OFDM system. The system use case was implemented by 16-QAM–OFDM modulated channels among 3.5 GHz, 7 GHz, and 10 GHz channel spacings scenarios. The ZCT precoding approach improves the nonlinear tolerance in UDWDM–PON systems. The performance in terms of error vector magnitude (EVM) and bit error rate (BER) is presented after transmission over 40 km of standard single mode fiber (SSMF). The results show a significant EVM improvement considering the aforementioned channel spacings and for different number of UDWDM channel groups in the nonlinear regime operation. The implemented ZCT precoding system outperforms the non precoded approaches in terms of EVM and BER results. & 2014 Elsevier B.V. All rights reserved. 1. Introduction To fulfill the requirements of future passive optical network (PON), several multiple access technologies have been proposed such as optical code division multiple access (OCDMA)– PON, time division multiple access (TDMA)–PON, ultra-dense wavelength division multiplexed (UDWDM)–PON [1], and orthogonal fre- quency division multiple access (OFDMA) – PON [2] as well as different hybrid options, which can be formed from one or more of the aforementioned technologies. Moreover, the WDM systems, either based on OFDM [3] or quadrature amplitude modulation (M-QAM) are usually affected by inter-channel cross phase modulation and four-wave mixing [4] that play a significant role in the signal quality degradation. For instance, when OFDM signals are employed, the latter is strongly correlated with the total peak-to-average power ratio (PAPR) for single, and multi-wavelength based network scenarios [5]. In this paper the precoding technique for PAPR reduction of OFDM signals through Zadoff–Chu (ZC) mathematical formulation that has been introduced in [6] will be further investigated and exploited for enhancing OFDM nonlinear tolerance in single and multi-wavelength optical communication networks. In fact a ZC sequence is a complex valued mathematical expression that exhibits remarkable properties such as constant amplitude and zero autocorrelation waveform. Both simulation and experimental results for coherent PONs show significant reduction in the PAPR between conventional and simple pre-coding approach with 16QAM constellations, i.e. the signal bandwidth is not changed. This PAPR reduction turned out to improve both the signal to noise ratio (SNR) and the fiber nonlinear tolerance. This enhancement of the optimum power can be used to increase the feeder power thus allowing extended reach in future PON applications. The paper starts with a system description of the ZCT precoding reduction techniques followed by the description of the experi- mental setup. Then, the achieved results are presented and discussed. Finally, conclusions are drawn at the end of the paper. 2. ZC sequence and ZC matrix transform (ZCT) The ZC sequence is used in recent technology such as long term evolution [7] and universal mobile telecommunications system. The ZC sequence presents an improvement in the Walsh–Hada- mard codes by generating a constant-amplitude output signal, resulting in reduction of cost and complexity of the radio's power amplifier [8]. Accordingly, constant amplitude zero auto-correlation ZC sequences become more attractive and are employed in many telecommunication areas due to their ideal periodic autocorrelation properties [9]. In addition, these properties also guarantee optimal Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/optcom Optics Communications http://dx.doi.org/10.1016/j.optcom.2014.04.014 0030-4018/& 2014 Elsevier B.V. All rights reserved. n Corresponding author. Tel.: þ351 234 377 900; fax: þ351 234 377 901. E-mail address: a.m.abdalla@ua.pt (A. Abdalla). Optics Communications 326 (2014) 88–93