# A Novel 64 QAM-OFDM Optical Access System Based on Bit Reconstruction
###### Abstract  
This paper proposes a novel orthogonal frequency division multiplexing (OFDM) optical accessmobile base stations, etc. [15; 16]. These characteristics of OFDM technology make it particularly suitable for application in access networks, with its service scheduling and flexibility. Recent internet access and traffic have greatly increased, and it is foreseeable that future communication system application scenarios will require higher data rates and a greater transmission capacity. For standard SMF optic transmission systems, multi-carrier modulation technology is an important means of system expansion [17]. At the same time, OFDM has a stronger dispersion robustness than the single-carrier modulation format, reasonably modulating the spectrum according to the characteristics of the subcarrier channel. It can achieve broadband optical access above 40 Gb/s with flexibility [18]. Because of its excellent digital signal processing characteristics and high spectrum utilization, OFDM optical access technology is widely used in access systems, so the advantages are even more obvious.  
Typically, OFDM technology is preferred for optical access systems; however, it brings some unavoidable problems. There is a high peak factor in the OFDM modulation which increases the complexity of digital-to-analogue converters (DACs) and analogue-to-digital converters (ADCs). [PERSON] et al. proposed a comprehensive non-linear compression-expansion technique to improve the discrete chord transform, effectively reducing the peak to average power ratio (PAPR) of the system [19]. Concomitantly, the OFDM system is very sensitive to phase noise and carrier frequency offset, causing inter-symbol interference (ISI). [PERSON] et al. innovatively proposed a robust decision feedback equalizer (DFE) for the OFDM system to overcome the serious ISI problem [20]. [PERSON] et al proposed a probabilistic shaping passive optical network technology based on symbol-level labelling and rhombus-shaped modulation, making deployment of the optical access networks flexible and robust [21]. This shows that some efforts have been put into the OFDM-PON to address the inherent effects of the system.  
The traditional one-dimensional coded modulation technique has low bandwidth utilization filtration, while the orthogonal modulation technique uses two-dimensional coded modulation, where both the amplitude and phase can carry information bits to achieve efficient transmission. However, at a given transmission power, the Euclidean distance between the constellation points of high-order QAM modulation is smaller than that of the constellation points of low-order QAM modulation, and therefore the noise tolerance of the signal is reduced [22]. Compared to 4/16/32 QAM, 64 QAM has higher spectrum utilisation. The complexity of this implementation is simpler compared to systems with higher-order QAM modulation formats. Therefore, we chose the 64 QAM-OFDM system as the approach to investigate.  
In this paper, a novel 64 QAM scheme suitable for an OFDM access network is proposed. Applying part of the response signal to the multi-amplitude modulation signal, the bit reconstruction part set in the OLT aims to generate a multi-level modulation signal and map the constellation according to the preset precept. The bit reconstruction portion realizes a strong correlation between the transmitter (TX) and the receiver (RX). According to the simulation results, compared with the general OFDM system, our project has a gain of about 1.3 dB and 0.51 dB at the threshold of SD-FEC (0.02) in a 10/20 km OFDM access system with 10 Gbaud transmission, respectively. Moreover, the system does not further deteriorate PAPR.