# RCLED Optimization and Nonlinearity Compensation in a Polymer Optical Fiber DMT System
## 1 Introduction  
Owing to the advantages of bend insensitivity, easy installation and robustness to the electromagnetic interference, the step-index polymer optical fiber (SI-POF) with a large core diameter becomes a popular transmission medium in short range communications, and can be used in a variety of applications [1; 2; 3; 4; 5]. Additionally, in the POF networking, the discrete multi-tone (DMT) modulation [6; 7; 8] with a bit-loading algorithm has been an efficient solution to maximize the system capacity [9]. However, the DMT signal is extremely sensitive to the nonlinear behavior of the SI-POF systems [10], and the low-cost resonant cavity light emitting diode (RCLED) is the main device that causes the nonlinearity due to the following two reasons [11]:  
1. The nonlinear transition noise caused by the electro-optical (E/O) converter;
2. The clipping noise caused by the minimum turn-ON voltage and the maximum saturation voltage (SAV) or maximum permissible voltage.  
To relieve the nonlinear distortions and improve the performance of the POF transmission, the RCLED must operate in the linear range. However, for the consideration of high power efficiency,the RCLED operating in the nonlinear region with signal compensation is preferred. In the existing works, the nonlinear distortion is mainly studied in the communication system with white LED [12; 13], but few studies focus on the POF system with the nonlinear RCLED. Additionally, the RCLED has a larger bandwidth than the white LED, and the characteristics of the E/O converter are also different. Therefore, these existing methods for the white LED cannot be adopted directly in the POF system with RCLED. A look-up-table (LUT) pre-distorter is proposed to compensate the RCLED nonlinearity [14], but the parameters are fixed, thus it is inflexible for different DMT transmissions, which should be further developed for practical transmission. In addition, the bit redundancy is very large in the training procedure. Moreover, the power back off, the peak-to-average power ratio (PAPR) reduction and the iterative signal clipping technique can also be also for the nonlinearity mitigation [15; 16], butthe system complexity is increased and the bandwidth efficiency is sacrificed.  
In this paper, the RCLED optimization and the nonlinearity compensation on the DMT modulation over a SI-POF link are first investigated. The transfer function of RCLED is measured and the nonlinear characteristics are theoretically analyzed. Joint optimizations are implemented to obtain the favorable linear region of RCLED and the appropriate operating points. To resist the RCLED nonlinearity at a high operating point, an improved digital LUT pre-distorter, based on an adaptive iterative algorithm is proposed in the DMT transmitter to compensate the nonlinear distortion. Moreover, a practical SI-POF with 50 m transmission is also realized to show the performance improvement with the proposed nonlinearity compensation method.  
The remainder of this paper is organized as follows. Section 2 presents the measurements of RCLED and introduces a polynomial fitting to model the static power transfer function. The E/O nonlinear transition noise and the threshold clipping noise are theoretically analyzed in Section 3. In Section 4, an improved digital LUT pre-distorter with a least mean square (LMS)-based iterative algorithm is proposed. Section 5 gives the results and discussions. Section 6 concludes this paper.