# Investigation of IEEE 802.16e LDPC Code Application in PM-DQPSK System
## 1 Introduction  
Since the inception of the first optical fiber, optical fiber communication has been dominating the communication market due to its numerous advantages. In recent years, with the rapid development of communication and information technology, optical fiber communication has emerged as the primary mode of data transmission. Optical fiber communication boasts not only large transmission capacity, high transmission speed, and extensive relay distance but also excellent security and immunity to external electromagnetic interference. Its lightweight and cost-effectiveness have cemented its status as a hallmark of the information age, playing an indispensable role in facilitating social information exchange [1]. In the current stage of rapid development, optical fiber communication has the advantages of long-distance transmission, dense wavelength division multiplexing, fiber tothe home, etc., and has become the key foundation to meet the needs of the Internet, 5G, Internet of things, and other applications for stable and efficient data transmission. In the future, with the continuous development of quantum communication technology, it will continue to play an important role in providing reliable support for the progress of digital society. However, with the development of optical fiber communication, the demand for transmission capacity and transmission distance is increasing due to the influence of noise, nonlinear effects of fiber, chromaticity dispersion, and polarization mode dispersion; the development and application of high-rate long-distance optical communication are limited [2; 3]. The high rate and large capacity optical fiber transmission system is mainly realized by high order modulation code pattern and intensive multiplexing technology. They have high requirements on optical signal-to-noise ratio (OSNR), especially high-order modulation techniques, which increase spectrum utilization while reducing the Euclidean distance between symbols, making phase states difficult to distinguish [4]. As a result, OSNR is the main factor limiting the transmission rate of the system. In recent years, the rapid growth of the optical fiber communication rate has been attributed to the strong support of advanced optical technology and forward error correction technology. Forward error correction (FEC) technology has long been used in electrical communication systems. It is the main technology to improve the reliability of digital communication systems and reduce the bit error rate. It has been widely used in deep-space communication, satellite communication, mobile communication, and computer networks. The FEC can greatly reduce the OSNR requirements of the system and reduce the bit error rate, so FEC is an indispensable part of the optical fiber communication system. The primary purpose of channel coding is to enhance the reliability of information transmission and fortify resistance against interference. Following source coding, a sequence composed of '0' and '1' undergoes encoding based on specific coding rules. By adding redundant code elements, the original random distributed code word sequence has a certain regularity. At the receiving end, the code is decoded based on the encoding correlation, errors are checked, and the original, error-free code word sequence is restored. In an optical transmission system, the forward error correction technique can eliminate the bit error rate (BER) plateau phenomenon in the system performance curve, and its coding gain also provides a certain amount of system prosperity so as to reduce the impact of linear and nonlinear factors in optical links on the system performance. Especially for the optical amplification system, the optical amplifier interval can be increased, the transmission distance can be extended, the channel rate can be increased, and the channel optical power can be reduced. There are many kinds of forward error correction codes.