# Investigation of IEEE 802.16e LDPC Code Application in PM-DQPSK System
## 3 PM-DQPSK System and Simulation Results
### PM-DQPSK Coherent Optical Transmission System  
The theoretical model of the PM-DQPSK coherent optical transmission system is shown in Figure 1. The system comprises three main parts: the transmitter, the fiber link, and the receiver.  
The optical transmitter is responsible for the conversion of electrical signals to optical signals and includes various components such as lasers, modulators, and interfaces. Fiber optic transmission links are part of an optical fiber communication system used to  
Figure 1: PM-DQPSK theoretical modeling of coherent optical transmission system.  
establish point-to-point data connections [19]. Among them, noise accumulation, chromatic dispersion (CD), polarization mode dispersion (PMD), and nonlinear effects are common signal impairments during optical fiber transmission, which will cause interference to the transmitted optical signals and affect the performance of the optical fiber transmission systems. Therefore, mitigating these effects is crucial to ensure reliable and high-quality data transmission.  
The fiber link represents the optical fiber channel through which the encoded signals propagate. This includes the physical fiber medium as well as any optical amplifiers or dispersion compensation devices along the transmission path [20]. The fiber link introduces various impairments, such as attenuation, dispersion, and nonlinear effects, which can degrade the quality of the transmitted signals.  
The noise in a high-capacity long-distance transmission system mainly originates from the semiconductor optical amplifier (SOA), and the magnitude of its noise can be evaluated by the OSNR [21]. When an optical signal passes through the SOA, it not only amplifies the signal and its inherent noise but also introduces extra amplified spontaneous emission (ASE) noise. It is impossible to completely eliminate noise, so ensuring that certain OSNR at the receiver becomes crucial for effective transmission within the optical system. The noise figure (NF) serves as a metric for quantifying the extra noise introduced by a given system or device during the transmission of a signal. Essentially, it indicates the extent to which the system or device contributes additional noise to the transmitted signal. A lower NF signifies that the system or device introduces less supplementary noise during signal transmission, thus minimizing the degradation of signal quality. Consequently, a lower NF typically correlates with a higher OSNR, reflecting improved signal integrity and a more favorable transmission environment.  
The optical receiver plays a crucial role in converting optical signals into electrical signals. This conversion is achieved through the utilization of a photodetector. The optical signals, transmitted via the optical fiber link, are received by the photodetector and transformed into electrical signals [22]. Initially, the electrical signal generated by the photodetector is relatively weak. An amplifier is employed to amplify the signal to an appropriate level. This amplification process ensures that the signal is robust enough for further processing. Once amplified, the electrical signal is forwarded to the DSP module.  
Overall, the PM-DQPSK coherent optical transmission system employs sophisticated modulation and detection techniques to achieve high-speed and robust data transmission over long distances. Each component plays a crucial role in ensuring the fidelity and reliability of the transmitted information across the fiber link.