# Investigation of IEEE 802.16e LDPC Code Application in PM-DQPSK System
## 3 PM-DQPSK System and Simulation Results
### Digital Signal Processing Algorithms  
At the receiving end of a coherent optical communication system, an analog-to-digital converter (ADC) is utilized to sample and quantize the electrical signals representing the in-phase (I) and quadrature (Q) components of each polarized light. This process results in the generation of four discrete digital sequences, which are then fed into a DSP unit for further processing.  
Specifically, the ADC operation results in the generation of four discrete digital sequences corresponding to the I and Q components of both the X and Y polarizations. These digital sequences capture the amplitude and phase information of the received optical signals, enabling subsequent signal processing and data recovery.  
Following ADC conversion, the digital sequences are fed into a DSP unit for further processing. The DSP unit performs various functions to enhance signal quality, mitigate impairments introduced during transmission, and extract the transmitted data accurately.  
The compensation module of the PM-DQPSK signal transmission system is shown in Figure 2, which is crucial for maintaining the integrity of the transmitted signals and ensuring reliable communication performance.  
The compensation modules within the PM-DQPSK signal transmission system play a crucial role in enhancing the quality and reliability of the received signals, which can mitigate impairments and distortions that may occur during signal transmission [23]. Compensation techniques such as orthogonal imbalance compensation, dispersion compensation, chromatic dispersion compensation, polarization mode dispersion compensation, and nonlinear distortion mitigation are applied to enhance the signal quality and reliability, thereby contributing to the overall performance of the communication system. Below is a detailed description of the compensation process:  
Firstly, the sampled signal is processed using the Gram-Schmidt orthogonalization procedure (GSOP). This procedure is utilized to orthogonalize the received signals, reducing interference and improving signal quality [24]. Subsequently, fixed dispersion coarse equalization techniques are applied to compensate for dispersion effects introduced during transmission. These techniques aim to mitigate signal distortions caused by dispersion, ensuring the integrity of the transmitted data [25]. Adaptive polarization demultiplexing algorithms are employed to separate and recover the original polarization states of the transmitted signals. This process is crucial for correctly decoding the data encoded in different polarization states [26]. And carrier frequency estimation compensation techniques are applied to estimate and correct any deviations in the carrier frequency of the received signals; this ensures accurate demodulation and recovery of the transmitted data [27]. Carrier phase estimation compensation techniques are utilized to estimate and compensate for variations in the carrier phase of the received signals. This step is essential for aligning the phases of the received signals with the reference phase, facilitating accurate demodulation [28]. Finally, the four discrete digital sequences generated by the compensation processes are sent to the symbol judgment module, which recovers the original valid data of the PM-DQPSK signal and allows for further data analysis or transmission.