Experimental study of non-binary LDPC coding for long-haul coherent optical QPSK transmissions

Area of Science:

• Optical Communications
• Information Theory
• Coding Theory

Background:

• Advanced modulation formats like Polarization Division Multiplexing Quadrature Phase Shift Keying (PDM-QPSK) are crucial for high-speed optical networks.
• Forward Error Correction (FEC) codes, particularly Low-Density Parity-Check (LDPC) codes, are essential for improving transmission performance and overcoming channel impairments.

Purpose of the Study:

• To evaluate the performance of a rate-0.8 4-ary LDPC code in a 40 Gb/s Dense Wavelength Division Multiplexing (DWDM) system.
• To investigate the feasibility of long-haul transmission using LDPC-coded PDM-QPSK signals over legacy dispersion-managed fiber.

Main Methods:

• The study employed a rate-0.8 4-ary LDPC code within a 50 GHz-spaced DWDM system utilizing PDM-QPSK modulation.
• Techniques such as time-interleaving polarization multiplexing and Maximum A Posteriori (MAP) detection were utilized.
• Transmission performance was assessed over 10,560 km of legacy dispersion-managed fiber.

Main Results:

• A net effective coding gain of 10 dB was achieved at a Bit Error Rate (BER) of 10⁻⁶.
• Error-free transmission over 10,560 km was demonstrated.
• The proposed nonbinary quasi-cyclic LDPC code exhibited an uncoded BER threshold of 4×10⁻².

Conclusions:

• The investigated LDPC code shows significant potential for enhancing the reach and reliability of high-speed optical communication systems.
• Implementation challenges, including phase ambiguity and optimal coding length selection for LDPC in coherent optical systems, require further consideration.