Simplified Soft-Output Direct Detection FTN Algorithm for 56-Gb/S Optical PAM-4 System Using 10G-Class Optics

In this paper, a simplified soft-output direct detection faster than Nyquist (SO-DD-FTN) algorithm is proposed to eliminate severe inter-symbol interference in optical interconnects. The feasibility of the proposed algorithm is verified in a C-band 56-Gb/s 4-ary pulse amplitude modulation (PAM-4) system using 10G-class optics, and the 3-dB system bandwidth is only 5.5 GHz. Compared to conventional SO-DD-FTN, the simplified SO-DD-FTN not only reduces 90% multiplication complexity and 80% addition complexity, but also has almost no performance degradation. Experimental results show that, for the back-to-back case, the achieved receiver sensitivity using simplified SO-DD-FTN has 4.5-dB improvement compared with conventional direct detection faster than Nyquist (DD-FTN). As for 20-km standard single-mode fiber (SSMF) transmission application, since the system suffers from both optical/electronic bandwidth limitation and dispersion-induced power fading, the conventional DD-FTN cannot achieve the bit error rate (BER) threshold of 10
^-3
, while the simplified SO-DD-FTN can achieve the BER threshold at received optical power of -14.5 dBm. In conclusion, the proposed simplified SO-DD-FTN algorithm achieves superior performance with low computational complexity and has the potential for applying to optical interconnects.