A Novel Baseband Faster-Than-Nyquist Non-Orthogonal FDM IM/DD System With Block Segmented Soft-Decision Decoder

In this paper, we systematically investigate our recently proposed faster-than-Nyquist non-orthogonal frequency-division multiplexing (FTN-NOFDM) generation technique for intensity modulation/direct detection (IM/DD) optical systems. The proposed scheme is based on modified fractional discrete Fourier transform (mFrDFT), and presents several advantages compared to other existing generation methods. Due to the exemption of guard bands, the proposed scheme achieves higher tolerance to narrow-bandwidth filtering. Besides, without the constraints on compression factor and modulation formats, the proposed system gives better signal flexibility and compatibility to various advanced signal processing techniques. To meet the demand on a large number of FTN-NOFDM subcarriers and high-order modulation formats, we further propose an efficient block segmented soft-decision decoder (BSSDD), which does not need to adjust the signal architecture or sacrifice the spectral efficiency. A 22.24-Gb/s 4-QAM FTN-NOFDM signal with up to 25% bandwidth saving has been successfully implemented over 20-km standard single mode fiber (SSMF) in an IM/DD optical communication system while keeping the bit error ratio (BER) below the hard-decision forward error correction (HD-FEC) limit. For both 4-QAM and 16-QAM FTN-NOFDM systems, the proposed BSSDD achieves better performance than the conventional decoder. All the results indicate that the proposed FTN-NOFDM technique has great potential to be used in high-speed IM/DD optical systems.