The ever-increasing bandwidth demand driven by new Internet applications such as cloud computing, three-dimensional video, and virtual reality stimulates an ultralarge capacity requirement for future fiber optical transmission. Therefore, given the limited bandwidth of optical fibers and amplifiers, high spectral efficiency (SE) transmission enabled by faster-than-Nyquist (FTN) techniques has recently attracted worldwide research interest. With the help of the FTN technique, multiple carriers can be packed tightly, narrower than symbol-rate carrier spacing with both manageable intersymbol interference (ISI) and intercarrier- interference (ICI). Duo-binary shaping is an effective approach to realize FTN transmission for the PDM-QPSK format.Toward higher order modulation formats, such as PDM-16QAM, FTN transmission becomes a tough challenge because of the lower tolerance of ISI and ICI. To the best of our knowledge, PDM-16QAM FTN transmission experiment over SSMF link has not been reported yet.
Prof. Songnian Fu andPh.D studentZhuopeng Xiao, Borui Li with school of optical and electronic information, Huazhong University of Science and Technology, experimentally demonstrate a FTN transmission for PDM-16QAM based on the proposed maximum likelihood sequence detection (MLSD) and multi-input multi-output (MIMO) algorithm. This is the first experimental demonstration for PDM-16QAM FTN transmission which achieves a record net SE of 7.68-b/s/Hz. The key concept of the MLSD and MIMO (M&M) algorithm is to mitigate the ICI by the crosstalk regeneration, with the help of RDE-based MIMO processing. The crosstalk regeneration is enabled by the receiver-side duo-binary shaping and MLSD. Due to the use of M&M algorithm for severely overlapped Nyquist-PDM-16QAM carriers, 5.4-dB optical signal-to-noise ratio (OSNR) improvement is experimentally observed under the scenario of 2 × 16-GBd PDM-16QAM back-to-back transmission with a carrier spacing of 15 GHz. Furthermore, FTN superchannel transmission with carrier spacings of 14 and 15 GHz are successfully realized over 960- and 1920-km SSMFs, respectively.
Fig. 1. FTN superchannel structure.
Fig. 2. (a) experiment setup, (b) transmitted FTN superchannel spectrum, (c) receiver-side DSP flow. DP-IQ Mod: dual-polarization in phase-quadrature modulator, SW: switch
Fig.3. Schematic of M&M processing for carrier A.
Fig.4. (a) BTB transmission results. (b) Transmission results with different distance.
Thispaper was published at Optics Letters (Vol. 42,No.6, pp. 1072-1075, 2017) andpartiallysupported by the 863High Technology Plan (2015AA015502), and National Natural Science Foundation of China (61575071, 61331010).
