Digital Coherent Superposition for Performance Improvement of Spatially Multiplexed 676-Gb/s OFDM-16QAM Superchannels

We demonstrate the use of digital coherent superposition to improve the performance of space-division-multiplexed (SDM) 676-Gb/s OFDM-16QAM superchannels, achieving ~4 dB improvement in OSNR by using two SDM copies and 1075-km (14x76.8km) transmission over a seven-core-fiber with an effective aggregate spectral efficiency of 23.7 b/s/Hz. Introduction Multi-core fiber (MCF) has been used to support space-division-multiplexed (SDM) transmission with record per-fiber capacities1-3 and spectral efficiencies (SEs)4. Aggregate capacities of 112 Tb/s2 and 305 Tb/s3 were demonstrated in a 76.8-km seven-core-fiber and a 10.1-km 19core-fiber, respectively, using polarizationdivision multiplexed quadrature phase-shift keyed (PDM-QPSK) signals. Recently, digital coherent superposition (DCS)5 has been proposed to improve the signal performance in MCF-based SDM transmission, especially for transmission links where link performance, rather than link capacity, is of primary concern. DCS was experimentally demonstrated in a 2688-km SDM transmission link with 128-Gb/s differentially-encoded PDM-QPSK signals, using an additional phase alignment technique to match the phases of the recovered SDM copies at the receiver5. Here, we demonstrate DCS in a coherent optical orthogonal frequency-division multiplexing (CO-OFDM) system where phase alignment among different SDM copies is automatically achieved, through the use of pilot subcarriers. We further show that with 16-point quadrature amplitude modulation (16-QAM) for 1 2 1 2 1