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. We further show that the performance improvement from the coherent superposition is retained in the nonlinear transmission regime through coordinated scrambling of signal constellations at the transmitter and appropriate unscrambling at the receiver, by using a series of simple scrambling functions.
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