Abstract
In this paper, a new four-dimensional (4D) constellation shaping scheme is proposed for long-haul fiber-optic system to approach the optimal performance. The scheme designs a non-uniform signaling for 4D 80-ary multisphere distribution (4D-80MD) constellation by combining constellation shaping of four dimensions with pragmatic low density parity check (LDPC) code modulation. Additionally, the design of multisphere distribution is introduced based on a bounded-energy enumerative amplitude trellis. The proposed 4D-80MD constellation has the same spectral efficiency (SE) as the conventional polarization-multiplexed 8-ary quadrature-amplitude modulation (PM-8QAM). We study the efficiency of 4D-80MD, combined with bit-interleaved coded-modulation with iterative decoding, and demonstrate in back-to-back system that the proposed 4D-80MD outperforms PM-8QAM about 1 dB For a long-haul optical fiber system, the optimized 4D-80MD is able to achieve a reach improvement of up to 21%. The results also show increased rates and reduced non-linear noise compared with 2D shaped signal with the same SE.
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