Variable length RNP-Huffman coding for short block-length shaping in optical communications

This paper introduces Recursive Node Partitioning (RNP)-Huffman coding scheme to address the demand for efficient short block-length shaping techniques in optical communications. The proposed scheme groups input bits into uniformly distributed symbols. Then, recursive partitioning of symbols with Huffman coding is used to map branch identifiers to Amplitude Shift Keying (ASK) amplitudes with a given target distribution. As a result, variable-length output shaped ASK amplitude sequence with pre-set distribution can be obtained. Analysis indicates that RNP-Huffman coding achieves low Kullback–Leibler (KL)-divergence and rate loss at very small average block-lengths. Simulation results show that the proposed scheme achieves a transmission distance improvement of 350 km compared to uniform 64-Quadrature Amplitude Modulation (64QAM), making it a promising solution for short block-length shaping in high-speed optical communications.