Four-dimensional QPSK modulation transmission scheme based on entropy-coding-aided symbol selection

We propose a four-dimensional quadrature phase shift keying (4D-SS-QPSK) modulation transmission scheme based on entropy-coding-aided symbol selection. The proposed scheme integrates Huffman coding with bit-interleaved coded modulation (BICM) to generate non-uniform signals and aggregates three QPSK signals with one BPSK signal, forming a four-dimensional phase unit. We select symbol subsets aided by source entropy (SE) and map signals onto a predefined four-dimensional symbol subset for transmission, thereby overcoming the limitation of traditional QPSK signals in terms of insufficient utilization efficiency in high-dimensional space. We demonstrate the proposed scheme in a 2-km intensity modulation/direct detection (IM/DD) seven-core fiber transmission system with a signal transmission rate of 14.175 Gb/s. Experimental results indicate that at the hard-decision forward error correction (HD-FEC) bit error rate (BER) limit of 3.8 × 10^-3, the receiver optical power of the 4D-SS-QPSK transmission scheme is -10.4 dBm, achieving approximately 0.7 dB receiver sensitivity gain compared with 3D-SS-QPSK signals, and approximately 0.5 dB gain compared with 4D-SS-QPSK signals without BICM.