Partitioned constellation shaping based on index-driven coding for OFDM transmission over seven-core fiber

This paper proposes a probabilistic shaping modulation scheme based on index-driven constellation partitioning. The 32QAM constellation is divided into eight non-overlapping energy-level patterns, with every three consecutive subcarriers grouped as a mapping unit controlled jointly by index bits through a predefined lookup table. By assigning more subcarriers to low-energy constellation modes located in the inner circle, the activation probability of low-energy symbols is increased, thereby reducing the average transmit power. Experimental validation is conducted over a 2 km seven-core fiber optic system. Experimental results demonstrate that at a bit error rate (BER) of 3.8 × 10^-3, the proposed index-driven constellation-shaped signal achieves up to 0.53 dB gain compared to uniform 32QAM signals. Meanwhile, the BER performance variation among different modes remains below 0.5 dB. With low implementation complexity, the proposed scheme exhibits promising potential in multi-user optical access scenarios.