Rotated 8QAM Probabilistic Shaping Scheme Based on DM-OFDM-IM over an Underwater Magnetic Induction Communication System

This paper introduces a rotated 8QAM probabilistic shaping (PS) scheme based on dual-mode orthogonal frequency-division multiplexing with index modulation (DM-OFDM-IM) over an underwater magnetic induction (MI) communication system. The key aspect of DM-OFDM-IM is its use of two distinct signal constellations to modulate the carrier signal independently. We categorize the 8QAM constellations into internal and external groups. We divide the subcarriers within the whole bit block into two groups: the index selection group and the constellation mapping group. The internal constellation transmission probability is advanced by making the index bits of the index selection group correspond to different constellation modulations' times and order. The modulation index is structured to achieve a 3:1 ratio between the internal and external constellation signals. This approach enhances the likelihood of transmitting low-energy internal signals, thereby lowering the bit error rate (BER). Comparative analysis with the constant composition distribution matching (CCDM) method reveals no significant performance differences. Additionally, constellation rotation is integrated to improve signal robustness in Rayleigh fading channels, increasing diversity gain without expanding bandwidth or power consumption. To validate the scheme's underwater performance, a practical experiment was conducted at 15 cm water depth, transmitting DM-OFDM-IM-based PS rotated 8QAM signals over a 23 cm distance at 62.18 Kb/s. Results demonstrate that constellation rotation significantly enhances signal reliability despite its simplicity. Compared to conventional 8QAM, the proposed method extends the communication distance by 4.6 cm at a hard-decision forward error correction (FEC) bit error rate (BER) limit of 3.8 × 10-3. This approach achieves high performance in MI communication with low complexity and ease of implementation, positioning it as a promising solution for future short-range underwater magnetic communication systems.