Joint Ultrasonic Index Modulation and Splitting Detection for Intra-Body Communications

The reliability of ultrasonic intra-body communication (IBC) is crucial for medical monitoring, diagnosis, and treatment. Recently, ultrasonic splitting receivers that jointly use coherent detection (CD) and non-coherent energy detection (ED) obtain superior bit-error rate (BER) performance compared to a single CD or ED receiver for reliable IBCs. However, this paper demonstrates that existing splitting receivers have performance gains in terms of mutual information and BER in the case of independent channel noises for CD and ED, while no performance gains exist in the case of correlated channel noises. To this end, we propose an ultrasonic index modulation-splitting-maximum ratio combination (UsIM-S-MRC) system. UsIM activates a part of transmission frames whilst S-MRC only processes active frames and treats inactive frames as zero, reducing the average channel noise in all frames. Therefore, the joint-designed UsIM-S-MRC effectively mitigates the impact of channel noise correlation on splitting detection. The theoretical mutual information, BER, and optimal splitting ratio are also derived. Extensive simulations validate the theoretical analysis and reveal that UsIM-S-MRC can attain performance gain in both independent and correlated channel noises, providing a reliable receiver structure for future ultrasonic IBCs.