Adaptive Parallel Transmission With Code Index Modulation and DCSK Over Ultrasonic Intra-Body Fading Channels

Ultrasonic intra-body communication (IBC) has essential application prospects in personalized medicine. However, due to the limitations of complex biological tissues in the human body, signal transmission faces challenges such as significant attenuation, multipath effect, and noise interference. In this paper, we propose an adaptive parallel transmission with code-indexed modulation and differential chaos shift keying (A-PT-CIM-DCSK), aiming to achieve the optimal data rate and system robustness in complex channel environments. To balance the data rate and bit error rate (BER), the number of parallel bit blocks is adaptively adjusted according to the channel state information. The proposed scheme extends the conventional DCSK dual time slot structure to realize the parallel transmission of multiple quasi-orthogonal chaotic sequences, using longer Walsh codes, which further improves the resistance to multipath effects in fading channels. In addition, this paper designs a new q-value search algorithm by adopting three classical diversity combining strategies. The theoretical BER of A-PT-CIM-DCSK over the generalized Nakagami fading channel is also derived. The simulation results verify the theoretical derivation and show that the proposed A-PT-CIM-DCSK scheme significantly improves the performance compared with the conventional CIM-DCSK and PT-CIM-DCSK systems while maintaining a lower BER for ultrasonic IBCs.