Ultrasonic Multidimensional Index Modulation With Low-Complexity Maximum Likelihood Receiver for Intra-Body Communications

Ultrasonic intra-body communication (IBC) demands high data rates and high reliability to achieve revolutionary medical and healthcare applications. This paper proposes an ultrasonic multidimensional index modulation (U-MIM) technique for IBCs. The core idea is to utilize ultrasonic pulses to transmit modulation bits whilst jointly utilizing indices in time and code domains to transmit index bits without adding extra pulses, significantly improving the data rate. Then, a maximum likelihood (ML) receiver is investigated to reveal the optimal bit-error rate (BER) performance. To address the high computational complexity of the ML receiver, a low-complexity ML (L-ML) receiver is proposed by employing maximum-minimum selection and joint estimation of index bits and modulation bits to reduce the number of template signals. Furthermore, this paper analyzes the theoretical BER, spectral efficiency (SE), and computational complexity of the proposed L-ML receiver considering the proposed U-MIM and the existing ultrasonic index modulation (UsIM) and UsIM with spread spectrum (UsIM-SS) systems. Theoretical and simulation results demonstrate that U-MIM achieves higher SE than UsIM-SS and lower BER than UsIM. Moreover, U-MIM with the L-ML receiver can attain the same BER as UsIM-SS but with significantly reduced complexity. Compared with traditional ML receivers, the L-ML receiver can reduce the complexity by up to twelve orders of magnitude for both U-MIM and UsIM-SS, while maintaining equivalent BER performance. The joint use of U-MIM and L-ML can provide a promising solution for versatile IBC applications requiring high data rate, high reliability, and low complexity.