An Enhanced Hard-Decision-Based ML Receiver for UsIM-SS Intra-Body Systems

Ultrasonic index modulation with spread spectrum (UsIM-SS) can achieve intra-body communications (IBCs) with high data rate and high reliability. However, existing maximum likelihood (ML) receivers, including soft-decision-based ML (S-ML) and hard-decision-based ML (H-ML), cannot provide a trade-off between complexity and reliability. To this end, this paper proposes an enhanced H-ML (E-H-ML) receiver for UsIM-SS. E- H - ML first estimates the received signal in each chip and uses the estimated chip signals to recover the modulation bits; then, the estimated modulation bits and estimated chip signals are jointly used to recover the index bits. The theoretical symbol error rate (SER) is derived and verified by Monte Carlo simulations, revealing that the proposed E- H - ML receiver can achieve lower SER than the H-ML receiver and much lower complexity than the S- ML receiver.