Ultrasonic index modulation can effectively improve the spectral efficiency and energy efficiency of intra-body communication systems. This letter proposes a codeword-based auto-correlation (C-AC) receiver for ultrasonic code-indexed modulation (UCIM) and ultrasonic time-code-indexed modulation (UTCIM) transmitters, employing Hadamard codewords. The core idea is to utilize the auto-correlation properties of the Hadamard codewords to demodulate the code index bits, where the demodulation of each code index bit is based on the sum of the dot products of every two sub-sequences, dispensing with precise synchronization and channel state information. The theoretical bit error rate (BER) is derived and verified by extensive Monte Carlo simulations over intra-body fading channels, revealing that the proposed non-coherent low-complexity C-AC receiver can achieve lower BER compared with the conventional transmitted-reference receiver for both the UCIM and UTCIM transmitters.

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.

Age of information (AoI) serves as a critical metric for evaluating information timeliness in health monitoring systems (HMS), particularly for real-time status updates. This paper investigates single-relay HMS with short-packet communications, proposing a two-hop transmission framework to analyze system’s average AoI under three distinct first-hop transmission protocols combined with classical automatic repeat request (CARQ) mechanism in the second hop. Expressions for average AoI are rigorously derived for each scheme, enabling a comprehensive evaluation of the joint impacts of coding length, signal-to-noise ratio, and destination node quantity on system performance. Numerical simulation results show that the use of non-ARQ for the first hop can minimize average AoI under better channel conditions, while CARQ is suitable for scenarios with high reliability requirements, and truncated ARQ shows better overall performance when dealing with the trade-off between information timeliness and reliability.

Peak age of information (PAoI) is an important performance metric for mobile edge computing (MEC) systems to characterize the freshness of information. This paper addresses MEC systems with joint sensing, transmission and computation, where sensors sense and transmit computationally intensive packets to local and edge servers. Three computation scenarios are considered: (1) a local computation scenario in which all computation tasks are handled by the local server, (2) an edge computation scenario in which all computation tasks are handled by the edge server, and (3) a partial computation scenario in which some of the computation tasks are handled at the local server, and the rest is handled by the edge server. Considering the exponential distribution of transmission and computation times and using a first-come-first-served queuing policy, a closed expression for the average PAoI is derived. Numerical simulation results show that a reasonable packet sensing rate can reduce the PAoI of the system. In addition, the PAoI of the system can be reduced by adjusting the server computation rate and offloading ratio, so that the MEC system can achieve better performance.

The traditional large-scale and low-power Internet of Things systems cannot meet the requirement of freshness of information. In this paper, a frame-slotted Aloha (FSA) based synchronization strategy is proposed to update information for multi-node access, which makes the information at the receiver side as fresh as possible. Firstly, a multi-point access model of the system from the perspective of FSA is studied, and the corresponding data flow and framework analysis are presented. Secondly, the age of information (AoI) analysis of FSA is conducted, and the expression of the average AoI and the optimal frame size under frame synchronization are derived. Finally, numerical simulation results show that the proposed strategy achieves the lower AoI in FSA based multi-node wireless sensor networks.

This paper focuses on the timely delivery of status updates for a multi-relay assisted energy harvesting (EH) Internet of Things system with short packet communications (SPC), where an EH-powered source transmits status updates to a destination with the aid of multiple EH-powered relays. To improve the energy efficiency and information freshness, an earliest-$\ell $partial relay selection scheme is proposed, in which the earliest$\ell $relays that successfully receive the status update from the source will be selected to forward the status update to the destination, and the selection combining (SC) and maximal-ratio combining (MRC) schemes are used at the destination. Firstly, we investigate the update packet delivery schemes of source-relay (S-R) and relay-destination (R-D) links, and present the packet error probabilities under the SC and MRC schemes based on SPC theory. Secondly, after characterizing the full charge time durations of both the source and relay, we investigate the statistical description of the number of the packet transmission attempts from the source to relay as well as the one from the relay to destination by using order statistics. With the analysis of the evolution of the instantaneous age of information (AoI), the expression for average AoI is derived. Finally, we investigate the average AoI minimization, which is formulated as a coupled triplet problem of the packet lengths at the source and relays and the value of$\ell $. To overcome the coupling, we present the convexity approximation of the average AoI as well as the concise proof. Then, an efficient gradient-based two-step optimal search algorithm is proposed to solve the optimization problem, which searches the local optimal packet lengths for S-R and R-D links by iteratively updating the value of$\ell $until the global optimal objective parameters are found. Numerical analyses give the effect of the key parameters on the average AoI, and the comparison demonstrates that the proposed gradient-based method converges quickly than the conventional greedy search method.

In recent years, reliable and timely data collection from underwater acoustic sensor networks (UASNs) has attracted widespread attention in academia. For this proposal, we study the autonomous underwater vehicle (AUV)-based real-time mobile relaying network in UASNs. Relay placement determines the reliability of communication in the relay network. We first formulate the relay positions that can meet a certain frame error rate (FER) requirement as the FER-restricted area (FRA), and approximate the FRA with a three-dimensional ellipsoid mathematical formula. The problem of reliable and timely data collection becomes planning a short AUV relaying trajectory under the different-sized FRA constraints. To this end, we propose a nearest-community (N-C) trajectory planning algorithm and further propose a member grouping method to form communities. Simulation results verify that the approximate FRA is more than 90% consistent with the real FRA and show that the proposed N-C can successfully receive more packets per minute and consume fewer sensors' energy than other algorithms.

Ultrasonic intra-body communication (IBC) is an emerging technique to enable revolutionary healthcare applications. This paper proposes an ultrasonic index modulation with spread spectrum (UsIM-SS) scheme for IBCs. The information bits consist of modulation bits and index bits. The core idea of UsIM-SS is that the modulation bits are spread by spreading codes and the index bits are leveraged to select the time-hoping sequence to transmit the spreading bits. We also find that the existing ultrasonic wideband (UsWB) and UsIM can be regarded as special cases of the proposed UsIM-SS. Furthermore, the soft decision-based maximum likelihood (S-ML) receiver and hard decision-based ML (H-ML) receiver are proposed for UsIM-SS, providing a trade-off between bit-error rate (BER) and complexity. Theoretical spectrum efficiency (SE) and BER expressions for UsIM-SS using S-ML and H-ML are also derived and verified by extensive Monte Carlo simulations. Both theoretical and simulation results show that UsIM-SS can achieve lower BER and higher SE than existing UsWB and Us 1M techniques.

Molecular communication (MC) is an emerging field aiming at realizing information exchange via chemical signals between nanomachines in nanonetworks. Information transmission that is energy-efficient and relies on relatively low-complexity transceiver techniques is of practical importance for MC systems. Based on the fact that constellation shaping can improve energy efficiency, in this paper, we propose a molecular shell mapping (MSM) scheme to implement the probabilistic constellation shaping for MC. The MSM method is designed to exploit the concentration sequences with the lowest sum sequence weight, which results in an energy-efficient signal constellation. Furthermore, we propose an algorithm for selecting and sorting the concentration sequences to mitigate inter-symbol interference. For information detection, we design a genie-aided maximum-likelihood (ML) detector and a realistic ML detector to leverage the constructive effect of intra-sequence interference, as well as derive their bit error rates and achievable rates. Additionally, for the applications using large blocklength sequences, a low-complexity ML detection method is proposed. Numerical simulation results confirm that the shaped signaling using the MSM method is more energy-efficient than the conventional equiprobable signaling, achieving shaping gains of up to 1.5 dB at an ultra-short blocklength of 4.

Relaying is an effective method to achieve reliable and timely data collection, which is one of the most important parts of underwater acoustic sensor networks (UASNs). Considering that the relay position determines the reliability of relay communication, we study the problem of relay placement and place an autonomous underwater vehicle (AUV) to mobile relay data transmission and realize the reliable, low-latency, and low-energy data collection in UASNs. First, we formulate the relay positions that can meet a certain frame error rate (FER) requirement as the FER-restricted area (FRA), and approximate FRA with a three-dimensional geometry formula. The problem of reliable and timely data collection becomes planning a short AUV relaying trajectory under the FRA constraint. To this end, we propose a nearest-community (N-C) trajectory planning algorithm to design the AUV relay trajectory. A member grouping method and the necessity of position (NoP) concept are proposed to further reduce the relay positions and trajectory length of the AUV. Simulation results verify that the approximate FRA is more than 90% consistent with the real FRA and show that the N-C using NoP-based member grouping can successfully receive more packets per minute and consume fewer sensors’ energy than other algorithms.

Ultrasonic intra-body communication (IBC) is a promising enabling technology for future healthcare applications, due to low attenuation and medical safety of ultrasonic waves for the human body. A splitting receiver, referred to as the splitting-detection separate-decision (SDSD) receiver, is introduced for ultrasonic pulse-based IBCs, and SDSD can significantly improve bit-error rate (BER) performance over the traditional coherent-detection (CD) and energy detection (ED) receivers. To overcome the high complexity and improve the BER performance of SDSD, a splitting-detection joint-decision (SDJD) receiver is proposed. The core idea of SDJD is to split the received signal into two streams that can be separately processed by CD and ED, and then summed up as joint decision variables to achieve diversity combining. The theoretical channel capacity and BER of the SDSD and SDJD are derived for M-ary pulse position modulation ( M-PPM) and PPM with spreading codes. The derivation takes into account the channel noise, intra-body channel fading, and channel estimation error. Simulation results verify the theoretical analysis and show that both SDSD and SDJD can achieve higher channel capacity and lower BER than the CD and ED receivers with perfect channel estimation, while SDJD can achieve the lowest BER with imperfect channel estimation.

Intra-body communication (IBC) plays a significant role in future health care systems. This article proposes a pulse-based ultrasonic index modulation (UsIM) to enable high data rate and low power IBCs. Two grouping-based UsIM strategies, namely ultrasonic grouped-chip indexed modulation (UsCIM) and ultrasonic chip-group indexed modulation (UsGIM), are proposed to implement multiple access for intra-body networks (IBNs). UsCIM uses the chip indices in a group to convey additional information bits for a biosensor node, while UsGIM uses the group indices and each node occupies one chip in a group. Three receivers, including the maximum likelihood (ML), the maximum ratio combination (MRC), and the energy detection (ED) receivers, are then proposed and studied. The theoretical bit-error rate (BER) upper bound for the ML receiver, and theoretical BER expressions for the MRC and ED receivers are derived. The system performance metrics in terms of complexity, throughput and energy efficiency are analyzed. The analytical and simulation results reveal that UsGIM achieves a lower BER than UsCIM under channel fading and multipath effect. Both UsCIM and UsGIM achieve higher throughput and energy efficiency than the existing IBC systems, and both strategies are important for energy-constrained implanted IBNs.

Ultrasonic intra-body communication (IBC) is a promising enabling technology for many future healthcare applications, due to the low attenuation and medical safety of ultrasonic waves for human body. This paper proposes a splitting-detection joint-decision (SDJD) receiver structure for ultrasonic pulse-based IBCs. The core idea is to split the received signal into two steams for the coherent-detection (CD) and energy detection (ED), and then the two detection outputs are summed up as joint decision variables to estimate the received signals. The theoretical channel capacity and bit-error rate (BER) expressions are derived, considering receive noise, tissue attenuation, and channel fading in intra-body channels. Both simulation and theoretical results show that the SDJD receiver outperforms the pure CD, ED, and splitting-detection separate-decision receivers in terms of BER performance. In addition, the proposed receiver has a low complexity requirement on the hardware and decision algorithm. Therefore, SDJD is an important structure to receive signals correctly and efficiently for ultrasonic IBCs.