Uncoordinated random-access protocols are attractive for underwater acoustic (UWA) networks due to their simplicity and low overhead, especially for data collection applications in scuba diving. However, the performance is limited by severe collisions and the challenging UWA channel, including rich multipath and time-varying channel (caused by Doppler effects and user movements). Existing UWA physical-layer waveforms struggle to resolve collisions while maintaining high data rates. This paper presents ZCMod, a Zadoff-Chu (ZC) sequence-based modulation that assigns unique ZC sequences to users to mitigate interference and encodes multiple bits via cyclic shifts for high data rates. To address UWA-specific challenges, ZCMod introduces two key designs: 1) shape-based demodulation, which tracks channel response shifts to combat multipath effects; 2) auxiliary modulation, where each symbol is modulated with two ZC sequences—one for channel estimation and the other for data transmission—to handle fast time-varying channels. Experiments and simulations demonstrate that a) ZCMod achieves more robust BER performance and eliminates error floors compared to state-of-the-art (SOTA) methods in slight time-varying channels; and b) ZCMod maintains stable throughput in fast time-varying channels, while SOTA approaches suffer significant degradation.

Uncoordinated random-access protocols are well-suited for underwater acoustic (UWA) networks due to their simplicity and low overhead. However, their performance is hindered by severe collisions and the challenging characteristics of UWA channels such as rich multipath and Doppler effect. Existing UWA physical layer waveforms struggle to resolve collisions while maintaining high data rates. This paper introduces ZCMod, a high-rate waveform allowing uncoordinated concurrent random access in UWA networks. ZCMod employs a Zadoff–Chu (ZC) sequence-based modulation that assigns unique ZC sequences to users to minimize inter-user interference and encodes multiple bits through cyclic shifts of the sequences to improve data rates. ZCMod further addresses the unique challenges of UWA channels via two new designs: 1) a shape-based demodulation approach that estimates the data-induced shift of channel response shape between the preamble and data symbols to handle rich multipath, and 2) an auxiliary modulation approach that modulates each data symbol with two ZC sequences, one for extracting current channel response shape and the other for data modulation, to handle the fast time-varying channel. Experimental results in a lake and a swimming pool and extensive simulation results show that a) ZCMod achieves around 100% higher throughput compared with the state-of-the-art (SOTA) approaches in quasi-static channels, and b) ZCMod maintains comparable throughput in fast time-varying channels as in quasi-static conditions, where the SOTA approaches experience significant degradation.

To facilitate responsive and cost-effective computing service delivery over edge networks, this paper investigates a novel two-stage double auction methodology via discovering an interesting idea of resource overbooking to overcome dynamic and uncertain nature of supply of edge servers (sellers) and demand generated from mobile devices (as buyers). The proposed auction integrates multiple essential goals such as maximizing social welfare as well as accelerating the decision-making process from both short-term and long-term perspectives (e.g., the time required to determine winning seller-buyer pairs), by introducing a stagewise strategy: an overbooking-driven pre-double auction (OPDAuction) for determining long-term cooperations between sellers and buyers before practical resource transactions as Stage I, and a real-time backup double auction (RBDAuction) for quickly coping with residual resource demands during actual transactions. In particular, by embedding a proper overbooking rate, OPDAuction helps with facilitating trading contracts between appropriate sellers and buyers as guidance for future transactions, by allowing the booked resources to exceed theoretical supply. Then, since pre-auctions may cause risks, our RBDAuction adjusts to real-time market changes, further enhancing the overall social welfare. More importantly, we offer an interesting view to show that our proposed two-stage auction can support significant design properties such as truthfulness, individual rationality, and budget balance. Extensive experiments demonstrate that our TwoSAuction achieves up to 76.8% reduction in decision-making time compared to conventional double auctions when considering 150 buyers and 25 sellers, while maintaining superior performance in social welfare and computational scalability over dynamic edge settings.

Affine Frequency Division Multiplexing (AFDM) is a brand new chirp-based multi-carrier (MC) waveform for high mobility communications, with promising advantages over Orthogonal Frequency Division Multiplexing (OFDM) and other MC waveforms. Existing AFDM research focuses on wireless communication at high carrier frequency (CF), which typically considers only Doppler frequency shift (DFS) as a result of mobility, while ignoring the accompanied Doppler time scaling (DTS) on waveform. However, for underwater acoustic (UWA) communication at much lower CF and propagating at speed of sound, the DTS effect could not be ignored and poses significant challenges for channel estimation. This paper analyzes the channel frequency response (CFR) of AFDM under multi-scale multi-lag (MSML) channels, where each propagating path could have different delay and DFS/DTS. Based on the newly derived input-output formula and its characteristics, two new channel estimation methods are proposed, i.e., AFDM with iterative multi-index (AFDM-IMI) estimation under low to moderate DTS, and AFDM with orthogonal matching pursuit (AFDM-OMP) estimation under high DTS. Numerical results confirm the effectiveness of the proposed methods against the original AFDM channel estimation method. Moreover, the resulted AFDM system outperforms OFDM as well as Orthogonal Chirp Division Multiplexing (OCDM) in terms of channel estimation accuracy and bit error rate (BER), which is consistent with our theoretical analysis based on CFR overlap probability (COP), mutual incoherent property (MIP) and channel diversity gain under MSML channels.

Linear chirp-based underwater acoustic communication has been widely used due to its reliability and long-range transmission capability. However, unlike the counterpart chirp technology in wireless - LoRa, its throughput is severely limited by the number of modulated chirps in a symbol. The fundamental challenge lies in the underwater multi-path channel, where the delayed signal may cause inter-symbol and intra-symbol interfere. In this paper, we present UWLoRa+, a system that realizes the same chirp modulation as LoRa with higher data rate, and address the multi-path challenge via the following new designs: a) we replace the linear chirp used by LoRa with the non-linear chirp to reduce the signal interference range and the collision probability; b) we design an algorithm that first demodulates each path and then combines the demodulation results of detected paths; and c) we replace the Hamming codes used by LoRa with the non-binary LDPC codes to mitigate the impact of the inevitable collision. Experiment results show that the new designs improve the bit error rate (BER) by 3 times, and the packet error rate (PER) significantly, compared with the LoRa's naive design. Compared with an state-of-the-art system for decoding underwater LoRa chirp signal, UWLoRa+ improves the throughput by up to 50 times.

Device-to-device (D2D) communications can allow different devices to communicate in short range, which greatly meets the demand of effective transmissions. In this paper, a spectral and energy-efficient carrier index keying-aided$M$-ary differential chaos cyclic shift keying (CIK-MDCCSK) system is proposed for D2D communications, where multiple cyclic-shifted$M$-ary information-bearing signals are superimposed and transmitted by a subcarrier, and extra keying bits are transmitted by indices of the selected subcarriers. Furthermore, a carrier keying detection and$M$-ary de-mapping algorithm is proposed to retrieve the information bits transmitted by$M$-ary symbols and indices of carrier keying. The bit error rate (BER) expressions of the proposed CIK-MDCCSK system are derived over the additive white Gaussian noise (AWGN) and D2D channels, and the optimal power coefficient is deduced for performance enhancement. Then, the spectral efficiency, energy efficiency, and complexity of CIK-MDCCSK are analyzed and compared to benchmark systems to show its superiority. Next, the proposed CIK-MDCCSK system is expanded into its enhanced version for more effective transmission without degrading the BER performance. It is demonstrated by performance comparisons that the proposed CIK-MDCCSK system can achieve more than twice energy efficiency and up to 3 dB gain in BER performance compared to state-of-the-art benchmarks.

Underwater acoustic channel (UAC) is featured as fast time-varying characteristic, and challenges the transmission designs. To countermine the time variation effect, we propose an adaptive design for orthogonal frequency division multiplexing (OFDM) transmission systems by utilizing the long-term stability of the second-order statistics of the channel state information (CSI). We derive the analytical expression of signal-to-interference-plus-noise-ratio (SINR) at each subcarrier to reach the target error performance based on the statistical information of the CSI. Thereafter, a new adaptive coding and bit-power loading algorithm with low computational complexity is proposed to pursuit the highest achievable bit rate with fixed error rate. The validity of SINR calculations and the effectiveness of the proposed adaptive algorithm are demonstrated under various conditions, wherein both simulated and measured channels have been tested.

Multi‐carrier (MC) and spread‐spectrum (SS) are two good technologies for underwater acoustic (UWA) communications. In this study, the authors propose a novel MC chaos modulation scheme, which combines code‐shifted differential chaos shift keying (CS‐DCSK) with orthogonal frequency division multiplexing (OFDM), namely MC‐CS‐DCSK. The proposed scheme exploits the orthogonal characteristic of the Walsh code to superimpose the chaotic reference chips and the information bearing chips in the same time/frequency unit. Besides, a cyclic‐shift interleaver is used on chips to harvest frequency diversity. To the in‐depth understanding of the proposed system, the bit error rate (BER) of the proposed system under Gaussian channels and multipath Rayleigh fading channels are derived and verified by simulation. Furthermore, the spectral efficiency of the proposed system is analysed and compared with some existing chaos‐based communication systems. Finally, they study BER performance of the system under UWA channels and compare it with traditional MC direct sequence SS and OFDM‐based MC‐DCSK systems. Simulation results show that the system has good robustness under time‐varying UWA channels.

In this paper, an M -ary orthogonal multilevel differential chaos shift keying system with code index modulation (CIM-OM-MDCSK) is proposed. In the proposed system, the multiple information bearing signals are modulated by the selected Walsh codes with specific indices and the M -ary information signals which are composed of modulated bits, chaotic signals, and their Hilbert transform. Since the reference signal along with the information bearing signals are overlapping in time domain and orthogonal in code domain by using Walsh code sequences, the spectral efficiency of the CIM-OM-MDCSK system is improved largely. By combining the M -ary modulation, orthogonal multilevel modulation, and code index modulation, the proposed CIM-OM-MDCSK system can achieve high data rate and superior bit error rate (BER) performance compared to its rivals. Moreover, we derive the BER expressions of the proposed system over additive white Gaussian noise and multipath Rayleigh fading channels. Finally, we make a performance comparison between the proposed system and other state-of-the-art non-coherent chaotic communication systems. The results confirm the competitive benefits of the proposed system.

Underwater hostile channel conditions challenge video transmission designs. The current designs often treat video coding and transmission schemes as individual modules. In this study, we develop an adaptive transceiver with channel state information (CSI) by taking into account the importance of video components and channel conditions. The design is more effective than the traditional ones. However, in practical systems, perfect CSI may not be available. Therefore, we compare the imperfect CSI case with existing schemes, and validate the effectiveness of our design through simulations and measured channels in terms of a better peak signal-to-noise ratio and a higher video structural similarity index.

In this paper, a novel chaos modulation scheme, which combines code-shifted differential chaos shift keying (CS-DCSK) with orthogonal frequency division multiplexing (OFDM) is presented, namely OFDM-CS-DCSK. In the proposed scheme, the reference signal and the information bearing signal of CS-DCSK are firstly formed by Walsh code sequences, then different chips of the CS-DCSK signal are mapped onto different subcarriers of OFDM. Utilizing the characteristics of OFDM and CS-DCSK, the proposed scheme has robust performance over time-frequency doubly selective fading channel. In addition, the proposed system has low complexity because channel estimation, channel equalizer and the radio frequency (RF) delay circuits are not required at its receiver. By the computer simulations, bit error rate (BER) performances of the proposed scheme are compared with OFDM-DCSK and CS-DCSK under various channel parameters. Simulation results indicate that the proposed scheme has significant performance advantages over OFDM-DCSK and CS-DCSK in time-frequency doubly selective fading channel.