This paper introduces a one-way ranging and time-alignment method for distributed multi-mode underwater communication networks that utilize both acoustic and optical front-ends. By capitalizing on the differences in propagation velocities between the optical and acoustic waveforms, the method enables precise distance and timing measurements between any two nodes by analyzing the time differences in signal arrivals. The theoretical foundation of the proposed technique is validated using the Cramer-Rao lower bound (CRLB), which serves as a performance benchmark. The method’s effectiveness is assessed through simulations and experimentally validated in a realistic underwater environment. The results demonstrate that the approach achieves excellent timing alignment with nanosecond precision and ranging accuracy at the centimeter level, utilizing only a single reference transmission. The results significantly enhance the reliability and efficiency of underwater communication systems, paving the way for advanced applications in marine exploration, environmental monitoring, and underwater robotics.

Direct optical communication between underwater and aerial nodes such as drones offers a transformative approach for accessing underwater devices and sensors in shallow water environments. Unlike traditional underwater communication systems, which rely on buoys or surface vehicles to relay data between underwater devices and ground stations. Direct waterair optical communication enables greater operational flexibility and cost efficiency by utilizing drones to directly collect data or transmit commands. However, a significant challenge in implementing such systems is their limited reliability and short operational range, typically restricted to a few meters in both water and air. To address this limitation, this paper proposes and implements a LoRa-based modulation scheme for optical water-air communication using software-defined radio (SDR) technology. LoRa, a widely recognized radio frequency (RF) technology, is specifically designed for energy-efficient, low-rate communication, making it a promising candidate for enhancing the reliability of optical links. This paper presents a systematic approach to optimizing system parameters to improve link reliability across various scenarios. The experimental results demonstrate that the implementation of optical LoRa-based modulation offers a significant improvement in performance, achieving a gain of 7 dB to 15 dB compared to traditional Binary Phase Shift Keying (BPSK) modulation. This substantial gain underscores the effectiveness of LoRa-based modulation in enhancing the reliability and robustness of optical water-air communication links.

This paper presents a proof-of-concept for the UniSDM architecture, which is capable of simultaneously operating across acoustic, optical, magnetic induction, and RF communication modes. A flexible firmware stack is developed that integrates the logic for modem operation and supports multiple underwater communication modes mentioned above and the protocols and standards such as OFDM and JANUS for acoustics, as well as OFDM, single carrier, and IRDA-like protocols for optical communication. Additionally, it provides various interfaces, including a socket-based API, Ethernet tun-neling, and a graphical user interface (GUI). The firmware architecture incorporates cross-layer frame and waveform processing, enabling advanced communication paradigms such as multimode cooperative MIMO.

This paper presents a distributed cell-free communication and radar system that operates in the uplink. The system schedules dedicated transmit (Tx) access points (APs) to transmit dedicated radar signals in the uplink together with the user equipment (UE). The receiving (Rx) APs decode the UE payloads while also detecting targets based on the Tx AP signals. To mitigate the added Tx AP interference, the Rx APs use multiuser processing to recover the UE payloads, while a combination of large processing gains, adaptive beamforming, spatial diversity, interference cancellation and power control is used to mitigate the UE interference impacting the radar. The radar introduces few changes to the physical layer and the additional computations needed are comparable to the communication system. The system is validated numerically by using Monte-Carlo simulations, where we highlight the inherent trade-offs between the various system parameters (such as the power control balancing, and the number of Tx APs scheduled and UEs cancelled) and show that both the communication and radar systems can be effectively integrated into the same network at a near optimal performance.

This paper presents a multiple-access procedure for joint radar and communication systems. Dedicated radar antennas transmit uplink pilots in the same time-frequency resources as the user pilots over several coherence blocks. The radar and communication channels are subsequently estimated wherein the respective systems proceed as conventional. In order to facilitate pilot reuse and mitigate interchannel interference, the radar and user pilots are modulated in time and frequency so that the channels are orthogonal in the range-Doppler domain. This allows the respective channels to be separated without mutual interference or the need to allocate dedicated radar pilots. The main advantage is the time-division duplex compatibility and the near optimal radar performance. The performance trade-offs are discussed and the method is demonstrated with numerical simulations, demonstrating a relatively unaffected EVM while simultaneously recovering the radar channel for sensing.

This paper considers the cell-free (CF) massive multiple-input multiple-output (MIMO) architecture with an added virtual uplink radar integration. We consider joint communications and radar (JCR) in the uplink payload of the time-division duplex (TDD) frame and formulate a set of linear interference constraints in order to limit the user equipment (UE) interference imposed on the radar system at each accesspoint (AP) through power control. The constraints are incorporated into the sum spectral efficiency (SSE) and the sum-log-SNR (SLS) policies. Furthermore, a largest large-scale fading (LLSF) heuristic is formulated as an approximate low-complexity solution. Numerical simulations indicate that all methods provide similar performance in terms of spectral efficiency (SE) and that power control allows the communication system to be controlled to satisfy a given worst case radar performance.

This paper considers the cell-free (CF) massive MIMO architecture from a joint communication and radar point of view. We propose a protocol for communication and sensing, where the network allocates a set of access-points (APs) to participate in the uplink together with the users (UEs) to serve the network with a radar signal. This realizes the radar system as a distributed bistatic radar system, where the objective is to recover the radar echoes from the multiuser interference. The imposed cost on the communication system is the loss of one AP and the need to schedule one additional virtual UE per allocated AP. We present two modes of radar sensing, occurring in either the uplink training segment or the data payload segment of the communication frame. A subspace signal model and its corresponding generalized likelihood ratio test is developed in order to evaluate the detection performance. We present expressions for the probably of detection and false alarm, and demonstrate the system numerically. Our main message is that coordinating the network to transmit in the uplink together with the UEs serves as an interesting approach in enabling radar sensing in CF massive MIMO systems.

This paper considers the problem of radar sensing by using a large number of antennas. We use the orthogonal frequency division multiplexing (OFDM) waveform, and show that the large arrays used in massive multiple-input multiple-output (MIMO) communications enable accurate localization in the array near-field, even at the narrow bandwidths typically encountered at low carrier frequencies. We validate our findings experimentally with a massive MIMO testbed operating at 3.5 GHz carrier frequency and 18 MHz OFDM bandwidth in an indoor environment. We consider a single moving cylinder, and demonstrate a median accuracy of (3.4, 5.6) cm in ($x$,$y$) in the near-field. We show that the accuracy is maintained with only a single subcarrier, and that the resolution increases with an order of magnitude when combining all antennas, effectively surpassing the 16.67 m bistatic range resolution set by the OFDM waveform. We use a radar symbol duration of$71.88~\mu $s at an effective transmission period of 2.5 ms, which indicates that the radar and communication systems can be implemented in time-division with a capacity loss of only 2.9%. Our results suggest that near-field radar sensing can be integrated into future massive MIMO systems operating at low carrier frequencies and narrow bandwidths.