Lina Pu

Challenges and Opportunities of Underwater Cognitive Acoustic Networks

In oceans, both the natural acoustic systems (such as marine mammals) and artificial acoustic systems [like underwater acoustic networks (UANs) and sonar users] use acoustic signal for communication, echolocation, sensing, and detection. This makes the channel spectrum heavily shared by various underwater acoustic systems. Nevertheless, the precious spectrum resource is still underutilized temporally and spatially in underwater environments. To efficiently utilize the spectrum while avoiding harmful interference with other acoustic systems, a smart UAN should be aware of the surrounding environment and reconfigure their operation parameters. Unfortunately, existing UAN designs have mainly focused on the single network scenario, and very few studies have considered the presence of nearby acoustic activities. In this paper, we advocate cognitive acoustic as a promising technique to develop an environment-friendly UAN with high spectrum utilization. However, underwater cognitive acoustic networks (UCANs) also pose grand challenges due to the unique features of underwater channel and acoustic systems. In this paper, we comprehensively investigate these unique characteristics and their impact on the UCAN design. Finally, possible solutions to tackle such challenges are advocated.

Effective Relay Selection for Underwater Cooperative Acoustic Networks

Cooperative communication has been studied extensively as a promising technique for improving the performance of terrestrial wireless networks. However, in underwater cooperative acoustic networks, long propagation delays and complex acoustic channels make the conventional relay selection schemes designed for terrestrial wireless networks inefficient. In this paper, we develop a new best relay selection criterion, called COoperative Best Relay Assessment (COBRA), for underwater cooperative acoustic networks to minimize the one-way packet transmission time. The new criterion takes into account both the spectral efficiency and the underwater long propagation delay to improve the overall throughput performance of the network with energy constraint. A best relay selection algorithm is also proposed based on COBRA criterion. This algorithm only requires the channel statistical information instead of the instantaneous channel state. Our simulation results show a significant decrease on one-way packet transmission time with COBRA. The throughput and delivery ratio performance improvement further verifies the advantages of our proposed criterion over the conventional channel state based algorithms.

Comparing underwater MAC protocols in real sea experiments

Underwater acoustic networks (UANs) have drawn significant attention from both academia and industry in recent years. Even though many underwater MAC protocols have been proposed and studied based on simulations and theoretical analysis, few work has been conducted to test and evaluate these protocols in a multi-hop real sea experiment. Due to the harsh acoustic channel condition caused by complex multi-path environment, fast varying acoustic channel and heterogenous channel quality, current simulators can hardly tell us how the protocols work in the real world. Along this direction, we conduced real sea experiments at Atlantic Ocean with 9 nodes deployed forming a multi-hop string network. In this experiment, the performance of three representative MAC protocols, random access based UW-Aloha, handshaking based SASHA, and scheduling based pipelined transmission MAC (PTMAC) are compared and analyzed at both packet behavior and node behavior levels. The end-to-end performance of these three protocols are also tested and studied in terms of throughput, delay, and packet delivery ratio. From field experiment results, the high packet loss rate and significant channel asymmetry, temporal and spatial transmission range uncertainty and delayed data transmissions are discovered to have evidential effects on the MAC performance. We provide some inspirations to address these observed issues in MAC design for real multi-hop networks.

CT-MAC: a MAC protocol for underwater MIMO based network uplink communications

Due to high bandwidth-efficiency, Multiple-Input Multiple-Output (MIMO) has emerged as a promising technique to address the low bandwidth challenges in underwater acoustic networks. Although extensive research has been conducted at the physical layer for underwater MIMO communications, the corresponding medium access control (MAC) is still largely unexplored, which makes it difficult to apply the existing underwater MIMO technologies in real applications. In this paper, we propose a distributed MAC protocol, called Coordinated Transmission MAC (CT-MAC), for underwater MIMO based network uplink communications. In CT-MAC, an efficient coordination scheme among immediate neighbors is designed for channel competition, which can avoid flooding overhead in the network. This scheme could also effectively address the long propagation delay problem and the collisions among long control packets in underwater acoustic communications. Protocol performance in terms of throughput and energy efficiency is evaluated via simulations which show significant improvements over handshaking based and random access based MIMO MAC approaches.

RSS-based secret key generation in underwater acoustic networks: advantages, challenges, and performance improvements

Due to the broadcast nature of acoustic channel, underwater acoustic networks (UANs) face threats of eavesdropping and fake data injection. How to secure acoustic communications in a UAN is becoming an important but challenging topic. Among different secret key approaches, received-signal-strength (RSS)-based key generation is particularly appealing, as it can eliminate the need to deploy an additional key distribution center, making it a more attractive method than conventional symmetric key cryptography in resource-constrained UANs. A variety of RSSbased key generation approaches have been designed for wireless radio networks. However, no attempt has been made to evaluate their performance in underwater environments. In this article, we provide an overview of the advantages of RSS-based key generation and explore the major challenges from the unique features of underwater systems through experiment results of sea trails. Meanwhile, we discuss viable solutions to improve the performance of RSS-based key generation in oceans.

Evaluating selective ARQ and slotted handshake based access in real world underwater networks

Medium Access Control (MAC) is an essential component of protocol stacks in Underwater Acoustic Networks (UANs). Numerous dedicated UAN MAC protocols have been proposed and studied via analysis and simulations. However, limited work has been done on evaluating these protocols in real ocean environments. To achieve a better understanding on how MAC protocols perform in real world UANs, we implemented Selective ARQ and Slotted Handshake based Access (SASHA) on UAN nodes. SASHA embraces some most essential and representative techniques in UAN MAC design, including selective ARQ, time slotting, handshake and collision avoidance. Moreover, a sea test was conducted at Atlantic Ocean to evaluate the performance of SASHA. With the experimental data, we are able to study how the aforementioned techniques affect the performance of SASHA. we also analyze the hop-by-hop and end-to-end behavior of SASHA. Specifically, we investigate the transmission delay and queuing delay of a data packet on one hop. From the findings, some issues are discovered and the corresponding design guidelines are emerged.

An efficient MAC protocol for underwater multi-user uplink communication networks

Multi-user uplink transmission has been proved to be a promising technique for spectrum-efficient communications. However, due to the unique features of underwater acoustic networks (UANs), such as the long propagation delay, low transmission rate and long preamble of acoustic modems, conventional medium access control (MAC) protocols proposed for terrestrial multi-user uplink communications need an overhaul to work efficiently in UANs. In this paper, we carefully consider these features and propose a new MAC protocol, called the competitive transmission MAC (CT-MAC), for underwater multi-user uplink networks. In CT-MAC, we aim to improve the channel utilization and energy efficiency of a network by using a parallel competition mechanism. With parallel competitions, the control packets produced by each user only need to reach the direct neighbors with a low transmission power to save energy. Meanwhile, the data generated by users in different time slots can join the competition transmission in parallel to improve the channel utilization. In addition, we propose two competition schemes for CT-MAC to achieve both the short-term and the long-term fairness in different network conditions. The theoretical analysis and simulation results illustrate that compared with the conventional multi-user uplink MAC protocols, CT-MAC can achieve higher channel utilization and much lower end-to-end delay in UANs, while maintaining comparable energy efficiency.

RISM: An efficient spectrum management system for underwater cognitive acoustic networks

Cognitive acoustic (CA) is emerging as a promising technique for environment-friendly and spectrum-efficient underwater acoustic networks (UANs). Due to the unique features of UANs, traditional spectrum management systems used for radio networks need an overhaul to work efficiently in underwater environments. In this paper, we propose a receiver-initiated spectrum management (RISM) system for underwater cognitive acoustic networks (UCANs). RISM seeks to significantly improve the performance of UANs through a collaboration of the physical layer and medium access control (MAC) layer. This system features collaborative spectrum sensing, efficient spectrum sharing and advanced spectrum decision algorithms. It aims to provide collision-free data transmissions and efficient spectrum utilization for CA users, while avoiding harmful interference with both “natural acoustic systems”, such as marine mammals, and “artificial acoustic systems”, like sonar users and other UANs. Simulation results show that, RISM can effectively operate in both tree topology and partially connected mesh topology networks and achieve collision-free data transmissions.

Traffic estimation based receiver initiated MAC for underwater acoustic networks

Due to the long preamble problem in underwater acoustic networks (UANs), traditional sender initiated handshaking MAC protocols are facing high overhead of control messages. To mitigate this problem, we propose a traffic estimation based receiver initiated MAC (FERI MAC) for UANs. In FERI MAC, a traffic prediction based adaptive data polling approach is used to help receiver request the data from neighbors at the right time. Via simulations, we evaluate the performance of FERI MAC, in terms of energy efficiency, channel utilization and one-hop delivery delay. FERI MAC shows a stable energy efficiency and channel utilization with arbitrary network traffic patterns. Our results also illustrate that, compared with existing receiver initiated MAC protocols, FERI MAC can achieve a higher energy efficiency while with some delay penalty. This confirms the strength of FERI MAC for delay tolerant underwater applications.

Aqua-Sim Next Generation: A NS-3 Based Simulator for Underwater Sensor Networks

Underwater Sensor Networks (UWSN) have grown immensely in popularity and research over the past decade. Alongside this comes the necessity to observe and test proposed work in di↵erent scenarios without the requirement of a large high-budget testbed. Simulation software becomes very desirable in this case to depict real-world design, architecture, and algorithms in an expansive, open-source tool. Aqua-Sim [1] was originally created in 2009 as an UWSN extension to NS-2 [2]. Aqua-Sim was created to meet the needs of researchers through constructing a highly controlled environment. With multiple version iterations of Aqua-Sim, additional protocols and features have been added. AquaSim 2.0 [3] reconstructed the design of this simulator to more closely match the code layout and functionality of real-world design. In this work, we further improve the quality of the overall architecture and design of Aqua-Sim 2.0 by creating a third version of Aqua-Sim named Aqua-Sim: Next Generation. Due to the shortcomings of NS-2, we first transition Aqua-Sim 2.0 to NS-3 [4]. The primary reasons for this core simulator shift include better memory management and one common programming language. Coupled with other improvements, this will lead to overall performance enhancement. Second, we port all preexisting protocols to AquaSim: Next Generation, to ensure continuous functionality. And finally, we implement additional features which illustrate enhanced underwater capabilities and phenomenons.