Northeastern University Wireless Networks and Embedded Systems Lab

Underwater Networks

The Internet Underwater: An IP-compatible Protocol Stack for Commercial Undersea Modems

Recent underwater sensor network research has focused on developing physical, medium access control, and network layer protocols to enable high data rate, energy-efficient and reliable acoustic communications. However, it is now essential to design and standardize architectures that will enhance the usability and interoperability of underwater networks. In this work we propose a networking architecture to efficiently provide interoperability with traditional TCP/IP protocol stacks for commercial underwater modems.

Internet Underwater

The proposal is based on an adaptation layer located between the data link layer and the network layer, such that the original TCP/IP network and transport layers are preserved unaltered to the maximum extent. The adaptation layer performs header compression and data fragmentation to guarantee energy efficiency. Furthermore, the proposed architecture includes mechanisms for auto-configuration based on router proxies that can avoid human-in-the-loop and save energy when broadcast is needed. The proposed architectural framework was implemented as a Linux device driver for a commercial underwater network modem SM-75 by Teledyne Benthos. Testing and simulation results illustrate that the architecture efficiently provides interoperability with TCP/IP.

The Internet Underwater architecture was tested extensively at Lake Erie, a few miles south of downtown Buffalo. We were able to demonstrated an IP-level connectivity by successfully sending and receiving a message from the deployed underwater sensor node to the boarder router using acoustic signals, which was relayed through a 3G/4G broadband Internet to a laptop and a smart-phone, connected to the traditional Internet.

Underwater Analog Network Coding


The performance of multi-hop underwater acoustic network is known to be limited by the long propagation delays and by the limited bandwidth of the underwater acoustic (UW-A) channel. Recent work on analog network coding (ANC) has shown that significant throughput gains can be achieved in multi-hop wireless networks. However, implementing ANC for UW-A communications is very challenging as the UW-A channel is severely affected by multipath.

Underwater Networking Protocols

We propose CE-CDMA, a collision-enabling direct-sequence code-division multiple-access scheme for multi-hop underwater acoustic sensor networks (UW-ASNs). In CE-CDMA two nodes, separated by two hops, are assigned the same code-division channel (i.e., spreading code) to communicate concurrently. The transmission of packets by the two nodes will therefore collide at the intermediate (relay) node. However, we show that by exploiting a priori information, i.e., the interfered packet previously received from one of the nodes, and an adaptive RAKE receiver that jointly estimates the two multipath-affected channels, the relay node can cancel the interference before decoding the packet of interest. Experiments demonstrate that for a 1−2dB tradeoff in signal-to-noise ratio (SNR) the proposed scheme can potentially improve the channel utilization of a unidirectional multi-hop linear network by up to 50%. We also outline the basic functionalities of a MAC protocol (CE-MAC) designed to support the proposed scheme.

Securing Underwater Acoustic Communications through Analog Network Coding

The underwater acoustic (UW-A) channel is considered one of the most challenging environments to establish reliable and secure communications. We propose a new secure underwater acoustic communication scheme designed to let a user (Alice) transmit a confidential message to another user (Bob) in the presence of an eavesdropper (Eve). A typical approach in conventional wireless physical-layer security is to rely on a friendly jammer to jam Eve through artificial noise (AN). Instead, for the first time, we propose a secure underwater communication scheme that relies on cooperative friendly jamming through CDMA-based analog network coding (ANC). The friendly cooperative jammer transmits information using the same spreading code used in the legitimate Alice-Bob link. The information transmitted by the cooperative jammer is known a priori to Bob, but not to Eve. Although the jammer’s packet will also interfere at Bob, we show that after jointly estimating the two multipathaffected channels, Bob can suppress the interfering packet and decode Alice’s packet, while Eve cannot.

Testbed for Underwater Networks

We also formulate the problem of joint optimal selection of friendly jammer and power allocation (for Alice and the jammer) that minimize Eve’s capability of intercepting the signal while guaranteeing a predefined level of quality of service (QoS) for Bob. The proposed scheme is implemented in a testbed based on Teledyne Benthos Telesonar SM-75 underwater modems and tested extensively in Lake LaSalle. Experiments and simulations demonstrate that, for a given energy budget, the proposed scheme can guarantee much higher bit error rate (BER) at Eve, while creating minimal BER disturbance at Bob, compared to the AN-aided approach.

A Low-cost Distributed Networked Localization and Time Synchronization Framework for Underwater Acoustic Testbeds

The underwater acoustic (UW-A) channel is characterized by slow propagation of acoustic waves, limited bandwidth, high transmit energy consumption, high and variable propagation delays, motion-induced Doppler spread, frequency selective fading and multipath. These characteristics pose severe challenges towards designing robust localization schemes that can achieve the following desirable properties: i) high accuracy, ii) fast convergence, iii) wide coverage, iv) low communication cost, and v) high scalability.

Localization and Time Synchronization Framework

Localization and time synchronization are both essential services for Internet-connected underwater acoustic testbeds. Although the two are mutually coupled, they are often treated separately. We propose a new low-cost distributed networked localization and time synchronization framework for underwater acoustic sensor network testbeds. The proposal is based on decoupling the two problems and solving first the time synchronization then localization using the same set of messages, i.e., with no additional overhead. A coarse, followed by a fine-grained localization algorithms are adopted to accurately estimate the location of an unknown node. The protocol is robust to noisy range measurements. The proposed scheme is implemented in a testbed based on Teledyne Benthos Telesonar SM-975 underwater modems and tested extensively in Lake LaSalle at the University at Buffalo. Experiments and simulations in terms of root mean square error (RMSE) demonstrate that the proposed scheme can achieve a high accuracy for a given energy budget, i.e., for a given number of message exchanges.

Stochastic Channel Access with Spatial and Temporal Interference Uncertainty

Designing medium access control protocols for underwater acoustic sensor networks (UW-ASNs) is a major challenge because of the spatial and temporal interference uncertainty caused by asynchronous transmissions and by the low propagation speed of sound, respectively. To deal with this uncertainty, in this project we propose a queue-aware distributed access scheme, in which each transmitter optimizes a transmission probability profile based on which it decides whether to transmit or to enqueue its packets over a series of time slots based on a statistical characterization of interference obtained through its past observations.

Testbed for Underwater Networks

To model the effect of unaligned interference, we propose a so-called L-measurement method, where interference is measured at multiple instants of time in each time slot to capture the effects of temporal uncertainty. We mathematically formulate the problem of dynamic transmission strategy optimization and propose an iterative distributed solution algorithm designed based on a best-response strategy. At each iteration, each node individually solves a nonconvex optimization problem of logarithmic complexity with the number of time slots jointly considered. We evaluate the performance of the proposed distributed solution by comparing it to several other distributed schemes and to the global optimum obtained through a newly-developed centralized globally optimal solution algorithm.

A Hybrid MAC Protocol with Channel-dependent Optimized Scheduling for Clustered Underwater Acoustic Sensor Networks

We propose a novel optimal time slot allocation scheme for clustered underwater acoustic sensor networks that leverages physical (PHY) layer information to minimize the energy consumption due to unnecessary retransmissions thereby improving network lifetime and throughput. To reduce the overhead and the computational complexity, we employ a two-phase approach where: (i) each member node takes a selfish decision on the number of time slots it needs during the next intra-cluster cycle by solving a Markov decision process (MDP), and (ii) the cluster head optimizes the scheduling decision based on the channel quality and an urgency factor. To conserve energy, we use a hybrid medium access scheme, i.e., time division multiple access (TDMA) for the intra-cluster communication phase and carrier sense multiple access with collision avoidance (CSMA/CA) for the cluster head-sink communication phase.

Hybrid MAC

The proposed MAC protocol is implemented and tested on a real underwater acoustic testbed using SM-75 acoustic modems by Teledyne Benthos. Simulations illustrate an improvement in network lifetime. Additionally, simulations demonstrate that the proposed scheduling scheme with urgency factor achieves a throughput increase of 28% and improves the reliability by up to 25% as compared to the scheduling scheme that neither use MDP nor optimization. Furthermore, testbed experiments show an improvement in throughput by up to 10% along with an improvement in reliability.

Cross-layer Protocols on Underwater Networks with MIMO Links


The MIMO transceiver technology has attracted considerable attention in radio-frequency (RF) communications since the early 2000’s. Instead of mitigating the impact of multipath fading, MIMO systems are able to exploit rich scattering and multipath fading to provide higher spectral efficiencies without increasing power and bandwidth. Hence, MIMO technology has the potential to take advantage of the rich scattering and multipath of the underwater acoustic environment to increase data transmission rates and improve link reliability.

Networking on Underwater Acoustic MIMO Links

The feasibility of MIMO systems and related spatial coding and modulation was tested for underwater acoustic communications and a significant performance improvement was demonstrated compared with the conventional SISO system architecture. However, the existing literature focuses mostly on experimental study, with very limited previous work on system performance analysis. Importantly, no previous effort has studied the impact of MIMO transceivers on the design of higher-layer communication protocols. Moreover, there is a growing literature on physical layer and coding aspects of underwater MIMO communications. However, to the best of our knowledge, no previous paper has addressed the design of protocols optimized for underwater MIMO communications.

Modeling Underwater Acoustic Channels in Short-range Shallow Water Environments


Statistical modeling of radio frequency (RF) wireless communication in air has been well studied, and widely accepted channel models are available. Because of the high complexity of the underwater acoustic channel it is very difficult to come up with a single statistical channel model that can be used to capture different underwater channel environments. Moreover, conducting underwater experiments and collecting data is very costly. Accordingly, there are only a few joint venture underwater experiments conducted to collect underwater data, which are not readily available. In addition to that, only very limited work has been done to study the characteristics of the underwater acoustic channel in shallow water environments.

Underwater Networking Protocols

We analyze the statistical channel properties of short to very shortrange shallow water communication environments based on real channel measurements taken in a water-tank, a swimming pool, very shallow and shallow lakes. More specifically, we estimate the channel impulse response (CIR), the probability density function (PDF) of channel fading and fit to Rayleigh, Nakagami, Weibull, Rician and Beta distributions. We compare the ‘goodness-of-fit’ of these distributions based on the Kullback-Leibler (KL) divergence criteria. From our experimental results, we confirm that the shallow water acoustic channel is highly time-varying and does not necessarily follow a Rayleigh distribution. Instead, we observe that in very-shallow water lake environments the channel fading exhibits close-to Weibull or Rician distribution. On the other hand, in shallow water lake the channel fading behavior is better captured by a Beta distribution.

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Related Publications

  • E. Demirors, J. Shi, R. Guida, T. Melodia, "SEANet G2: Toward a High-Data-Rate Software- Defined Underwater Acoustic Networking Platform," in Proc. of ACM Conference on Underwater Networks and Systems (WUWNet), Shanghai, China, October 2016.

  • E. Demirors, T. Melodia, "Chirp-Based LPD/LPI Underwater Acoustic Communications with Code-Time-Frequency Multidimensional Spreading," in Proc. of ACM Conference on Underwater Networks and Systems (WUWNet), Shanghai, China, October 2016.

  • E. Demirors, G. Sklivanitis, G.E. Santagati, T. Melodia and S. N. Batalama, "Design of A Software-defined Underwater Acoustic Modem with Real-time Physical Layer Adaptation Capabilities," in Proc. of ACM Intl. Conf. on Underwater Networks & Systems (WUWNet), Rome, Italy, November 2014. [pdf] [bibtex]

  • H. Kulhandjian and T. Melodia, "Modeling Underwater Acoustic Channels in Short-range Shallow Water Environments," in Proc. of ACM Intl. Conf. on Underwater Networks & Systems (WUWNet), Rome, Italy, November 2014. [pdf] [bibtex]

  • H. Kulhandjian and T. Melodia, "A Low-cost Distributed Networked Localization and Time-synchronization Framework for Underwater Acoustic Testbeds," in Proc. of IEEE Underwater Communications Conf. and Workshop (UComms), Sestri Levante, Italy, September 2014. [pdf] [bibtex]

  • H. Kulhandjian, T. Melodia, D. Koutsonikolas, "Securing Underwater Acoustic Communications through Analog Network Coding," in Proc. of IEEE Intl. Conf. on Sensing, Communication, and Networking (SECON), Singapore, June 2014. [pdf] [bibtex]

  • Y. Sun, T. Melodia, "The Internet Underwater: An IP-compatible Protocol Stack for Commercial Undersea Modems," in Proc. of ACM Conference on Underwater Networks and Systems (WUWNet), Kaohsiung, Taiwan, November 2013. Best Student Paper Award [pdf] [bibtex]

  • J. Jagannath, A. Saji, H. Kulhandjian, Y. Sun, E. Demirors, T. Melodia, "A Hybrid MAC Protocol with Channel-dependent Optimized Scheduling for Clustered Underwater Acoustic Sensor Networks," in Proc. of ACM Conference on Underwater Networks and Systems (WUWNet), Kaohsiung, Taiwan, November 2013. [pdf] [bibtex]

  • H. Kulhandjian, T. Melodia, D. Koutsonikolas, "CDMA-based Analog Network Coding through Interference Cancellation for Underwater Acoustic Sensor Networks," in Proc. of ACM Intl. Conf. on UnderWater Networks and Systems (WUWNet), Los Angeles, CA, USA, November 2012. [pdf] [bibtex]

  • Z. Guan, T. Melodia, D. Yuan, "Stochastic Channel Access for Underwater Acoustic Networks with Spatial and Temporal Interference Uncertainty," in Proc. of ACM Intl. Conf. on UnderWater Networks and Systems (WUWNet), Los Angeles, CA, USA, November 2012. [pdf] [bibtex]

  • L. C. Kuo, T. Melodia, "Cross-layer Routing on MIMO-OFDM Underwater Acoustic Links," in Proc. of IEEE Conf. on Sensor, Mesh and Ad Hoc Communications and Networks (SECON), Seoul, South Korea, June 2012. [pdf]

  • L. Kuo, T. Melodia, "Distributed Medium Access Control Strategies for MIMO Underwater Acoustic Networking," in IEEE Transactions on Wireless Communications, vol. 10, no. 8, pp. 2523-2533, August 2011. [pdf]

  • L. Kuo, T. Melodia, "Tier-Based Underwater Acoustic Routing for Applications with Reliability and Delay Constraints," in Proc. of IEEE Intl. Workshop on Wireless Mesh and Ad Hoc Networks (WiMAN), Maui, HI, USA, July 2011. [pdf]

  • L. Kuo and T. Melodia, "Medium Access Control for Underwater Acoustic Sensor Networks with MIMO links," in Proc. of ACM Intl. Conf. on Modeling, Analysis and Simulation of Wireless and Mobile Systems (MSWiM), Tenerife, Canary Islands, Spain, October 2009. [pdf]