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Underwater Sensor Networks:Applications and Challenges

Jun-Hong Cui

Computer Science & Engineering

University of Connecticut

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Why Underwater? The Earth is a water planet

– About 2/3 of the Earth covered by oceans• Uninhabited, largely unexplored• A huge amount of (natural) resources to discover

Many potential applications– Long-term aquatic monitoring

• Oceanography, marine biology, deep-sea archaeology, seismic predictions, pollution detection, oil/gas field monitoring …

– Short-term aquatic exploration• Underwater natural resource discovery, hurricane

disaster recovery, anti-submarine mission, loss treasure discovery …

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What are the Application Requirements?

Desired properties– Unmanned underwater exploration

– Localized and precise data acquisition for better knowledge

– Tetherless underwater networking for motion agility/flexibility

– Scalable to 100’s, 1000’s of nodes for bigger spatial coverage

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Underwater Sensor Networks (UWSNs)

The Ideal Technique:

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Application Scenario I

Submarine Detection

Buoys

Radio

Acoustic

Data Report

Sonar Transmitter

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Why UWSN for Submarine Detection? Existing Approaches

– Active or passive sonar– Cons: submarine anti-detection techniques (e.g.,

sonar absorption) make them less-effective Using UWSN

– Collaborative detection• Multiple sensors, and/or multi-modal data

– Large coverage– Timely reporting– High reusability

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Application Scenario II

Estuary Monitoring

Fresh

Salty

Fresh Water Current

Salty Water Current

BuoyancyControl

BuoyancyControl

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Why UWSN for Estuary Monitoring? Existing Approaches

– Ship tethered with chains of sensors moves from one end to the other

– Cons: no 4D data, either f(x, y, z, fixed t), or f(fixed (x, y, z), t); and cost is high

Using UWSN– Easily get 4D data, f(x, y, z, t), sensors move– Reduce cost significantly– Increase coverage– Have high reusability

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Research Issues (I)

Sensor node system design– Sensing, computing, communication integration – Power management: energy saving, life time

Autonomous network system design – Communication, multiple access– Routing, forwarding, reliable transfer– Localization, synchronization– Security, robustness– Energy efficiency

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Research Issues (II)

Applications and data management– Application classification & characterization– Data sampling, structure, storage

Collaborative estimation & detection– Data fusion, dissemination, tracking

Modeling, simulation, evaluation– Network simulator– Sensor node simulator

Hardware, middleware, software design

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Research Personnel Sensor Network and Systems research

– Jun-Hong Cui, Computer Science & Engineering (Director)– Yunsi Fei, Electrical & Computer Engineering– Jerry Zhijie Shi, Computer Science & Engineering– Bing Wang, Computer Science & Engineering– Peter Willett, Electrical & Computer Engineering– Shengli Zhou, Electrical & Computer Engineering (Co-director)

Algorithmic and Performance support– Reda Ammar, Computer Science & Engineering– Lanbo Liu, Civil & Environmental Engineering– Sanguthevar Rajasekaran, Computer Science & Engineering

Context and Applications consultation– Amvrossios Bagtzoglou, Civil & Environmental Engineering– Thomas Torgersen, Marine Sciences

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UWSN Lab @ UCONN

http://uwsn.engr.uconn.edu/

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Networking Issues in UWSNs

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Underwater Transmission Characteristics

Narrow bandwidth channels– High-frequency waves rapidly absorbed by water

radio not applicable in water– Must use acoustic channels - low bandwidth, fading

High attenuation– Bandwidth X Range product = 40 Kbps x Km– Very low compared to RF channels (1:100)

• 802.11b/a/g yields up to 5Mbps x Km

Very slow acoustic signal propagation– 1.5x103 m / sec vs. 3x108 m / sec– Causes large propagation delay

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State-of-Art Underwater Acoustics

Reported by Modulation Method Bandwidth Bandwidth Carrier Data Rate Range

Kaya&Yauchi,Oceans'89 16QAM 125kHz 1000kHz 500kbps 60mJones et al.,Oceans'97 DPSK 10kHz 50kHz 20kbps 1kmCapellano et al.,Oceans'97 BPSK 0.2kHz 7kHz 0.2kbps 50km

Courtesy: Kilfoyle & Baggeroer

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Research Challenges UnderWater Acoustic (UW-A) channel:

– Narrow band: hundreds of kHZ at most– Huge propagation latency– High channel error rate

Random topology and sensor node mobility (1--1.5m/s due to water current)

– Existing protocols in terrestrial sensor networks assume stationary sensor nodes;

– In mobile sensor networks, these protocols weakened

Mobility & UW-A channel limitations open the door to very challenging networking issues

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UWSN Protocol Stack

UWSNs must require:– Reliable data transfer (tolerating high error-prone

acoustic channels)– Efficient data delivery (should be energy-efficient) – Localization (for geo-routing or meaningful data) – Time synchronization (for sleep cycle schedule, multiple

access protocol schedule, etc)– Efficient multiple access (sensors are densely deployed)

Design Objective: – Build efficient, reliable, and scalable UWSNs

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Reliable Data Transfer TCP like end-to-end approach does not work

– Large propagation delay large end-to-end delay large bandwidth x delay product– High error-prone acoustic channels high loss rate

Pure ARQ type (hop-by-hop) does not work well– Performance degraded because of frequent ACKs

Possible solutions: – FEC-based hop-by-hop approach with infrequent ACKs

• Have obtained some initial results

– Network coding utilizing multiple paths for robustness• Have started investigation on this

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Efficient Multi-Hop Routing Existing routing protocols in terrestrial WSNs do

not work well in UWSNs – Node mobility changes node neighborhood– Directed diffusion requires too frequent route

enforcement Existing routing protocols in terrestrial ad-hoc

networks do not work well in UWSNs– Proactive: too much overhead to maintain updated topo– Passive: flooding is not efficient, also causes contention

Possible solution: location-based routing– VBF: Vector-Based Forwarding (Networking’06)– More work to improve energy efficiency & robustness

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Localization & Time-Synchronization GPS-free and Mobility are main challenges Existing GPS-free localization & time-sync

schemes (range-based & range-free) – Nodes are usually immobile– Multi-hop schemes usually suffer from

• Poor precision due to high error probability & dynamic network topologies

Considering underwater GPS-like approach – Using multiple surface reference points

Range-based approaches are possible– Need dedicated devices to measure distances

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Efficient Multiple Access Challenges: Large prop. delay & low bandwidth Examine existing MAC protocols

– Scheduled protocols• TDMA: ? CDMA: ?• FDMA: not feasible due to narrow band

– Contention-based protocols• Random access: ALOHA/Slotted ALOHA ?• Carrier sensing access: CSMA (meaningless in UWSN) • Collision avoidance with handshaking: MACA/MACAW ?

Suggested solutions– A cluster architecture:

• CDMA between clusters, TDMA inside clusters

– An adaptive approach • Exploring random access & handshaking

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Acoustic Physical Layer The key is to increase bandwidth

– Current limit: range x rate < 40 kbps x km Explored approaches:

– FSK, PSK, QAM Advanced technologies to examine

– MIMO transmission– Multicarrier communication

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UWSN Lab Testbed @ UCONN

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Testbed Overview

Equipment List:

– Acoustic modem– Underwater speaker– Hydrophone– Sound mixer– Sound receiver– Speaker/microphone– Aquarium

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Micro-Modem Designed and Implemented by WHOI (Woods Hole

Oceanographic Institution)

A Low-power Acoustic Modem

Based on the TMS320C5416 DSP from TI

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Receivers/Speakers

Control-1 150 Watt Two-Way Loudspeaker System

– Good performs in recording studios– Low distortion reproduction – Frequency Range: 70 Hz - 20 kHz

Sony STRDE197 Stereo Receiver

Sennheiser MKE 300 Microphone

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Underwater Speakers

Frequency range: 200 Hz to 32 KHz

Directional at higher frequencies

A completely passive, non-powered device

Can be used as an air speaker or a receive hydrophone

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Aquarian Hydrophone

Output: – 300mW, short-circuit-proof– 3.5mm (mini) phone jack

Power Requirements: – 7mA quiescent current

Usable Frequency Response:

– 20Hz - 100KHz

Polar Response:

– Omni directional

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Behringer SL2442FXPRO Eurodesk 24-Channel Mixer

Ultra-Pure Sound and Crystal-Clear Audio

99 special sound effects: – Reverbs– Delays– Tube distortion– And More!

24 channels

Could simulate different underwater environments

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Water Test Setting Distance between the underwater speaker and hydrophone: 1 meter

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Conclusions and Future Work UWSN

– Is a challenging and promising new area– Requires interdisciplinary efforts from

• Acoustic communication• Signal processing• Network design

Future Work– A long to-do list …– Conducting research at different layers

• reliable transfer, routing, localization, multiple access, acoustic communication, …

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