Underground Sensor Networks

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• Chapter 17
• Underground Sensor Networks

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Wireless Underground Sensor NetworksI.F.
Akyildiz and Erich Stuntebeck, Wireless
Underground Sensor Networks Research
Challenges, Ad Hoc Networks (Elsevier) Journal,
Nov. 2006.
Sink

• Soil Condition
• Sensor
• Water
• Salinity
• Temperature

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APPLICATIONS

• Soil condition monitoring for agriculture,
  landscaping
• Toxic substance monitoring near wells and
  aquifers
• Earthquake and landslide prediction and
  monitoring
• Security underground pressure sensors can be
  used to detect intruders
• Coal Mines
• Diamond Mining

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FURTHER APPLICATIONS

• Sports field monitoring
• Golf courses
• Soccer fields
• Baseball fields
• Grass tennis courts

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FURTHER APPLICATIONS

• Infrastructure monitoring
• Pipes
• Electrical wiring
• Liquid storage tanks, underground fuel tanks,
  septic tanks
• Monitoring the structural health
• Building, bridge, or dam
• Border Patrol and Security

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Major Undetected Pipe Leak in 2006New York
Times, March 15, 2006

• The largest oil spill occurred on the tundra of
  Alaska's North Slope
• 270 K gallons of thick crude oil spilled over two
  acres
• Oil escaped through a pinprick-size hole in a
  corroded 34-inch pipe
• Most of the oil seeped beneath the snow without
  attracting the attention of workers monitoring
  alarm systems
• The spill went undetected for as long as five days

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Underground Pipeline Monitoring
Sink
Sensor (powered by fluid flow)
Flow Direction
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Existing Underground Sensor Technology

• Large number of sensors wired to an above-ground
  data-logger, which uses wires, cellular, or
  long-range single-hop wireless for backhaul of
  data
• PROBLEM
• Wired sensors are costly to deploy
• Datalogger units are expensive
• Above-ground antennas and equipment may be
  unsightly
• SOLUTION Underground wireless
  sensor nodes

Datalogger
Moisture sensor
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Advantages of WUSN

• Concealment (versus visibility)
• Ease of deployment
• Timeliness of the data
• Reliability
• Potential for coverage density

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Network Topology Examples
Multi-Depth Terrestrial Hybrid
Single-Depth
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UNDERGROUND CHANNEL CHALLENGES

• Dynamic Channel
• Soil properties highly spatially variant
  (sand/clay makeup, water content)
• Temporal variance in the channel due to rain,
  irrigation

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Underground Channel Challenges

• Power Constraints
• Difficult/impossible to change the batteries for
  underground devices
• High radio power necessary due to extreme path
  losses
• Low data rate
• Channel conditions are best at low carrier
  frequencies
• Less bandwidth is available at lower frequencies

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Underground Channel Challenges

• Antenna Design
• Extremely Lossy Environment
• Strong FEC needed to help overcome weak signals,
  but must not use excessive energy in processing
• A comprehensive channel model for the underground
  does not yet exist

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Antenna Challenge

• Lower frequencies are necessary to achieve
  practical propagation distances of several
  meters.
• The lower the frequency used, the larger an
  antenna must be to efficiently transmit and
  receive at that frequency.
• At a frequency of 100 MHz, a quarter-wavelength
  antenna would measure 0.75 meters.
• Challenge for WUSNs!!!

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Antenna Directionality

• Omni-directional antenna or a group of
  independent directional antennas?
• A single omni-directional antenna ? challenges
• Sensors may be in different depths and common
  omni-directional antennas experience nulls in
  their radiation patterns at each end
• With a vertically oriented antenna, communication
  with devices above and below would be impaired

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Antenna Directionality

• This issue may be solved by equipping a device
  with antennas oriented for both horizontal and
  vertical communication.
• Antenna design considerations will also vary
  depending on the physical layer technology that
  is utilized.
• We have focused on EM waves here
• Open research ? Are other technologies better
  suited to this environment?

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Environmental Extremes

• Water, temperature extremes, animals, insects,
  and excavation equipment all represent threats to
  a device
• Processors, radios, power supplies, and other
  components must be resilient to these factors.
• Physical size of the sensor device should be kept
  small, as the expense and time required for
  excavation increase for larger devices.
• Battery technology must be chosen carefully

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Underground Channel Modeling Analysis
L. Li, M. C. Vuran, I. F. Akyildiz,
Characteristics of Underground Channel for
Wireless Underground Sensor Networks, in Proc.
Med-Hoc-Net (Mediterranean Ad Hoc Networks)
Conference, Corfu, Greece, June 2007.
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Underground Signal Propagation Path Loss

• Path loss due to material absorption is a major
  concern when using EM waves for underground
  communication.
• Losses are determined by both
• The frequency of the wave
• The properties of the soil or rock through which
  it propagates

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Underground Signal Propagation Path Loss

• Friis equation gives us the received signal
  strength Pr in free space at a distance r from
  the transmitter

where Pt is the transmit power Gr and Gt
are the gains of the receiver and transmitter
antennae. Lo is the path loss in free
space.
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Underground Signal Propagation Path Loss

• Include a correction factor to account for the
  effect of the medium - soil

where Pt is the transmit power Gr and Gt
are the gains of the receiver and
transmitter antennae. Lo is the path loss
in free space. Lm is the additional path
loss due to soil
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Path Loss due to Soil

• Lm can be calculated by considering
• Difference of the wavelength of the signal in
  soil
• Difference in attenuation

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Underground Signal Propagation Path Loss

• Total path loss in the soil

where d is the distance in meters (m) a is the
attenuation constant. 1/m b is the phase
shift constant. radian/m
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Peplinski Principle
N. Peplinski, F. Ulaby, M. Dobson, Dielectric
Properties of Soils in the 0.3-1.3GHz Range,
IEEE Tr. in Geoscience and Remote Sensing, pp.
803-807, 1995.

• Given the 0.3-1.3GHz band, dielectric properties
  of soil can be obtained

where e is the dielectric constant of soil e
and e are the real and imaginary parts of the
dielectric constant
mv - the volumetric water content of the soil rb
- the bulk density in grams per cubic
centimeter rs - specific density of the solid
soil particles a an empirically determined
constant b, b - empirically determined
constants dependent on soil-type efw, efw -
real and imaginary parts of the relative
dielectric constant of water
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Underground Signal Propagation (Path Loss)

• Peplinski principle governs the value of the
  complex propagation constant of the EM wave in
  soil

Where a is the attenuation constant. ß is the
phase shift constant is the angular
frequency µ is the magnetic
permeability e and e are the real
and imaginary parts of the
dielectric constant

• (values dependent on dielectric
• properties of soil)

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Interpretation

• The complex propagation constant g of the EM wave
  in soil is dependent on
• operating frequency
• composition of soil in terms of sand, silt, and
  clay fractions
• bulk density
• volumetric water content
• ? Path loss also depends on these parameters.

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Simulations

• Soil composition parameters 50 sand, 15clay
  and 35 silt
• Frequency 400MHz
• Water content 5
• Distance between two sensors 3m

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Path Loss vs. Frequency and Distance
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Path Loss vs. Frequency and Distance

• Distance has an important impact on the path
  loss, which increases with increasing distance,
  d, as expected.
• Increasing operating frequency, f, also increases
  path loss, which motivates the need for lower
  frequencies for underground communication.

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Path Loss vs. Frequency and Water Content
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Path Loss vs. Frequency and Water Content

• The attenuation significantly increases with VWC
• Increase of 30dB is possible with a 20 increase
  in the VWC of the soil.

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Underground Channel Characteristics

• Reflection from ground surface
• Total path loss changes in shallow area (depth lt
  2m)
• Multi-path fading
• Two-path location dependent Rayleigh fading
  channel in shallow area (depth lt 2m)
• One-path location dependent Rayleigh fading
  channel in deep area

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Reflection from Ground Surface
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Reflection from Ground Surface

• In shallow area (depthlt2m), total path loss
  changes to

?, ? - the amplitude and phase angle of the
reflection coefficients at the reflection point
P ?(r) - the difference between two paths (r-d) ?
- the attenuation factor ? - the wavelength in
soil (2?/?)
where Lf is the total loss of two-path channel
model Lp is the path loss due to the single
path. VdB is the attenuation
factor due to the second path in dB
(10 logV)
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Multi-Path Fading

• The surface of the ground is not ideally smooth
  and, hence, not only causes reflection, but also
  refraction.
• Usually there are rocks or roots of plants in
  soil and the clay of soil is generally not
  homogeneous, which causes refraction.

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Multi-Path Fading

• The underground channel is relatively stable when
  the composition of soil is considered
• Randomness is due to the locations of the nodes
  rather than time, which still obeys the Rayleigh
  probability distribution
• The envelope of the signal from each path is
  modeled as an independent Rayleigh distributed
  random variable

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Multi-Path Fading

• One-path Rayleigh Distribution
• Two-path Rayleigh Distribution

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Bit Error Rate

• 2PSK modulation
• Channel model
• Two-path Rayleigh model (depthlt2m)
• One-path Rayleigh model (depthgt2m)
• SNR

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Bit Error Rate

• Noise -103dBm (measurements)

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Simulations

• Soil composition parameters 50 sand, 15clay
  and 35 silt
• Frequency 400MHz
• Water content 5
• Distance between two sensors 3m
• Depth 0.5m
• Transmit signal strength 10dBm

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Simulations

• One Path BER depthgt2m
• Two-Path BER depthlt2m

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One-Path BER vs. Distance and Transmitting Power
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One-Path BER vs. Distance and Transmitting Power

• The transmit power increases, the BER decreases.
• However, this decrease is minimum since even when
  the transmit power increases to 30dBm, the
  horizontal distance can be extended to 4 meters
  with the limitation that BER is below 10-3

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One-Path BER vs. Frequency and Water Content
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One-Path BER vs. Frequency and Water Content

• Volumetric Water content (VWC) has an important
  impact on the BER compared to other parameters.
• An increase from 5 to 10 results in almost an
  order of magnitude increase in BER.
• This result confirms that VWC is one of the most
  important parameters for underground
  communication.

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Two-Path BER vs. Distance and Transmitting Power
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Two-Path BER vs. Distance and Transmitting Power

• The communication distance can be extended for
  low depth applications due to the constructive
  effects of the reflected rays from the ground
  surface.

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Two-Path BER vs.Depth at Different Frequencies
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Two-Path BER vs.Depth at Different Frequencies

• There is a fluctuation on BER.
• As the burial depth increases, the fluctuation
  decreases and the BER becomes more stable
• For a particular operating frequency, there is an
  optimal depth for communication where BER is
  minimum.

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Two-Path BER vs.Frequency with Different Water
Content
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Two-Path BER vs.Frequency with Different Water
Content

• Compared to the single-path model results, higher
  VWC is acceptable when the operation frequency is
  low.

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Underground Experiments _at_ UNL
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Alternative Physical Layer Technologies
Magnetic Induction (MI)

• Multi-path fading is not an issue for MI
  communication
• Since communication is achieved by coupling in
  the non-propagating near-field, a transmitting
  device will be able to detect the presence of any
  active receivers via the induced load on the coil
  (MAC)
• Solves the issue of antenna design since
  transmission and reception is accomplished with
  the use of a small coil of wire

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Seismic WavesK. Ikrath and W. Schneider,
Communication via Seismic Waves employing 80Hz
resonant seismic transducers, IEEE Tr. on
Communications, pp.439-444, 1968.

• Successfully demonstrated in both soil and rock
  at ranges of up to 1km
• Seismic waves have many drawbacks.
• Frequencies even lower than those needed for EM
  communication are
• necessary for their propagation over any useful
  distance.
• 80 Hz carrier, and has only 3 to 5 Hz of
  bandwidth.
• Higher frequencies of seismic waves may produce
  audible coupling
• to the air, and generating seismic waves
  requires a large amount of energy.

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PHYSICAL LAYERJ. Vasquez, V. Rodriguez, D.
Reagor, Underground Wireless Communications
Using High-Temperature Superconducting
Receivers, IEEE Tr. on Applied
Superconductivity, pp. 46-53, 2004.

• Selection of an appropriate modulation scheme is
  a challenge and unexplored!!!
• Reported success using QPSK, QAM-16 and QAM-32
  modulation schemes with a 4 kHz carrier and 10
  watts of transmit power.
• A data rate of 2 kbps is achieved.

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Open research issues at the physical layer

• Electromagnetic, magnetic induction (MI), and
  seismic communication in the underground needs to
  be carried out to identify the most appropriate
  physical layer technology.
• Some combination of these technologies may be
  optimal
• A power-efficient modulation scheme suitable for
  the dynamic high-loss underground channel must be
  chosen
• Research into varying the modulation scheme
  depending on underground channel conditions is
  needed

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Open research issues at the physical layer

• The trade-off between reliability and capacity
  must be examined.
• An information theoretical study of the capacity
  of underground wireless communication channels
  is needed

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Higher Layers

• ?