1 Wireless Sensor Networks Akyildiz/Vuran Chapter 17: Underground Sensor Networks 2 Wireless Sensor Networks Akyildiz/Vuran Soil Condition Sensor - Water - Salinity - Temperature Wireless Underground Sensor Networks I.F. Akyildiz and Erich Stuntebeck, Wireless Underground Sensor Networks: Research Challenges, Ad Hoc Networks (Elsevier) Journal, Nov Sink 3 Wireless Sensor Networks Akyildiz/Vuran 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 4 Wireless Sensor Networks Akyildiz/Vuran FURTHER APPLICATIONS Sports field monitoring Golf courses Soccer fields Baseball fields Grass tennis courts 5 Wireless Sensor Networks Akyildiz/Vuran 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 6 Wireless Sensor Networks Akyildiz/Vuran Major Undetected Pipe Leak in 2006 [New 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 7 Wireless Sensor Networks Akyildiz/Vuran Sink Flow Direction Sensor (powered by fluid flow) Underground Pipeline Monitoring 8 Wireless Sensor Networks Akyildiz/Vuran 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 9 Wireless Sensor Networks Akyildiz/Vuran Advantages of WUSN Concealment (versus visibility) Ease of deployment Timeliness of the data Reliability Potential for coverage density 10 Wireless Sensor Networks Akyildiz/Vuran Network Topology: Examples Single-Depth Multi-Depth Terrestrial Hybrid 11 Wireless Sensor Networks Akyildiz/Vuran UNDERGROUND CHANNEL CHALLENGES Dynamic Channel Soil properties highly spatially variant (sand/clay makeup, water content) Temporal variance in the channel due to rain, irrigation 12 Wireless Sensor Networks Akyildiz/Vuran 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 13 Wireless Sensor Networks Akyildiz/Vuran 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 14 Wireless Sensor Networks Akyildiz/Vuran 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!!! 15 Wireless Sensor Networks Akyildiz/Vuran 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 16 Wireless Sensor Networks Akyildiz/Vuran 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? 17 Wireless Sensor Networks Akyildiz/Vuran 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 18 Wireless Sensor Networks Akyildiz/Vuran 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 Underground Channel Modeling & Analysis 19 Wireless Sensor Networks Akyildiz/Vuran 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 20 Wireless Sensor Networks Akyildiz/Vuran Underground Signal Propagation Path Loss Friis equation gives us the received signal strength P r in free space at a distance r from the transmitter where P t is the transmit power G r and G t are the gains of the receiver and transmitter antennae. L o is the path loss in free space. 21 Wireless Sensor Networks Akyildiz/Vuran Underground Signal Propagation Path Loss Include a correction factor to account for the effect of the medium - soil where P t is the transmit power G r and G t are the gains of the receiver and transmitter antennae. L o is the path loss in free space. L m is the additional path loss due to soil 22 Wireless Sensor Networks Akyildiz/Vuran Path Loss due to Soil L m can be calculated by considering Difference of the wavelength of the signal in soil Difference in attenuation 23 Wireless Sensor Networks Akyildiz/Vuran Underground Signal Propagation Path Loss Total path loss in the soil where d is the distance in meters (m) is the attenuation constant. 1/m is the phase shift constant. radian/m 24 Wireless Sensor Networks Akyildiz/Vuran Peplinski Principle Given the GHz band, dielectric properties of soil can be obtained N. Peplinski, F. Ulaby, M. Dobson, Dielectric Properties of Soils in the GHz Range, IEEE Tr. in Geoscience and Remote Sensing, pp , where e is the dielectric constant of soil and are the real and imaginary parts of the dielectric constant m v - the volumetric water content of the soil b - the bulk density in grams per cubic centimeter s - specific density of the solid soil particles an empirically determined constant , - empirically determined constants dependent on soil-type fw, fw - real and imaginary parts of the relative dielectric constant of water 25 Wireless Sensor Networks Akyildiz/Vuran Underground Signal Propagation (Path Loss) Peplinski principle governs the value of the complex propagation constant of the EM wave in soil: Where: is the attenuation constant*. is the phase shift constant* is the angular frequency is the magnetic permeability and are the real and imaginary parts of the dielectric constant *(values dependent on dielectric properties of soil) 26 Wireless Sensor Networks Akyildiz/Vuran Interpretation The complex propagation constant 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. 27 Wireless Sensor Networks Akyildiz/Vuran Simulations Soil composition parameters: 50% sand, 15%clay and 35% silt Frequency: 400MHz Water content: 5% Distance between two sensors: 3m 28 Wireless Sensor Networks Akyildiz/Vuran Path Loss vs. Frequency and Distance 29 Wireless Sensor Networks Akyildiz/Vuran 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. 30 Wireless Sensor Networks Akyildiz/Vuran Path Loss vs. Frequency and Water Content 31 Wireless Sensor Networks Akyildiz/Vuran 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. 32 Wireless Sensor Networks Akyildiz/Vuran Underground Channel Characteristics Reflection from ground surface Total path loss changes in shallow area (depth < 2m) Multi-path fading Two-path location dependent Rayleigh fading channel in shallow area (depth < 2m) One-path location dependent Rayleigh fading channel in deep area 33 Wireless Sensor Networks Akyildiz/Vuran Reflection from Ground Surface 34 Wireless Sensor Networks Akyildiz/Vuran Reflection from Ground Surface In shallow area (depth2m Two-Path BER: depth