31-05-2012, 01:46 PM
Node Positioning in ZigBee Network Using Trilateration Method
Based on the Received Signal Strength Indicator (RSSI)
[attachment=23688]
Abstract
This paper investigates the possibility of implementing node positioning in the
ZigBee wireless sensor network by using a readily available Received Signal Strength
Indicator (RSSI) infrastructure provided by the physical layer of 802.15.4 networks. In this
study the RSSI is converted to the distance providing the basis for using the trilateration
methods for location estimation. The software written in C# is used to solve the trilateration
problem and the final results of trilateration methods are mapped using Google maps.
Providing node positioning capability to the ZigBee network offers an enormous benefit to
the Wireless Sensor Networks applications, possibly extending the functionality of existing
software solution to include node tracking and monitoring without an additional hardware
investment.
Keywords: Positioning, ZigBee, Trilateration, Received Signal Strength Indicator (RSSI)
1. Introduction
Satellite-based Global Positioning System (GPS) has been used previously alongside the GIS
applications to provide the spatial information to map real world location to the computer
representation, thus making the planning and management of resources more informed. The GPS is a
space-based global navigation satellite system that provides reliable location and time information in
all weather conditions and at all times and anywhere on or near the Earth when there is an unobstructed
line of sight to four or more GPS satellites. The positioning system that uses the GPS receiver adds
cost to the solution infrastructure, increases power requirements to operate the on-board receiver chip
and increases infrastructure maintenance points, making the deployment of such solution not
commercially viable.
Wireless Sensor Networks, particularly the ZigBee devices offer a more viable positioning
method that uses existing infrastructure without escalating the operational expenses. This paper
explains the method used to do node positioning in the ZigBee network.
Node Positioning in ZigBee Network Using Trilateration Method
Based on the Received Signal Strength Indicator (RSSI) 49
2. Problem Statement
The ZigBee node's physical hardware mainly comprises of low specification and low cost component
to facilitate mass production, which makes it affordable to be deployed intensively in a monitoring
zone. This has created a challenge in mapping the locations of sensor nodes as the hardware cannot
provide precise timing in calculating time of flight of a packet, an important parameter in estimating
distance between transmitting node and receiving node for time-based positioning methods such as
Uplink-Time Difference of Arrival (U-TDOA) and Time of Arrival (TOA). The common use of cheap
single Omni directional antenna in most of the ZigBee deployments also ruled out the possibility of
using techniques that rely on packet Angle of Arrival (AoA) for estimating the location. This paper
investigates positioning methodology that is based on received signal strength to estimates the distance
to node and performing trilateration of those distances. This approach does not require any hardware
modifications to the sensor node, providing a more viable positioning methodology in ZigBee
networks.
3. Positioning In Wireless Sensor Networks
Wireless networks uses radio microwave to communicate with each other and particularly with the
ZigBee network, it uses the 2.4 GHz radio frequency that can be measured for positioning purposes.
The ZigBee network infrastructure provides radio signal properties as part of the Quality of Service
such as the Time of Arrival (ToA), Time Difference of Arrival (TDoA), Angle of Arrival (AoA) and
the Received Signal Strength Indicator (RSSI) [1].
i. Received Signal Strength Indicator (RSSI)
In an open environment such as a cattle breeder farm, with high probability of Line of Sight (LoS) and
low multipath effect, it is possible to use the simple RSSI-based location algorithm if coarse accuracy
is acceptable. The received signal strength can be measured for every packet received, the value
indicating the signal strength is provided as part of the Link Quality Indicator (LQI) as a RSSI value
and available at the PHY layer in IEEE 802.15.4 network. Using the RSSI value, a distance to node can
be measured and trilateration calculation can be performed against other nodes with known positions.
ii. Uplink-Time Difference of Arrival (U-TDOA)
The signal time difference received by the device from cells antenna tower is being calculated in
determining the positioning of the device. Using the time difference information gathered, the distance,
on the other hand, from cells antenna tower or base station to the device could be easily estimated, thus
leading to the coordinate positioning of the device.
iii. Time of Arrival (TOA)
It is similar to the U-TDOA; however, the only difference is that it uses the absolute time of arrival at a
cells antenna tower or base station rather than the difference between two stations. As a result the
distance can be calculated from the time of arrival as signals travel with a known velocity which in this
case is the speed of light (300,000 kilometres per second). The different data of time arrival from two
cells antenna tower or base station will formulate a position to two circles and the third cells antenna
tower or base station is required to determine the precise device position.
50 R, Mardeni and Othman, Shaifull Nizam
iv. Angle of Arrival (AoA)
The AoA in ZigBee networks can be achieved by grouping together three or four nodes involved in a
typical radio interferometric to form an antenna array, which acts as an anchor node. The bearing of the
target node can then be estimated by computing the angle of hyperbola asymptote [2]. In this method,
the AoA mechanism locates the node at the point where the lines along the angles from each cells
antenna intersect.
4. Positioning Using Zigbee Infrastructure
The positioning of node using the RSSI value provided by LQI of the ZigBee node offers possibility of
providing spatial data without any additional hardware requirements to the existing solution. The
localisation process involves the use of trilateration calculation for intersection of three spheres of
which the radius is obtained from the distance estimated from the RSSI value; to work this model
requires that the transmitting node must be inside the intersection of three other receiver nodes of
which the locations are known. One possibility to acquire a distance is measuring the received signal
strength of the incoming radio signal. The idea behind RSS is that the configured transmission power
at the transmitting device ( ) directly affects the receiving power at the receiving device ( ).
According to Friis’ free space transmission equation [3], the detected signal strength decreases
quadratically with the distance to the sender.
PRX = PTX X GTX X GRX
2 (1)
4d
Where;
PRX = Transmission power of sender
PTX = Remaining power of wave at receiver
GTX = Gain of transmitter
GRX = Gain of receiver
= Wave length
d = Distance between sender and receiver
In embedded devices, the received signal strength is converted to a received signal strength
indicator (RSSI) which is defined as ratio of the received power to the reference power Pref [3].
Typically, the reference power represents an absolute value of Pref =1mW.
The RSSI formula can be shown as in eqn. (2) as below;
RSSI = 10 X log PRX dBm (2)
Pref
An increasing received power results a rising RSSI. Distance (d), is indirect proportional to
RSSI. In practical scenarios, the ideal distribution of PRX is not applicable, because the propagation of
the radio signal is interfered with a lot of influencing effects. The RSSI value is provided by the PHY
layer of the ZigBee network.
Figure 1: Relationship between the Transmit power and the distance
PRX
Distance
Node Positioning in ZigBee Network Using Trilateration Method
Based on the Received Signal Strength Indicator (RSSI) 51
There are few factors that degrade and impact the RSSI values in the wireless networks and
ZigBee in particular:
• Reflections on metallic objects
• Superposition of electro-magnetic fields
• Diffraction at edges
• Refraction by media with different propagation velocity
• Polarisation of electro-magnetic fields
• Unadapted MAC protocols
The RSSI based distance estimation of the target node can then be used for positioning
calculation using the trilateration or multilateration formula. Few multilateration methods has been
proposed to solve the localisation problem in 3D space, such as Semidefinite Programming [4], MDSMAP
[5] for centralised algorithm approach and Diffusion based Multilateration [6] and Gradient
based Multilateration [7].
Based on the Received Signal Strength Indicator (RSSI)
[attachment=23688]
Abstract
This paper investigates the possibility of implementing node positioning in the
ZigBee wireless sensor network by using a readily available Received Signal Strength
Indicator (RSSI) infrastructure provided by the physical layer of 802.15.4 networks. In this
study the RSSI is converted to the distance providing the basis for using the trilateration
methods for location estimation. The software written in C# is used to solve the trilateration
problem and the final results of trilateration methods are mapped using Google maps.
Providing node positioning capability to the ZigBee network offers an enormous benefit to
the Wireless Sensor Networks applications, possibly extending the functionality of existing
software solution to include node tracking and monitoring without an additional hardware
investment.
Keywords: Positioning, ZigBee, Trilateration, Received Signal Strength Indicator (RSSI)
1. Introduction
Satellite-based Global Positioning System (GPS) has been used previously alongside the GIS
applications to provide the spatial information to map real world location to the computer
representation, thus making the planning and management of resources more informed. The GPS is a
space-based global navigation satellite system that provides reliable location and time information in
all weather conditions and at all times and anywhere on or near the Earth when there is an unobstructed
line of sight to four or more GPS satellites. The positioning system that uses the GPS receiver adds
cost to the solution infrastructure, increases power requirements to operate the on-board receiver chip
and increases infrastructure maintenance points, making the deployment of such solution not
commercially viable.
Wireless Sensor Networks, particularly the ZigBee devices offer a more viable positioning
method that uses existing infrastructure without escalating the operational expenses. This paper
explains the method used to do node positioning in the ZigBee network.
Node Positioning in ZigBee Network Using Trilateration Method
Based on the Received Signal Strength Indicator (RSSI) 49
2. Problem Statement
The ZigBee node's physical hardware mainly comprises of low specification and low cost component
to facilitate mass production, which makes it affordable to be deployed intensively in a monitoring
zone. This has created a challenge in mapping the locations of sensor nodes as the hardware cannot
provide precise timing in calculating time of flight of a packet, an important parameter in estimating
distance between transmitting node and receiving node for time-based positioning methods such as
Uplink-Time Difference of Arrival (U-TDOA) and Time of Arrival (TOA). The common use of cheap
single Omni directional antenna in most of the ZigBee deployments also ruled out the possibility of
using techniques that rely on packet Angle of Arrival (AoA) for estimating the location. This paper
investigates positioning methodology that is based on received signal strength to estimates the distance
to node and performing trilateration of those distances. This approach does not require any hardware
modifications to the sensor node, providing a more viable positioning methodology in ZigBee
networks.
3. Positioning In Wireless Sensor Networks
Wireless networks uses radio microwave to communicate with each other and particularly with the
ZigBee network, it uses the 2.4 GHz radio frequency that can be measured for positioning purposes.
The ZigBee network infrastructure provides radio signal properties as part of the Quality of Service
such as the Time of Arrival (ToA), Time Difference of Arrival (TDoA), Angle of Arrival (AoA) and
the Received Signal Strength Indicator (RSSI) [1].
i. Received Signal Strength Indicator (RSSI)
In an open environment such as a cattle breeder farm, with high probability of Line of Sight (LoS) and
low multipath effect, it is possible to use the simple RSSI-based location algorithm if coarse accuracy
is acceptable. The received signal strength can be measured for every packet received, the value
indicating the signal strength is provided as part of the Link Quality Indicator (LQI) as a RSSI value
and available at the PHY layer in IEEE 802.15.4 network. Using the RSSI value, a distance to node can
be measured and trilateration calculation can be performed against other nodes with known positions.
ii. Uplink-Time Difference of Arrival (U-TDOA)
The signal time difference received by the device from cells antenna tower is being calculated in
determining the positioning of the device. Using the time difference information gathered, the distance,
on the other hand, from cells antenna tower or base station to the device could be easily estimated, thus
leading to the coordinate positioning of the device.
iii. Time of Arrival (TOA)
It is similar to the U-TDOA; however, the only difference is that it uses the absolute time of arrival at a
cells antenna tower or base station rather than the difference between two stations. As a result the
distance can be calculated from the time of arrival as signals travel with a known velocity which in this
case is the speed of light (300,000 kilometres per second). The different data of time arrival from two
cells antenna tower or base station will formulate a position to two circles and the third cells antenna
tower or base station is required to determine the precise device position.
50 R, Mardeni and Othman, Shaifull Nizam
iv. Angle of Arrival (AoA)
The AoA in ZigBee networks can be achieved by grouping together three or four nodes involved in a
typical radio interferometric to form an antenna array, which acts as an anchor node. The bearing of the
target node can then be estimated by computing the angle of hyperbola asymptote [2]. In this method,
the AoA mechanism locates the node at the point where the lines along the angles from each cells
antenna intersect.
4. Positioning Using Zigbee Infrastructure
The positioning of node using the RSSI value provided by LQI of the ZigBee node offers possibility of
providing spatial data without any additional hardware requirements to the existing solution. The
localisation process involves the use of trilateration calculation for intersection of three spheres of
which the radius is obtained from the distance estimated from the RSSI value; to work this model
requires that the transmitting node must be inside the intersection of three other receiver nodes of
which the locations are known. One possibility to acquire a distance is measuring the received signal
strength of the incoming radio signal. The idea behind RSS is that the configured transmission power
at the transmitting device ( ) directly affects the receiving power at the receiving device ( ).
According to Friis’ free space transmission equation [3], the detected signal strength decreases
quadratically with the distance to the sender.
PRX = PTX X GTX X GRX
2 (1)
4d
Where;
PRX = Transmission power of sender
PTX = Remaining power of wave at receiver
GTX = Gain of transmitter
GRX = Gain of receiver
= Wave length
d = Distance between sender and receiver
In embedded devices, the received signal strength is converted to a received signal strength
indicator (RSSI) which is defined as ratio of the received power to the reference power Pref [3].
Typically, the reference power represents an absolute value of Pref =1mW.
The RSSI formula can be shown as in eqn. (2) as below;
RSSI = 10 X log PRX dBm (2)
Pref
An increasing received power results a rising RSSI. Distance (d), is indirect proportional to
RSSI. In practical scenarios, the ideal distribution of PRX is not applicable, because the propagation of
the radio signal is interfered with a lot of influencing effects. The RSSI value is provided by the PHY
layer of the ZigBee network.
Figure 1: Relationship between the Transmit power and the distance
PRX
Distance
Node Positioning in ZigBee Network Using Trilateration Method
Based on the Received Signal Strength Indicator (RSSI) 51
There are few factors that degrade and impact the RSSI values in the wireless networks and
ZigBee in particular:
• Reflections on metallic objects
• Superposition of electro-magnetic fields
• Diffraction at edges
• Refraction by media with different propagation velocity
• Polarisation of electro-magnetic fields
• Unadapted MAC protocols
The RSSI based distance estimation of the target node can then be used for positioning
calculation using the trilateration or multilateration formula. Few multilateration methods has been
proposed to solve the localisation problem in 3D space, such as Semidefinite Programming [4], MDSMAP
[5] for centralised algorithm approach and Diffusion based Multilateration [6] and Gradient
based Multilateration [7].