ZigBee positioning solution
Imagine that you burst into the shopping centers, eager for your other half of the purchase him (her) bed of roses for a birthday present. You are naturally pulled out cell phone or PDA to resolve the issue of how to buy a birthday present. At this point, your mobile handheld device will display a shopping mall Daogou map and marked on the map need to go to the Procurement area. Mobile handheld devices will be shown at any time you may be interested in commodities.
Radio Frequency (RF) technology is expected to make the vision become a reality. Some ZigBee rf devices embedded with the indoor GPS positioning engine, the system can be comparable to its built-in ZigBee networks, positioning engine uses to calculate the RF infrastructure, the location of things or people. Compared with the GPS positioning engine in a single-chip RF transceiver with mcu integrates GPS hardware, the cost of less than 1 / 10, power is only a small part of the GPS hardware. The kinds of positioning engine can be used indoors can also be used outdoors, but as long as there are existing ZigBee networks, no need to install the mobile receiving antenna.
Typical applications include:
· Remote on / off the light all the rooms in houses;
· Track wharf start shipment of container;
· Tracking site equipment.
In addition, when the new access network equipment, the positioning engine able to identify its physical location, so that it can also be used to simplify wireless network settings.
Most wireless sensor networks require the availability of a method of determining the location of network nodes. Therefore, during installation, need to find out what nodes to exchange data directly between each other, or to determine which node directly with the central data collection point for data exchange.
When the adoption of software-based calculation method to determine the location of network nodes, we need to consider market-based solutions (market solution). These specific method of calculation is: the first node, the node reads the location of the calculation parameters, and then transmit relevant information to a central data collection point, the calculation of the node location, finally, the node location of the relevant parameters and then pass back to the node. This is a typical data-intensive computing, and the need to configure a PC or a high-performance MCU.
This calculation approach is the location of the node is only applicable to small networks and a limited number of nodes because of the associated traffic will calculate the required number of nodes increases as the exponential rate. Therefore, high traffic load coupled with lack of bandwidth limitations of this method in battery-powered network applications.
In response to these issues, CC2431 uses a distributed positioning calculation. This calculation is based on a reference from the nearest node (whose location is known) received information, the local computing nodes to determine the location of the relevant node. Therefore, the amount of network traffic from the node under test range in the number of nodes in the decision. In addition, because network traffic will increase as the number of nodes under test in proportion to increase, therefore, CC2431 also allows the same network there is a large number of nodes under test.
This article provides the results are based on measurements obtained ZigBee network, however, the results of these measurements also apply to based on the IEEE 802.15.4 protocol to build a simpler network.
Positioning Engine Technology
Positioning engine under the wireless networks near the RF received signal strength indication (RSSI), calculate the required position as shown. In different environments, RSSI between the two RF signals significant changes will occur. For example, when a pedestrian between the two radio frequency, the received signal will be reduced 30dBm. To compensate for this difference, as well as the results for accuracy considerations positioning, positioning the engine will be based on up to 16 from the RF RSSI value associated position calculation. Based on the theory is: When using a large number of node, RSSI of the change will eventually reach the average.
In the RF network, with a known location positioning engine, known as the RF reference node, while the need to calculate the location of the node location is called the node under test.
Required in the reference node and the node under test is the only information transferred between the reference node of the X and Y coordinates. Positioning the engine according to the received X and Y coordinates, combined with the data according to reference nodes derived from RSSI measurements and calculating the location position.
Positioning technology into the network protocol
Some use of the application of positioning engine may require a number of reference nodes placed to serve as an integral part of the infrastructure set up. ZigBee technology can achieve the right home, office and industrial applications such as wireless control. With the ZigBee devices in building infrastructure, increasing the number of installations, ZigBee will be in the home and office automation, has a broader application prospects.
A typical office space will be configured ZigBee devices, through the offices and meeting rooms in the temperature sensor, control, temperature control devices and A / c ducts. At the same time, each room will be installed by the ZigBee-controlled light switches and equipment, the equipment is also easy as the positioning engine reference node. The ZigBee RF ZigBee protocol stack as the reference node, the code required for the capacity is typically less than 1 Kb.
Positioning engine reference from 3 ~ 16 nodes collect data and use these data to derive targeted locations. If the positioning engine from more than 16 nodes when data is received, it will be will receive a reference node location categories, and then use 16 reference nodes in the strongest RSSI signal value.
Positioning engine coverage of 64m × 64m, however, the majority of applications require greater coverage. To expand the coverage of positioning engine can be achieved in two ways:
* Enhance the reference node, the output power, while reducing the accuracy of positioning engine results;
* In a larger scale layout reference node, and use the strongest signal associated reference node position calculation.
As the second method can locate the engine at the same time without sacrificing coverage of the positioning accuracy, and therefore more desirable. Specific works are: the network under test nodes send broadcast messages, and a reference from the adjacent nodes collect data, select the strongest signal of the reference nodes X and Y coordinates. Then, calculation and reference to other relevant nodes in the node coordinates. Finally, the positioning engine to process data and to consider the offset from the nearest reference node value node under test in order to obtain the actual location of large networks (see Figure 1).
Figure 1 shows the location and positioning information of the ZigBee network path
In order to achieve the best positioning range, when the arrangement of reference nodes, should also take into account the indoor and outdoor environments ceiling / floor of the adsorption. The best option is to make each node in the same height, and away from the floor, ceiling and walls. In the actual deployment process to achieve such a request, it is more difficult. Therefore, as far as possible reference to the node attached to the ceiling height or lower than the ceiling height, and make the antenna upside down so that the RF signal transmission outward and downward, while the node under test (hand-held or fixed on the equipment) placed in a person's waist above the head the following locations (here referred to a high degree are people standing in the environment for the standard). This means the node is set to achieve the ceiling and floor minimize adsorption, and will achieve in the place of the pedestrian or object to minimize interference with each other.
The accuracy of positioning engine
In order to ensure the indoor positioning engine performance, will be used in the office environment, with eight reference node network. According to available reference nodes will be placed on the surface of the corner office location, such as office furniture or other surface, between people's surface between the waist and shoulders. In Figure 2, the eight reference nodes A ~ H 8 were used to name letters.
Figure 2 indoor office environment
In the six locations selected location estimates collected data, positioning data for each location an average of 20 read points. Relevant test results as shown in Table 1.
Table 1 8 reference nodes indoor test results (All figures are in units of m)
8 reference nodes in the collection of data, will be six new reference node added to the system. Next, in 4 the same location, re-locate the estimated calculations, observation of these new reference node location estimate impact.
14 reference nodes in the measurement results as shown in Table 2.
Figure 2 14 reference nodes indoor test results (All figures are in units of m)
When the node position when entering the network coverage, the positioning accuracy will be significantly improved. Moreover, when the network node in an increase in the reference set, the positioning accuracy will also be rising. In this experiment, an additional 6 months after the reference node, four locations will be improved positioning accuracy, while reducing the location of each position reporting standard deviation (consistency).
Positioning engine reference from nearby nodes using RSSI measurements to calculate the location of the node under test. RSSI will be with the antenna design, the surrounding environment, and includes a number of other factors, including other nearby RF sources varies. Positioning Engine will be a number of node location information to be average. Increase in the number of nodes, each node can reduce the dependence of the specific test results, while comprehensively improve accuracy.
No matter under what circumstances to set reference node, will influence the positioning accuracy, mainly because when the reference node is set away from the surface close to the relevant places, will have ceilings or floors adsorption. Therefore, we should make full use of all the parties have the same launch capabilities bit of omni-directional antenna.
Positioning engine implements ZigBee network radio "room-style" accuracy, and low power consumption, communication overhead also realized minimized. In addition, the positioning engine technology also allows use of the existing ZigBee infrastructure to determine the network location. Information such as these can be used to track the purpose of easy access to central data collection point, users can also use the location engine technology and the navigation within the completed building work.
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