This article explains the logic behind location tracking and calibration.
- Mojo Wireless Manager version 7.0 or later
- Sensors / APs (with background scanning)
- Location map where Sensors/APs are placed
What is Location Tracking?
For location tracking to work, we need to have sensors (or APs with background scanning mode) placed at correct location on the floor plan. The location tracking algorithm takes following input:
- Co-ordinates of placed sensors
- Signal strengths observed by placed sensors for the device being located
- Results of calibration
- RF properties of the environment
- If layout is imported from SPM file (generated by AirTight Planner), then RF properties are specified by the SPM file.
- If layout is not imported from SPM file, then RF properties are specified by global policy under Configuration >System Settings > Advanced Settings > RF Propagation.
The layout is divided into a number of cells according to the selected accuracy. For higher accuracy, number of cells is higher. For each cell, we calculate the probability of the device physically being in that cell. To calculate the probability, we predict the signal strengths that the placed sensors would observe if the device were actually present in the cell. The predicted value is then compared with the observed value (using cumulative normal distributive function) to derive the probability. Note that to predict signal strength we have to assume transmit power of the device. As we do not know the transmit power of the device, we find probabilities by assuming 7 different transmit powers and then use the highest probability of the results.
Now that we have probability distribution across all the cells, we color the cells accordingly. Color of the cell is derived by the ratio of probability for that cell to maximum probability across all the cells.
What is Calibration?
While predicting the value of signal observed by a sensor sent by a transmitter at some distance, we use path loss model and do not consider existence of obstacles. The RF propagation constants used in the calculation also may not reflect the actual RF environment. This results in inaccuracy in location tracking. To overcome this, calibration can be performed. Calibration requires some APs to be placed at known location on floor plan along with the sensors. This way the system can predict the signal strength that the sensors should see according the placement on floor plan and current RF parameter. The prediction is then compared with the actual observations. The two lines (actual vs predicted) should be as close together as possible. If there is a difference between the two lines, it means that RF parameters considered by the system are not accurately reflecting the actual environments. By adjusting the RF parameters, user can bring the two lines close. After calibration is performed and the results have been saved, those calibration factors are then used in location tracking for better prediction of the signal strength.
Steps for Calibration
Deploy Mojo Sensors at their respective positions.
Place the Sensors at or above 7 ft and in open space (center of the room/hall, away from the walls and metallic obstacles) to improve the Location Tracking and coverage prediction accuracy.
- Identify the APs which would be used for calibration
- Find out the Transmit Power for these APs
- Deploy the identified APs
Make sure that each Mojo Sensor sees three or more APs. Distribute the visible APs uniformly around the Mojo Sensors. The distance between the Mojo Sensors and the APs is presented in Table 1 below.
|Typical Office Space||15m ? 20m|
- Make all the identified APs Authorized by using the appropriate option in the Devices tab.
- Drag and drop all the APs and Mojo Sensors to their respective positions on the floor map.
- Verify that the actual Transmit Power of each AP is accurately reflected on its corresponding properties screen (by double clicking the AP icon on the Location Viewer screen).
- On the Calibration screen, manually adjust the signal decay coefficients so that the observations match the predictions. Clicking on Update Graph reflects the difference between the two curves.
- If floor objects are modeled
|Typical Office Space||2||2.5||-4||0.08|
- If floor objects are not modeled
|Typical Office Space||2||2||-4||0.08|
Press the Calibrate button to automatically tune the Mojo Sensor calibration factors. Repeat Step 7 through Step 8 till you are satisfied with the predictions.
Automatic tuning of Mojo Sensors calibration factors is optional. Mojo Sensors calibration factors can also be manually tuned by double clicking Sensor icon on Location Viewer screen.
- Click Apply.
- If any APs were made Authorized purposely for Calibration, then remove those APs from the floor map and delete them from the Authorized AP list.
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