Choosing the right tool can significantly
save time, money, and enhance efficiency. Just as an adjustable wrench excels
at tightening nuts but falters at driving nails, the choice of technology
matters for specific tasks.
Wi-Fi technology, popular for its
affordability and widespread manufacturer support, undoubtedly excels in home
and office networking. However, when it comes to industrial applications, it
might not always be the optimal solution. While Wi-Fi can be applied, its
efficiency in these scenarios is akin to driving nails with an adjustable
wrench.
In my next blog posts I want to describe
applications where I would not use the Wi-Fi.
Real Time Location System
The RTLS is the good example of application where I would not use Wi-Fi as the technology. Wi-Fi-based RTLS faces two significant problems:
- Low Accuracy: Location accuracy typically ranges from 3 to 10 meters with a 90% probability.
- Slow System Response: The system response time can take minutes.
Location Accuracy
Wi-Fi-based RTLS encounters challenges in achieving precise location accuracy due to several inherent technological limitations. These factors contribute to the degradation of location accuracy:
- Logarithmic Nature of RF Propagation. The use of Received Signal Strength Indicator (RSSI) relies on the logarithmic decrease of RF waveform amplitude with distance. The logarithmic nature of Free Space Path Loss causes accuracy to degrade as the distance between the client and Access Points (APs) increases. Increasing accuracy by densifying APs escalates project costs and introduces Co-Channel Interference.
- RF Fluctuations. Natural fluctuations in every RF environment lead to random changes in signal levels, making it challenging to predict accurate location information.
- Nonlinear Frequency Response of Antenna Systems. Each antenna system can be perceived as a broadband filter, and the smaller the implementation of the filter, the more challenging it is to maintain a constant frequency response across the spectrum. Consequently, the Equivalent Isotropic Radiated Power (EIRP) for Wi-Fi clients varies depending on the channel, causing two APs at the same distance to the client but on different channels to measure different received signal strength (RSS).
- Inconstant RF-Environment. Any changes in the room, such as alterations in occupancy or rearrangement of furniture, lead to fluctuations in RF signal propagation. The measured RSS in an empty room and in a crowded room will differ due to these environmental changes.
- Lack of Calibration. APs are not measuring devices, and they lack calibration. Each Wi-Fi AP measures RSS in its unique way. Wi-Fi engineers conducting site surveys using multiple Ekahau Wi-Fi dongles can attest to the fact that each dongle measures different RSS values.
These factors collectively contribute to the challenges faced by Wi-Fi-based RTLS in achieving consistent and accurate location information.
System Response
The slow system response arises from the communication behaviour between clients and APs. As clients communicate solely with connected APs, neighbouring APs on different channels cannot measure the client's signal. The only frame, which every client broadcast on all channels it the prob request frame. The interval of probing depends on communication conditions, and it can be up to 5 minutes is the client stay close to the AP. This resulting in a slow system response time.
To improve system response, reducing the number of enabled channels is an option, but it comes at the cost of degrading WLAN performance.
Summary
Wi-Fi proves ineffective for positioning in RTLS compared to alternatives like BLE and UWB, primarily due to its inherent limitations in accuracy and system response time. Understanding these limitations is crucial when evaluating the suitability of Wi-Fi for applications that demand high precision in real-time location tracking.
