WiFi for Indoor Positioning

This article discusses using WiFi for indoor asset tracking. Whether you are at your local coffee shop, home, or office, WiFi is everywhere. Being able to track assets using WiFi is very exciting because the network infrastructure is already ubiquitous and standardized. WiFi networks are relatively cheap, easy to set up, and inconspicuous. For spacious indoor locations where GPS is inaccurate, WiFi offers a great alternative. We will be taking a deeper dive into what WiFi is and how indoor positioning systems leverage WiFi connectivity.

What Is WiFi?

WiFi describes a local wireless network that uses radio waves to communicate data, typically from the Internet. To be considered WiFi, the radio signal must use the IEEE 802.1 standard to communicate. There are multiple versions of WiFi as defined in the IEEE specifications, which include common ones such as 2.4 GHz and 5GHz frequency radio waves. WiFi is typically used to transfer large amounts of information. It has transfer speeds of up to several gigabits per second for certain versions.

How Does WiFi Track Location?

There are multiple methods for tracking an indoor WiFi-enabled asset. The two primary methods are RSSI and fingerprinting. RSSI (or Received Signal Strength Indication) of a WiFi signal is inversely proportional to distance. In combination with trilateration, RSSI can be used to approximate an asset’s location. However, the measured RSSI of a signal can be decreased due to obstructions and/or reflections, leading to inaccurate RSSI measurements and, therefore, location determinations.

A technique called fingerprinting can be used to improve this accuracy significantly. Fingerprinting uses historical RSSI information as well as known locations to determine the position of an asset given its current RSSI values. Fingerprinting is highly dependent on the environment, so if a chair or table is moved, the fingerprint will need to be updated. In addition to RSSI and fingerprinting, Angle of Arrival (AoA) and Time of Flight (ToF) can be used to determine location. While these methods are accurate, they require expensive equipment and methods such as multiple antennae and clock synchronization.

Advantages

The major advantages of WiFi are accessibility, large range, and a high data throughput rate. WiFi is also widely available. In some cases, there may be several WiFi access points in a room or floor. In general, the more WiFi access points there are, the higher the accuracy of the indoor tracking will be. For some applications, there could be little to no infrastructure costs for implementing a WiFi network for asset tracking. Additionally, WiFi has a very long range up to 150 meters, which is much greater than Bluetooth. Compared to Bluetooth, WiFi also has a much higher data transfer, which could be useful if the asset needs to transfer a large amount of data (e.g. images, video, or uncompressed audio).

Disadvantages

One of the major disadvantages of WiFi is its accuracy. With an accuracy at the higher end of 15 meters, it may not be as suitable for precise location applications, where Bluetooth excels by comparison with its 2-4m accuracy. However, accuracy can be as low as 2-3m with fingerprinting, a significant amount of WiFi access points, and/or combining tracking data with other technologies (also known as sensor fusion). Oregon State University, in collaboration with Intel, is working on SAIL, a WiFi indoor tracking technology that “achieves a location accuracy better than 1m.” Another disadvantage of using WiFi is security concerns. WiFi is such a common technology that it has already experienced numerous hacks and resulting security patches. Security will always be a concern for WiFi, but given this, there will always be companies working to harden WiFi from malicious attacks.‍

Conclusion

WiFi is definitely a viable solution for indoor asset tracking. For a business office, shopping mall, or university campus, WiFi would be an ideal low-cost solution to track assets indoors. While accuracy and security issues may hinder WiFi, these drawbacks can be mitigated using techniques such as fingerprinting, sensor fusion, and security hardening. Hopefully, indoor tracking will be more prevalent given the widespread use of WiFi.