Wi-Fi Positioning Explained

Each Wi-Fi access point relays data to allow users to access the internet. Wi-Fi positioning can take advantage of the signals emitted from Wi-Fi routers to gain some insights into the users location.

When a Wi-Fi router sends a wireless signal, Wi-Fi receivers (e.g., smartphones) can accept those signals. Each signal contains some metadata which can be processed and used to determine the users’ location.

Wi-Fi Metadata:

MAC address (unique ID for the router)
RSS (received signal strength)

The MAC address is unique for each Wi-Fi router and the RSS provides a rough measurement of how far away the device is. Intuitively, a strong RSS would show that the user is close to that Wi-Fi router and a weak RSS would indicate that the user is far from the Wi-Fi router.

The challenge for indoor environments is that the signal strength is weakened as it passes through obstacles such as walls, furniture, or people’s bodies. Therefore, a weak RSS could imply that either the user is far away or that they may be close but there are obstacles in the way (e.g., walls or furniture).

There are two main ways Wi-Fi MAC address and RSS can be used for positioning.
The first is Wi-Fi trilateration and the second is Wi-Fi fingerprinting.

Wi-Fi trilateration requires knowledge of the location of each Wi-Fi router. Then, based on the RSS, it estimates a distance between the user and each visible Wi-Firouter and calculates its position based on those distances.

Wi-Fi fingerprinting does not need to know the locations of the Wi-Fi routers but instead creates a large fingerprint map of what the RSS of each MAC address looks like at various locations and then in real-time compares the measured RSS to the fingerprint map to determine the users position.

Assembling the W-Fi fingerprint map can be time-consuming and requires a site-survey, but tends to provide more accurate results than Wi-Fi trilateration.

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Pros:

  • Already deployed in most large/popular indoor venues
  • Wi-Fi has a relatively large range so it can cover a lot of area

Cons:

  • iOS devices cannot be used for Wi-Fi positioning (blocked by Apple)
  • Accuracy alone is not sufficient for indoor positioning (10-15 meters)
  • Wi-Fi signals propagate through various walls and obstacles leading to high signal variance and ambiguity which makes it difficult to extract distance

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