RTLS technology

Radio Frequency Identification (RFID) is one of the oldest methods of tracking and involves attaching a tag, usually passive, to the device you wish to track. RFID tags are generally inexpensive and earlier generations looked like small coils of wire on a sticky pad. Tags have evolved and are now designed differently to suit varied applications and environments. Tags have decreased in size and are now produced from various materials, supporting use in varied environments such as extreme temperature, humidity, and the ability to better withstand sources of radio interference, shock and damage.

Radio transmitters, at key positions around the building (such as doorways), emit radio waves which induce a responding signal from the tag, containing their ID, which identifies the RFID tags passing the antenna.  As the tag emits a low energy response, RFID has a very limited range, generally up to 5 meters. 

An example of this is theft detection systems in shops where items have RFID tags attached, and RFID antennas which detect the tags, are mounted by the building exit.

With Active RFID the tag can send out its identification at given time intervals making it more accurate as it moves around the site in real time. Powered tags are capable of holding and sending more information than passive tags.

Devices that are Wi-Fi enabled, such as phones and laptops, can be tracked by existing Wi-Fi infrastructure. The device needs to be able to listen to the Wi-Fi AP but does not need to connect to it. You may need additional licensing and software to enable this service.

If you want to track assets that are not Wi-Fi enabled, then Wi-Fi tags can be attached to them. These are similar in design and application as tags used by other RTLS technologies, but as Wi-Fi can be power intensive, these tags tend to be larger and require larger batteries than those using Bluetooth Low Energy (BLE) or Ultra-Wide Band (UWB).

Of the technologies listed, using Wi-Fi tracking for location is the least accurate and typically can locate a device’s location to within 5 to 10 metres. The location accuracy will depend on the density of APs deployed and the physical environment.  Precision improves with more APs and less obstructions within a given area.

Consideration will also need to be given to MAC address randomisation. MAC address is a unique identifier for each wireless radio that is used to track devices over Wi-Fi. MAC address randomisation has become the default behaviour for consumer mobile devices  (such as phones and tablets) from Apple and Android to enhance privacy This  setting can usually be switched off in the devices settings to prevent interference with tracking of the device.

Bluetooth Low Energy, or BLE, is a radio-frequency technology for wireless communication that can detect and track the location of tags attached to people, devices, and assets indoors. It is independent of classic Bluetooth and not compatible.

BLE tags can be small, inexpensive and have good battery performance, with the potential to last several years. The location tracking accuracy is typically within 1 to 5 metres.

To implement BLE, beacons need to be installed around the building and the assets you want to track need to be BLE enabled or have a BLE tag attached. Wi-Fi access points purchased post 2020 are highly likely to have BLE beacons built in.

Any deployment of RTLS using BLE tracking via existing Wi-Fi infrastructure should take into consideration the impact of the additional requirements on existing services and capabilities, such as guest and corporate Wi-Fi networks. Depending on local deployment of infrastructure and the number of devices to be tracked the density (number in an area) of access points you have deployed may need to be increased to meet your RTLS accuracy and reliability requirements. See Wi-Fi metrics and measures for guidance on understanding Wi-Fi capabilities, including access point density and identifying requirements.

Those interested in enabling BLE should start by  contacting your access point supplier(s) to confirm the capability is in place and understand what  additional licensing and software is required to enable this service.

This is the latest technology of those listed and as its name suggests uses a wide band of radio frequencies. Benefits of UWB over Wi-Fi and BLE include faster data transfer rates and improved penetration of walls and other obstacles inside buildings.  

UWB tags are comparatively cheap and have very good battery performance, with some lasting several years.  UWB is able to track a tags location to a precision of centimetres. 

At the time of writing UWB is not yet widely deployed within NHS environments and as such is likely to require new UWB specific network infrastructure to be deployed to implement the capability.  

RFID, BLE, UWB and Wi-Fi all make use of a combination of the below techniques to determine the location of a tracked tag.  The techniques used will depend on the device manufacturer.

RSSI works by an access point, or receiver, sending out a signal to the tag or device being tracked and calculating its proximity based on the strength of the signal.

If the device is detected by three or more receivers, then each one will see it with different signal strengths and this information can be used to triangulate its position and increase accuracy.  Physical environmental factors such as furniture, walls and other radio congestion can affect signal strength causing the accuracy to drop.

There are techniques that can help improve RSSI accuracy in congested environments, such as Fingerprinting.  With Fingerprinting the impact of the known physical environment is added to the RTLS system allowing this data to be factored in when calculating signal strength to determine the location.  Fingerprinting can significantly improve accuracy but relies on the Fingerprinting data being updated every time something is changed within the building so can be high maintenance.

This method works by measuring the time it takes for a signal to be sent and detected. As radio waves travel at a constant speed (the speed of light) then knowing the time it takes for the signal to travel means you can calculate the distance travelled.  If you measure this time difference from several fixed access points, then you can triangulate the distance and calculate the location of the device or tag.

There are two common implementations of time-based measurement, Time of Flight (ToF) and Time Difference of Arrival (TDoA).  The main difference between them is how the signal time is sent and recorded.

In very simple terms the receiver/access point has multiple antennas built within it.  When the signal from the device/tag hits the antennas, the system looks at the “phase” or angle of the signal hitting each antenna and the time. From this information the direction it came from can be determined based on the timings.

The data rates that devices can send/receive is also an important consideration.  With faster data rates more data can be sent which will improve accuracy.