Ultra Wide-band Technology

What is UWB Technology?

Ultra-wideband (UWB) is a radio technology which typically transmits short pulses at a low power level over a large portion of the radio spectrum (greater than 500 MHz).

FCC regulations, established in 2002, dictate that UWB signals may not exceed a spectral density (power level per unit of spectrum) of -41.3 dBm/MHz. UWB signals often do not exceed the level of background noise (the “noise floor“) and hence are almost indistinguishable from noise for most receivers, making it difficult to intercept without specialised equipment, and ensuring they do not interfere with other radio signals such as WiFi.

UWB can transmit information at faster data rates than Bluetooth, while also consuming less battery power. The technology is becoming more mainstream, with Apple’s introduction of a dedicated UWB chip in the iPhone 11 in 2019, and BMW’s planned use of UWB to develop more secure and reliable digital key systems.

The technology has the ability to detect distances or locations down to a 10 centimeter accuracy by detecting the time it takes for signals to travel between connected devices, or the difference in the times that a signal is received at multiple fixed anchors.

Distance Detection (Ranging)

Two Way Ranging (TWR) algorithms are used to allow UWB devices to accurately detect distances to other devices that are in range.

Typically, the process is as follows:

1. Device A sends a Poll message which is received by device B

2. Device B waits for a known fixed period, and then sends a Response message back to device A

3. Device A then sends a Final message to device B

At this point device B has enough information to calculate the distance to device A knowing the exact Time of Flight (TOF) and the speed of transmission (which is the speed of light). Three messages are necessary in order to overcome sources of error such as device clock drift when measuring such short periods of time on the order of nanoseconds (billionths of a second).

Device B can then send the measured distance to device A, or device A can determine this separately when it receives a Poll message from device B.

Advantages of Two Way Ranging include:

• Device clocks do not need to be synchronised.

• The method can work in a true peer to peer environment where there are no fixed anchors in place.

Disadvantages include:

• All devices need to be listening for Poll messages for the majority of the time from all other devices, which is a drain on battery life.

• Scaling to a large number of devices is challenging as the number of signals between the devices multiplies exponentially, so an advanced algorithm along with the correct hardware is required to ensure that an accurate distance is calculated reliably between every pair of devices in the environment.

Location Detection (RTLS)

A UWB Real-time Location System (RTLS) consists of hardware and software which allows precise location tracking (down to 10 centimeter accuracy) of assets in an indoor environment. Positioning systems consist of two main components: anchors and devices.

• Anchors are devices with a fixed and known position. These are normally installed under the ceiling in the corners of the tracked area, in order to have full visibility of devices within the area.

• Small wearable devices communicate with anchors in order to allow accurate position tracking (the same device hardware can either operate in RTLS mode or alternatively can be used in pure peer to peer distance detection solutions without any anchors in place).

The position of a device is determined by first measuring the distance between the device and 3 or more anchors. In a similar process to how GPS positioning works, these distances can then be used to determine the precise location of the device using a technique called trilateration.

There are two main techniques used to detect the exact position of each device:

• Two Way Ranging (TWR) using Time of Flight (TOF) measurements

• Time Difference of Arrival (TDOA)

Two Way Ranging is already described above in the Distance Detection section, but it can also be used between devices and anchors as a technique to detect distances in order to allow position calculations. Time Difference of Arrival is the second method which can be used in RTLS systems.

Positioning using Time Difference of Arrival (TDOA)

In this scenario each device just needs to send a short pulse (or “blink” signal) at regular intervals, and can immediately switch itself off until the next time interval (or until it detects movement).

Each anchor listens for these pulses, records their exact Time of Arrival and sends the information to the central RTLS server.

With this information from at least 3 anchors, the RTLS server can calculate the exact position of each device. However, in order for this to work, the anchors need to have clocks which are always precisely synchronised.