Real-time positioning using Bluetooth Angle Estimation

Angle of arrival and departure explained

Radio waves travel in one of two ways: planar or spherically. Location devices predominantly reply on planar waves to relay signals. A transmitter typically has one antenna that sends out a sine wave to receivers with a multiple antenna array. The line on which receivers and transmitters lie will determine how a signal is received. The receiver will pick up all incoming sine waves from transmitters that lie on a normal line that is perpendicular to its antenna array or channel. Conversely, receiving antenna will pick up phase differences if a transmitter lies outside the normal line. Sample are taken from each channel and consist of sample pairs made up of “In-phase” and “Quadrature-phase” readings. Receivers use ADC channels or RF switches to interpret received information. Complex value pairs have phase and amplitude data that is used to estimate an arrival angle algorithm. Radio and light waves travel at the same speed. Frequencies of 2.4GHz equivalent wavelengths are 0.125m. Antennas mustn’t be more than half a wavelength away from one another. This will distort the signal data. This is the process of angle of arrival (AoA).

Angle of departure (AoD) operates in the same way as angle of arrival, in terms of measuring phase differences; however, the roles of each device is reversed. Transmitters now have many antenna, while receivers only have one. In AoD, transmitting antenna are switched in sequence. The receiver knows the switching order, which means it can determine its own location by using the angle and position data from multiple beacons. This is how triangulation works. AoA tracks angles from single objects to determine their location. Bluetooth uses a combination of both these techniques to transmit and receive data. Using this method yields greater location accuracy, but its effectiveness largely depends on the structure of the locating system. Using these methods has transformed the asset tracking practices. Bluetooth’s latest trilateration algorithms enable direction finding on top of location signalling, further enhancing asset tracking systems.

Challenges with angle of arrival and angle of departure

The most challenging aspect of calculating angle estimates is the environment. These calculations are easy to configure in an ideal environment, but most facilities have many obstacles, like walls and shelves, that reflect the flow of radio waves. Coherent signals are delayed because of these obstacles, which can impact the accuracy of location estimations. Furthermore, signal polarisation can occur in real-world applications; something that is uncontrollable. Location systems have include solutions to this problem, particularly because mobile devices are the main disrupters. Signal noise and propagation create further delays and create more variables that need consideration. Some systems require powerful CPUs because angle estimation algorithms are conducted in real-time. The introduction of different devices like Bluetooth tracking tags and smart beacons has helped resolve some of these challenges. Applications that can be used on mobile devices somewhat alleviate the need for large computational power.

Emerging technologies to improve asset tracking

Bluetooth’s angle of arrival and angle of departure technologies have applications in more than just asset tracking. Wayfinding, thanks to the direction finding feature, and indoor positioning systems will also benefit from this method of location estimation. Effective RTLS systems require RF switches, antenna arrays and algorithms for accuracy. There are other location technologies such as RSSI. The Received signal strength indicator (RSSI) is used to measure the power of a radio single. When used in combination with AoA and AoD, trilateration algorithms can be used to determine the direction of a signal. AoA and AoD are the standard in Bluetooth location services. Asset tracking tags and smart beacons use this phase-based method to locate assets in real-time.