Signal strength RSSI positioning
RSSI (signal strength) positioning is a way to estimate the distance between two devices based on the attenuation of the signal strength during the transmission distance. Take Bluetooth Beacon as an example. Bluetooth Beacon is a type of RSSI positioning. Different from the positioning method of a single gateway base station, Bluetooth Beacon is a triangulation solution, that is, through three or more base station signals, more accurate location information is cross-calculated, which can range from 1m to 3m.
AoA/AoD positioning
DF (Direction Finding) direction finding positioning includes the AoA (Angle of Arrival) and AoD (Angle of Departure) positioning algorithms. It is a positioning method introduced in the Bluetooth 5.1 standard. The Bluetooth device identifies the arrival/transmission direction of the transmitting node signal, thereby calculating the relative angle or orientation between the receiver and the transmitter.
AoA is a method that uses antenna arrays and phase differences to estimate the angle of arrival. The transmitter uses a single antenna to transmit the signal, and the receiver has at least two antenna arrays built in. When the signal passes, a phase difference will be generated due to the different signal distances received by each antenna, and the direction of the signal is calculated.
The principle of AoD is the same, and the angle is calculated using the signal phase difference. It is just that the roles of the transmitter and the receiver are exchanged. The transmitter has multiple antennas, while the receiver has a single antenna. The receiver uses the angle relative to the transmitter like the angle of attack. The direction of the wave can be calculated through the received signal, and then the positioning is performed.
Theoretically, the Bluetooth AoA/AoD positioning accuracy can reach sub-meter level (within 1m).
Channel Sounding CS Positioning
The new version of the Bluetooth standard that includes the Channel Sounding (CS) function has not yet been released. However, it is understood that the Bluetooth Technology Alliance (SIG) included this technology in the draft technical specification (Change Request R02) released in November 2022, which is expected to be officially released in 2024-2025, and may be updated in Bluetooth 5.5 or Bluetooth 6.0. The draft specification can be downloaded from the official website of the Bluetooth Technology Alliance: Click to view
Channel sounding was previously called high-accuracy distance measurement (HADM), which is a technology that uses round-trip time (RTT) and phase measurement (PBR) to estimate distance and correct each other, with higher accuracy and safer protection mechanisms.
* Round-trip time (RTT) refers to the duration required for a signal to travel from a transmitter to a receiver and back again.
* Phase ranging (PBR: phasebased ranging) is a way to determine distance by using the phase change of an RF signal. At frequency f, PBR measures the phase shift of a signal as it propagates over a distance D to calculate the distance. When two or more signals of different frequencies are used, the phase difference between the signals can be measured to accurately estimate the distance.
Channel detection defines 79 radio frequency physical channels in the 2.4GHz frequency band, and 72 of them are actually used. See the following table:
From the table above, we can see that the definition of CS physical channels is the same as the 79 physical channels of classic Bluetooth. The 7 channels that are not actually used are or are close to the main broadcast physical channels of low-power Bluetooth (2402/2426/2480MHz), and are not used to avoid interference.
When using channel detection to estimate distance, the two devices are a transmitter and a receiver. The two devices exchange information through 72 RF physical channels. First, the transmitter transmits a signal (Iinitiator) and the receiver completes simple time synchronization. Then, the receiver reflects the signal without modifying the phase (Reflector). When completing multiple transmissions at frequencies fi, the Initiator measures the phase change between the transmitted signal and the received signal, and performs algorithm estimation to achieve accurate distance measurement in this alternating transmission and reception process.