Bluetooth technology plays an important role in our lives and is currently the most widely used short-range low-cost wireless connection technology. Today, we will briefly summarize the three technologies of Bluetooth positioning:
- Signal strength RSSI positioning
RSSI (signal strength) positioning is a way to estimate the distance based on the attenuation of the signal strength between two devices in the transmission distance. Taking Bluetooth Beacon as an example, Bluetooth Beacon is a type of RSSI (signal strength) positioning. Unlike 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/sending 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 signals, 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 can be calculated.
The principle of AoD is the same, and the angle is calculated by using the signal phase difference. It's just that the roles of the transmitter and receiver are swapped. 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, and can calculate the direction of the wave through the received signal, and then locate it.
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:
https://www.bluetooth.org/DocMan/handlers/DownloadDoc.ashx?doc_id=553104
Channel Sounding (CS) 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.
Channel Sounding (CS) defines 79 RF physical channels in the 2.4GHz band, and 72 are actually used. As shown in 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 all or close to the main broadcast physical channels of low-power Bluetooth (2402/2426/2480MHz), and are not used to avoid interference.
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: phase based 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 using two or more signals of different frequencies, the phase difference between the signals can be measured to accurately estimate the distance.
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 sends a signal (Iinitiator) and the receiver completes simple time synchronization. Then the receiver reflects the signal (Reflector) without modifying the phase. When completing multiple transmissions at frequencies fi, the Initiator measures the phase change between the transmitted signal and the received signal and performs an algorithm estimate to achieve accurate distance measurement in this alternating transmission and reception process.
- Compared with RSSI and AoA/AoD, what is the future of CS?
Compared with RSSI, CS has significantly improved positioning accuracy. Under ideal conditions, the positioning accuracy of CS can be within 1m; in addition, the stability of CS is much improved compared with RSSI, because RSSI positioning is based on signal strength, and if the signal strength is unstable, the positioning is also unstable.
Compared with AoA/AoD, the biggest advantage of CS is that it does not require the addition of array antennas. CS is a phase radar mode that can be achieved with a single antenna.
In addition, CS provides enhanced built-in security features to reduce the risk of "man-in-the-middle" attacks. When using RSSI, AoA/AoD technology for distance measurement, it is very vulnerable to "man-in-the-middle" attacks because these technologies infer distance through signal strength. Therefore, if an attacker can artificially enhance the signal, the receiving device will detect a stronger signal and judge that the target is closer. The phase-based ranging algorithm and link layer data encryption process of Bluetooth channel detection can prevent relay attacks.
It can be speculated that Bluetooth channel detection will play an important role in high-precision positioning services in the future.