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02/27/2025Bluetooth, as a low-power, highly compatible short-range communication technology, has been widely applied in the field of digital car keys. By enabling seamless coordination between vehicle-end and mobile-end devices, Bluetooth technology can accurately detect the location of digital car keys, thereby facilitating seamless lock/unlock functions and significantly enhancing the user experience. This paper briefly evaluates three mainstream Bluetooth positioning technologies—Received Signal Strength Indication (RSSI), Angle of Arrival/Departure (AoA/AoD), and Channel Sounding (CS)—exploring their respective advantages and limitations in the context of digital car keys.
RSSI positioning is a way of estimating distance based on the attenuation of signals over distance during wireless transmission between two devices. Taking Bluetooth Beacon as an example, Bluetooth Beacon is a type of RSSI positioning, which can use either multilateration or fingerprint positioning solutions. The positioning scheme of multilateration is to calculate the position of the target based on the ranging results of at least three base station signals, but it is susceptible to environmental influences; The fingerprint positioning scheme calculates the target position by matching pre-collected fingerprint libraries, which has good adaptability to complex environments but is easily affected by environmental changes.
The AoA/AoD positioning algorithm is a positioning method introduced in the Bluetooth 5.1 standard. It calculates the relative angle between the receiver and transmitter by calculating the signal phase difference between multiple antennas of Bluetooth devices, and uses multi angle intersection to calculate the target position.
AoA is a method of estimating the Angle of Arrival using the phase difference of the receiving antenna array. The transmitter uses a single antenna to transmit the signal, while the receiver has an array of at least two antenna units built in. When the signal passes through, a phase difference is generated as the signal reaches each antenna at different distances, which then calculates the direction of the signal target. The principle of AoD is the same, except that the roles of transmitter and receiver are reversed.The transmitter uses a multi antenna array for transmission, while the receiver uses a single antenna for reception.
In theory, the positioning accuracy of Bluetooth AoA/AoD can reach sub-meter level (within 1m). However, in practical applications, the positioning accuracy is greatly affected by the accuracy of the anchor installation position and attitude angle, and decreases as the distance to the anchor increases.
The new Bluetooth 6.0 standard, which includes CS functionality, released in August 2024. CS is a technique for distance estimation and mutual correction through Round Trip Time (RTT) and Phase Based Ranging (PBR), offering higher ranging accuracy and enhance security mechanisms.
• RTT refers to the duration required for a signal to travel from the transmitter to the receiver and return again.
• PBR is a method of determining distance by utilizing the phase variation of RF signals. PBR calculates distance by phase shifting the signals at both ends, but there is a problem of multiple solutions. The use of multiple signals with different frequencies can measure the phase difference between signals to eliminate multiple solutions.
When using CS technology to calculate distance, the transmitter (initiator) and receiver (Reflector) exchange information through 72 RF physical channels. Firstly, simple time synchronization is achieved by the transmitter and receiver, and then the signal is reflected by the receiver without modifying the phase. After completing the information exchange process on multiple frequencies, the transmitter measures the phase change between the transmitted signal and the reflected signal of the receiver, achieving accurate distance calculation.
Overall, compared to RSSI, CS has significantly improved positioning accuracy, and ideally, the positioning accuracy of CS can be within 1m; In addition, CS has significantly improved stability compared to RSSI.
Compared to AoA/AoD, the biggest advantage of CS is that it does not require the use of antenna arrays, thereby reducing device size.
In addition, CS provides enhanced built-in security features – reducing the risk of relay attacks through interactive phase ranging algorithms and RTT data encryption.
The following figure shows the performance test of Bluetooth CS using NXP KW45 chip. The curves in the figure represent the deviation distribution of distance measurement results with actual distance for each distance, and the curves in different colors represent the results of different distance calculation methods. From the figure, it can be seen that the Bluetooth CS has good consistency in ranging deviation and good dispersion of ranging results within a range of 25 meters.
Watchdata has accumulated profound R&D capabilities in the field of Bluetooth positioning technology, covering RSSI, AoA/AoD, and Channel Sounding. Leveraging extensive project implementation experience, we have successfully delivered high-quality solutions. Looking ahead, Watchdata will continue to focus on technological innovation and service upgrades, actively addressing market changes and customer needs to create more competitive products and services. We look forward to collaborating with more partners to drive industry development and jointly create a brighter future.