Overview
With the development of Bluetooth Low Energy (BLE), a variety of Bluetooth ranging and positioning algorithms have emerged, including RSSI (received signal strength indication), AoA (angle of arrival), AoD (angle of departure), ToA (time of arrival), TDoA (time difference of arrival), and ToF (time of flight). These methods differ in implementation complexity, power consumption, and accuracy, so system designers choose an approach that matches budget and accuracy requirements. Among these, RSSI-based positioning is widely used because it offers low power consumption, low cost, and simple implementation. This article explains the principles of an RSSI positioning system.
What RSSI Represents
RSSI stands for Received Signal Strength Indication. It is an indicator of the received signal power at the radio link layer and can be used to assess link quality or adjust transmit power. RSSI-based positioning estimates the distance between a transmitter and a receiver by measuring signal attenuation and then uses those distance estimates for location computation. In typical BLE deployments, the transmitter is a Bluetooth beacon and the receiver is usually a smartphone.
As a BLE device broadcasts, the farther the receiver is from the transmitter, the weaker the received RSSI value; the closer the receiver, the stronger the RSSI. Received RF signal levels are usually reported in dBm and are typically negative values: larger (less negative) values indicate stronger signals. RSSI values commonly fall in the range from 0 dBm to -100 dBm, where 0 dBm represents an idealized case that does not occur in practical deployments.
Detection Modes: Presence and Trilateration
Depending on application requirements, Bluetooth positioning is often classified into presence detection and trilateration. Presence detection is used for tasks such as patrol verification or attendance logging: a single transmitter and a single receiver can be sufficient to demonstrate presence within a defined area, and the underlying principle is simple.
Trilateration Using RSSI
Trilateration assumes fixed transmitter positions. For example, suppose three beacons are deployed at known coordinates BS1(x1, y1), BS2(x2, y2), and BS3(x3, y3). A receiver at point E measures the RSSI values from each beacon. By mapping those RSSI values to distance estimates and applying the three-point trilateration principle, the receiver position E(x, y) can be inferred from the intersections of the distance circles centered at each beacon.
Practical Considerations
Real-world implementations are more complex than the idealized description above. Algorithms must account for environment-dependent path-loss factors, multipath effects, and temporal noise. Filtering techniques are commonly applied to smooth RSSI measurements, and some systems combine RSSI-based estimates with inertial sensors to improve stability. Even with these measures, positions computed solely from Bluetooth RSSI are approximate and subject to errors introduced by the propagation environment.
