RSSI (Received Signal Strength Indicator) is an indicator of the received radio signal power, measured in decibels (dB). The closer the number is to zero, the stronger the signal, and values like -80 dB indicate significant attenuation. In the systems internal positioning (indoor navigation) RSSI serves as one of the key tools, especially where GPS is unavailable. It is most often used with technologies such as Bluetooth Low Energy (BLE) and Wi-Fi (IEEE 802.11).
In BLE networks, RSSI helps to estimate the relative distance between the transmitter (tag, beacon) and the receiver, for example, to assess the quality of the connection or the power consumption of devices. In Wi-Fi networks, this indicator is used not only for positioning, but also for analyzing coverage, network diagnostics, and solving connection problems.
Here, the "noise level" is a background electromagnetic "blockage", and the calibration coefficient adjusts calculations for specific equipment. Since devices from different manufacturers may interpret the RSSI differently, the measurement scale may vary. The typical operating range is approximately -80 dB to -30 dB (the minimum level is when the device is very close to the transmitter).
How RSSI is used for indoor positioning and asset tracking
In the systems indoor asset tracking (asset tracking within RTLS — Real-Time Locating Systems) RSSI is one of the simplest and most affordable solutions. Its popularity is explained by the fact that many devices already support BLE or Wi-Fi, meaning there is no need to introduce expensive additional equipment.
To build a tracking system, you need:
- Beacons or tags (e.g. BLE tags, sensors, smartphones) that emit a signal;
- Receivers (gateways) that register the signal strength;
- A software platform (RTLS) that collects RSSI, analyzes data, and calculates asset positions.
Using the drop in signal power as you move away, the system can estimate the distance between the beacon and the receiver.
Algorithms based on RSSI
The classical mathematical model used to calculate distance based on a signal is power loss (attenuation) model:
Where:
- P(d) — measured signal strength;
- d is the distance between the devices;
- A and B are constants specific to a particular transmitter.
By default, A=−82A = -82A=-82, B=3B = 3B=3 are often used for BLE devices, but in practice, engineers "calibrate" these parameters for a specific environment, which significantly increases accuracy.
Then, to determine the position of the object, you can apply partial filter (particle filter)The system creates a set of hypothetical "particles" — potential object positions, each particle is assigned a weight depending on the discrepancy between the measured RSSI and the expected one according to the model. The final position is the weighted average of these particles.
Possibilities and limitations of the RSSI method
Advantages:
- Cost-effectiveness — you can often use an existing Wi-Fi or BLE infrastructure;
- Versatility — the technology is not tied to any one standard: it works with BLE, Wi-Fi and even RFID.
- Ease of implementation — Easily scales to different scenarios: tracking equipment, employees or other assets.
Disadvantages:
- Signal interference — Walls, furniture, and metal structures affect RSSI, causing reflections, attenuation, and multipath;
- Limited accuracy — RSSI can most often give a "zonal" localization, with an error of several meters;
- Dense infrastructure is needed — especially for large rooms; lack of receivers can create blind spots;
- Height problems (Z-axis) — the method does not determine the vertical location well (for example, on which floor the tag is located).
Comparison: RSSI vs AoA
One of the alternatives to RSSI is AoA (Angle of Arrival, the angle of arrival of the signal). Here are the key differences:
AoA often provides greater accuracy, especially in complex environments, but it is more expensive and requires equipment that supports Bluetooth 5.1 and an antenna array.
When should I choose RSSI for indoor navigation?
If it is important cost-effectiveness and you want to use an existing infrastructure (Wi-Fi or BLE).
If allowed accuracy within a few meters: for retail analytics, geomarketing, tracking personnel or assets in a warehouse, this level of accuracy is often sufficient.
If the project does not require strict submeter accuracy: in such cases, RSSI can be combined with other technologies (for example, AoA) — RSSI covers a large area, and AoA “adjusts” coordinates in critical areas.
RSSI remains one of the most common ways to indoor localizationespecially when simplicity, versatility, and low cost are important. However, it has its limitations: accuracy, sensitivity to the environment, and the need for calibration.
If the task requires high precision (for example, for robotics, AR navigation, or warehouse automation), it's worth considering methods like AoA. But in most cases, RSSI is the optimal choice for asset tracking, analytics, and indoor navigation.
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