Bluetooth signal strength is measured using a metric called Received Signal Strength Indicator (RSSI).
Understanding Bluetooth RSSI
The Bluetooth radio provides RSSI measurements whenever it scans for devices. Here's a breakdown of key characteristics of these measurements:
- Logarithmic Scale: RSSI values are on a logarithmic scale.
- Negative Values: These values are always negative.
- Unit: They are measured in decibels relative to a milliwatt (dBm).
- Signal Strength: Higher (less negative) RSSI values indicate a stronger, more robust signal.
RSSI Measurement Details
| Feature | Description |
|---|---|
| Scale | Logarithmic |
| Sign | Negative |
| Unit | decibels relative to a milliwatt (dBm) |
| Interpretation | Higher (less negative) values indicate stronger signals. |
Practical Insights and Examples
- An RSSI of -30 dBm is a very strong signal, usually indicating devices that are very close.
- An RSSI of -70 dBm represents a much weaker signal, often near the edge of reliable connection.
- An RSSI value can fluctuate based on environment, obstacles, and even device orientation.
What RSSI Doesn't Tell You
While RSSI gives a good idea of signal strength, it doesn't directly translate to exact distance. This is because:
- Environments affect signal strength (e.g., walls can weaken signals)
- Devices have varying antenna designs that affect signal strength.
Example Use Case
- A Bluetooth device scanning nearby devices would receive RSSI readings for each device detected. This helps the device decide which signals are strongest and thus the best to try and connect with.
In essence, Bluetooth measurement relies on RSSI, a logarithmic, negative dBm metric that indicates the strength of the received signal, with higher (less negative) values representing stronger signals.
