Understanding RSSI and LQ on ExpressLRS
If you’re into RC flying or DIY radio projects, you’ve likely come across the terms RSSI and LQ, especially when working with ExpressLRS (ELRS). These two metrics are crucial for knowing how well your radio link is performing. But what do they actually mean, and how does ExpressLRS use them? Let’s break it down.
What is RSSI?
RSSI stands for Received Signal Strength Indicator. It’s a measure of the power level your receiver detects from the transmitter’s signal. Think of it as the "volume" of the radio signal reaching your receiver. A higher RSSI means a stronger signal, which is generally good news for maintaining control over your craft.
However, RSSI alone doesn’t tell the whole story. A strong signal doesn’t always mean the link is reliable if there’s lots of noise or interference.
How Does ExpressLRS Handle RSSI?
ExpressLRS receivers measure RSSI internally from the radio chip itself, often the ESP32 or an SX1280/868/915 module, depending on your setup. This measurement is updated frequently and sent back to your transmitter or flight controller, so you can see signal strength in real-time.
In ELRS, RSSI is usually reported as a percentage or a dBm value. Typical values range from about -120 dBm (very weak) to -30 dBm (very strong). The firmware converts these raw signal levels into user-friendly numbers.
What About LQ (Link Quality)?
LQ stands for Link Quality. Unlike RSSI, which measures signal strength, LQ estimates how clean and reliable the signal is. It factors in how many packets are being received correctly versus those that are lost or corrupted.
In simple terms, LQ is a measure of the connection’s integrity. A high LQ means your control commands and telemetry data are getting through reliably, even if the RSSI isn’t maxed out.
ExpressLRS’s Approach to LQ
ExpressLRS calculates LQ based on the percentage of successfully received packets over a short time window. It smooths out momentary glitches to avoid flickering numbers, giving you a stable readout.
ExpressLRS firmware typically displays LQ as a percentage from 0% (no link) to 100% (perfect link). This helps pilots quickly assess if their model is flying safely or if they’re approaching a risk of signal loss.
Summary: RSSI vs LQ — What Should You Watch?
Both RSSI and LQ are important, but they serve different purposes:
- RSSI: Indicates how strong the signal is at your receiver.
- LQ: Shows how reliable and clean that signal is.
For example, you might have a decent RSSI but low LQ if there’s interference or multipath reflections causing packet loss. Conversely, a slightly lower RSSI with high LQ often means a solid, clean link.
Why This Matters for UK RC Hobbyists and Makers
In the UK, where the 868 MHz band is popular for ExpressLRS, understanding these metrics helps you optimise your setup. It can inform antenna placement, transmitter power settings, and flight planning. By monitoring RSSI and LQ, you reduce the risk of losing control due to weak or noisy signals.
If you’re building your own ExpressLRS modules or tweaking firmware, knowing how these values are derived helps you customise your system for the best performance.
Final Tips
- Keep an eye on both RSSI and LQ during flight for a complete picture.
- Use quality antennas and proper orientation to improve both metrics.
- Remember that LQ is often a better indicator of real-world link reliability.
- Experiment with ELRS settings like power output and packet rate to optimise link quality.
With these insights, you’re better equipped to get the most from your ExpressLRS setup, whether you’re flying FPV or building custom radio gear.
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