How RSSI and LQ Actually Work on ExpressLRS: A Simple Guide for RC Makers

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Introduction

If you’re into RC flying or DIY drone projects, you’ve probably heard of ExpressLRS (ELRS). It’s a popular open-source radio control system known for its long range and low latency. Two crucial terms you’ll encounter when flying with ELRS are RSSI and LQ. But what exactly do these mean, and how do they work under the hood?

In this article, we’ll break down RSSI and LQ in a straightforward way, helping you understand how they impact your flying experience.

What is RSSI?

RSSI stands for Received Signal Strength Indicator. In simple terms, it’s a measurement of how strong the radio signal is from your transmitter (TX) when it reaches your receiver (RX) on the model.

Think of RSSI like the bars on your mobile phone — more bars mean a stronger signal and a better connection. In ExpressLRS, RSSI is measured in dBm (decibels relative to 1 milliwatt), with values typically ranging from around -120 dBm (very weak) to -30 dBm (very strong).

How RSSI Works in ExpressLRS

ExpressLRS receivers continuously measure the power of the incoming signal. This value is used internally to:

  • Help the receiver decide if it’s receiving a usable signal.
  • Send RSSI data back to your flight controller or OSD (On-Screen Display).
  • Inform failsafe and range testing decisions.

It’s important to note that RSSI is a raw indicator of signal strength, but it doesn’t tell you directly about the quality or reliability of the link.

What is LQ (Link Quality)?

LQ stands for Link Quality, a metric unique to ExpressLRS. Unlike RSSI, which measures signal strength, LQ gives you an idea of how clean and error-free the communication link is between your transmitter and receiver.

LQ is expressed as a percentage, from 0% (no link) to 100% (perfect link). It takes into account:

  • Packet loss — how many data packets fail to arrive correctly.
  • Bit errors — mistakes detected in the received data.
  • Retries — how often the receiver asks for the same packet again.

How LQ Works in ExpressLRS

ExpressLRS uses a fast, robust error-checking and correction algorithm. The receiver tracks how many packets it successfully receives without errors versus how many need correction or retransmission.

The LQ value is then calculated based on this packet success rate, giving you a real-time snapshot of how reliable your link is. A high LQ means you can expect smooth control and telemetry, while a low LQ warns of potential dropouts or control lag.

RSSI vs LQ: What’s the Difference?

It’s easy to confuse RSSI and LQ because both relate to your radio connection, but they measure different things:

  • RSSI measures signal strength.
  • LQ measures signal quality and reliability.

For example, you might have a strong RSSI reading but poor LQ if there’s interference or multipath distortion affecting packet integrity. Conversely, a slightly weaker RSSI with a clean environment can still give you a high LQ.

Practical Tips for UK Hobbyists

Here are some handy tips to get the most from RSSI and LQ readings on ExpressLRS:

  • Use LQ as your main guide: It’s a better indicator of how well your control link is performing.
  • Check RSSI during range tests: It can help you understand signal fade and antenna placement.
  • Keep antennas clear: Obstructions and orientation affect both RSSI and LQ.
  • Update firmware regularly: ELRS improvements often enhance link stability and metrics accuracy.

Conclusion

RSSI and LQ are both important tools in your ExpressLRS toolkit. Understanding that RSSI tells you how strong your radio signal is, while LQ tells you how good the connection quality is, can help you troubleshoot and optimise your RC setup.

Next time you fly, keep an eye on these values via your OSD or transmitter display to ensure a safe and enjoyable flight experience!

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