LQ and RSSI can be confusing for first time TBS Crossfire users. In this tutorial I will try to explain what the differences are between LQ and RSSI in Crossfire. I will also show you how to set it up in Betaflight and OpenTX on the Taranis TX to monitor your link quality and range.
If you are a new Crossfire owner, check out my guide on how to setup TBS Crossfire with Betaflight and Taranis X9D.
Link Quality (LQ) and Signal Strength (RSSI) are two different ways of measuring how reliable your radio link is.
Normally you can display one of them, or both of them if you want. I personally prefer using LQ over RSSI, but sometimes it’s useful to have both readings. For example, if you have low LQ, but high RSSI, it means you have lots of interference around you. But if you have low RSSI, it’s an indicator that maybe something is wrong with your antennas, or maybe you simply fly too far away.
A Crossfire LQ between 300 and 200 means 150 Hz Mode, between 200 and 100 it‘s 50 Hz Mode and then it switches to 4 Hz Mode below 100 LQ. If your LQ drops below 70% you should turn back immediately!
To learn more, make sure to read the rest of the article.
Differences between LQ and RSSI
RSSI is an indicator of received signal strength, measuring the power present in a received radio signal.
Besides RSSI, Crossfire receivers can also output LQ, which stands for “Link Quality”. It’s based on the percentage of signal data received.
You can also output a combined RSSI/LQ value. The RSSI/LQ value will always show the worse value between the two. This is useful if the used OSD only has one input for radio link statistics.
What is LQ?
The Crossfire LQ is scaled from 0% to 300%, not 0% to 100% like other traditional signal indicators. This is because of the increased data link in Crossfire.
Traditional remote controls can only do 50Hz, which means 50 commands per second. The Crossfire can transmit at 150Hz maximum, that’s 150 commands per second. If we assume 50 commands per second is 100% link quality, then 150 commands per second would be 300%.
Why use LQ for monitoring Link Quality?
To know the range limit based on RSSI, you also need to know the noise floor. Heavy noise level can screw up the RC link all the same regardless how strong the signal is. Noise floor depends on many factors including the environment and the electronic components in your quadcopter.
The general recommendation is to use LQ over RSSI and signal to noise ratio (SNR) as your primary metric for whether your radio link is healthy or not.
In many situations I’ve found to have bad RSSI while LQ is still high and working well.
LQ just seems to be more reliable for me than RSSI. You can also use RSSI/LQ if you want to be more conservative about range.
Display LQ (From Betaflight 4.1)
Since Betaflight 4.1, displaying LQ or RSSI from Crossfire is made super easy. You no longer need to assign LQ or RSSI to an AUX channel in the receiver (LUA script). It just appears in Betaflight now, all you have to do is:
- In configuration tab, Disable RSSI_ADC
- In Receiver tab, RSSI Channel, select Disable
- In OSD tab, you can now select Link Quality, RSSI in dBm, or RSSI
Link Quality also displays correct, from 300 to 0. LQ between 300 and 200 means 150 Hz Mode, between 200 and 100 it‘s 50 Hz Mode and then it switches to 4 Hz Mode below 100 LQ
Note that you need Crossfire firmware version 2.94 or newer for this to work. Otherwise you will get 0 LQ and RSSI.
Update (June 2020): Betaflight changed the way LQ is displayed in Betaflight 4.2. Now it shows the RFMD value (0-2) followed by the LQ value (0-99). While the RFMD value is at 2 (150Hz latency mode) you don’t even need to worry about the LQ value. Once the RFMD values drops down to 1 (50Hz latency mode) you need to start paying attention to the LQ value because anything below 80 while in RFMD 1 is warning territory and 70 is in the critical danger zone (you need to turn back immediately). If you ever see an RFMD value of 0 then your quad is already falling from the sky.
If you use an older version of Betaflight prior to BF4.1, then you’d have to following the instructions below.
Display LQ (Before Betaflight 4.1)
Assign Channel to Output LQ
Execute the “Crossfire” LUA script on your Taranis. (How do I do that?)
Scroll down to “Channel Map” and pick a spare Dst. channel and change it to “LQ”, for example channel 8.
Display it on OSD
You can display this value on Betaflight OSD.
Go to the “Receiver” tab in Betaflight Configurator, select the Aux channel you are using for LQ. For Ch8, you should pick “AUX 4”. (minus the throttle, yaw, pitch and roll channels)
In the OSD tab, you also need to enable RSSI to display the value on screen and that’s it :)
Setup Low LQ Warning in Tarains
LQ in Crossfire can output a wider range than OpenTX can interpret (0-300%). Therefore it’s split into 2 separate values in OpenTX (sensors in the telemetry tab): RLQY and RFMD.
RLQY stands for Received Link Quality. It’s the amount of the transmitted signal received by the receiver in percentage (0% to 100%).
RFMD means the Received Frequency Mode, and there are 3 RF modes:
- RFMD = 2 , 150Hz Mode,
- RFMD = 1, 50Hz Mode
- If RFMD = 0, 4Hz Mode (no telemetry)
Basically RFMD 2 is the Low latency mode for short distance flying and it provides the fastest update rate of 150Hz. Crossfire goes into RFMD 1 when you go for long range. If signal gets really weak, it will go into RFMD 0 and drop the update rate to 4Hz without telemetry.
By the way LQ can drop for a split second when it changes mode.
Here are what the logical switches are doing:
- L01 – this switch is triggered when we are in RFMD 1 or 0
- L02 – This is the warning switch. it’s turned on if L01 is on, and LQ is lower than 80%. There is a 0.5 second in case it’s just a glitch when mode is changing
- L03 – This is the critical warning switch. It’s turned on if L01 is on, and LQ is lower than 70%. There is also a 0.5 second delay for the same reason
Here are the Special Functions that play the audio warnings:
- SF1 – it plays the sound “Warn1” when L02 is switched on (80% LQ)
- SF2 – it plays the sound “Siren” when L03 is on (70% LQ)
You can use function “Play Track” to play any audio files you upload to the radio, here is a tutorial to teach you how.
!1x at the end of a function means it won’t play when you turn on the radio, and it only plays it once.
What’re the Ideal LQ and Lowest LQ
When you are in RFMD 2 (150Hz), the maximum LQ will be 300%. But really, the percentage of LQ in this mode is kind of irrelevant and you can ignore it. Because the system will switch to RFMD 1 (50Hz) if it requires better link quality.
You should start to pay attention to LQ in RFMD 1. If the signal quality continues to deteriorate it will change to the super slow RFMD 0 which only gives you 4Hz of update rates. It’s probably not great for acro flying on a quad, but should be okay to bring a wing/plane home.
A healthy long range link (RFMD 1) should stay above 90%.
LQ of 70% of lower is considered bad, If you see this you should turn back asap! If it’s consistently lower than that then you should check your antenna and frequency setting for potential setup errors.
- Apr 2018 – article created
- Jan 2020 – added tldr section, added instructions to display LQ for Betaflight 4.1