As you might know, most of the FrSky receivers and transmitters are available in two versions, they are sometimes referenced as EU or non EU or EU LBT and FCC.
FrSky themselves calls the EU version LBT and the non EU version FCC. When you download firmware files for flashing your receiver they will be named accordingly.
For example if you download the ZIP package for the R-XSR receiver you will find two files inside:
Which version you will use depends on your transmitter and your geographical location. If you are from any other region than the EU, you will most probably have a transmitter that uses the FCC firmware and will thus use the FCC firmware on your receivers. If you are from the EU you will have to use the LBT version on both your transmitter and receivers.
Keep in mind, that the receiver firmware has to match your transmitter firmware. Radios that have the FCC firmware will only work with receivers that are running the FCC firmware and the other way around for the LBT version.
What kind of firmware are there?
It is important to note, that there are three different places where firmware is installed, when it comes to FrSky Transmitters and Receivers.
- The firmware for the receiver - this can either be FCC or LBT
- The firmware for the transmitter - this will most likely be OpenTX
- The firmware for the XJT module (the actual hardware that is responsible for RF transmission) of the transmitter - this can again be either FCC or LBT.
On some newer version of FrSky transmitters that come with the LBT firmware, users have reported that they can not flash the FCC version to the XJT module and can thus not make an international version of their transmitter - the other way around does not seem to be a problem, so the XJT module of an international, FCC transmitter can be flashed with the LBT firmware.
What is the FCC?
The FCC is the Federal Communications Commission of the United States. One of its tasks is to regulate the use of radio frequencies within the United States. Radio equipment must be certified for compliance with certain technical standards designed to limit interference to other devices. This certification has to be done by the manufacturer. Once this certification is obtained, the user is allowed to operate those devices without any special licence. It is in the responsibility of the user to make sure that he is allowed to operate this device without any additional license.
Devices that have FCC certifications come with an ID issued by the FCC, the so called FCCID. There is a database that can conveniently be searched to check such an ID.
Each manufacturer gets an ID and every product by that manufacturer gets another ID. So based on the FCCID you can identify the manufacturer and the exact product.
For example: FrSky has the grantee ID XYF and the Taranis X-Lite has the product ID XELITET, so the total FCCID for the Taranis X-Lite by FrSky is XYFXELITET. This ID is placed somewhere on the product and allows the user to easily verify that the device complies to FCC regulations.
What does LBT mean?
LBT stands for Listen Before Talk is also know as listen before transmit. With LBT the device is required to first listen to its environment before it is allowed to transmit. The device has to listen for a free channel before it is allowed to transmit any data. In this context free means that the signal strength on this channel has to fall under a certain threshold before the device is allowed to transmit on this channel. This technology is used to make sure that multiple devices on the same band can broadcast safely without interfering with each other.
LBT is also part of ETSI (European Telecommunications Standards Institute) to which all RF devices in the EU have to be compliant to.
Advanced Continuous Channel Shifting Technology (ACCST)
When transmitting via frequency hopping a certain amount of data is transmitted on a hop frequency, then the hop frequency is switched and the next chunk of data is transmitted on the next hop frequency and so on. Both, transmitter and receiver know the pseudo-random order in which every hop frequency is used. So as soon as the receiver receives it’s data chunk on frequency n it switches to hop frequency n + 1 and waits for the next chunk. Should data not arrive in a defined interval it will also switch to the next hop frequency.
Since every frequency is only used for a very short time, the amount of data lost in the worst case of interference is kept to an absolute minimum.
Frequency hopping allows hundreds of receiver/transmitter pairs to operate at the same time on the same frequency band.
LBT and frequency hopping
As we can see from the previous paragraphs, frequency hopping might not be possible on all frequencies when it has to comply with LBT - there might be a broadcast on one of the hop frequencies, which then in turn can not be used during frequency hopping. Instead, Adaptive Frequency hopping using LBT has to be implemented according to ETSI specifications.
The transmitter has to do a Clear Channel Assessment (CCA) check on the hopping frequency to make sure there is no other broadcast happening. If a broadcast is happening on this frequency, the frequency can not be used. The check duration has to be 0.2% of the dwell cycle and at least 20 micro seconds long.
Further it is specified that a hopping frequency is not allowed to be occupied for longer than 60 milli seconds and has to be followed by a minimum idle time of 5% of the channel occupancy time and at least 100 micro seconds.
Why not just use LBT altogether?
It seems, that LBT is a technology that only does good - so why does FrSky not generally ship with LBT?
This question is easily answered:
Because there are already so many FCC devices out in the wild that it would be insane to ask all their users to update all the receivers they already own, just because they upgraded to a new transmitter.
Disadvantages of LBT
Technically there are non in comparison to the FCC version - the link is reliable on both versions.
The only downside is, that you will not be able to run receivers in D8 mode, but this can be easily fixed by de-selecting the EU check-box in the OpenTX companion - no need to upgrade the firmware of the XJR module - still you would be operating in a gray area within the EU.
Chris is a Vienna based software developer. In his spare time he enjoys reviewing tech gear, ripping quads of all sizes and making stuff.
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