Question about RSSI details.

[Reid Williams]

Hello everyone,

I've been following this thread on RCGroups and it has me wondering about the RSSI that I use on my fpv ships. Does anyone know if Frsky's RSSI is simply an indication of power on frequency like the spektrum guy claims or if it is an actual measure of link quality?

Supposedly nearby WiFi routers or other devices on 2.4 can cause RSSI to be high while link quality is low (bad packets/no packets) however I haven't experienced this with Frsky gear so perhaps they've already thought of this. Just wondering, as if it is just "dumb" RSSI perhaps it would be worthwhile to implement the more useful data in our telemetry receivers to get a better awareness of actual link quality.

 

[PressAllTheKnobs]

I would not worry. FrSky have had an RSSI alert since the DJT, their first telemetry capable module released (in 2010 I think) RSSI telemetry is probably one of the most popular features the FrSky system is known and loved for...after just working very well. With FrSky, RSSI is predictable enough that the alert is built in requiring no setup assuming you are using a Telemetry receiver which most are. I think if RSSI was "useless" then that would have showed up by now...

What I took from that discussion is that it seems that measuring RSSI with DSSS (which Spektrum uses) is problematic because the Cypress chip measures the power across the broadband DSSS transmission and so will capture the power of any interference in that band resulting in higher RSSI values that one would expect. The main claim of DSSS is that it can tolerate interference by spreading the data across a broad band.

Pure FHSS (which FrSky uses) uses narrow band transmissions which are less tolerant of direct interference but mean that the RSSI measurement will not include adjacent interference and any interference strong enough to seriously distort the RSSI measurement will likely kill the packet. Consequently it is reasonable to assume the pure FHSS RSSI measurement is a much more accurate measurement of the received TX power than is the case with DSSS. Consequently it should corresponds reasonably well with link "reliability". Remember, if you are receiving telemetry then the link quality must be reasonably good since Telemetry is returned by the RX after it receives a good packet. Both RSSI and Telemetry lost alerts indicate that you may not have a reliable link and should move your model closer.

This is a fairly simplistic take but it seems reasonable to me anyway.

 

[Reid Williams]

I see, thanks for the info. I wasn't aware that Frsky FHSS and Spektrum DSSS worked in such a different way. I wasn't worried about it as I've never had an issue, just curious as to how things work. Guess I have some homework to do on DSSS and FHSS.