Reference: https://meshtastic.org/docs/overview/radio-settings/#presets
What does the link budget, expressed in dB, actually refer to?
Does that mean I can expect to have a usable signal down to the RSSI listed as the link budget? e.g. LongFast will be usable down to -153 dB RSSI where ShortFast will need a received signal strength of at least -143 dB?
I’m debating on whether to deploy my mesh in LongFast mode (default) or ShortFast. I’ve received advice that, in the wild, ShortFast has “close enough” range to LongFast but with the benefits of being roughly 10x faster and reducing the airtime per client (which produces other benefits). They said terrain, height, and line-of-sight are more critical factors (which makes sense).
Looking at the link budget table, there’s also a 10dB difference between ShortFast and LongFast. I do know that the decibel scale is logarithmic, so that difference is fairly significant.
Basically, I need to make sure I’m understanding link budgets correctly so I can put the proper values into the site planner.
You must log in or register to comment.
I still don’t fully understand how it’s applied to the real world, but I’m a bit closer after reading this:
https://lora.readthedocs.io/en/latest/#link-budget
A link budget is the sum of all of the gains and losses from the transmitter, through the medium (aka free space), to the receiver in a telecommunication system. It is a way of quantifying the link performance.
The receiver sensitivity is the lowest power level at which receiver can receive or demodulate the signal.
Keep in mind it’s very unlikely that you will ever get down to -150 dBm and be able to decode just because of ambient noise. For example, where I live, my RSSI of noise is about -109, so the best possible I could decode would be a -129 signal.
You could also do what most others do and deploy in long fast and then keep an eye on the average percentage of airtime utilization and move it up to a higher speed once there’s enough clients in the area to need it. I have heard it’s best to keep your channel utilization under 30%. And so if your average started getting close to 30%, you could then change your mode to one of the faster ones.
Good to know this, thanks! Mine has never been higher than 2.0 so I’m assuming that’s fine lol.
Yeah, then you’re good. It’s not really a problem if it peaks above 30% sometimes, but if it’s up there on average, it could start to become a problem. So that’s why I say to keep an eye on the averages instead of the peaks themselves.
As an example, in my local mesh, there are about 10 nodes. And sometimes when people are talking, the channel utilization can get to 15% or so on long fast. But on average, it’s more like 3%.
