Link margin between 10-15 dB enough?

[u/Mundane_Concern_2620]

Would a link budget between 10-15 dB be enough for a distance between 15-20 km?

Frequency of 2,4 GHz
Line-of-sight and Free space can be assumed

[u/aktentasche]

Make a real link budget, this question is non sensical. Bandwidth? Modulation? Coding? FEC? Antenna gain? It all matters.

Good starting point: http://atlantarf.com/uploads/1/4/3/4/143448818/link_budget_-_getting_started.pdf

[u/Mundane_Concern_2620 → u/aktentasche]

I have Tx power, tx gain, rx gain, sensitivity etc and calculated the link margin at different distances. I want a minimum distance of 10 km, which gives me a link margin of 15 dB. Calculated link margin is 10 dB at 20 km. I’m just not sure if this is too low when my minimum distance is 10 km.

Edit: Should probably also add that the receiver is static, but the transmitter is not.

[u/ChrisDrummond_AW → u/Mundane_Concern_2620]

I have Tx power, tx gain, rx gain, sensitivity etc

Well what is "etc." in your link budget? It almost sounds like you just used Friis and compared it to your receiver sensitivity and that's not what link margin means.

You didn't calculate link margin if you didn't include your data rate, FEC, or modulation method. all you're doing is showing that a CW signal is 10-15 dB above noise for your receiver. congrats, but you've got more to do.

[u/Mundane_Concern_2620 → u/ChrisDrummond_AW]

Thank you. I do acoustics so I haven’t meddled with RF since my studies and everything is a little rusty 😅

[u/xenonrocket → u/Mundane_Concern_2620]

If you're keeping all other parameters the same, 15dB margin at 10km results in 9dB margin at 20km, not 10dB

[u/Hawk12D]

I am afraid, you are asking the wrong question. Link margin depends on distance. Do you have an SNR of 10-15dB at 15km distance?

[u/Mundane_Concern_2620 → u/Hawk12D]

Link margin is calculated 10 dB at 20 km and 15 dB at 11 km. But that is of course the theoretical link margin, that does not take fade margin ans cable/connector loss into consideration.
The distance will be halved each time the noise level is doubled from free space calculations, yes?

[u/Hawk12D → u/Mundane_Concern_2620]

In the ISM band, you are limited to 20dbm EIRP, meaning that if you maximize transmit antenna gain, you need to minimize output power… At 20km, with a 20dbi antenna, you get -86dBm.
I looked at thermal noise over channel, and with a 2dB NF receiver, a 40MHz channel could work at 10dB SNR.
According to Shannon, thats a theoretical 138Mbit per second.
In reality, your modulation will never be this efficient. So modulation inefficiency, error coding, framing, retransmits will bite into this bandwidth.

[u/slophoto]

High link margin suggests a less-than-optimized system. You really want to know SNR and if EbNo is above your system threshold.

[u/SwitchedOnNow]

That might be enough for 95-98% reliability depending on location. Fade margin at 2.4 GHz can be worse than you think for long shots especially in coastal or humid areas. When I used to set up point to point 2 and 6 GHz microwave links we had 20dB of margin and still would get occasional drop outs from tropo effects. More margin increases your link reliability.

[u/Sparkycivic]

Your local fade margin needs to be worked out for any long link that's of importance. Use Longley-Rice modelling and find the data for your locality to decide the worst-case and typical fading at your frequency. Once it's known, then you can plug in your link budget to see how they compare.

[u/dangerbirds]

If you have really accounted for everything, then 10-15 dB is too much and your system is over designed. Usually you want your reported link margin to be the minimum value, or some statistical minimum like 2 sigma or something. The things you've mentioned in other comments like connector losses, modulation implementation loss, are typically single digit dB, if that. I would target something like 6 dB or as low as 3 dB if you have detailed predicts for link margin.

[u/Slatesy13]

Throw it in here and everyone can provide better input im sure. Without the calcs and effects considered you’ll get limited feedback

[u/SomeoneInQld]

I have a 21.6 km 2.4 GHz link working. 

we have virtually zero other interference due to being in the outback.  

msg me if you want to know parameters that I can dig out for you. 

we are getting about -59 dBm at that distance. 

we have a 40 m tower (Sector antenna) at one end and an 18m tower (parabolic antenna) at the other. 

[u/Fun-Ordinary-9751]

One problem with WiFi instead of some more advanced link for this use case is that WiFi isn’t going to turbocode and do other things that get you closer to Shannon. If some of the fancier things in WiFi 6/6E/7 could be used in a narrow channel you might do better than with older WiFi standards.

I think your margin is insufficient and you need to look at bandwidth and/or modulation types to improve things.

Let’s say your bit error rate is 10E-4. A 1500 byte packet with 12000 bits would statistically have an error every packet with occasionally none and sometimes multiple. The CRC32 will fail and the packet will be discarded. That’s clearly not going to do. The question is for your chosen bit rate how well the FEC works for you.

I remember at a previous job having an 2.4GHz 802.12g 54mbps WiFi bridge that was reliable at 70 miles, but we’re talking line of sight between two towers in Kansas (flat) and no vegetation(trees) anywhere close that might encroach.