Home-Made Optical P2P Data Link (2003)

FCC Part 15 allows for intentional radiators and is the basis for WiFi. With directional antennas, one can make a P2P link that reliably exceeds 10km. A world record has been set at 381km [https://www.wired.com/2007/06/w-wifi-record-2] but I don't know if the power level into the antenna was low enough to be Part 15 compliant. In college, I set up a 40km link with compliant power and antenna gains (using offset feed Primestar satellite dishes), and we use Part 15 compliant P2P wifi in St. Louis for our ham radio repeater control and data network with links between 2 and 5km.

Even at maximum allowed tx power? Two of these should do 1km LoS easily at 90-150 mW: https://www.amazon.com/TP-Link-Directional-Parabolic-Connect... Of course, more power, more range (but also noise, so in reality you don't want more than ~200mW with standard routers)

I used one of them to get a good 54Mbps connection with an AP ~400 meters away (5th floor to 2nd floor, only some treetops between). And that's basically the antenna just picking up the unboosted signal from the router (Linksys WRT54gs @ 85mW iirc)

Unlicensed point-to-point links are permitted in the US provided that the Part 15 EIRP limits are respected. You are allowed to push up to 53 dBm of EIRP on the 5.8 GHz ISM band (30 dBm power + 23 dBm antenna gain), which gives a lot of link margin to play with.