We recently setup a 1 mile client bridge between a warehouse and an office. years ago back before even G came out, we setup a Point to Point between two schools which were about 1,000 feet apart. Back then there was no 1G Ethernet, and dial up was still pretty common. So I had the privilege to experience the setup of two extremely different wireless connections. Both setups had the same principles hold true, over 10 years apart from each other but some things never change. Line of Sight and interference.
The thing that anyone who has worked with WiFi in any sort of long range scenario, granted 1 mile is not long range, but its not from floor 1 to floor 2, its different, is line of sight is not what it sounds like. Well maybe it is, just not how we initially think of it. Line of sight is actually more math and Geo spacial info than it is “Can I see the place where the signal is going.” Especially when visibility does not allow you to see the other location because you are 5 miles away. To learn what line of site is look at this article on Fresnel Zone.