What could be the possible causes of wireless interference for outdoor implementation using the latest technology 802.11ac. Are trees considered as interference?.
Trees (also books or anything made out of cellulose) are a nightmare, they block most of the signal even from 2.4Ghz frequencies.
It sounds like you call interference both losses from propagation and real interference. But, theres a difference, because you can lose signal power to a level that the SINR becomes too low and the channel noise becomes a significant problem. Also situations where multipath signal reflection generates collisions at the receiver or situations where other microwave sources uses the same channel of your AP, for example.
802.11ac is restricted to 5Ghz frequencies. Higher the frequency, lower the possibility to go through walls, for example. 5Ghz signals might even be "trapped" inside the room where the access point is located.
It's a too generic question.. but the pack of usual problems, same as earlier 802.11s: Refraction, Defraction, Reflection, Propagation losses (related to distance), Channel interference (rarer in 5Ghz), Collisions, Low signal power from client -> AP compared to AP -> client
This is a very broad question, so I'll give you a very broad answer: anything in the propagation path will reduce the signal. So yes, trees buildings, people, etc will interfere. Other objects will reflect signals and cause mutlipath interference. Other radios operating on the same or nearby channels will also interfere.
At higher data rates, a higher SNR is needed and tolerances are tighter, generally speaking. So in that sense 802.11ac is more susceptible to interference.
Another, often overlooked, problem with outdoor Wi-Fi implementations is distance. Wi-Fi performance degrades with distance, even at distances within the specifications, and especially at higher frequencies. Wi-Fi was designed to operate up to 100 meters, just like UTP, but it will not deliver full performance for the full 100 meters.
I have seen it work at much longer distances, but the users complained that they were not getting advertised data rates at several hundred yards. They didn't realize that they had far exceeded the specifications and asked how to boost the power so that it works better. They wanted advertised data rates regardless of the distance from the single AP they had installed.
It's not just the radio(s) in the AP(s); the radios in the clients need to be able to reach the AP(s), too. Boosting the AP radio power doesn't doesn't change the capabilities of the radios in the clients.
Just remember, when you want density (sustainable service to lots of people) bring your channels down to 20MHz. When you want performance (Less people, higher speeds) you may span your channels to 40MHz-80MHz to take full advantage of AC's capacity. As you move down to a short frequency (2.4->5Ghz) you will lose penetrating power in materials; this is general as many materials will interact and obscure differently compared to the frequency range you choose. Trees have lots of water and 2.4 Ghz loves to absorb into polar molecules; working like your microwave.