Controllerless Networks

Is it possible to do wireless backhaul with a single IAP or does this require an Aruba AP, router or controller downstream?  In a situation where a client has another vendor's AP already installed, is it possible to do 802.11ac outdoors with an Aruba IAP or is it necessary to install two IAPs or a controller based solution?

You need either (1)  two Aruba Instant APs or  (2) two Aruba Campus APs with a controller.

Generally you need two of the same vendor's APs to do a mesh link/wireless bridge. If the other vendor's device supports 'client bridge mode', then you may be able to get away with just one Aruba AP/IAP and convert the other device to a client bridge.