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With in the limitations you've got you may want to look at setting up a Point-To-Point link using 2 directional antennas. Here google is your friend as there any number of folks tinkering around with 802.11 class WiFi stretching signal ranges well beyond the RFC standards. That your up high can help, think about clear line of sight and pin-point accuracy in aiming the antennas at each other. The only other wireless option would be to look into using wireless broadband hardware outside of the 802.11 WiFi Class of equipment. Then there is the option of fiber, but there is the cost.