If you want to cover a 15km radius then, as far as I know, that is impossible with a single device (directional antenna wouldn't cover radius) what you will need is multiple access points
Take a look at this site http://www.radiolabs.com/products/wireless/wireless.html, they sell powerful directional antennas and amplifiers. Look at the WiFi Parabolic Antennas, the higher the stage, the more distance you will get.