We want to enable wireless lan connection between two building in long distance (e.g 500 meters).

Dear friend to establish a connection like this mean meters,it is much too far to be in usable range for WiFi. WiFi is designed to, in the best case, transmit throughout one small household. Also, physical barriers, like walls, cars, other buildings will badly effect  the signal. You might be able to get WiFi through one or maybe even two walls if you're lucky, but to get it meters, you'd have to transmit at power higher than allowed by the standards bodies, and use an extremely directional antenna, with line of sight (no barriers in between).

i can help u with this issue 

 what is your bandwidth speed?