[yongkimleng]

For starters you may want to look into changing the wireless link into a 802.11x based system. That way you can legally use YAGIs or omnis and and higher power units.

I'm not clear if you are allowed to modify the antenna or not. From my understanding FCC regulates the power emissions of wireless devices. E.g. 200mW for user 802.11 devices. What antenna, using RX amps, or how many of such devices within an area shouldn't be a problem.