Please refer to the following link:
https://supportforums.cisco.com/discussion/11967891/wap-321-best-practice
Also, a site survey would be helpful. Please refer to:
http://www.cisco.com/c/en/us/support/docs/wireless/5500-series-wireless-controllers/116057-site-survey-guidelines-wlan-00.html
http://www.cisco.com/c/en/us/support/docs/wireless-mobility/wireless-lan-wlan/68666-wireless-site-survey-faq.html
Hope that helps.