Hello Nelson Chai,
Answers to your questions:
May i know if i use Ruckus 7341 901-7341-WW00 without zone director --> Yes, ZF7341 CAN be used in standalone mode.
Will AP it self will change channel base of spectrum from time to time --> Yes, if AP is set to use "smart select" in channel setting. Smart select feature enables channelfly feature. You can read more on channelfly -->
http://c541678.r78.cf2.rackcdn.com/fe...
Can it function will at environment noise level -80 dBm? I am not sure on this however if channelfly is enabled which is primarily programmed to benefit in high density and interference ridden challenging environments.
Hope this helps.