Antenna simulation of Tag

HI,

I am trying to do power and link budget analysis for UHF RFID Tags.
I am trying out for FCC regulated 865~867MHz operation and 4W EIRP.
In order to do power budget analysis for the RFID Tag, I need to know how much power is received by the tag if reader transmits 4W EIRP.
I have to do circuit level simulation of the blocks of chip like Rectifer, Mod-Demod.
What is the best way to integrate both Tag antenna and Tag chip part?
What are the steps to be considered while simulating antenna? Which antenna has to be selected?
Is it possible to simulate the path loss? What are the different aspects to be considered while calculating path loss?
I am newbie to antenna simulation and none are familiar to antenna simulation in the place I work.
Request you to share and suggest your ideas.

Thanks!