rfid tag antenna in uhf band 868Mhz using ads agilent or cst studio

Hello people here ;
I want to design a miniature antenna(20mm*20mm) for rfid tag using cst studio or ads agilent and higgs 3 chip in uhf band . I'm new in this field of design but i have basic notions about rfid principle ...
Can someone help to getting started designing this antenna .
Thank You