Are there any RFID major patents limit other company's devel

Only see big guns like TI, Philips etc doing RFID quite well....is this because some RFID patents stop other companies from doing the similar designs, such as RFID Tag?

Thank you.

EM-MArin , Atmel , Micrel etc etc,
even trolleyscan have own patented tecnology.
there is not any special patent , which may forbid rfid development by slightly different way.