Here's an interesting paper about actually implementing this (sorry for paywall). http://ieeexplore.ieee.org/xpl/articleDetails.jsp?tp=&arnumber=5515168&queryT

Here's an interesting paper about actually implementing this (sorry for paywall). http://ieeexplore.ieee.org/xpl/articleDetails.jsp?tp=&arnumber=5515168&queryText%3Dchipless+rfid http://ieeexplore.ieee.org/xpl/articleDetails.jsp?tp=&arnumb...