Impedance measurement

Sir/Madam,

  I have been designing a Differential drive rectiifer for UHF passive RFID in 130nm CMOS process. As the design of rectifier completes I got stuck with the impedance measurement, i.e. for finding input impedance of rectifier using Cadence Virtuoso. As it is a non-linear circuit which simulation should I choose to find or how should I find the input impedance of rectifier in Cadence Virtuoso, which is essential for matching network design.

 Thanks and Regards,

jithin