simulation of passive rfid

Hi.

I am designing a wireless sensor which is very similar with 13.56 Mhz passive RFID which must need a charge pump that harvests power from RF signal in order to feed the rest of components in a chip.

My question is what kind of power (or voltage) you receive on the tag antenna as minimum, but still enough to operate the chip? It will definitely depend on the distance between the tag and system, and design of the tag antenna, but let's say a reasonable distance (maybe up to 10cm) with a typical antenna design.

My question comes from this. I have made a charge pump in Cadence. It generates Vdd as below

Input (sine wave, peak-to-peak), Output (Vdd)
200mV, 0.7V
300mV, 1.1V
400mV, 1.5V
500mV, 2V
800mV, 4.6V

What if the maximum input from the tag antenna is below 300mV, my circuit may not work, I have to know what power or voltage I get from tag antenna side so that I, at least, approximate my charge pump criteria.

My second question is what could be the easiest way to simulate power efficiency vs Number of stage. I am using a typical Dickson's charge pump.

I do appreciate your precious time for leaving your clever opinions