Tag modulation in UHF RFID

Can anyone suggest a link to read more about Tag modulation in UHF rfid?(backscattering)
I want to know how the tag is modulation the carrier wave?

Book: The RF in RFID

Could you tell me Back scatter modulation is same as ASK or PSK?
As per EPC Gen 2 protocol a tag can respond back to reader using ASK or PSK.
So which is same as Back scatter modulation in the case of PASSIVE tag?

The tag can choose between ASK or PSK. The changes the load impedance that is connected to the antenna. One approach is to change between matched and a short, then an ASK modulation is created. If the load changes its imaginary part a PSK modulation could achieved.

You could look for the EPC Gen 2 standard, probably it is available on the internet. The standard describes the modulation.