Investigation of UHF RFID tag backscatter

The results of this paper show that the backscatter from an RFID tag can be strongly influenced by the presence of other tags. This occurs even if the tag spacing is not very small. In fact, for tag spacings approaching normal antenna array spacings, the group of tags appears to behave like a parasitically coupled antenna array. The change in tag backscatter amplitude or phase, which is used to modulate tag response onto the reader signal, can be less than or greater than that observed with a single tag. When the change is smaller, there may be a need to reduce the data rate or accept an increase in bit error rate. In some cases, the resulting change becomes small enough that the tag may not be read.