Degrees of Freedom and the Bounds on Information Density in Chip-Less RFID

High bit density and, simultaneously, elimination of reading errors are desired, e.g., in chipless radio frequency identification (RFID).This technology aspires to be cheap and easy to manufacture (by printing the tags with conductive ink).As such, chipless RFID aims to replace classical QR and bar codes and, in some cases, chipbased RFID tags.There are many associated issues on which the community is working, namely low bit density, limited reading angles, polarization selectivity, and others.This work deals with maximizing the bit density of planar passive scatters.The design of chipless RFID opens an interesting fundamental question: Having a piece of metal, how many different shapes can be found to be distinguishable in the far field considering a given reading precision?As such, we ask about the information represented by a perturbation of the initial shape.