Reduction of Proximity Effects on UHF Passive RFID Systems by Using Tags With Polarization Diversity
A new technique that enables passive ultra high frequency (UHF) radio frequency identification (RFID) tags to be read when they are placed in close proximity in an array is presented. This paper demonstrates that, in a linear tag array with a tag separation of 1 cm, the interaction between the backscattered waves and incident wave causes a significant degradation in tag sensitivity. It is found that the use of tags that have polarization diversity can improve the read performance when they are placed in close proximity to one another compared with conventional linear tags. Two ways of achieving polarization diversity are studied in this paper, namely: 1) using a circularly polarized tag and 2) using a cross-polarized tag pair. Both methods show an improvement in close proximity read performance and it is experimentally demonstrated that by using cross-polarized tag pairs in an array, one achieves on average a 2.6-dB increase in read power margin for a 57-tag array with 1 cm separation compared with using conventional linearly polarized tags.
A Solution to Low Read Rate Problem in RFID Scattering Networks
High interference and low read rate have repeatedly been reported by radio frequency identification (RFID) consumers as the main drawback of passive RFID technology. We investigate a solution to this problem by employing low scattering antennas. We propose a new state of scattering state for RFID tags in that tags will switch to low scattering states to suppress their interference to an established and ongoing backscattering link between an RFID reader and a target tag. We evaluate the efficiency of our proposed solution by random deployment of tags in a network of ten antennas. We show that by using our proposed solution the read rate is 93.76% when the threshold of detecting signal at the reader is set to <inline-formula> <tex-math notation="LaTeX">$0.75{\delta }$ </tex-math></inline-formula> where <inline-formula> <tex-math notation="LaTeX">${\delta }$ </tex-math></inline-formula> is the magnitude of the backscattered signal from the target antenna when it is alone in the field.
Circularly polarised travelling-wave printed line antennas
The development of a broadband, circularly polarised antenna is presented. The configuration is based upon a travelling-wave microstrip line antenna. The experimental results are in good agreement with the numerical analysis. The experimental measurements show pattern bandwidths of 10 to 16% with input impedance bandwidths greater than 20%. The axial ratio is less than 3dB within the operating bandwidths.