Theory and design of next-generation retrodirective tags and their channels

Abstract

Passive and semi-passive backscatter communication systems such as radio-frequency identification (RFID) experience several challenges that limit their proliferation especially at microwave and millimeter-wave (mm-wave) frequencies, a consequence from the round-trip and low-powered nature of these systems. These challenges manifest themselves in the forms of backscatter-communication range reduction, deep spatial nulls caused by the rapid change in the received power within a small area, or both. To overcome these challenges, a retrodirective-array-equipped backscatter transponder (an RFID tag) is used to replace the standard single-antenna transponder. The benefits of using retrodirective tags are twofold: First, since retrodirective tags that operate at microwave and mm-wave frequencies have similar propagation properties--in terms of power losses and field-of-view--to the current single-antenna RFID tags, which operate at ultra-high frequency (UHF) band, the higher-frequency retrodirective tags maintain the same coverage distance as the UHF tags and permit faster data rates by leveraging the spectrum availability at microwave and mm-wave regimes. Second, retrodirective tags reduce the randomness of the backscatter channel by changing the small-scale statistical behavior of the channel from double- to single-fading statistics, much like current one-way wireless channels--an original contribution of this research. This work presents a compact, novel, and high spectral-efficiency microwave structure using a ring-based retrodirective array. Furthermore, this research investigates, theorizes, and measures the small-scale statistical characteristics of retrodirective backscatter channels. In fact, a two order of magnitude reduction in the channel fade margin is measured when a retrodirective tag replaces its single-antenna counterpart--a significant improvement in the reliability of the backscatter link. The analyses, results, and designs in this research are key enablers for next-generation microwave and mm-wave, ubiquitous, and power-free RFID and Internet-of-things (IoT) systems.