Project C09 - RFID Tags for Sub-mm Localization
Current radio-based methods for locating mobile devices (e.g. robots) within buildings or rooms are affected by multipath fading, resulting in inaccurate positioning with position errors of several decimeters. Due to this inaccuracy, collisions among mobile devices as well as between mobile devices and stationary facilities can occur. Highly accurate positioning is mandatory in order to realize material characterization with mobile devices because even small inaccuracies can lead to large characterization errors.
The central aim of this project, in direct cooperation with the project S04 and with strong links to other projects within MARIE, is the exploration of an innovative THz-radio-based indoor localization method with high accuracy under consideration of minimal costs for the infrastructure. To achieve the targeted accuracy and unambiguity, several known positions are equipped with THz radio beacons/landmarks that serve as position reference. Instead of using radio transmitters (interconnected for energy supply and reference frequency distribution), the usage of cheap, energy-autarkic and compact Radio Frequency Identification tags (RFID) based on “chipless tag” technology is envisaged in this project, allowing for autonomous positioning of the mobile device. The RFID tags to be developed, also referred to as passive chipless mm-Wave/THz racon (radar beacon) for identification and μm positioning, cracons (for chipless radar beacons) for short, or simply “tags”, are therefore key components for the realization of the localization system and the main subject of this project (C09). “Passive” refers thereby to the lack of active transmission at the beacon and “chipless” describes the chipless RFID technology the cracons are based on.
The mobile device stimulates the (e.g. wall-mounted) cracons with the interrogation field for a short period of time during the localization procedure. The cracon reflects a signal by means of a backscattering technique to transmit an encoded identification number. The readout of identification and thus positioning data of several tags with additional distance and angle determination should lead to localization accuracies well below 1 mm.
While the concept of the system is well understood in principle, the accuracy, complexity and cost of the system are mostly unknown. The mobile device shall be equipped with a group of adaptive antennas, which can read the fixed RFID tags using adjustable beamforming techniques and an FMCW-like radar interrogation scheme.
The tag responds to the interrogation with a high-Q resonant backscatter at several frequencies, and the presence or absence of the backscatter signal at distinct frequencies encodes the tag’s ID. Due to the high Q factor, the resonant backscatter outlasts the radar clutter and can therefore be separated by time-gating techniques. This scheme poses new challenges for the localization, since only fractions of the interrogated bandwidth are available as backscatter signal due to resonant coding, and the feasible range will be limited to several meters [J]. To account for these challenges, not only the multi-resonant tags are investigated but also the concept of retroreflectivity. This allows for high signal magnitude and large bandwidth backscatter.
Several steps will lead to useable chipless landmarks. First, the resonant tag structure is investigated and, in a second step, it is combined with a conventional retroreflector (corner reflector) into a hybrid retroreflective tag. The resonant structure and the corner reflector are separated such that the low power ID backscatter and the high power broadband backscatter can be stimulated independently. The quirks of such a hybrid system will be investigated on all levels from manufacturing technology, resonator and tag design up to readout algorithms and system planning aspects in close cooperation with S04. With the experience gained in this hybrid resonant/retroreflective concept, a more complex approach will be targeted that integrates the resonant ID generation and the concept of retroreflectivity into an integrated tag solution. Such a tag would allow the ID to be collected within a highly increased range. Within the first project phase of 4 years, fundamental foundations for such an integrated tag will be laid by a preinvestigation in the second half of the first phase’s duration.
The several parts of the overall localization system translate to the project structure as follows: C09 focuses on the tag and the fundamental readout procedure, whereas the system aspects of the localization system, the reader signal processing and the computational aspects will be taken care of in S04, and the reader frontend will be based on the results from project C03. Since there are strong interdependencies among the parts of the localization system that are beyond interface specifications, several work packages will be executed together by several part projects.
Besides its integration into MARIE’s vision and the strong interaction with the accompanying projects S04 and C03, the research project tackles current challenges in several research areas, especially in chipless RFID, indoor positioning/localization, and mm-Wave and THz technology. The novel chipless racon technology, being the heart of the project, in combination with the step towards THz frequencies, is thereby able to provide a cheap tagging technique for accurate indoor localization.