Krigslund, Rasmus

English

This thesis focus on passive Radio Frequency IDentication (RFID), a tech- nology designed for automated identication of tagged objects. The tags are small passive devices powered by the wireless signal from the reader. Their simple operation thus solely allows them to reply with their unique ID when requested by the reader, which is a high power handheld or stationary transceiver. The RFID technology has matured over the past two decades, but still there exist ambiguities regarding the reading, or interrogation, of tags. Due to the passive nature of the tags, they are severely aected by the environment and the object they are attached to. Hence they may be rendered unable to reply to the readers request, in spite of being within the expected read range of the reader. This is referred to as False Negative Detections (FNDs). Similarly, constructive eects in the environment can en- able a tag outside the expected read range to reply. This is denoted a False Positive Detection (FPD). It is desired to reduce the occurrence of these false positive and negative detections hence in this thesis it is proposed to impose interference on the communication between reader and tag. Interference has a well-known blocking eect, and the experimental investigations show how tag replies outside the expected read range are blocked, as expected. More- over, tuned and detuned tags alike are aected by the decreased Signal to Interference and Noise Ratio (SINR) due to the imposed interference, hence their read ranges are thus eectively equalized. From the investigations it can therefore be concluded that the imposed interference has a constructive eect as it enables design of well dened and sharply bounded interrogation v zones, where tags are read with high probability regardless of the level of detuning they might experience. Moreover, this thesis proposes to use passive RFID tags as orientation sensors. This is an innovative application for RFID technology in itself, but orientation sensing can also help existing RFID applications that relies on the Received Signal Strength (RSS) in the tag reply. Many Ultra High Frequency (UHF) RFID tags utilize a dipole antenna, and the RSS metric of the tag reply is thus prone to polarization and gain mismatches between tag and reader antennas. Being able to estimate the orientation can help mitigate this eect. It is proposed to let a reader antenna collect information about the polarization of the received tag reply by decomposing the RSS into the horizontal and vertical dimensions using a dual linearly polarized reader antenna. The method is evaluated experimentally, and when the movement of the tag is limited to two dimensions, i.e. a plane, the results for track- ing the tag orientation are promising. Tag movements in three dimensions complicates however the orientation estimate. Multiple reader antenna are therefore utilized in order to collect samples of the tag reply from multiple directions, and their data are fused using Kalman ltering. The nal results show good potential, as even simple data processing enables recognition of gestures, i.e. predened motions with the tag. However, the estimate of the three dimensional orientation vector oers relatively low precision, and does therefore not allow dedicated motion capture of the tag. But it has been shown through experimental investigations that the orientation infor- mation is available in the tag reply, and using more advanced methods for data processing, a reliable orientation estimate can be obtained

Experimental Assessment of Orientation Sensing and Constructive Interference in Passive RFID Systems

https://vbn.aau.dk/ws/files/69995564/PhdThesisRasmusKrigslund.pdf

Experimental Assessment of Orientation Sensing and Constructive Interference in Passive RFID Systems

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