tailieunhanh - Radio Frequency Identification Fundamentals and Applications, Bringing Research to Practice Part 4

Tham khảo tài liệu 'radio frequency identification fundamentals and applications, bringing research to practice part 4', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | The Approaches in Solving Passive RFID Tag Collision Problems 53 continuous wave the radar cross section RCS of the tag the polarization of the tag antenna the propagation loss in the tag-to-reader link and so on. In order to mitigate the near-far problem a sophisticated power control scheme is required. Unfortunately the implementation of power control mechanism on each individual tag is impractical due to the limitations of the tag power and cost. Maina et al. apply Walsh codes to spread the tag backscatter signals Maina et al. 2007 which allows multiple tags to respond simultaneously. However the paper does not reveal how to assign the spreading sequences to the tags which reply in the same time slot. In addition the demodulation and decoding process are not clearly mentioned in the paper. Fukumizu et al. presents a scheme by applying both Walsh code and pseudo random sequence to solve the tag collision Fukumizu et al. 2006 . The approach is very similar to a modern wireless transmitter such as a mobile phone. However the scheme may not be suitable for a RFID system with passive tags due to the complexity constraint of the passive tags. Liu and Guo proposed a method by applying Huffman sequences which is nearly orthogonal to its delay version to passive backscatter signals Liu Guo 2008 . The Huffman sequences are more near-far resistant and can preserve code orthogonality without precise synchronization of received signals. Unlike aforementioned system where each tag uses a unique spreading sequence the proposed RFID anti-collision scheme uses only one Huffman sequence system for all tags. Consequently the reader can have the knowledge of the Huffman sequence used to spread the tag backscattering signal a priori which can significantly reduce the complexity of reader design. Furthermore the tag backscatter signal spread by the sequence can be easily generated using a set of preset circuit with corresponding reflection coefficients because only a Huffman sequence