Research On High Efficient RFID Tag Anti-collision Protocols

With the development of IoT(Internet-of-Things)technology,RFID(Radio Frequency Identificatin)technology,as one of the most important auto-identification technologies,has been widely used in many areas.Taking the advantages of non-contact,non-line-of-sight,high reliability,large storage and low manufacture cost,RFID is very suitable for large-scale warehouse and logistics applications.In RFID systems,all the tags share the same wireless channel,and they are not able to detect the channel conditions and communicate with other neighboring tags.When multiple tags transmit messages simultaneously,the signals from different tags overlap and cancel each other out.In such a situation,tag collision occurs and the reader cannot correctly decode the super-imposed signal,resulting in increased time and energy costs.In this thesis,we investigate one of the most challenging problems,e.g.,the tag collision problem,and provide high efficient tag anti-collision protocols for various RFID systems.Firstly,we propose a new assigned tree slotted Aloha(ATSA)tag anti-collision protocol to improve the system efficiency of passive RFID systems.Through assigning a unique prefix to each slot,the reader identifies tags with less numbers of collision and empty slots than previous works in dense tag environment.Simulation comparisons also demonstrate this result.However,the performance of ATSA is affected by duplicated collisions,especially when tags' IDs are sparsely distributed.Taking advantage of Manchester coding in the identification process,an improved ImATSA protocol is proposed to eliminate the duplicated collision slots.Both theoretical analysis and simulation results prove that the performance of ImATSA is not affected by tag ID distribution and its optimal system efficiency is about 58%,compared with previous works the proposed ImATSA can reduce the average identification by at least 40%;Secondly,in large-scale RFID systems the number of tags are usually unknown.Without any prior knowledge about the tag number,the reader will waste many collision or empty slots,especially in the early stage of the identification process.In order to reduce the number of such slots,an accurate and easy-to-implement tag number estimation algorithm is proposed.Making use of the binary query tree structure,the new estimation algorithm splits tags into small groups and estimates the number of tags in each group.Theoretical analysis demonstrates that the estimation algorithm is of high accuracy in each groupp.Next,by combining the new estimation algorithm and the proposed ImATSA algorithm,we propose a new adaptiveATSA(AdATSA)protocol to further reduce the identification time.Both theoretical analysis and simulation experiments are conducted to prove that the system efficiency of AdATSA is always 61% in large-scale RFID systems,and compared with ImATSA the new adaptive protocol can identify tags with 23.5% less time.Next,the effect of unreliable channels on the tag anti-collision protocols is analyzed,and some remedies are introduced;Finally,with recent development in the RFID technologies active tags,which are powered by batteries,receive increasing attention.Comparing with the limited reading range of passive tags,active tags can be read at a much longer distance,which makes them suitable for applications covering large areas.To prolong active tags' lifetime and reduce the frequency of recharging batteries,a new bit-detecting tag anti-collision protocol is proposed to save the energy cost and reduce the identification time for the active RFID systems.Through a specially designed response message string,the reader can effectively predict the states of several slots and skip all the empty slots.Both theoretical analysis and simulation results are presented to show that the new bit detecting protocol can effectively reduce the energy and time costs,compared previous works the reduced average time and energy costs are 46.9% and 20%,respectively.