Interference on the RFID Interrogation Range 최신 유비쿼터스 기술 기반 원전 유지보수 도구 프로토타입 개발 및 검증 Interference on the RFID Interrogation Range Korea Advanced Institute of Science and Technology Seung Min Lee 19 MAY 2008

CONTENTS Introduction Definitions and Solutions of Interference Tag-tag collision Tag-reader collision Reader-reader collision Summary & Conclusion References

INTRODUCTION To introduce RFID system in Nuclear Power Plant, lots of RFID tags are needed. ex) about 120,000 RFID tags for inventory management. To construct simple RFID system, an amount of readers are needed. To construct USN environment as well as simple RFID system, many fixed and mobile readers are needed. Deploying RFID systems on large scale (=dense mode) results in an unwanted effect, interference. As multiple readers and tags may exist in a working environment and communicate over shared wireless channels, frequency interferences would happen.

DEFINITIONs and SOLUTIONs of COLLISION 1) TAG-TAG COLLISION It occurs when multiple tags respond to the same reader simultaneously. Due to multiple signals arriving at the same time, the reader may not be able to detect any tag.

DEFINITIONs and SOLUTIONs of COLLISION ALOHA algorithm ISO/IEC 18000-6, air interface at 860MHz-960MHz EPCglobal Class-1 Generation-2 UHF Protocol at 860MHz-960MHz Fig1. IDs collection sequence of RFID using ALOHA

DEFINITIONs and SOLUTIONs of COLLISION 2) READER-TAG COLLISION It occurs when the signal from a neighboring reader interferes with tag responses being received at another reader. Passive tag has no battery and no right to choice frequency received from reader , and interference would happen even if two readers send different frequency. TAG READER R2 Read Range R1 Read Range R2 R1 T2 T1 T3 Fig2. Reader-to-tag interference in UHF RFID system

DEFINITIONs and SOLUTIONs of COLLISION transmitted wave is very strong and has a great interrogation range compared with back-scattered wave. (reciprocity) In fixed RFID system, there is no problem of interference if tags are included in only one reader`s interrogation range. But in mobile RFID system, this is not easy to be solved . the reader-tag collision problem is mitigated by allowing Frequency Hopping in the UHF band or by Time Division Multiple Access. reader tag Transmitted wave Back-scattered wave Fig3. Reciprocity

DEFINITIONs and SOLUTIONs of COLLISION 3) READER-READER COLLISION It occurs if an interfering reader transmit a signal to a tag when a continuous wave signal from the desired reader is converted into backscattered signal from the tag and transmitted to the desired reader again. Due to reciprocity, this type of interference can happen even if the interrogation range of the two readers has no intersection. In this situation, the tag might be unable to respond to any reader at all. CCI (Co Channel Interference) & ACI(Adjacent Channel Interference) Fig4. Reader-to-reader interference in UHF RFID system

DEFINITIONs and SOLUTIONs of COLLISION SIR (Signal to Interference Ratio) IRRR (Interrogation Range Reduction Ratio)

DEFINITIONs and SOLUTIONs of COLLISION Frequency Hopping Fig5. IRRR in the multiple reader envirionment Channel bandwidth : 500kHz

DEFINITIONs and SOLUTIONs of COLLISION Path loss Free Space Path Loss Terrestrial Pass Loss d : separation distance d0 : reference distance (=1m) n : value that depends on the surroundings and building type Result on minimum distance for Interfering Reader to interfere with the tag reply. Table 1. The effect of tag distance on multi-reader interference n : 3.5 d0: 1m

DEFINITIONs and SOLUTIONs of COLLISION LBT (Listen Before Talk) Reader must “listen” and confirm that a particular channel is not occupied before it can use that particular channel to interrogate any tag. LBT will effectively shut down many of the channels, thought those channels might have been, in the absence of the LBT provision, freely available for interrogation between readers and tags.

SUMMARY and CONCLUSION Fixed RFID system (invetory system, smart box, logistics) Interference between tag and tag solved by ALOHA algorithm that is already placed on sale. Interference between multi-reader and tag able to be solved by methods of path loss, frequency hopping and TDMA. no clear solution yet. Interference between reader and reader able to be solved by path loss and LBT not yet clear. Smart tool box Inventory system Mobile RFID system ( USN, RTLS) Real Time Location System To integrate these contents with real environment that will be constructed.

REFERENCES S.Jain, S.R.Das, “Collision Avoidance in a Dense RFID Network”, WiNTECH, 2006 B.Zhen, M.Kobayashi, M.Shimizu, “ Framed ALOHA for Multiple RFID Objects Identification”, Transactions on Information and Communication Engineers, 2005 D.Y.Kim et al., “Effect of Reader Interference on the RFID Interrogation Range”, Proceedings of the 37th European Microwave Conference, 2007 K.S.Leong et al., “The Reader Collision Problem In RFID Systems”, Proceedings of IEEE International Symposium on Microwave, Antenna, Propagation and EMC Technology for Wireless communications, 2005