1. 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

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

3. 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.

4. 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.

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

6. 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

7. 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

8. 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

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

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

11. 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

12. 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.

13. 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.

14. 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