Efficient Multi-Access Methods in RFID Systems

Muiti-Access methods in RFID System Student :Yi-Shyuan WU Adviser : Kai-Wei Ke Date : 2006.12.5

Overview • Multi-Access Procedures (Anticollision) • Anticollision Algorithms in RFID System • ALOHA algorithm • (Pure) ALOHA • Slotted-ALOHA • Frame-Slotted ALOHA • Binary search algorithm • Binary search algorithm • Dynamic binary search algorithm • Improved Dynamic Binary search algorithm • Conclusion • References

RFID System • Interrogator(Reader) • Transponder(Tag) • Application Software

Multi-Access Procedures • The operation of RFID systems often involves a situation in which numberous transponders are present in the interrogation zone of a single reader at the same time. • Can differentiate between two main forms of communication. • Broadcast mode • Multi-access to a reader

Multi-Access Procedures —broadcast and multi-access

Multi-Access Procedures (Anticollision)

Multi-Access Procedures — SDMA used for RFID applications • SDMA • These consist of several dipole antennas, and therefore adaptive SDMA can only be used for RFID application at frequencies above 850MHz(typical 2.45 GHz) as result of size of the antennas. • A disadvantage of the SDMA technique is the complicated antenna system. The use of this type of anti-collision procedure is therefore restricted to a few specialised applications.

Multi-Access Procedures — FDMA used for RFID applications • FDMA • For RFID systems is to used various independent subcarrier frequencies for the data transmission from the transponders to the reader. • A disadvantage of the FDMA technique is the relatively high cost of the readers, since a dedicated receiver must be provided for every reception channel. This anti-collision procedure, too, remains limited to a few specialised applications.

Multi-Access Procedures — TDMA used for RFID applications

Multi-Access Procedures (Anticollision) • Unfortunately, all these methods can not be used in a RFID system directly because they are much too complicated. • The first limiting factor for RFID systems is the constraint on memory and computation capabilities. • Secondly, several regulatory bodies restrict the readers’ maximum operating field strength . • The inability to sense the medium preventing tags to be aware of each others’ presence and transmissions. • For reasons of competition, system manufacturers are not generally prepared to publish the anticollision precedures that they use.

ALOHA algorithm • Transponder-driven stochastic TDMA procedure (Tag-Talks-First). • The procedure is used exclusively with read-only transponders. • The implicit start of the exchange between the tags and reader, with the tags automatically sending their IDs upon entering a powering field.

Tag transmits upon data ready Detect success or collision Tag retransmits after random backoff timefollowing collision Tag’s can’t detect/sense carrier. Collision is: Determined by listening for Reader’s (N)ACK …undetected ALOHA algorithm

Switch-off If Tag response successfully decoded, Tag automatically enters Quiet state Slow-down compromise between Aloha and Switch-off Reader overwhelmed by responses „Slow-down“ command sent, Tag adapts its (random) backoff algorithm Goal is to diminish Tags‘ reply frequency „Carrier Sense“ MUTE signal to all Tags when start of transmission is detect ALOHA algorithm extensions

Slotted ALOHA algorithm • Interrogator-driven stochastic TDMA procedure (Reader-Talks-First). • Transponders may only begin to transmit data packets at defined, synchronous points in time (slots). • Packet either collide completely or do not collide at all • Synchronization overhead: • Reader SOF(start of frame), EOF(end of frame)

Slotted ALOHA algorithm

„Terminating“ If Tag response successfully decoded, Tag automatically enters Quiet state Avoids collisions due to Tags replying indefinitely Tags re-enter Active state upon next “Wake-up” from Reader Failure to recognize “Wake-up” a problem: Tags time-out of sleep mode automatically Also called “Muting” „Early End“ Slot delimited by Reader SOF, EOF Reader issues „Next-Slot“ command on no responses received Slotted ALOHA algorithm extensions

Further discretisation of time: Medium access grouped into Frames,with N slots per frame Tags transmit at most once in a randomly selected slot, within maximum N Little extra synchronization overhead: Reader SOF, EOF for slots maximum slot number N set in Tag as default Framed Slotted ALOHA algorithm

Adaptive Reader can temporarily expand / contract number of slots for upcoming round Number of slots in a round varies with number of Tags in field Previous extensions also applicable: Terminating / Muting (slotted) „Early End“ Framed Slotted ALOHA algorithm extensions

Perspective

Binary search algorithm • The implementation of a binary search algorithm requires that the precise bit position of a data collision is recognised in the reader. • Manchester code is used in order to recognize the bit where there is a collision.

Binary search algorithm

Binary search algorithm • After the completion of the read/write operations, transponder 2 can be fully deactivated by an UNSELECT command, so that is no longer responds to the next REQUEST command.

Dynamic binary search algorithm • In the binary search procedure both the search criterion and the serial numbers of the transponders are always transmitted at their full length.

Dynamic binary search algorithm

Improved Dynamic Binary search algorithm • Has two differences compared with Dynamic Binary search algorithm • If there is only one collided bit no matter where it is, the reader does not need to sent REQUEST command again and can automatically identify two tags once. • After successive collisions have been detected, ever correlative bit but the last in REQUEST command will be set to zero.

Improved Dynamic Binary search algorithm

Comparison Binary search algorithm • Binary search algorithm • L : The average number of iterations • N : The number of transponders in the interrogation zone of the reader • L(N) = log(N) / log(2) +1 • Improved Dynamic Binary search algorithm • N-bit successive collisions are detected, tags can be recognized at best with only +1 commands transmitted.

Conclusion • Qualitative description of collision-resolution algorithms. • Comparisons difficult, meaningful? • Different application used different multi-access procedures .

Conclusion

References • RFID Handbook : Fundamentals an Applications in Contactless Smart Cards and Identification , Second Edition, Klaus Finkenzeller. • Luc André Burdet, "RFID Multiple Access Methods," ETH Zürich, Summer semester 2004, Seminar "Smart Environments". • Leian Liu, Zhenhua Xie, Jingtian Xi and Shengli Lai, "An improved anti-collision algorithm in RFID system," 2005 2nd International Conference of Mobile Technology, Applications and Systems. • Jae-Ryong Cha and Jae-Hyun Kim, " Novel anti-collision algorithms for fast object identification in RFID system," 11th International Conference of Parallel and Distributed Systems, 2005.

Efficient Multi-Access Methods in RFID Systems