# RFID Reader Collision Problem ( RCP) Rana Abaalkhail Uottawa Instructor: Professor : ivan stojmenovic Fall 2010 CSI 5148.

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RFID Reader Collision Problem ( RCP) Rana Abaalkhail Uottawa Instructor: Professor : ivan stojmenovic Fall 2010 CSI 5148

Outline :  Brief description ( RCP)  Reader collision types  Reader collision Algorithms: Classify the algorithms, explain some of the algorithms, how they work, disadvantages, some comparisons  References  Questions

How Reader collision problem could happen? In many applications there are number of RFID readers deployed in the same area, and they form dense RFID Systems Readers have to work near to each other to read many tags This means there will be interfere between RFID readers operations

As a result, there will be lack of communication between the readers and some tags This will lead to inefficient and incorrect operation of a RFID system How Reader collision problem could happen? (cont.) the RCP is considered to be the bottleneck of the system throughput and reading efficiency. So we have to solve the problem as much as we can

Reader collision types: 1- Reader- to – Reader : Occurs when two or more readers communicate on the same frequency at the same time. Even when the readers ranges do not overlap

R1 lies in the interference region of reader R2, the reflected signal reaching reader R1 from tag T1 can easily get distorted by signal from R2. 1- Reader- to – Reader ( cont.) EXAMPLE

2- Reader-to –tag : occurs when one tag is simultaneously located in the interrogation zones of multiple readers. Every reader attempts to communicate with the tag in the same time. Reader –Reader collision types: (cont.)

3- Tag Hidden Terminal Problem Signal from R2 interferes with signal from R1 to a tag T1 but the signal from R2 does not reach R1, which is communicating with T1 and thus R1 does not recognize the collision that occurs at T1. Reader –Reader collision types: (cont.)

1-DCS: Distributed Color Selection  It is TDMA based ( Time Division Multiple Access) : The RFID reader and tags are allocated at different time slots in order to avoid simultaneous transmission  TDMA slots is relatively easier in the fixed RFID readers’ scenario. But, not in the mobile scenario.  When new reader join the other readers ; there should be recalculation of the time assigning.

DCS( cont.) Frame structure of TDMA-based anti-collision algorithm In the reader-to-reader communication period, each reader decides its time slot for transmission. The reader-to-tag period is used for reading the tags

DCS scenario: ( cont.) The frame time which consist the time slots( max-color) is fixed Each reader select a time slot( no two interfering readers has the same time slot) (color?!!) Each reader transmit in its time slot If the transmission collides with another reader, the transmission request is discarded and the reader chooses a new color and reserves it. Readers keep track of what is the current color ( time)

If any of neighbouring readers has the same color, it has to choose another color. DCS scenario: ( cont.)

Subroutines for DCS: Manages transmit ion Manages collision and the reservation for new time slot Manages kick resolution

Did you figure out why the algorithm called Distributed Color Selection ? This algorithm is similar to the coloring graph theory; where the adjacent nodes can not have the same color. In this algorithm, the readers that may have collision between them can not have the same time slot (color).

Disadvantages of DCS : DCS with the fixed max -color is simple BUT If the number of the adjacent readers is much larger than the max- color, many readers may experience collisions. If the max_ colour size is greater than the number of readers, many colours may be wasted DCS is a “greedy” algorithm – a node’s chances of colliding immediately after experiencing a collision are minimized at the expense of its neighbours.

DCS (cont.) Example assume the following: There are 10 time slots R7, R6 select time slot (1) What will happen based on the subroutines of the Algorithm?

DCS (cont.)Example 1- R7, R6 will start the communication 2- collision will happen between R7, R6 3- R7, R6 will select new time slot 4- send messages for the neighbours R6 will send for R4 R7 will send for R5 5- when the neighbours receive the kick message, they check if the time slot= their time, then they should select different time slot.

Improvement of DCS (cont.) : The Colorwave algorithm based on DCS but:  Max-colors is increased if the collisions become more than certain threshold  Max-colors is decreased if the collisions fall below certain threshold

2-HiQ: A Hierarchical Q- Learning Algorithm HiQ: is a hierarchical, online learning algorithm that finds dynamic solutions to the Reader Collision Problem in RFID systems. Q – learning Based on : Reinforcement Learning : a branch of Artificial Intelligence. It allows machines and software agents to automatically determine the ideal behaviour within a specific context, in order to maximize its performance

HiQ attempts to minimize reader collisions by learning the collision patterns of the readers and by effectively assigning frequencies over time to ensure neighbouring readers do not experience collisions from one another. HiQ(cont.)

 Hierarchical : It is composed of three basic hierarchical layers: the reader, the reader-level server and the Q- learning server. Q- Server: The highest level; the most intelligent level. Has global knowledge of all available frequencies and time slot for the readers. R-server: receive request from readers to communicate. Assign frequencies and time slots that are reserved by Q-server for the readers that are capable to communicate.

HiQ(cont.) Readers: the lowest level, it has limit capabilities. Ask the R-server to communicate with its tags. When the reader receive the frequency and time slot it can start the communication. Readers are responsible to detect and inform the collision number and types to R-server

HiQ structure Root Q-server: has Global knowledge for the constrains in the network to make resource allocation Q- server : each Q-server responsible for group of R-sever R-server: : each R-server responsible for group of Readers

HiQ(cont.) Example: By looking at the previous figure: readers R1, R2, R3 and R4 are within the interrogation range of reader R5  Whenever reader R5 has to read the tags, it sends the request for frequency and time for R-server.  It starts reading after getting frequency and time. At the same time it also pings other readers in the same interrogation zone.  After getting the ping, readers R1, R2, R3 and R4 send back the response with their current state, that is, whether they are reading or not and which frequency and time slot they are using.

How The server determine if the algorithm good or bad ? That based on the Cost Function : The cost function is essential to the success of the Q learning agent. From the feedback of the current state the algorithm can learn the goodness or badness of the current state. The cost function is a function of the total number of resource requests, request rejections, request grants, experienced frequency collisions, possible tag collisions, the actual number of readers that experienced frequency interference, and the actual number of readers that experienced tag interference HiQ ( cont.)

Disadvantages: Additional management of the overall hierarchy is required for even a slight change in the lower layer ( new readers, remove readers) HiQ assumes collision detection for readers which are not in the sensing range of each others If the number of readers are large, it may be hard to find the optimal solution due to large complexity. Thus, the HiQ algorithm may not be suitable for a network with mobile readers in which neighbouring readers are frequently changing.

3-MCMAC: Multi-Channel MAC Protocol:  There are N Channels without overlap: N-1 Data Channels : deliver the data packets 1 Control Channel : to exchange the control packets  Reader that may have collision with other readers can talk in the control channel to inform them that it now communicate with the tags

Since R1, R2and R3 interfere with each other on the data channel, they will be able to communicate on the control channel. when R2 prepare to read tags, it first informs R1, R3 or other readers in the R2 interference range that it will communicate with tags. MCMAC (cont.) EXAMPLE:

MCMAC (cont.) :  MCMAC is similar to Listen before talk protocol (LBT)  Listen Before Talk: a communication protocol that requires a reader to listen on a channel before transmitting to determine if that channel is being used by another device. If the channel is already in use, the reader will switch to a different channel.

Disadvantages of MCMAC : the reader to- tag problem still exists with this approach. Since passive RFID tags are unable to distinguish between two data channels, multiple data channels cannot be used directly in a passive tag environment.

4-CC-RFID : Central Co-operator RFID Algorithm IDEA: In RFID systems, the tags read by adjacent readers contain exactly the same information. If the same tags could be read by one reader and shared with others, the reading efficiency in RCP environment will be further increased.

CC-RFID System Architecture Central device is used to communicate between tags and readers. Multiplex readers’ queries

CC-RFID Working Scheme : First working Scheme: appropriate to apply in scenarios where tags have high mobility

Second working Scheme: appropriate to apply in scenarios where tags change slowly CC-RFID( cont.) CC Search: The CC receives readers’ requests and looks through the CC Storage Module for the corresponding tags’ ID requested by readers Tags’ information CC sends updating query to read tags and tags respond by sending their latest information to CC

 In CC-RFID system, with the introduction of the central operating device –Central Cooperator (CC), the present ‘multiple points to multiple points’ (MP2MP) collision problem is converted into two ‘multiple points to one point’ (MP2P) classical collision problems  The reading queries from several readers could be multiplexed by CC and the same tag information could be stored and shared among adjacent readers. CC-RFID ( cont.)

Disadvantages: This approach need to use an extra device, namely the central co-operator. It might have a scalability problem, because the central device is mounted in a specific place and has a fixed cooperation capacity. CC-RFID ( cont.) Advantages: It takes in consideration the tag side for collision because the central device communicate with the tags not the readers

Compare between the CC-RFID Working Schemes the first working scheme in CCRFID system consumes a little more time slots than the second one. This is due to the fact that, in the first working scheme, CC reads tags based on readers’ requests

RCP solutionScheduling basedCoverage basedCentral co- operator based Reduce the problem by assigning different time and frequency. however it requires the system to establish and maintain information over the network, which will be time and energy consuming. The reading ranges of readers are adapted dynamically to reduce the overlapped area between adjacent readers as much as possible, but it usually needs a central node to calculate the distance between every two readers and adjust their reading ranges, which will increase the complexity of realization. Take in consideration the redundant tags information between multiple readers, but it requited special Hardware, and the scalability problem. Comparison between different solutions of RCP

Algorithm Function Used Major overheadDynamic Channel Assignment Carrier Sensing Tag Side For Collision Avoidance DCS Color number Time synchronization YesNo HiQ CostManagement overhead YesNo MCM AC LBTAdditional channel Yes No CC- RFID MP2PSpecial hardware No Yes Comparison of reader anti- collision Algorithms

References:  Book : RFID Systms Resarach trends and challenges,2010.  RFID JOURNAL http://www.rfidjournal.com/http://www.rfidjournal.com/  A Multi-Channel MAC Protocol for RFID Reader Networks, Hongyue Dai, Shengli Lai, Hailong Zhu, Hongyue Dai, 2007.  HiQ: A Hierarchical Q-Learning Algorithm to Solve the Reader Collision Problem, Junius Ho, Daniel W. Engels, Sanjay E. Sarma, 2005.  A Novel Solution to the Reader Collision Problem in RFID System, Dong Wang,2006.  Colorwave: An Anti-collision Algorithm for the Reader Collision Problem, James Waldrop,2003.  Introducing Probability in RFID Reader-to-Reader Anti-collision, Filippo Gandino, Renato Ferrero, Bartolomeo Montrucchio, Maurizio Rebaudengo,2009.  GENTLE: Reducing Reader Collision in Mobile RFID Networks, Jieun Yu and Wonjun Lee,2008

QUESTIONS: Q1: Based on the following figure. What is the type of the reader collision problem? Explain how the collision happened? The type is Reader- to – Reader Interface. R1 lies in the interference region of reader R2, the reflected signal reaching reader R1 from tag T1 can easily get distorted by signal from R2.

Q2: According to Distributed Color Selection algorithm, let assume the following: There are 10 time slots R1, R4 select time slot (1) Trace the subroutines for R1,R4 and see What will happen ?

Answer: 1- R1, R4 will start the communication 2- collision will happen between R1, R4 3- R1, R4 will select new time slot 4- send messages for the neighbours R1 will send for R2 R4 will send for R6, R5 5- when the neighbours receive the kick message, they check if the time slot= their time, then they should select different time slot.

Q3: According to Multi-Channel MAC Protocol, after the reader listen to the control channel and concludes that the control channel is free, the reader enter the contention phase. What is the goal from this phase? the reader enter the contention phase waiting for control message to decide which data channel to use to communicate with the tags.

THANK YOU

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