The Tag Anti-Collision Algorithm using the Power Increasing Method ...

ICEIC 2008
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June 24-27, 2008
Tashkent, Uzbekistan
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7-3 Cross-Layer Designed MAC Protocol for Wireless Sensor Network 168
7-4 Mobile Widget based Advertisement System using Push Message Control 172
7-5 An Enhanced Tag Anti-collision Algorithm in Passive RFID Systems by using
QueryAdjust Command
7-6 The Tag Anti-Collision Algorithm using the Power Increasing Method in the UHF
RFID System
WE8: Analog/Digital Circuits and Systems II
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Proceedings of ICEIC2008, June 24-27, 2008, Tashkent, Uzbekistan
The Tag Anti-Collision Algorithm using the Power
Increasing Method in the UHF RFID System
You Seok Yim, Joo Kwan Lee, Sung Hwan Shon, and Jae Moung Kim
INHA-WiTLAB, INHA University
253 Youghyun-dong, Nam-gu Incheon 402-751, Korea
ohsomays@naver.com, miraju81@nate.com, kittisn@naver.com, jaekim@inha.ac.kr
Abstract – RFID is that the reader detects the tag
information attached on the objects without contact.
Particularly, the negative effects, we called the tag-collision,
may occur because there are lots of tags in the UHF RFID
system. We should solve these effects and the Slotted
Random Anti-collision algorithm is presented to prevent the
tag-collision effect in the EPCglobal Gen2 protocol. In this
paper, in order to minimize the tag-collision effect and bring
on the system efficiency, we propose the Power Increasing
Method that controls the transmission power of the reader to
prevent the tag collision effects and verifies the improved
performance.
1. Introduction
The RFID(Radio Frequency Identification) is the wireless
technology which detects a lot of tags attached on the
objects(e.g. things, livings etc.) without contact. The RFID
system consist of the reader(which detects the tag), its antenna,
the tag and the software which can process the information
detected from tags. And there are several types of tags which
are active, passive and semi-types. Basically, the active tags
have their own power and the passive tags don’t have own
power. In case of the active tags, they have longer
interrogation range but their size is little big due to the battery.
On the other hand, the passive tags use the power transmitted
from the reader and we can make them small and their price
low. Therefore, these days the passive tags are more popular
in the UHF RFID systems for these reason.[1]
Especially, the UHF(900MHz) RFID technology attracts
our attention as an alternative of conventional Bar-code
system due to long interrogation distance and the ability of
detecting multiple objects simultaneously. But there are lots
of tags to detect in the UHF RFID system which applies to the
Logistics & the Distribution industry. If the reader tries to
detect multiple tags, the tag-collision will occur because more
than two tags respond in the same time. These effects cause
the loss of the system efficiency, thus the anti-collision
algorithm is essential to detect multiple tags efficiently.
Although there are lots of anti-collision algorithms and
CSMA etc, the slotted random anti-collision algorithm is
introduced in the EPCglobal class1 Gen2 protocol.[2]
In this paper, we propose the tag anti-collision algorithm
that the reader controls its transmission power to detect the
tags more efficiently and to improve the system performance
through changing its recognition coverage. We verify these
effects via the computer simulations.
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2. Conventional Anti-Collision Algorithm
In the UHF RFID system, the multiple tags may be
distributed in certain area as shown in the Figure 1. If there
are lots of tags within the reader coverage, the tag-collision
may occur because the tags respond simultaneously to the
signal which the reader transmitted. This effect makes the
system efficiency low and the reader can’t detect the tags
correctly. Therefore, the anti-collision algorithm is required
and the slotted random anti-collision algorithm mentioned in
the EPCglobal class1 Gen2 protocol.
Figure 1: Environment of the UHF RFID system
The Frame Slotted Aloha(FSA) concept, which is adopted
in the RFID system, is shown in the Figure 2. Figure 2 shows
us the procedure of the tag detection partially. Basically, the
FSA algorithm accesses the object by the time order and the
tags determine their own slot to respond to the Query that the
reader transmit to recognize the tag. There are three type of
the slot state in the FSA algorithm. At first, if only one tag’s
information is inserted in one slot, we called ‘Success’ as you
can see at the second slot of the frame (N1). And if more than
two tags try to insert their information into one slot, we called
‘Collision’. We can see the collision state of the slot at the
third slot of the frame (N1) in the Figure 2. And if there is no
information on a slot, we called ‘Idle’. We can know the idle
state of the slot at the first slot of the frame (N1). As you see
in Figure 2, if the tag-collision occurs at the third slot of the
first frame (N1), the reader retransmits the query to detect
collided tags and repeats the procedure until detecting the all
tags. We call the Query retransmission and the reader can
detect the collided tags. Although the reader tries to detect the
collided tags again in the frame (N2), the reader repeatedly
transmits the Query if the all tags are not recognized.[3][4]

Proceedings of ICEIC2008, June 24-27, 2008, Tashkent, Uzbekistan
Figure 2: The Frame Slotted Aloha in the RFID system
However, there are some limitations on the conventional
FSA algorithm. Let us analyze these reasons mathematically.
Assuming the number of assigned slots is ‘S’, the probability
‘p’ that the tags try to respond at one Frame is defined as
‘p=1/S’.[5] As a view of the Binomial Distribution, the
probability ‘Ps’ that just one tag among the total ‘n’ responds
successfully can be expressed as (1). The probability ‘Pi’ that
slot will be the idle state and the probability ‘Pc’ that slot will
be the collision state are also expressed as (2) ~ (3).
Figure 3: Comparison of the theoretical results about the
collision slot in one frame(S=16)
According to (3), when ‘n’ tags try to respond through ‘S’
slots, the number of the collision slot in the one frame is
(1)
(2)
(3)
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defined as (4). And then Figure 3 shows the results about
calculating the equation (4) with 16 slots.
As shown in Figure 3, if the number of tags increases, the
number of collision slots increases radically. The more tags
exist, the more slot usage for detecting all the tags will be
needed due to large number of repetition(retransmission)
frames. It causes not only the delay of detecting time in the
system but also the large decline of the system efficiency.
3. Proposed Scheme
In conventional anti-collision algorithm(FSA), the reader
repeatedly transmits the Query using 1W transmission power
to detect all tags including the collided ones. But, in order to
overcome the limitation of the conventional algorithm which
is described in the second section, we propose a step by step
tag detection scheme defined as the Power Increasing
Method(PIM). In order to describe the proposed algorithm in
detail, the assumed environment and the detection procedure
of detecting the tags are drawn in Figure 4 and the Figure 5.
First of all, we assume that the tags are distributed uniformly
and the tags which are successfully detected once do not
respond upon retransmission of the reader.
Figure 4: RFID system environment for the PIM
Briefly saying, the reader transmits the Query with certain
power up to dotted-line area for the first step and tries to
detect remained tags by transmitting the Query up to dashedline
area. In the second step, the reader increases its
transmission power and transmits the Query up to broken-line
area. And then, the reader increases its transmission power to
1W up to the solid line which is maximum interrogation area
of the reader. But if the collided tags are remained in the
reader’s interrogation area, the reader transmits the Query
with 1W again.
First of all, in order to realize as remarked above
practically, the recognition distance of the reader in the sense
(4)

Proceedings of ICEIC2008, June 24-27, 2008, Tashkent, Uzbekistan
of the free space model(multipath propagation isn’t
considered due to the short distance) is expressed as
where ‘Preader’ is the reader transmission power, ‘Greader’ and
‘Gtag’ are antenna gains of each reader and tag, ‘Ptmin’ is
receiving sensitivity of tag and ‘ is wavelength.[6]
From (5), the required reader transmission power which can
be used to transmit the Query up to distance ‘dreader’ is
expressed as (6).
Table 1: The reader transmission power with 4 steps for PIM
Steps Each Distance[m] Power[W]
1 st
1.3 0.062
2 nd
2.6 0.248
3 rd
3.9 0.557
4 th
5.2 1
According to the (6), we can calculate the required power
for the each step of the PIM and result of the calculation
under the four power step of the PIM is indicated at Table 1.
At this time, the maximum recognition distance is calculated
5.2m under the Table 2 conditions. And we assume that the
each recognition distance for the PIM is 1.3m.
Table 2: The parameters for the computer simulation
Parameters Value
Frequency 914 MHz
Reader Transmission Power 1 W(Maximum)
Reader Antenna Gain 6 dBi
Tag Antenna Gain 0 dBi
Tag Receiving Sensitivity 0.1 mW
The number of tags 1 ~ 100
The number of slots 16
And then, in order to understand the PIM algorithm in
detail. Figure 5 shows us the procedure of the detecting the
tags with PIM. Examining Figure 5 carefully with Figure 4,
the reader tries to detect the tags in the 1 st line domain(dottedline
in the Figure 5). And the reader detects the tags up to the
2 nd line domain. At this step, the collided tags in the 1 st
domain also have to be detected. The reader increases its
transmission power up to maximum and tries to detect the
remained tags in the whole interrogation area. But if all tags
aren’t detected until this step, the reader transmits the signal
with its maximum transmission power again to detect the all
tags perfectly.
(5)
(6)
183
Figure 5: Comparison with the results of the mathematical
derivation and the simulation for the slot usage for PIM
4. Performance Analysis
In order to verify the performance of the PIM, the
computer simulations are made via the Table 2. We have the
three kinds of the simulation results and they can describe the
better system performance.
First of all, in order to compare the slot usage(that the
reader totally used during detecting the all tags) for the
conventional FSA and the PIM, the results are indicated at the
Figure 6.
Figure 6: Comparison of the slot usage for the conventional
FSA and the PIM
As you can see in the Figure 6, the PIM algorithm
indicates the better performance more than 33 tags.
Particularly, in case of the 90 tags, the PIM help the RFID
system improve about 34% in the sense of the slot usage.
For the next step, let us look into another simulation result
which is related to the collision frequency. The collision
frequency is how many collisions occur during recognizing
the all tags in the reader’s coverage. The high collision
occurrence means that the number of the retransmission is
increasing and the time is delayed to detect the tags perfectly.

Proceedings of ICEIC2008, June 24-27, 2008, Tashkent, Uzbekistan
The Figure 7 shows us the results about the collision
frequency in case of the conventional FSA and the proposed
algorithm.
Figure 7: Comparison of the collision frequency for the
conventional FSA and the PIM
According to the Figure 7, the PIM indicates the better
performance more than the FSA. The PIM algorithm starts to
perform better over about 14 tags. And in case of the 100 tags,
the PIM can bring on the RFID system’s efficiency about
35% comparing to the FSA algorithm. So we can know that
the PIM algorithm lead the system efficiency higher.
Figure 8: Comparison of the power consumption for the
conventional FSA and the PIM
Finally, in aspect of the power consumption, we can think
that lots of retransmissions in the certain system make the
system inefficient and also consume the large amount of the
power. Figure 8 shows us these kinds of the situation.
Because the PIM uses less power, it can give the RFID reader
184
more long using time. We can confirm that the reader with
PIM algorithm can consume less power than conventional
FSA over about 12 tags. Although the performance of the
PIM isn’t good for very small number of tags, the PIM gives
us the great improvement for the large number of the tags.
Therefore, we can think that these results will support the
UHF RFID system more efficiently.
5. Conclusions
In this paper, we propose the tag anti-collision algorithm
with the Power Increasing Method. As you can confirm via
the three kinds of the simulation results, the proposed
algorithm is able to make the system efficiency enhanced up.
And then, it can be adopted easily for Logistics &
Distribution Industry because the proposed algorithm
indicates high performance in large number of tags. But the
proposed algorithm does not indicate the better performance
than the FSA under the small number of the tags. Thus, in
order to make the overall RFID system more efficient, the
additional researches under the small number of tags have to
be executed.
Acknowledgments
This work was supported by Korea Science &
Engineering Foundation(KOSEF) through the National
Research Lab. Program funded by the Ministry of Science
and Technology(M10600000194-06J0000-19410).
And this research also was supported by the
MKE(Ministry of Knowledge Economy), Korea, under the
ITRC(Information Technology Research Center) support
program supervised by the IITA(Institute of Information
Technology Assessment) (IITA-2008-C1090-0801-0019).
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Proceedings of ICEIC2008, June 24-27, 2008, Tashkent, Uzbekistan
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