Collision Free RFID Tag

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34 JOURNAL OF COMMUNICATIONS AND NETWORKS, VOL. 14, NO. 1, FEBRUARY 2012

Collision-Free Arbitration Protocol for Active RFID

Systems

Honggang Wang, Changxing Pei, and Bo Su

Abstract: Collisions between tags greatly reduce the identifica-

tion speed in radio frequency identification (RFID) systems and in-

crease communication overhead. In particular for an active RFID

system, tags are powered by small batteries, and a large num-

ber of re-transmissions caused by collisions can deteriorate and

exhaust the tag energy which may result in missing tags. An ef-

ficient collision-free arbitration protocol for active RFID systems

is proposed in this paper. In this protocol, a new mechanism in-

volving collision detection, collision avoidance, and fast tag access

is introduced. Specifically, the pulse burst duration and busy-tone-

detection delay are introduced between the preamble and data por-

tion of a tag-to-reader (T-R) frame. The reader identifies tag colli-

sion by detecting pulses and transmits a busy tone to avoid unnec-

essary transmission when collision occurs. A polling process is then

designed to quickly access the collided tags. It is shown that the use

of the proposed protocol results in a system throughput of 0.612,

which is an obvious improvement when compared to the framed-

slotted ALOHA (FSA) arbitration protocol for ISO/IEC 18000-

7 standard. Furthermore, the proposed protocol greatly reduces

communication overhead, which leads to energy conservation.

Index Terms: ALOHA, collision, radio frequency identification

(RFID).

I. INTRODUCTION

In a radio frequency identification (RFID) system, a reader is

required to identify tags as quickly as possible. All tags share

the same wireless channel, which leads to collisions between

multiple tags. Therefore, a tag arbitration protocol, also called

an anti-collision protocol, is important for fast identification of

multiple tags.

Tag arbitration protocols can be simply categorized into tree-

based protocols and ALOHA-based protocols. The tree-based

tag arbitration protocols are deterministic [1], [2], while theALHOA-based ones presented in [3][8] are statistical. In [5],

combinatory model and frame-size update methods are pre-

sented for analyzing object identification using passive and ac-tive RFID technology. In [7] and [8], a binary tree is intro-

duced to deal with collision tags such that a better throughputis achieved. In [8], improved framed-slotted ALOHA (FSA)

with robust estimation and binary selection (EB-FSA) is used

to achieve a maximum system throughput of about 0.42. On

one hand, given the fact that the performance of FSA is lim-

Manuscript received November 9, 2010; approved for publication by Homay-oun Yousefizadeh, Division II Editor, June 17, 2011.

This work was supported in part by NSFC (No. 61072067), the 111 Project(B08038), the Project ISN 1001004, and Shaanxi Industrial Science and Tech-nology Project (2009K01-46).

The authors are with the State Key Laboratory of Theory and Key Technolo-

gies of Integrated Services Networks, Xidian University, Xian, China, email:[email protected], [email protected], [email protected].

ited by the initial frame size, the performance of EB-FSA is

also affected by this factor. On the other hand, EB-FSA re-

quires an estimation phase with a fixed framed size before iden-

tification phases. While using a large frame size in the esti-mation phase leads to the wastage of time slots, the use of a

very small frame size requires an increased number of time

slots in the estimation phase. Hence, a simple and effective

method is required for choosing an appropriate frame size for

unknown tags in the estimation phase. In [9], a reservation-based protocol is used to achieve a high system throughput,

but a long reservation-bit sequence significantly increases the

complication of tag design. In addition, too many reservation

bits overlap temporally, which greatly deteriorates the demod-

ulation performance of the reservation-bit sequence. In [10],a capture-aware access control method for improving the sys-

tem throughput of slotted-ALOHA-based arbitration protocol is

proposed. The use of this method results in a limited improve-

ment in the system throughput. Slotted ALOHA is adopted

for the standard ISO/IEC 18000-6 standard [11]. In [13] and

[14], RFID-arbitration-protocolperformance evaluation and im-

provement based on the ISO/IEC 18000-6 standard type C are

presented.

The aforementioned protocols are more effective for a pas-

sive RFID system. A passive tag backscatters reply signals, de-pending on the continuous waves transmitted by a reader. A pas-

sive tag can not work independently without utilizing the reader

radiation. In other words, a passive tag cannot listen indepen-

dently to a wireless channel and actively transmit data. However,when powered by a battery, an active tag can listen to a wireless

channel as well as can transmit data actively. Currently, framed-

slotted ALOHA is adopted as the arbitration protocol for the

ISO/IEC 18000-7 standard for active RFID applications [12].

The usual application of active RFID are in logistics, assetmanagement, real-time location system (RTLS), and so on. For

these applications, fast tag identification and collision arbitra-

tion are still the key technical challenges. In a general wireless-

network node device, the media access control (MAC) layer di-rectly controls the actions of the wireless communication mod-ule, which is regarded as the main component responsible for

energy dissipation. An active tag is similar to a wireless net-

work node with limited energy, and the arbitration protocol in a

RFID system functions as a MAC protocol in a general wireless

network system. Therefore, a good arbitration protocol shouldhave low energy consumption, high system throughput, and be

as simple as possible.

To decrease communication overhead and better system

throughput for an active RFID system, an efficient collision-free

arbitration protocol is presented in this paper. Our protocol is

different from slotted ALOHA. In brief, a new method for colli-

1229-2370/12/$10.00 c 2012 KICS


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WANGet al.: COLLISION-FREE ARBITRATION PROTOCOL FOR ACTIVE RFID SYSTEMS 35

Fig. 1. Modified T-R frame structure.

sion detection and collision avoidance is introduced. In our pro-

posed protocol, a reader can determine whether tags are collid-

ing by detecting the pulses, that are added into the tag-to-reader

(T-R) frame. When a collision occurs, the reader transmits abusy tone to avoid unnecessary transmission and uses a polling

process to access the collision tags quickly by using pulse po-

sition ID. In accordance with the description given above, the

proposed arbitration protocol is named collision-free ALOHA(CFA).

The rest of this paper is arranged as follows. In Section II,

slotted ALOHA arbitration protocol for ISO/IEC 18000-6 and

18000-7 are introduced. The proposed arbitration protocol is de-

scribed in Section III. In Section IV, a theoretical throughput

analysis and simulation results are presented. Finally, Section Vconcludes the paper.

II. SLOTTED ALOHA ARBITRATION PROTOCOL

The slotted ALOHA arbitration protocol is adopted for the

ISO/IEC 18000-6 standard type C [11]. In this standard, a readerstarts an inventory process by broadcasting a command Queryafter initialization. Query contains a parameter Q, which im-

plies that the tag population estimated by the reader is2Q. UponreceivingQuery, participating tags pick a random number in the

range[0, 2Q 1]. Tags that pick zero reply immediately in thecurrent slot, where as those that pick a nonzero number do not

reply. If only one tag replies, it is successfully identified by the

reader. If more than one tag reply in the same time slot, a colli-

sion occurs and no tag is identified. The reader may adjustQ to

obtain optimum performance. An algorithm that can be used bythe reader to choose a appropriate Qaccording to tag responses

in the previous time slot, referred to as Q algorithm in the fol-

lowing sections, is presented in [11].A variation of slotted ALOHA, framed-slotted ALOHA, is

adopted for the ISO/IEC 18000-7 [12]. A reader initiates a tag

collection process through standard command collection. Tags

receiving collection randomly select a slot to respond. The selec-

tion is determined by a pseudo-random number generator. Uponselecting a slot number, a tag waits for a time delay before re-

sponding; this delay is equals to the time of slot number multi-

plied by the slot delay. The total number of slots is determined

by the current window size (frame length). The reader dynam-

ically chooses an optimum window size for the next collectionround on the basis of the number of collisions in this round.

The use of slotted ALOHA and framed-slotted ALOHA canresult in an optimum throughput of 0.368 [4], [6], [13]. The use

Fig. 2. Frame structure based on the ISO/IEC 18000-7 standard.

of the framed-slotted ALOHA arbitration protocol adopted for

the ISO/IEC 18000-7 standard results in a system throughput of

about 0.328. The deference of 0.04 from the theoretical value of

0.368 is mainly due to the estimation error in the tag population.

System throughput is usually defined as the ratio of the totalnumber of successfully accessed tags and the total number of

time slots used by the reader.

III. PROPOSED ARBITRATION PROTOCOL

A. Collision Detection and Collision-Free Mechanism

In our proposed protocol, collision detection and a collision-

free mechanism are realized by a new data frame in the T-

R link. As shown in Fig. 1, the new T-R frame is a modifi-

cation of the T-R frame based on the standard ISO 18000-7,

which is shown in Fig. 2. The differences between the two T-Rframes are that a pulse burst duration (PBD) and a busy-tone-

detection delay (BTDD) are introduced between the preamble

and data. However, in the new T-R frame, the first data byte for

data communication timing is still put on the head of the data

part [12]. The PBD is divided into several small time slots called

subslots. After receiving a command from the reader, for exam-ple, collection, tags reply with the proposed frame in which a

pulse is transmitted in a random subslot. When the transmis-

sion of the preamble and the pulse is finished, tags switch their

transmitter to their receivers and wait for a busy tone in theBTDD. These pulses are detected and recorded by the reader,

and the number of pulses represents the number of collision

tags. When more than one pulse is detected in the PBD, the

reader transmits a busy tone in the BTDD. After receiving busytone, tags that want to reply in the current slot halt the transmis-

sion of the real data part. This is done to avoid the unnecessary

transmission. If the busy tone is not received, tags again switch

to their transmitter and data transmission continues. The aboveprocess is termed as a collision-free mechanism.

For easy detection, pulses can be shaped or modulated before

transmission. Protection intervals are added at the beginning and

end of the PBD to avoid inter-symbol interference. Accordingto the ISO/IEC 18000-7 standard, the pulse width can be set as

18s or 30 s, and other values can also be determined eas-

ily. The width of BTDD should satisfy the minimum time re-

quirement of the transceiver switch and the pulse detection.

B. Protocol Description

The proposed arbitration protocol is based on slotted ALOHA

and has the following steps.

1) The reader initiates an inventory round through the command


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(a)

(b)

Fig. 3. Inventory flow chart for the proposed protocol: (a) Inventoryprocess on the reader side and (b) inventory process on the tag side.

collection that contains the parametersQ and Ws. Upon re-

ceiving this command participating tags pick a random value

in the range[0, 2Q 1]. Tags that pick zero reply immedi-ately. Tags that pick a nonzero value try to reply in the nextslot. Q algorithm defined in [11]is used to choose the value

ofQ. The reader refreshes Q each time before sending col-

lection.

2) Tags that reply in the current slot should transmit a pulse in a

randomly selected subslot in the range [0, Ws 1].Ws rep-resents the number of subslots in the PBD.

3) The reader receives the reply signals from tags and detectsthe pulse number in the PBD. The number of detected pulses

is then considered as the estimation value of the number ofcollisionsNc in the current slot. If no pulse or signal is de-

tected, the reader starts a new inventory. If only one pulse is

detected, i.e., Nc = 1, the reader waits for the data transmis-sion. It means that a tag completes the frame transmission

when a busy tone is not received. When a tag is identified, itdoes not participate in the next inventory round [11], [12].

4) When more than one pulse is detected, i.e., Nc > 1, it indi-cates Nc tags in the current slot, and the reader transmits a

busy tone in the BTDD. IfNc is more than a given thresh-

old Nth, it results in step 1) being repeated and the whole

procedure is started over again. Otherwise, the reader startsa polling process through command polling that contains

pulse position ID, i.e., subslot sequence numbers. The reader

records the sub-slot sequence numbers of all the subslots in

which the pulses are located. The reader then accesses the

collision tags by polling these subslot sequence numbers oneby one. When a received position ID is equal to the selected

subslot sequence number, the tag responds topolling. To en-

sure that only the collision tags respond to polling, one-bitpolling ID (PID) is set into all tags. Ifcollectionis received,

tags initiate the PID with zero, i.e., PID = 0, and if a busytone is received, it implies that tags collided in the last slot,

i.e., PID = 1. Whenpollingis received, tags with PID = 1reply with the frame shown in Fig. 2 instead of the proposed

frame shown in Fig. 1 to reduce the transmission delay.

5) If collisions still occur during the polling process, which can

be detected by a cyclic-redundancy check, the reader gives

up these collision tags in this round of polling. After polling

all detected subslot sequence numbers, the reader repeats

the procedure from step 1) and starts a new inventory round

throughcollection.The inventory flow chart for CFA is shown in Fig. 3. Here,

A and B represent the frames in Figs. 1 and 2, respectively.

It should be noted that the reader is authorized to terminatethe polling process and start a new inventory round at any time.

IV. ANALYSIS AND SIMULATION

A. Theoretical System Throughput

GivenN slots andn tags, the probability of having qtags in

one slot is given by a binomial distribution:

p(N , n , q ) =n

q1

Nq

N 1Nnq

(1)

where p(N,n, 1) is the probability of a successful tag access.As is well known, under the only condition of N = n, theprobability reaches a maximum value of 0.368, which is the

maximum theoretical system throughput of slotted ALOHA or

framed-slotted-ALOHA-based arbitration protocols.For a given Nth(Nth 2), the theoretical system throughput

of the proposed protocol is given by

Ps= p(N,n, 1) + Np

1p(N,n, 0) + 1p(N,n, 1) + 1P(Nc > Nth) + Sp(2)

where Np is the average number of tags successfully accessedduring the polling process. Sp is the average number of occu-


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Fig. 4. Theoretical system throughput for different threshold values.

pied time slot during the polling process. The expression for Npis

Np =nci=2

p(N , n , i)Wsp(Ws, i, 1) (3)

where nc is defined as the actual number of participating tags

when Nccollision tags are detected using pulse detection in one

slot andnc Nc. In this case, there is the following relation:

Nc = Ws(1 p(Ws, nc, 0)) (4)

Spis given by

Sp =nci=2

p(N , n , i)(Ws(1 p(Ws, i, 0)) + 1) (5)

To obtain the upper bound of the theoretical system through-

put, let Ws . Thus,we can get Nc = nc from (4). Thiscondition implies that there are no pulse collisions in subslots

in our theoretical analysis. Then, in (3) and (5), the value ofncis equal to Nth because the polling process is performed when

Nc Nth. Moreover,

P(Nc > Nth) =n

i=(Nth+1)

p(N , n , i) (6)

The system throughput can then be calculated using (2). For

example, for n = 1000 (it should be noted that the algorithmcan achieve a constant system throughput for different number

of tags), the theoretical throughput for different threshold value

shown in Fig. 4. For Nth 6, the theoretical throughput isalmost constant at 0.612.

B. Simulation Results

To analyze the performance of the proposed protocol, we

focus on two metrics: System throughput and communication

overhead.

The average number of collisions in one collision slot is 2.39for the slotted ALOHA arbitration protocol [4], [15]. Therefore,

Fig. 5. Collision probability versus number of tags.

Fig. 6. Relation betweenNc and nc.

Nth should be larger than 2.39 in order to prevent collisions in

slots for most cases. The probability of different number of tags

colliding in one slot is shown in Fig. 5. When the number of col-

lision is more than 6, the collision probability is almost zero. Bythe analysis results obtained from Figs. 4 and 5, we set Nth= 6

as the optimal value in our simulation.The value ofWs should be large enough to decrease the col-

lision of pulses in a subslot. However, too many subslots will

introduce an additional access time delay. The relationship be-tweenNcand ncis described by (4). Fig. 6 shows the relation-

ship between the detection results ofNcand ncfor Ws= 8andWs = 16. The average number of colliding tags (simulationresult) is about 3 when Q algorithm is adopted to estimate the

number of tags. Thus, the value ofWs = 8and Ws = 16cansatisfy the purpose of low collision probability, as well as a low

additional access time delay.

The simulation results for the proposed protocol, CFA, are

shown in Fig. 7. By setting Ws= 16andNth= 6in our simu-lation, the use of CFA result in system throughput value of 0.6,


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Fig. 7. System throughput of CFA.

which is very close to the theoretical value 0.612. This differ-ence is ascribed to the estimation errors in the determination of

the number of tags from Q algorithm and pulse detection. We

compare CFA with the existing ALOHA based arbitration pro-tocols for an active RFID system, for example, EB-FSA (= 1)[8] and framed-slotted ALOHA (FSA) [12]. CFA shows an ob-

vious improvement in system throughput. In our simulation, the

channel conditions and capture effect are not taken into account.

CFA not only improves the system throughput but also re-duces the communication overhead.

The communication overhead contains all the transmission

data by the reader and tags. For CFA, the overhead includes

reader commands, for example, collection and polling, thepreamble of the T-R frame, the data part of the T-R frame, andthe added parts of PBD and BTDD in the proposed frame ar-

chitecture. For FSA, the overhead includes reader commands,

for example,collection, the data part of the T-R frame, and the

preamble of the T-R frame. We record the transmission times

for all types of transmission packets from the reader and tagsto determine the communication overhead. We set Ws = 16andNth = 6in the CFA simulation. It is assumed that readercommands have the same length. Fig. 8 illustrates the commu-

nication overhead for CFA and FSA in our simulation. When

compared with FSA, CFA reduces reader-command transmis-sion times from 3221 to 1667 and reduce data transmission more

efficiently, which decreases from 7541 to 1096 for an inventoryof 1000 tags.

The only additional overhead is a small amount of data per-

taining to PBD and BTDD transmission. Both the time delayand energy consumption corresponding to PBD and BTDD are

very low, and the delay is dependent on the pulse width and the

number of subslots. For CFA, setting the number of sub-slots as

8 or 16 is enough for the protocol to perform well. For example,

whenWs = 16and the pulse width is set as 18s, the PBD isonly 288s (several bits). When compared to the length of the

T-R data which can possibly be hundreds of bits, the time-delay

cost is negligible.

Furthermore, we defi

ne the overhead ratio (Oratio) for analyz-ing the communication overhead for the arbitration protocols,

Fig. 8. Communication overhead for CFA and FSA.

Fig. 9. Communication overhead ratio for CFA and FSA.

which is given as follows

Oratio=packet transmission times

number of identified tags (%). (7)

The overhead ratio defined by (7) is shown in Fig. 9. CFA

obviously reduces all types of transmission overhead, which re-

sults in low time delay and low energy consumption.

Moreover, CFA established a waiting-time threshold forempty-slot detection. If no signal is detected within the time

threshold, the reader will start a new round to speed up the in-

ventory process. In our system throughput analysis, empty slotsaccount for about one third of the total slots. Empty-slot detec-

tion can increase the relative system throughput.

V. CONCLUSION

We proposed a CFA protocol to prevent the collision of tags

in active RFID systems. For the given parameters, CFA achieved

a system throughput of about 0.612, as shown by theoretical

analysis and simulation results. When compared with the sys-tem throughput of FSA for the ISO 18000-7 standard, and even


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the EB-FSA ( = 1) [8], CFA showed a significant improve-ment in the system throughput. Moreover, collision avoidance

and empty-slot detection by CFA greatly reduced the communi-

cation overhead and access time delay, which are very important

for battery-powered active RFID tag.

The proposed arbitration protocol can be backward-compatible with a slotted-ALOHA-based RFID system that in-

cludes the Q algorithm for the estimation of the number of

tags. Under this circumstance, the proposed protocol only per-

forms the basic process besides the collision resolve process. In

short, CFA is a very simple protocol, the use of which results inlow improvement cost for current products.

REFERENCES

[1] C. Law, K. Lee, and K. Y. Siu, Efficient memoryless protocol for tag iden-tification, inProc. Int. Workshop on Discrete Algorithms and Methods for

Mobile Computing and Communications, USA, Aug. 2000, pp. 7584.[2] R. Don and C. W. Hush, Analysis of tree algorithms for RFID arbitration,

inProc. IEEE ISIT, USA, Aug. 1998, pp. 107116.[3] H. Vogt, Efficient object identification with passive RFID tags, in Proc.

Int. Conf. Pervasive Comput., Swiss, Aug. 2002, pp. 98113.[4] J. Cha and J. Kim, Novel anti-collision algorithms for fast object identi-

fication in RFID system, in Proc. Int. Conf. Parallel Distrib. Syst., Japan,July 2005, pp. 6367.

[5] B. Zhen, M. Kobayashi, and M. Shimizu, Framed ALOHA for multipleRFID objects identification, IEICE Trans. Commun., vol. E88-B, no. 3,pp. 991999, 2005.

[6] C. Floerkemeier, Bayesian transmission strategy for framed ALOHAbased RFID protocols, inProc. IEEE Int. Conf. RFID, USA, Mar. 2007,pp. 228235.

[7] M. A. Bonuccelli and F. Lonetti, Tree slotted aloha: A new protocol fortag identification in RFID Networks, in Proc. IEEE Int. Symp. World ofWireless Mobile and Multimedia Networks, USA, June 2006, pp. 603608.

[8] J. Park, M. Y. Chung, and T. J. Lee, Identification of RFID tags in framed-slotted ALOHA with robust estimation and binary selection, IEEE Com-mun. Lett., vol. 11, no. 5, pp. 452454, May 2007.

[9] C. P. Wong and Q. Y. Feng, Grouping based bit-slot ALOHA protocol for

tag anti-collision in RFID systems,IEEE Commun. Lett., vol. 11, no. 12,pp. 946948, Dec. 2007.

[10] W. J. Shin and J. G. Kim, A capture-aware access control method forenhanced RFID anti-collision performance, IEEE Commun. Lett., vol. 13,no. 5, pp. 354356, May 2009.

[11] ISO/IEC, 18000-6 Radio frequency for item management, Part 6: Param-eters for air interface communications at 860 MHz to 960 MHz, p. 110,2006.

[12] ISO/IEC, 18000-7 Radio frequency for item management, Part 7: Param-eters for an active RFID air interface communications at 433 MHz, p. 3,2004.

[13] Y. Maguire and R. Pappu, An optimal Q-algorithm for the ISO 18000-6CRFID protocol, IEEE Trans. Autom. Sci. Eng., vol. 6, no. 1, pp. 1624,Jan. 2009.

[14] Y. C. Ko, S. Roy, J. R. Smith, H.-W. Lee, and C.-H. Cho, RFID MAC per-formance evaluation based on ISO/IEC 18000-6 type C, IEEE Commun.

Lett., vol. 12, no. 6, pp. 426428, June 2008.

[15] F. C. Schoute, Dynamic frame length ALOHA,IEEE Trans. Commun.,vol. 31, no. 4, pp. 565568, Apr. 1983.

Honggang Wang was born in 1977, and is a Ph.D.candidate. He received the M.S. degree in commu-nication and information system from Xidian Univer-sity in 2005. He is currently working on the energyefficiency physical layer and communication protocoldesigns of RFID and WSN.

Changxing Pei was born in 1945, Professor, Advi-sor for doctoral students at Xidian University. His re-search interests include wireless communication andnetwork measurement, quantum communication, andanti-jamming communication.

Bo Su was born in 1982, Ph.D. candidate. He com-pleted his B.Sc. from Xidian University. Now, he iscurrently studying at the College of Telecommunica-

tion at Xidian University for his Ph.D. and his majorresearch fields include wireless ad hoc & sensor net-works.

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