An Overview of Anti-Collision Protocols for Radio Frequency ...

China Communications • November 2014 44

ers focusing on RFID technology, it began to be commercialised, in field such as animal tagging. Come the 1990s, and RFID technol-ogy had mature; for example, RFID-based electronic toll collection systems appeared in America and Europe. In the 21st Century, RFID technology, as one of the key technolo-gies of the Internet of Things, has found more applications and received more attention, es-pecially in service industries, manufacturing, process/factory systems, etc. [2-7].

The RFID system mainly includes two parts: the reader and a tag. Usually, the reader is powered equipment bearing enough memory and read/write range used to identify tags in the read/write area. The tag types, which are used for object identification, mainly include: active tags, semi-active tags, and passive tags. Passive tags have the advantages of low cost, simple processing, long service life, etc. and are thus widely used in practical applications [8]. The communication between reader and tag is real-ised by data transmission through a single chan-nel. A tag is only recognisable by the reader in the case of one-on-one communication between tag and reader. The major key technologies of RFID involve: antenna design, frequency selection, data integrity, data security, standard unification, etc. At present, antenna is designed mainly aiming at miniaturisation and reduced power consumption [9]; frequency selection mainly includes selecting the actual application scenarios for frequency adaptability and the

Abstract: Radio frequency identification (RFID) i s a new type of non-contac t automatic identification technology. Due to its low energy consumption, low cost, and its adaptability to harsh environments, it has been applied to many fields. In the RFID systems, data collision is inevitable when the reader sends a communication request and multiple tags respond with simultaneous data transmission. Data collision is prone to causing problems such as: identification delay, spectrum resource waste, a decreased system throughput rate, etc. Therefore, an efficient, stable anti-collision protocol is crucial for RFID systems. This research analysed the current research into RFID anti-collision protocols and summarised means for its improvement through the mechanism of implementation of different types anti-collision protocols. Finally, a new direction is proposed for the future development of RFID anti-collision protocol systems.Keywords: radio frequency identification; automatic identification; data collision; anti-collision protocol.

I. INTRODUCTION

As a new non-contact automatic identification technology, radio frequency identification was initially proposed by Stockman in an article called “Communication by means of reflected power” [1]. In the 1970s, with many research-

An Overview of Anti-Collision Protocols for Radio Frequency Identification DevicesQIAN Zhihong*, WANG Xue

College of Communication Engineering, Jilin University, Changchun, Jilin, China, 130012

COMMUNICATIONS SYSTEM DESIGN

China Communications • November 201445

time slots by multiplexing. Upon collision, a tag generates a random back-off slot number, i.e. it resends the data after several time slots. When the data are correctly recognised by the reader, the tag enters its HALT state [17, 18]. The anti-collision method of the tree-based deterministic algorithm (tree algorithm) uses only one ID number for identifi cation. In detail, the ID number is fi rstly used for tag di-versity. Upon tag collision, the reader changes the inquiry commands sent so that the tag can respond to the reader by self-adaption: correct tag identifi cation is thereby achieved [19, 21].

This study analysed and investigated a number of existing anti- collision protocols for RFID systems. By comparing the imple-mentation principle, identifi cation delay, hard-ware requirements for reader and tag, overall performance, and scalability of different algorithms, it pointed out the advantages and disadvantages of each algorithm. The paper is arranged as follows: Section 2 introduces the basic theory of RFID systems; Section 3 in-dicates the state-of-the-art with regards RFID anti-collision protocols. Moreover, it classifi es the Aloha and tree protocols in detail and out-lines the implementation mechanism of new protocols. Finally, it concludes with ideas for the improvement of different protocols; Sec-tion 4 predicts novel development directions for RFID protocol research.

II. RFID SYSTEMS

An intact RFID system is composed of a central processor, middleware, reader and antenna, and tag and antenna. As shown in Figure 1, the central processor is mainly used for data processing and storage; middleware is connected in series with the CPU and reader; reader and tag communicate with each other through their own antennas [22].

2.1 Communication modes

The energy sensing mode of RFID system contains inductive coupling and backscatter coupling. For different working distances (ranges), RFID systems use different sensing

application technology of ultra high frequencies [10]; data integrity aims for stable and effi cient anti-collision protocols to achieve error-free transmission in the system [11]; design for data security mainly focuses on commercial secrets and individual privacy [6,7]; no RFID stan-dard has yet been globalised and harmonised. The Economic and Planning Council (EPC) and International Standardisation Organisa-tion (ISO) have listed standards for the coding system (target identification), radio frequency identifi cation system (data acquisition), and in-formation network system (specifi c application) respectively. However, the standards are in-compatible. Thus RFID standards are generally oriented towards achieving globalisation and harmonisation [12-15].

The non-contact automatic identifi cation of RFID technology is achieved by information transmission through the spatial coupling of radio frequency signals in a single channel. When multiple tags are communicating with the reader, data collision, namely tag collision, is inevitable. Tag collision, as a key problem in RFID systems, can waste bandwidth and energy, while increasing identification delay. Therefore, a stable, effi cient anti-collision pro-tocol is essential for RFID systems [16].

At present, anti-collision protocols for RFID systems mainly contain an ALO-HA-based indeterminate algorithm or a tree-based deterministic algorithm. The two algorithms are consistent in concept, name-ly, that in the case of any identification tag collision at the reader, the identification tags are gradually divided into small subsets in a specific way until single tag communication is achieved. The anti-collision method of the ALOHA-based indeterminate algorithm (ALOHA algorithm) proceeds as follows: the communication channel is divided into several

Fig.1 Structure of RFID

This research analyzed the current research into RFID anti-collision protocols and sum-marized means for its improvement through the mechanism of im-plementation of diff er-ent types anti-collision protocols.


2.3 Tag and reader

According to the type of energy supply, a tag is categorised as: passive, active, or semi-ac-tive. The passive tag is energised by the reader. It can only be passively activated and supplied with the energy to communicate as required by the reader. It has a short working distance, is simple to process, cheap, and has a long service life. Active tags are equipped with batteries and can actively send communication requests to the reader. Its working distance is longer, while the service life is determined by its battery capacity. Although provided with batteries, a semi-active tag does not actively send communication requests to the reader. It only sends data when entering the read/write range of the reader and when receiving a com-munication request from the reader.

According to type, readers include: read-only, write-only, and read-write variants. The reader and background central processing unit cooperate to complete the tag identifica-tion, data acquisition, information exchange, etc. [25].

III. THE ANTI-COLLISION PROTOCOLS OF AN RFID SYSTEM

In the case of a reader communicating with a tag in multi-access mode, that is, multiple tags simultaneously respond to requests from the reader, collision occurs due to the failure of the reader to precisely identify the tag IDs, as shown in Figure 2. The collision leads to an increased identification time, a waste of ener-gy and spectral resources. Therefore, an effi-cient, stable anti-collision protocol is essential for RFID systems [8, 15, 26].

At present, data collision is mainly solved by four protocols, including: frequency di-vision multiple access (FDMA), time divi-sion multiple access (TDMA), code division multiple access (CDMA), and space division multiple access (SDMA). FDMA [15, 27] di-vides the channels into multiple sub-channels according to frequency. Different sub-channels are used for data transmission. This protocol

modes. For low-to-medium frequency sys-tems, they use an inductive coupling mode due to the short distance involved, while for systems operating at high frequency (HF) and ultra high frequency (UHF), they use back-scatter coupling mode due to the long working distance [23, 24].

The inductive coupling mode requires an RF carrier frequency of 13.56 MHz and one less than 135 kHz. The inductive coupling mode is mainly used for sensing passive tags (almost all of them will be passive) at low-to-medium frequencies and at a system working distance of less than 1 m. The reader acts as the energy supplier to the tag. Data are transmitted by way of negative load modula-tion, namely, a tag changes the load according to the voltage of its binary data coding signals [8, 23].

Backscatter coupling is theoretically based on radar technology: the scattered energy produced by the transmitted signal from the antenna is partially reflected thereto. By am-plifying and processing the received signals, information related to the target is obtained. Backscatter coupling is suitable for RFID systems at high or ultra high, frequencies and at a working distance of more than 1 m. Sim-ilarly, the reader supplies the energy to each tag. Data transmission is mainly achieved by changes in the antenna reflection performance induced by load variations on a tag antenna [3, 15].

2.2 Forms of communication

RFID systems have three communication types: (1) wireless radio, that is, the commu-nication request sent by the reader is received by multiple tags within read/write range of the reader simultaneously; (2) multiple access, namely, a plurality of tags within read/write range of the reader transmit data to the reader simultaneously; (3) the simultaneous com-munication and data transmission of multiple readers and tags. Wireless radio and multiple access are commonly used methods of com-munication in RFID systems [22].


At present, the TDMA-based anti-collision protocol mostly inc ludes the Aloha-based in-determinate protocol and the tree-based deter-ministic protocol. The two methods, following different anti-collision mechanisms, both have advantages and disadvantages.

3.1 The Aloha protocol

The Aloha-based protocol is a random TDMA technology, that is to say, as reader sends a communication request, a tag responds to the reader at random and transmits its data. Upon data collision, the tag retreats for a random time before resending its data. Aloha-based protocols include: pure Aloha, slotted Aloha, frame-slot-ted Aloha, dynamic Aloha, and various im-proved protocols. In the following paragraphs, the workings, advantages, and disadvantages of each protocol are discussed [31-50].

3.1.1 Pure Aloha (PA) protocol

The basic idea of the pure PA protocol is that, in case of a communication request from the reader, the tag sends the data by a certain time interval repeatedly as long as it has data to be sent. The communication between reader and tag maybe involve one of two conditions: (1) the label is correctly identifi ed in the case of the tag transmitting the data and receiving the confi rmation information (ACK); (2) data col-lision is produced in the case of the tag failing to receive confi rmation information (NACK). In such a condition, the tag suffering the data collision randomly retreats for a certain time before resending its data. Figure 3 shows the communication process.

PA has a lower channel utilisation rate. Mathematical analysis points out that channel throughput rate G and frame production rate G are mathematically related by:

(1)

By taking the derivatives of the equation above, it is seen that throughput rate G is max-imised by G=0.5, which is , various researchers have also made some im-provements to PA.

The Aloha-Muting protocol [31] is one of

requires co mplex re ad/write ability in the reader; for CDMA [28, 29], it is necessary to add code to the tag with a random orthogonal code before transmission. This process calls for very high communication costs and energy consumption. SDMA [30] uses the array direc-tion antenna to divide the reading range into multiple subspaces, with only one tag in each subspace. However, this protocol demands complex antenna design. Moreover, in cases involving many tags, it is unable to ensure that there is only one tag per sub-space.

Based on the time dimension of channel, TDMA enables both the tag and reader to transmit their own data at different times during communication. The anti-collision protocol of most RFID systems uses TDMA. Meanwhile, there have since been many novel and classical variant algorithms developed to improve the performance of RFID systems.

Fig.2 Problem of collisions

Tag 1

Tag 2

Tag 3

Reader

t

t

t

t

Collision

T1

T2

T3

Identified

Fig.3 Communication process of PA


of each slot. In the case of an empty slot, there are no tag data to be transmitted; in the case of a collision slot, more than two tags transmit their data in this slot. Thus the reader fails to make a correct identifi cation and leaves these tags to the next inquiry round; in the case of a successful slot, only one tag transmits its data in this slot. The reader can correctly re-ceive the tag and accurately identify it. The preservation sequence, which is assumed to be Q bits in length and to correspond to Q slots, is constructed by labelling the position of the slot fi rstly selected by the tag as “1” and other positions as “0”. Mathematical analysis suggests that the GBBSA protocol reaches its maximum throughput rate when Q=15. To improve the performance of the system, the Q value can fl uctuate with the dynamic varia-tions in the collision slots, as shown in Figure 5. In this Figure, Q is initialised at a value of 15.

The Q protocol [34, 35] is a randomly ac-cessed SA protocol. When the reader sends a communication request, the tag randomly yields a 16-bit random sequence and preserves

the improved PA variants enhanced by add-ing silence commands in the Aloha protocol, namely, after being recognised, one tag enters a silent state and no longer responds to the request of the reader. The data transmission times of the tag are reduced and the perfor-mance of the RFID system is improved. Figure 4 shows the communication process behind the Aloha-Muting protocol.

3.1.2 Slotted Aloha (SA) protocol

SA divides the Aloha protocol [32] into dis-crete time periods (slots). Each tag initiates its data transmission synchronously only when its slot begins and completes its data transmission when its slot ends. The whole process requires time synchronisation. The slots have three possible states, including: empty, collision, and identification slot types. An empty slot refers to the case where there is no data trans-mission in the slot; a collision slot occurs as a data collision is induced when more than two tags respond to the reader and transmit their data simultaneously. Identifi cation slots appear when only one tag responds to the reader and the reader makes a correct identification of this tag. In SA, the data collision problem is solved as follows: the collision tag randomly yields a random number. By retreating from this random number for several slots, the tag resends the data until the reader gives the correct identifi cation result for the tag data. In SA, the relationship between throughout rate and frame production is expressed as:

(2)

By taking the derivatives of the equation above, it is seen that the throughput rate G of SA is maximised at G=1, which is G=63.8%, and is double that for PA.

The grouping based bit-slot aloha protocol for tag anti-collision in RFID systems [33] (GBBSA) is a modified SA protocol. Before the slot distribution at the reader, the tag ran-domly generates a reservation sequence and transmits the data according to that reservation sequence. The reader detects the received data using the Manchester code and judges the state

Fig.4 Communication process of PA-Muting

Tag 1

Tag 2

Tag 3

Reader

t

t

t

t

Collision

T1

T3

Identified

Fig.5 Adjustment rules of Q

The number of colliding bit-slots Adjusting the Q

<11(the lower boundary) Q=Q-(Q>0)

11~20. inclusive. Q=Q >20(the upper boundary) Q=min{Q+1.15}


The EFSA protocol [39, 40] is a rapid an-ti-collision protocol used in circumstances where the target tag must be recognised repeat-edly. After a correct identification of the tag by the reader, the tag is allocated a slot number for its next identification. In cases involving empty or collision slots, the reader neglects these slots and estimates the quantity Q of col-lision tags. According to this estimate, the tags were subjected to Q-tree decomposition until the tags are all provided with a slot number protocol. The repeated identification time is thereby reduced. The improved FSA protocol [41] is completed in two stages: tag estimation and tag identification. Being different from the usual tag estimation, the tag estimation here uses the probabilities of the three slot states (empty, collision, and success), instead of the collision slots, to predict the tag quantity. In this way, the stability of the system is im-proved by reducing large variations in the tag quantity induced by small errors in prediction. For the DJ protocol [42], the identification process is divided into two steps: collision de-tection and skip-reading. Before skip-reading by the reader, a tag randomly sends a random 4-bit sequence. The reader then calls a 4-bit detection frame with the tag’s random se-quence to detect the skipping position. Since the size of the skip-frame corresponds to the number of frames successfully transmitted in the detection frame, slot collision is avoided and the identification time is reduced.

As a milepost of the Aloha protocol, the dynamic frame slotted Aloha (DFSA) proto-col significantly improves the performance of RFID systems. In brief, the DFSA protocol is a slotted Aloha protocol with dynamically varying frame lengths. It is constructed on the basis that the throughput rate is maximised when the frame length in the FSA protocol is equal to the number of tags to be recognised. This protocol aims at reducing the number of empty, and collision, slots. It reduces iden-tification time and improves the throughout rate of the system by self-adaptively altering the length of the next frame using an estimate of the number of tags [43-48]. The DFSA

this sequence in the internal counter. Only those tags with a random figure of 0 are al-lowed to be transmitted in this slot. The slot state is judged by the reader: (1) in an empty slot, there are no tag data to be transmitted and the random figure held by the tag counter is re-duced by 1; (2) in the event of a collision slot, the counter of the extra tag displays 0. Thus this tag cannot be recognised by the reader and is left until the next inquiry round. Meanwhile, the figure in the tag counter is reduced by 1; (3) in cases involving a successful slot, only one tag counter displays 0, thus the reader cor-rectly recognises the tag and meanwhile, the figure in the tag counter is reduced by 1.

Other improved protocols include the RSMBA protocol [36], the SA-Early end pro-tocol [37], etc. The RSMBA protocol uses a multi-bit preservation sequence. During its judgement of the slot state, empty slots are excluded from the reservation operation to reduce the identification time; the SA-Early end protocol [37] has additional SOF and EOF commands. When the slot begins, the reader detects the slot state using SOF. For an empty slot, it sends the EOF command to end the slot in advance so as to improve the performance of the system.

3.1.3 Frame slotted Aloha (FSA) protocol

The FSA protocol [38] divides the discrete time slots into frames. In each frame, each tag has one time for data transmission. When the reader sends a communication request, the tag randomly selects the slots in one frame for its response. Then the reader judges the slot states in that frame. Similarly, the slots have three potential states: empty, collision, and success-ful. The tags involved in collision are only allowed to select slots randomly from the next frame for data transmission. Since the sum of empty slots and collision slots is less than the number of successful slots, the performance of the RFID system is improved. To reduce fur-ther the number of empty slots, collision slots, and the identification time in turn, researchers have made a variety of improvements and in-novations to RFID systems.


The Aloha algorithm is a probability-based anti-collision protocol. Research into the Alo-ha algorithm mainly focuses on two aspects: how to allocate tag data transmission at dif-ferent times to avoid data collision and ensure data integrity to the greatest extent, and, when a collision occurs, what strategies should be adopted to eliminate the collision to achieve the reduction of identification time and the improvement of the system’s throughout rate. The PA protocol, SA protocol, and the relat-ed improved algorithms mainly use random transmission data from a tag. Upon colliding, the tag retreats for a random time period and then resends its data. Moreover, these proto-cols also use the silence command, early end, accelerated modules, etc. to improve system performance. The FSA and DSFA protocols, and related improved algorithms, package the discrete slots into frames. In each frame, the tag is only allowed to transmit data once. Moreover, by predicting the number of unrec-ognised tags using specific methods, the length of the next frame can be dynamically adjusted to improve the throughput rate of the system to the greatest extent.

3.2 Tree protocol

A tree protocol is an ID number-based search protocol. In this type of protocol, each tag is endowed with only one number for identifi-cation. The communication between reader and tag is achieved by certain specific rules or sequences. In other words, the reader changes the inquiry commands to progressively group the tags until only one tag is left to communi-cate with the reader. The reader then makes a correct identification of the tag. The tree pro-tocol contains TS protocol, BS protocol, and QT protocol. Moreover, it needs global syn-chronisation of the system and the silencing function of the tag to work.

3.2.1 Tree splitting (TS) protocol

The TS protocol is an anti-collision protocol based on random numbers: tags are grouped by the reader according to the random num-bers they generated. Then using a recursive

protocol is a hotspot in current Aloha proto-col research. Improvement of the protocol is achieved by searching for new tag estimation models. Classical mathematical estimation models are obtained using collision slots and the equation below. In this equation, n is the number of unrecognised tags and ncoll is the number of the collision slots.

n ＝ 2.39ncoll (3)

The BE-PDFSA protocol [45] optimises the DSFA protocol using bias estimation and a probability mechanism. The bias estima-tion can make accurate, real-time estimates of the number of unrecognised tags, that is

, where n# is the number of unrec-ognised tags; n* is the number of the tags as predicted by bias estimation; js is the number of collision slots. In the identification phase, a probability mechanism is used to enhance sys-tem performance. The PDFSA protocol [46] integrates the idea of the PFSA protocol [47] into the DFSA protocol to solve the anti-col-lision problem in the face of a large number of tags. Using the PZE and PCE methods in the PFSA protocol, the probability of each tag transmission data being detected is calculat-ed to estimate the number of the tags and the length of the next frame is then dynamically adjusted. Deng et al. [48] treated the slot se-lection process of a tag as a Bernoulli exper-iment governed by the binomial distribution. Moreover, they proposed the concept of the transmission time Trep. By substituting Trep into the communication time, the throughout rate of DSFA protocol was maximised when

ra ther than

when the frame length was equal to the num-ber of the tags to be recognised. In the formu-la above, N is the frame length and n is the number of the tags to be identified. Only when Trep is neglected, N=n. The MAP protocol [49] presents an accurate tag estimation mechanism with low computational complexity. In this protocol, the tags are accurately estimated us-ing the empirical data detected by each tag and the priority distribution of the number of tags.


random number 0, including tags A, B, and D, transmit their data in slot 2. Similarly, in case of a collision in slot 2, the random numbers of tags B and D are updated to 1, while those of tags A and C are updated to 0 and 10 re-spectively. Therefore, tag A, with its random number of 0, transmits its data in slot 3 and is recognised by reader. Meanwhile, the reset random numbers were reduced by 1; thus tags B and D, with their new random number of 0, are allowed to transmit their data in slot 4, and so on, until a total of 9 slots are needed to cor-rectly recognise all four tags.

The allocated BTS (ABTS) protocol [52, 53] solves the problem of collision and re-duces the number of empty slots. The tags in this protocol are provided with two counters, including a progressed slot counter (PSC) and an allocated slot counter (ASC). As the reader correctly recognises one tag, the number in the PSC is increased by one. The ASC func-tions consistently with the counter in the BTS protocol. Data transmission is permitted only when the two counters are equal. The number of empty slots is thereby reduced. Both EMBT [54] and OS protocols [55] belong to M-ary tree decomposition anti-collision protocol class. In addition to that, the range of the ran-dom number is changed from {0, 1} to {0, 1, …, M-1}, the basic anti-collision mechanisms of the two protocols are in agreement with that of the TS protocol. Moreover, the two protocols present a calculation method for the optimal Mopt. The M here determines the sys-tem performance. An overly large M value is prone to lead to superfluous empty slots and thus degrade the system performance, while an overly small M value may result in a failure to improve the system performance.

3.2.2 Binary tree(BT) protocol

The BT protocol [15] is a tag ID-based bitwise arbitration search protocol. In this protocol, the reader initiates its search from the highest bit of the tag and arbitrates as to the state of the slot. In collision slots, the reader sends ei-ther a 0 or a 1 (bearing the specifi c bit number of the ID number). According to the number of

method, the branch is divided into two subsets once collision occurs. The subset size is grad-ually reduced until the final subset contains merely one tag or is empty.

For the basic TS (BTS) protocol [51], colli-sion tags generate random numbers and record these in their counters when the collision is detected by the reader. Then the reader selects different slot transmission data according to the random number to solve the data collision problem. The random number generator in the tag merely yields 0 or 1: only tags with a 0 are allowed to transmit data, those with a 1 enter a “wait” state. After each round of queries, the reader will detect the slot state: if the time slot indicates a collision, collision tags then generate random numbers to be added to the original random number, plus one, to label the random number in the “wait” state; when the time slot is empty or successful, all label ran-dom have one subtracted from their counter. According to this rule, reader and tag are com-municating with correct identifi cation tags. To understand TS protocol, we assume that there are four tags to be recognised. Figure 6 shows the identifi cation process. In case of a collision in Slot 1, the colliding tags yielded random numbers. Tag C, with a random number value of 1, enters its “wait” state, while that with

Fig.6 Identifi cation process of TS

Timeslots 1Tag (A, B,C,D)

Timeslots 9Tag C

Timeslots 2Tag (A, B,D)

Timeslots 4Tag (B,D)


Timeslots 3Tag A

Timeslots 6Tag B

Timeslots 7Tag D

Timeslots 8

Collision

Idle

Succession

1

1

1

1

0

0

0

0


between the two protocols lies in the fact that the reader and tag in the DBS protocol merely communicate useful information (for anti-collision). As for the tags, the information after collision of the inquiry commands sent by the reader is useless. For the reader, the pre-collision ID information returned by the tags is useless. The DBS protocol eliminates redundant information to improve system per-formance. The BLBO protocol [59] increases the bit-locking commands. According to the ID information returned for a tag, the reader labels the collision bit as 1 and other bits as 0. The binary number string thus formed is used as a bit-locking command. According to the bit-locking command, a tag extracts the information related to the tag collision bit and forms the information into a new ID num-ber. In the anti-collision process, the new ID number is used to reduce redundancy in the communicated information. Moreover, this protocol also applies a back-tracking strategy to decrease the number of collision slots. The EBS protocol [60] stipulates that tags respond to the reader in different time segments and different slots by judging the slot as either a

this bit (0 or 1), collision tags respond accord-ingly. In the case where empty slots are also successful slots, the reader restarts the search from the highest bit.

The ID-binary protocol [56] integrates the ID-binary tree stack into the BT protocol. According to the position (depth) of the stack storage tag on the binary tree and the stacks of the tag, the positions of the reader commands on the binary tree are stored. The ID-BTS pro-tocol uses the first-in last-out rule to control the stack. When a tag is correctly recognised, the reader presents the command from its stack. Then the tag responds according to the information in its stack records to reduce the inquiry time. The GBT protocol [57] divides the identification process into several small BTs and solves the tag collision problem using a capture effect. The tags remaining unrecognised in one cycle are hidden using the capture effect and then recognised in the following BT cycles.

3.2.3 Binary search (BS) protocol

The BS protocol [3] is a non-memory search protocol based on the tag ID. The reader can detect the bit where collision occurs. When reader sends the commutation request contain-ing the UID, the tags will compare the sizes of their ID with the transmitted UID. Those tags with IDs less than or equal to the trans-mitted UID transmit their ID numbers to the reader. Upon collision, the reader detects the highest bit (the bit on the left of the ID) using Manchester code and generates a new UID using the 0 of the highest collision bit and the 1 of other collision bits. Then the reader sends the new UID and begins the next round of in-quiries until a tag is successfully recognised. Subsequently, the protocol starts a new inquiry in a non-memory state. Assuming that there are four tags to be recognised, including tag A (100), tag B (010), tag C (110), and tag D (011), the identification process is shown in Figure 7.

The DBS protocol [58] is consistent with the BS protocol with regards its identification mechanism. However, the largest difference Fig.7 Identification process of BS

UID Tag reply

Round（slot）1 111 XXX（A, B, C, D）

Round（slot）2 011 01X（B, D）

Round（slot）3 010 010（B)

Round（slot）4 111 XXX（A, C, D）

Round（slot）5 011 011（D）

Round（slot）6 111 1XX（A, C）

Round（slot）7 101 100（A）

Round（slot）8 111 110（c）


identifi cation process is shown in Figure 8.The BQT protocol [62, 63] accurately de-

tects the collision bit using Manchester code, that is to say, the BQT protocol is a QT proto-col bearing bit detection capabilities. In addi-tion, this protocol shows a different updating rule for command set Q from that in the QT protocol. When collision occurs, the reader correctly receives the ID sequence segments from the highest bit to the highest collision bit of the tags. By adding 0 or 1 on the back of the collision segments, new commands are constituted. As BQT accelerates the search penetration, empty slots are avoided and the identifi cation time is reduced.

The optimal query tracking tree protocol (OQTT) divides the tags into multiple subsets by estimating the number of tags to estimate the number of collision slots. This protocol comprises three steps: ① bit estimation: this step detects the ID number state rather than the slot state and estimates the number of tag to be recognised according to the distribution of collision bits; ② optimisation and group-ing: this step divides the tags into groups and proves that this grouping method is optimal by calculation; ③ tracking inquiry, this steps firstly records the grouping condi-tion of the tags in each round (querying tree structure). When a tag is correctly identifi ed, the reader inquires of other tags according to the record.

The RN16QTA protocol [64] is a type of EPC Class1 Gen 2 protocol that replaces the EPC code of the tag with a temporary ID (TID). It reduces the collision frequency by using TID in the anti-collision stage and EPC code in its identifi cation stage. The MQT pro-tocol is an M-ary tree splitting QT protocol which calculates the M value for the minimum average communication time.

The tree protocol is a type of deterministic protocol based on tag IDs: each tag is provid-ed with only one ID number for identifi cation. Although using a random number of tags for decomposition and grouping, based on tag IDs, the BT protocol realises tag transmission in communication with the reader and thus

1 or a 0 type of slot. The prefi x overhead and iterative cost are thereby reduced to improve the performance of the system.

3.2.4 Query tree (QT) protocol

The QT protocol [61], which is a non-memory anti-collision protocol based on an inquiry tree, acts as a milepost for RFID anti-collision protocols. The commands sent by the reader in this protocol differ from the one-bit com-mand in the BT protocol and the multi-bit (ID length) command in the BS protocol. In the identifi cation process, the reader yields an in-quiry command set Q and reserves this set in its stack. For tags, there is no need to memo-rise any information except their ID numbers. The command from the reader is initialised as “NULL”. The reader reads all the tags within range and gives a corresponding judgment on each. Upon collision, the reader updates the command set Q, that is, the collision com-mand in the command set is supplemented (suffixed) with a bit valued at either “0” or “1”. For successful slots, the reader presents the commands from the command set in turn for searching. It is assumed that there are four tags to be recognised, including tag A (100), tag B (010), tag C (110), and tag D (011). The

Timeslots 1Tag (A, B,C,D)



Timeslots 3

Timeslots 5Tag B

Timeslots 6Tag D

CollisionIdleSuccession

q=0

q=00

q=1

q=011 q=010

Timeslots 7Tag (A,C)

q=01

Timeslots 8Tag C

Timeslots 7Tag A

q=NULL

q=10q=11

Fig.8 Identifi cation process of QT


and edge (0 or 1). The LoF algorithm over-comes the problem of low time efficiency in conventional algorithms and shortens the cal-culation required for the identification of each tag.

Wu et al. [76] found a value approaching the tag number by adjusting the frame length using dynamic, self-adaptive, splitting methods. The new method is advantageous in that there is no need to estimate the tag number and the effi-ciency is unaffected by variations in the number of tags. The initial frame is 2Q long: the count-er of each tag selects a random integer from 0 to L-1. The tags with their counters showing 0 respond first; in the event of a collision, the reader stops on this frame and broadcasts a frame with a single bit increase, i.e. Q=Q+1; in cases involving empty slots, the reader termi-nates on this frame and broadcasts a frame with a bit reduction, i.e. Q=Q-1.

3.4 New algorithms (tag signal separation in the physical layer)

From the original premise of the desire to harmonise RFID standards, a new receiver was designed to support the realisation of new algorithms. The existing solution methods for anti-collision protocol are all executed on MAC layer, while the new algorithm is conducted on the physical layer. The new al-gorithms use the collision information in the physical layer to recover tag information and improve the system throughput rate and its performance [77].

Angerer et al. [77] first proposed a new algorithm, that is, one in which the collision signal recovers the physical layer and they designed a new receiver to support their algo-rithm. Their study analyses the characteristics of the information in the physical layer and proved that the tag information can be separat-ed and recovered from collision information in the physical layer. Moreover, their study explained the collision model in the physical layer in detail and designed two receivers, including a single antenna receiver and a multi-antenna receiver. The former eliminates the subspaces that divide the signals received

detects collisions. Tree protocols mainly aim at preventing the generation of empty slots, re-ducing the number of collision slots, decreas-ing data redundancy during communication, and accelerating the slot search. The main methods included are as follows [64-74]: (1) bit detection: the reader detects the collision bit according to Manchester code and merely completes the decomposition of the highest collision bit. The targets of reducing collision slots, avoiding empty slots, and accelerating the search are thereby achieved. (2) Bit lock-ing: this method extracts the information from the collision bits using Manchester code and eliminates the information that is useless for anti-collision purposes. (3) Stack increasing: all useful information in the identification pro-cess is recorded and used to recognise tags. (4) N-ary tree splitting: according to the estimated number of tags, the N-ary value of the tree is self-adaptively adjusted to reduce the number of collision slots.

3.3 Hybrid algorithms

The tree-based anti-collision protocols achieve an identification rate of 100 %, while Alo-ha-based algorithms show less time delay. Therefore, researchers around the world pro-pose new algorithms, in an attempt to integrate the advantages of each type to achieve better system performance. At present, two combina-tions are commonly used: the first combination applies the idea of time-separation to the tree-based algorithm (i.e. it uses double slots); the second applies a binary separation algorithm to those collision slots detected by the Aloha algorithm.

Using probability graph calculation tech-nology in an application database, the LoF protocol [75] treats the Aloha frame as a bit-graph and further converts the spatial concepts in the probability graph into time concepts (slot graph). Moreover, the ID number of each tag is used as its identification criterion, while the tags are allocated to the Aloha slots ac-cording to the Hash function’s geometrically distribution. Moreover, the Aloha slots are divided into three parts: suffix (1), prefix (0),


system rapidly decreases. In reference to other industries, such as stock market forecasting, historical information may be fused into the label predictions. However, the complexity of the system (both hardware and software) will also increase as a consequence.

The QT protocol will be a focus for tree protocol-related research in the future. Re-searchers mainly focus on the updating of the rules for command set Q. In addition, they also propose to eliminate redundant informa-tion in the communication. The identification of a large number of tags is unavoidable for the Internet of things in the future. However, under such circumstances, tree protocols pres-ent some problems such as the deterioration of system performance, long identification de-lays, etc. Thus the pre-grouping of large num-ber of tags may be helpful, to a certain extent, for the alleviation of these problems. Before identification, the reader divides the tags into subsets in priority sequence according to cer-tain rules. Research teams have proposed new protocols using information about tag IDs, that is, the differences and similarities of the ID number of the group of tags concentrated on the reader. By increasing the number of inqui-ry commands made by the reader, the tags are grouped in advance. The reader then makes an identification of each group. The deterioration of system performance caused by increased tag numbers is thereby avoided. Mathematical analysis and simulation trials also suggest that the proposed protocol significantly increases the performance of an RFID system. EPC forecasts that each object has a unique identi-fication code, namely, its EPC code. With an increased ID number length, the redundant in-formation in an anti-collision protocol also in-creases. Redundant information about any tag ID should be eliminated according to certain rules since they account for a large proportion of the tags on certain occasions. Researchers have proposed an anti-collision protocol by in-creasing the bit-locking instruction. According to the collision information sent by the tags, the reader produces this bit-locking command instruction and subsequently, tags extract the

by the reader into two orthogonal signals using zero forcing filters and successive interference patterns. Moreover, it distinguishes the sources of two tags on the I/Q plane. This receiver is only applicable to that case where two tags are collocated in the same slot at the same time: the latter uses the spatial characteristics of two different signals to resolve tags.

Fyhn et al. [78] proposed a time-dependent model based on the continuous signal subspace parameters of each tag to design and analyse the physical layer of the receiver in a passive UHF RFID reader. They used a single antenna to achieve multi-packet reception ability and a fast, efficient arbitration tag collision algorithm to solve multiple tag problems more rapidly. On the physical layer, they achieved a reply from more than two UHF RFID tags based on the intrinsic diversity of tag parameters.

IV. CONCLUSIONS AND FUTURE RESEARCH

As a fundamental network information collec-tion technology, RFID technology is develop-ing rapidly with increasing development of the Internet of things, and also following increases in market scale [79, 80]. Data integrity, as one of the key technologies innate to RFID, will draw the attention of a large number of re-searchers. Anti-collision protocols may appear in more rapid and stable forms. According to the analysis of the status of anti-collision algo-rithms, we make the following conjectures:

The DSFA protocol will be a future research hotspot amongst those interested in the Aloha protocol. Related research may mainly focus on innovation in forecasting models. Classical DSFA protocols are all statistically-based and they apply these statistics individual sample. However, this induces errors and may degrade the performance of an RFID system. There-fore, real-time, accurate estimation methods provide a guarantee of the reliability of the DSFA protocol and can also solve the “down-hill problem” triggered by estimation error, that is, when the frame length exceeds the number of tags and the throughput rate of the


analysis, protocols will become more practical in the breadth of their potential application.

ACKNOWLEDGEMENTS

The authors would like to thank the reviewers for their detailed reviews and constructive comments, which have helped improve the quality of this paper. This paper is supported by the National Natural Science Founda-tion of China (No. 61371092), the Doctoral Fund of Ministry of Education of China (No.20130061120062), and the China Postdoc-toral Science Foundation (No. 2014M551184).

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BiographiesQIAN Zhihong, doctoral supervisor, graduated with the Ph.D. in Communication and Information Systems at Jilin University. His research work focuses on radio frequency identification (RFID), wireless network and communication system, including key technologies of wireless sensor networks (WSN), internet of things (IOT), wireless ad hoc networks, theory and appli-cations of wireless personal area network based on bluetooth and ZigBee, signal analysis and process of wireless network system. He is recipient of several awards, and referee or reviewer for more than 20 journals and numerous conferences. He is one of Editorial Board Members for Journal of Communi-cations, Journal of Electronics & Information Tech-nology, Journal of Electronics (China), and Study on Optical Communications. He is the senior member of IEEE, the senior member of Chinese Institute of Electronics, and Institute of Communications. He also served on several institute committees, such as the Electronic Circuit and System Council of the Chinese Institute of Electronics, the National Higher Education Teaching Guidance Committee for Internet of Things Teaching, and the National University Students’ IoT Application Innovation Competition Committee. He hosted CSIE 2011, ITA2013, SECS-2013, AICE2014 and 2nd SECS-2014 as the General Chair, and more-over, delivered keynote speech for U-World-2011 and the 22nd Academic Annual Meeting of Electronic Circuit and System Council of the Chinese Institute of Electronics, ISEEIP 2012, and ITA2013 respectively in the latest 3 years. *The corresponding author. Email: [email protected]

WANG Xue, lecturer in the department of commu-nication engineering at Jilin University. She received doctor’s degree in Communication and Information Systems at Jilin University in the year of 2012. She focuses on the research on radio frequency identi-fication (RFID), ultra wideband (UWB), wireless net-work communication systems, and routing theories in internet of things (IOT), wireless ad hoc networks. Email: [email protected]

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