Radio Frequency Identification (RFID) is a contactless automatic identification and data capture (AIDC) technology that uses RF signals for communication. Data is stored on silicon chips (tag memory), which are attached to targets such as books, parcels, humans, animals, or other objects. This section presents the components of an RFID system and explains how it operates to provide a better understanding of RFID.
1.1.2.1. RFID Components
As illustrated in Fig. 1.2, a typical RFID system consists of a reader, multiple tags/transponders, and the middle-ware software (application) [25].
Data Clock Energy
Application Coupling element (coil, microwave antenna)
Figure 1.2: Components of an RFID system.
RFID Tag: Tags (or transponders) are the actual data-carrying devices that are attached to objects for identification. A tag harvests energy from reader interrogation,
Contactless data carrier = Transponder
Reader
performs lightweight computation, and transmits data in response to reader queries.
Due to their simple structure, small size, and low manufacturing cost, tags serve as an economical and competitive method for managing massive objects, such as inventory control, object tracking, activity monitoring, authentication, localization, and more.
Figure 1.3: RFID tag.
The basic components of a tag include a microchip containing non-volatile memory and an antenna to collect and transmit radio waves as shown in Fig. 1.3 [26]. The chip contains circuitry that stores a unique binary number in called an electronic product code (EPC) [27], while the antenna serves as the receiver and transmitter of inform a- tion. EPC is a universal identifier (normally, 64 or 96 bits) that provides a unique identity to a specific physical object. The antenna, which is much larger than the microchip, typically consists of loops or coiled wire extending out from the chip. It receives signals from an RFID reader and backscatters the signal with required data.
RFID tags can be broadly classified in three types: passive, active, and semi-passive[1].
• Passive tags do not have their own power source, and they rely on the energy emitted by the RFID reader to power them up. When the RFID reader sends a signal to the tag, the tag absorbs the energy and uses it to transmit the data back to the reader. Passive tags can be further classified as low-frequency (LF), high-frequency (HF), and ultra-high frequency (UHF) tags. LF tags are suitable for short-range applications, such as access control systems, whereas HF tags are ideal for mid-range applications, such as payment systems. UHF tags are suitable for long-range applications, such as inventory management and supply chain management. Fig. 1.4 shows the EPC tag data structure of the 96-bit passive tag [28, 29].
• Active tags have an onboard power source, usually a battery, and are equipped with a powered receiver and transmitter. This enables the reception of very weak
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Figure 1.4: A passive RFID tag having 96 bits of memory to represent an EPC number. Header identifies the version of EPC itself; EPC Manager number identifies an organization; Object class refers to a unique type of product produced by an EPC manager; Serial number uniquely identifies each item within an object class.
signals and transmission of signals over long distances or through interference.
Moreover, active tags are capable of detecting collisions and sensing the channel, thereby improving their overall performance. They are particularly useful in sce- narios where the tag needs to operate in harsh environments or over a long range.
Active tags are often larger and more expensive than passive tags due to their additional components and power source. However, they offer greater flexibility and functionality in terms of their communication capabilities
• Semi-passive tags combine elements of both active and passive tags. They pos- sess an onboard power source that is used to power the microchip and a passive receiver. The semi-passive tag communicates using backscatter and can commu- nicate over a longer range than passive tags. Semi-passive tags are typically used in applications where longer read ranges are needed, such as in logistics or as- set tracking. The use of a battery allows for the tag to transmit data at higher power levels than passive tags. This feature is particularly useful in applications where tags are embedded in metal or other materials that can interfere with the radio signal. Semi-passive tags are also capable of sensing their environment and collecting additional data such as temperature, humidity, or motion.
Table 1.1: Tag characteristics
Characteristics Passive tag Active tag Semi-passive tag
Frequency LF, HF, UHF, Microwave UHF, Microwave UHF
Internal power No Yes Yes
Transceiver on broad No Yes No
Bit rate (Kbps) 246 20/40/250 16
Memory (KB) 128 128 4
Multi-tag collection 3 sec. to identify 20 tags 1000 tag/sec at 100 mph 7 tags/sec at 3 mph
Read Range (m) 0.1-7 More than 100 60-80
Tag size Thin, flexible Large, bulky Thin, flexible
Cost (USD) 0.15-1.00 10-100 0.75-2.00
Life Time (years) 3-10 0.5-5 0.5-5
In brief, Table. 1.1 technically summarizes various characteristics a according to tag types [1, 30].
An Reader or interrogator is an electronic device that communicates with RFID tags using radio waves. The reader consists of a radio frequency module, an antenna, and a processor that performs the tag detection and data communication functions.
The reader sends out a radio signal, and when a tag is within range, the signal is reflected back to the reader’s antenna. The reader then processes the data received from the tag and sends it to the middle-ware software for further processing [26].
RFID readers can be either fixed or mobile, depending on the application require- ments.
• A fixed RFID reader is a stationary device that is typically mounted in a fixed location, such as a wall, desk, or portal. It is designed to read RFID tags as they pass by the reader’s antennas. These readers are designed suitable for indoor ap- plications with a low-to-moderate traffic of tagged items. Fixed RFID readers can operate on a variety of frequencies, including low, high, and ultra-high frequency, and support various communication protocols. Fixed readers can be connected to a computer or network via serial, USB, Ethernet, or Wi-Fi connections. They are commonly used in applications such as inventory management, asset tracking, and access control.
• A mobile RFID reader, also known as a handheld RFID reader or portable RFID reader, is a battery-powered device. The device can be carried by a person and moved around as needed, making it suitable for use in a wide range of applications.
Mobile RFID readers are commonly used in inventory management, asset tracking, and supply chain management applications. These devices typically feature an integrated antenna, a display, and a keypad or touchscreen interface. They may also include wireless connectivity options such as Bluetooth or Wi-Fi for data transmission. The mobility of these devices enables real-time data collection and tracking, improving the efficiency and accuracy of business operations.
Middle-ware software: is an essential component of an RFID system that enables efficient and effective management of data collected by RFID readers. It acts as a bridge between the reader and backend systems, providing a layer of abstraction that simplifies data integration and processing [31]. This software also provides data management capabilities, such as storing, filtering, sorting, and routing data to different applications and systems.
1.1.2.2. Operating Frequencies
There are four major frequency ranges as briefly discussed below [1, 32, 33].
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• Low frequency (LF): RFID systems using low frequency tags operate at a fre- quency range below 135 kHz and typically have a read range of up to 10 centime- ters. These passive tags draw power from the reader and have a data transfer rate of less than 10 kbps. They are commonly used in animal tagging, access control, and waste management, among other applications.
• High Frequency (HF): High frequency RFID technology uses 13.56 MHz frequency and has a read range of 10 to 20 centimeters. The data transfer rate is less than 100 kbps, and the tags are mostly passive. These tags are suitable for applications that require moderate range and are used in various settings such as access control, item tagging, and baggage control.
• Ultra-High Frequency (UHF): Ultra-high frequency (UHF) RFID tags operate in a frequency range of 860 MHz-960 MHz and have an average read range of 5 to 6 meters, but modern larger tags can reach up to 30+ meters under ideal conditions.
The data transfer rate is 100 kbps, and UHF RFID systems support all three types of tags. UHF RFID technology is used in various applications, including baggage handling, toll collection, and pharmaceutical serialization.
• Microwave: RFID systems that use microwave tags, also known as super-high frequency tags, operate at 2.45 GHz or 5.8 GHz frequency range. These tags have a large read range of up to 100 meters and a data transfer rate of less than 200 kbps. However, microwave systems are more expensive than other types of RFID systems. They are commonly used in electronic toll collection, real-time tracking of valuable goods, and production line tracking.
1.1.2.3. Communication Principle
In order to establish the communication between tags and readers, RFID technology uses either magnetic or electromagnetic coupling, which are briefly described in the following.
Magnetic Coupling
Reader Antenna Tag Antenna
Figure 1.5: A magnetic coupling RFID system.
Reader Tag
Reader Tag
In magnetic coupling systems, a reader generates a time-varying magnetic field by passing a high-frequency current through its coil, as shown in Fig. 1.5. The magnetic field produced by the reader’s coil then induces an alternating current in the coil of the tag. This alternating current is rectified to a direct current to power the tag’s microchip, which then modulates the magnetic field and sends data back to the reader [25, 1]. The communication between the reader and the tag is based on the strength of the magnetic field, which decreases with distance. The tag must be close enough to the reader’s coil to receive enough energy to operate, but not so close that the magnetic field becomes distorted and communication is affected [25, 34].
Reader Antenna (Dipole) Tag Antenna
RF signal
Backscatter signal
Figure 1.6: An electromagnetic coupling RFID system.
The electromagnetic coupling systems, on the other hand, are called backscatter sys- tems. As depicted in Fig. 1.6, a reader emits a radio signal whose energy is transferred to a tag’s antenna through an electromagnetic field. This causes the tag’s antenna to resonate at the same frequency as the reader’s antenna, which in turn induces an electrical current in the tag’s antenna, which powers the tag’s circuitry. The tag then responds by modulating the signal and reflecting it back to the reader. The reader receives the modulated signal and decodes the information stored in the tag [25, 35].
The strength of the coupling between the tag and the reader depends on several fac- tors, including the frequency of the radio waves, the distance between the tag and the reader, and the size and orientation of the antennas. By adjusting these parameters, RFID systems can be optimized for different applications and environments [25, 34].
1.1.2.4. Communication Protocols
As mentioned in Section 1.1.2.1, readers and tags are the two key components of an RFID system, while they are, in any practical RFID systems, usually large. Commu- nication protocols are understood as the set of communication rules/methods between one or more readers and tags to perform necessary tasks, such as identification and missing-tag detection. The most widely used protocol in RFID is EPC (Electronic Product Code) C1G2, which is based on a master-slave style architecture. The reader acts as the master and initiates all communication, providing power for the tags to
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operate. The protocol is half-duplex, requiring three major steps for a reader to access a tag [28, 36].
• First (Query): The reader sends a Query command to tags, which includes in- formation such as the session, target, and Q value. The session determines the level of security and the target identifies specific tags to be accessed. The Q value determines the number of (time) slots that the tag should wait before responding to the reader.
• Second (Acknowledgment): The tags respond to the reader with an Acknowledg- ment (ACK) or Not Acknowledgment (NACK) response, depending on whether it is ready to communicate or not. If a tag responds with an ACK, it enters the inventory round and waits for the reader’s next command.
• Third (Data Exchange): The reader sends a series of commands to the tag, such as Read or Write commands, to exchange data. The tag responds with the requested data, and the reader continues to send commands until it has finished accessing the tag. After this inventory round is completed, the tag goes back to sleep mode to conserve energy.
More generally, current communications protocols are mainly based on the principle of time division multiple access (TDMA) in which timeslots are used to control the number of tags’ responses. They are classified into two types i.e., tree-based and (frame slotted) Aloha-based.
No response Identified Collision
Figure 1.7: An illustration of Tree-based protocol.
In tree-based protocols, collided tags i.e., the tags respond to a reader at the same timeslot, are divided (by the reader) into smaller subsets until there is only one tag
Random number
Timeslot 1 (A,B,C)
0 1
Timeslot 2 (A,C)
Timeslot 7 (B)
0 1
Timeslot 6 Timeslot 3 (A,C)
0 1
Timeslot 4 (A)
Timeslot 5 (C)
−
N
1 − f
left. To do it, each tag is required to equip itself a counter or a random generator. In other words, tags are organized in hierarchical tree structure by which collisions and interference between tags are minimized, which improves the efficiency and accuracy of the communication process. Despite these advantages, tree-based protocols have some limitations. One of the main challenges is the overhead associated with managing the hierarchical tree structure. As the number of tags in the system increases, the size of the tree and the complexity of the communication process also increase, which can lead to increased latency and reduced system performance. A simple tree structure of this kind of protocol is depicted in Fig. 1.7 with three tags, i.e., A, B and C.
On the other hand, in frame slotted Aloha (FSA) protocols, multiple tags can ran- domly respond to the reader in a frame of size f of fixed-length timeslots [26, 28, 37, 38].
It is a random access approach that enables a tag to communicate with a reader in a designated slot without any predefined order. In particular, at the start of each frame, the reader sends a query command to the tags, broadcasting the parameters ⟨f, R⟩, where R is a random seed. Upon receiving the query command, each tag selects a slot in the frame to reply to by computing H(ID, R) mod f , where H (.) is a hash function that has been pre-deployed by the reader and tags, and ID is the tag’s identification.
This generates a slot counter SC that ranges uniformly in [1, f − 1]. The reader then processes every slot sequentially by broadcasting a “Slot end” command to terminate the current slot and begin the next one. After receiving the command, each tag decre- ments its slot counter SC by one. In any given slot, when a tag’s slot counter is equal to 0, it can transmit its messages to the reader. Once a frame is executed, the reader has the ability to categorize each slot into three distinct types. These include: (i) an empty slot without any tag replies, (ii) a singleton slot with only one tag reply, and (iii) a collision slot with multiple tag replies.
Here, if we assume there are totally N tags in the considered systems, the probability that k tags transmit simultaneously in one slot can be expressed as
N
1 k 1 N −k
Equation. 1.1 reflects a fact that the average number of identified tags per timeslot during the frame or the throughput (defined by à) in other words, can be found as
N 1 1 1
à = 1 − N 1 N
e− N 1
36.8%, (1.2)
1 f f
N −
≈ f e ≤
e ≈
where f e e reaches its maximum value when N = f . The FSA throughput is also plotted in Fig. 1.8 for different values of the frame size.
f
P (k) = k . (1.1)
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Tags’ response
S E C
Figure 1.8: FSA throughput for different frame sizes.
The FSA protocols have a number of advantages over other RFID protocols. For examples, they are simple, making them easy to implement and use, while they can provides high throughput and efficiency. They are also our focus in this dissertation.
However, one of the main limitations of FSA protocols is determining the optimal frame length for a given system, especially for those with unknown and huge number of tags.
This requires balancing the need for high throughput with the need to avoid collisions, minimize idle/empty timeslots, and also hardware constraints [32].
QUERY with f 5
Reader
Observations
E C
Timeslot status E: Empty S: Singleton C: Collision
Figure 1.9: An illustration of FSA protocol.
Fig. 1.9 shows a simple example of Aloha-based protocol with a reading round, where the frame size f = 5 and there are five tags. Each tag randomly selects one of five timeslots to transmit their information. The resulting state of timeslots 1 and 4 are empty slot, indicating that no tags selected them. Timeslot 3 is singleton slot because it is occupied by tag 3. Timeslot 2 and 5 are occupied by two tags, resulting in a collision slot.
Tag 1 Tag 2 Tag 3 Tag 4 Tag 5
frame
1 2 3 4 5
1 2
frame
3 4 5
Tag 1
Tag 4 Tag 3 Tag 2
Tag 5