Radio Frequency Identification (RFID) is a technology for identifying objects that employ radio frequency signals, and it is one of the Internet of Things’ most important technologies. This technology allows communication between both the reader and the tags, the two main RFID devices.
The tags share a communication channel. As a result, if multiple tags try to send data simultaneously, the reader would be unable to differentiate between them. This situation is described as the tag collision problem. As a consequence, the time required for system identifiers and energy consumption increases. RFID solutions should use an anti-collision methodology to reduce tag collisions.
What causes RFID tag collisions?
When multiple tags transmit to a reader simultaneously, their diffracted signals deactivate each other, leaving the reader with an indecipherable message. This is known as collision, resulting in a loss of identifying time and an increase in the reader’s power consumption.
The general problem of RFID collision can be divided into two categories:
- When a reader tries to communicate with tags in another reader’s communication range, a reader collision is detected.
- When many tags try to transfer their ID simultaneously, a tag collision occurs: the reader receives a mix of messages and cannot comprehend them fully.
What are the existing anti-collision protocols?
Multi-Access Methods
To physically differentiate the transmitters’ signals, each anti-collision protocol employs various multi-access identification techniques. Space Division Multiple Access (SDMA), Frequency Division Multiple Access (FDMA), Code Division Multiple Access (CDMA), and Time Division Multiple Access (TDMA) are the four different types.
SDMA stands for space division multiple access. The term “multiple access” refers to the division of a network’s capacity into distinct areas. The beam can be pointed at different places to recognize tags in protocols relying on this method. Complex directional antennas are used to differentiate the channel spatially. The use of multiple readers is another way to accomplish this. As a result, adjacent readers’ channel capacity is increased.
FDMA—Tags that transmit in many different frequency channels, necessitating an advanced and powerful reader. As a result, different frequency ranges could be used for interaction: 135 kHz from the reader to the tags and 433–435 MHz from the tags to the reader. However, this method is costly and only suitable for a limited number of applications.
CDMA tags must multiply their ID by a pseudo-random sequence (PN) before transmitting data. CDMA excels in several areas, including the security of communication channels between RFID tags and readers, as well as multiple tag identifiers. It adds a lot of intricacies and costs a lot of money for RFID tags. Besides that, this method consumes a lot of energy and can be categorized as a high-demand group.
Aloha Protocols
To effectively detect the number of tags in an execution area, Aloha-based protocols employ a random-access tactic.
Because tags convey their IDs randomly to the chosen slots in a frame to minimize the risk of a collision, they are classified as probabilistic protocols.
- One of the most basic anti-collision protocols is Pure Aloha (PA). It is based on the TDMA protocol. When tags access the investigation zone, they choose a frequency to transfer their data at random.
- Slotted Aloha (SA) was created to prevent unfinished collisions. The time in SA is split into several slots, so each tag must choose a slot at random to transfer its data. The reader and the tag are now communicating synchronously.
- Framed Slotted Aloha, here, time is divided into a varying number of frames in Framed Slotted Aloha (FSA), with each frame containing several slots. All tags should send information into a fixed-length frame, but each tag can only transfer data into one slot per frame. Because tags can only reply once per frame, this protocol greatly reduces the likelihood of a collision.
- When the number of tags is small, and the frame size is significantly larger, the main drawback of FSA is slot waste. The Dynamic Frame Slotted Aloha (DFSA) protocol was created to address this issue. DFSA can adjust the frame size based on an estimate of the number of tags. The reader informs the tags of the frame length at the start of each frame.
It is impossible to conclude from these protocols that only certain protocol types rise. However, it should be mentioned that the most recent protocols became more refined and achieve better simulation results. This is also in contrast to the ability to put these solutions into actual devices.
