UHF RFID Anti-collision algorithm

Multi-tag collision: multiple tags are in the field of action of the reader. When more than two tags are sending data at the same time, there will be communication collision and data interference (collision).

In order to prevent these conflicts, certain related commands need to be set in the radio frequency identification system to solve the conflict problem. These commands are called anti-collision commands or algorithms. It is divided into the following two types, the deterministic algorithm based on the deterministic polling mechanism and the non-deterministic algorithm based on the random mechanism (mainly the ALOHA algorithm).

ALOHA algorithm is a random access method. The basic idea is to adopt the way that the tag speaks first. When the RFID electronic tag enters the recognition area of the reader, it will automatically send its own ID number to the UHF reader. During the process of sending data from the tag, if there are other tags Data is also being sent, overlapping signals will occur, causing collisions. The reader detects whether there is a conflict in the received signal. Once a conflict occurs, the reader will send a command to stop the tag from sending, and wait for a period of time before resending to reduce the conflict.

1. Pure ALOHA algorithm

In the pure ALOHA algorithm, if the read-write device detects that there is mutual interference in the signals, the reader-writer will send a command to the tag to stop transmitting signals to the reader-writer; after the tag receives the command signal, it will stop sending information, and will enter the standby state during a random period of time, and only after the period of time has elapsed will the information be sent to the RFID Reader again. The length of the standby time segment of each RFID electronic tag is random, and the time to send the signal to the reader again is also different, so as to reduce the possibility of collision.

When the UHF reader successfully recognizes a certain tag, it will immediately issue a command to the tag to enter the dormant state. The other tags will always respond to the commands issued by the reader, and repeatedly send information to the reader. When the tags are recognized, they will enter a dormant state one by one until the reader recognizes all the The algorithm process ends only after the labels in the region are selected. There will be no collision in sending frames, and it can be analyzed that the probability P of successful sending is related to the throughput rate and the amount of data contained.

Features: packet length (equal length), large conflict area, simple implementation, suitable for scenarios with low packet transmission density

Summary: When a conflict is detected, enter the standby state, wait for a random period of time, and then send

2. Time slot ALOHA

The slotted ALOHA algorithm divides the time into multiple discrete time slots, the length of each time slot is equal to or slightly larger than one frame, and the tag can only send data at the beginning of each time slot. In this way, the tags are either sent successfully or completely collide, avoiding partial collisions in the pure ALOHA algorithm, halving the collision period, and improving channel utilization. The slotted ALOHA algorithm requires the reader to calibrate the time of the tags in its identification area. Because the tag only transmits data in a certain time slot, the collision frequency of this algorithm is only half of that of the pure ALOHA algorithm, but the data throughput performance of the system will be doubled.

Features: The conflict area is limited to the time slot, correct reception: no conflict, correct verification, collision: reception error, empty time slot

Summary: Divide the channel into several time slots (greater than or equal to one frame), each terminal can only start to transmit information in each time slot, the conflict area is limited to the time slot, and the result is only success and collision (failure) , the throughput of slotted ALOHA is twice that of pure ALOHA.

3. Framing time slot ALOHA

In the framing time slot algorithm, the time is divided into multiple discrete time slots, and the electronic tag can only start transmitting information at the beginning of the time slot. The reader/writer sends query commands in a frame cycle. When the electronic tag receives the request command from the reader, each tag sends information to the reader by randomly selecting a time slot. If a time slot is only selected by a unique tag, the information transmitted by the tag in this time slot is successfully received by the Honglu reader, and the tag is correctly identified. If two or more tags choose the same time slot to send, conflicts will occur, and these tags that send information at the same time cannot be successfully identified by the reader. The recognition process of the entire algorithm will be repeated in this way until all tags are recognized.

Features: The disadvantage of this algorithm is that when the number of tags is much larger than the number of time slots, the time to read tags will be greatly increased; when the number of tags is much smaller than the number of time slots, time slots will be wasted.

Summary: Several time slots form a frame, and all tags select time slots to send in the frame.

Binomial Model of ALOHA Algorithm

Binary tree search algorithm: The binary tree search algorithm is controlled by the reader. The basic idea is to continuously divide the electronic tags that cause collisions and reduce the number of tags to be searched in the next step until only one electronic tag responds.

Basic idea: After multiple tags enter the reader's workplace, the reader sends an inquiry command with restrictions, and the tags that meet the restrictions reply. If a collision occurs, modify the restrictions according to the bit where the error occurred, and send the query again commands until a correct answer is found and the read and write operations to the tag are completed. Repeat the above operations for the remaining tags until the read and write operations for all tags are completed.