1.2 RFID System Anti-Collision Technology
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there are multiple RFID tags in the reading range of a reader, which is called tag
collision. Another situation is that a tag may be read by multiple readers at the same
time, meaning that the scope of the readers overlaps.
In order to realize simultaneous recognition of multiple objects, it is necessary
to solve the problem of signal interference when multiple tags correspond to one or
more readers. This is the collision problem of RFID systems. As mentioned above,
the collision problem can be divided into tag collision and reader collision. Reader
collision refers to the fact that the working frequency of the reader overlaps so that the
tag cannot select the appropriate reader. When multi-tag corresponds to multi-reader,
the communication link between the tag and the reader cannot be established. Tag
collision refers to that when multiple tags correspond to a reader, the tag sends data to
the reader at the same time, and the signals collide with each other, making the reader
unable to obtain relevant information correctly. As readers are active devices, their
performance design is powerful, and readers can communicate with each other. Thus,
reader collisions can be resolved by communicating between readers and following
uniform read rules. It is tag collision that really troubles the RFID system to identify
multiple tags at the same time.
The essence of the tag collision problem is channel contention. Collision prevention is to improve the utilization of channels. In order to explain the problem more
clearly in principle, this chapter starts from the basic concept of the channel and
introduces the basic thinking of solving the problem.
1.2.1 Information Channel and Channel Capacity
Information channel is one of the indispensable parts of signal in a communication
system. A channel is a physical medium for transmitting signals from the sender to the
receiver, which can be divided into the wired channel and wireless channel. Channel
quality affects signal reception and modulation in two ways. On the one hand, when
the signal is transmitted in the actual channel, the channel characteristic is not ideal,
which will cause the distortion of the signal waveform; On the other hand, there are
various noises in the channel, which will affect the signal transmission. Information
channels can be divided into additive white Gaussian noise channels, Rayleigh fading
channels, and so on [19].
In a broad sense, a channel can be divided into modulation channels and a coding
channel according to its functions. The so-called modulation channel refers to the part
of the output of the modulator to the input of the demodulator. From the point of view
of modulation and demodulation, the modulation channel is all the transformation
devices and transmission media from the output of the modulator to the input of the
demodulator. Whatever the process, it is simply a transformation of the modulated
signal. We only care about the result of the transformation, not the detailed physical
process. Therefore, it is convenient to adopt this definition when studying modulation
and demodulation.
9
there are multiple RFID tags in the reading range of a reader, which is called tag
collision. Another situation is that a tag may be read by multiple readers at the same
time, meaning that the scope of the readers overlaps.
In order to realize simultaneous recognition of multiple objects, it is necessary
to solve the problem of signal interference when multiple tags correspond to one or
more readers. This is the collision problem of RFID systems. As mentioned above,
the collision problem can be divided into tag collision and reader collision. Reader
collision refers to the fact that the working frequency of the reader overlaps so that the
tag cannot select the appropriate reader. When multi-tag corresponds to multi-reader,
the communication link between the tag and the reader cannot be established. Tag
collision refers to that when multiple tags correspond to a reader, the tag sends data to
the reader at the same time, and the signals collide with each other, making the reader
unable to obtain relevant information correctly. As readers are active devices, their
performance design is powerful, and readers can communicate with each other. Thus,
reader collisions can be resolved by communicating between readers and following
uniform read rules. It is tag collision that really troubles the RFID system to identify
multiple tags at the same time.
The essence of the tag collision problem is channel contention. Collision prevention is to improve the utilization of channels. In order to explain the problem more
clearly in principle, this chapter starts from the basic concept of the channel and
introduces the basic thinking of solving the problem.
1.2.1 Information Channel and Channel Capacity
Information channel is one of the indispensable parts of signal in a communication
system. A channel is a physical medium for transmitting signals from the sender to the
receiver, which can be divided into the wired channel and wireless channel. Channel
quality affects signal reception and modulation in two ways. On the one hand, when
the signal is transmitted in the actual channel, the channel characteristic is not ideal,
which will cause the distortion of the signal waveform; On the other hand, there are
various noises in the channel, which will affect the signal transmission. Information
channels can be divided into additive white Gaussian noise channels, Rayleigh fading
channels, and so on [19].
In a broad sense, a channel can be divided into modulation channels and a coding
channel according to its functions. The so-called modulation channel refers to the part
of the output of the modulator to the input of the demodulator. From the point of view
of modulation and demodulation, the modulation channel is all the transformation
devices and transmission media from the output of the modulator to the input of the
demodulator. Whatever the process, it is simply a transformation of the modulated
signal. We only care about the result of the transformation, not the detailed physical
process. Therefore, it is convenient to adopt this definition when studying modulation
and demodulation.