Mancerster code is one of the signal FM coding. He represents a binary value with a transient value of the half cycle in each bit. Therefore, the intermediate rising edge of the symbol time corresponds to the binary value 1 in the binary value 0. The bit rate is equivalent to the communication bandwidth. The intermediate jump of the symbol time is very important when receiving the synchronization signal, especially when multiple cards are in the operating range of the RFID, the anti-conflict detection process is more important. However, Mancerz has a higher bandwidth than other coding methods. In addition, Manchester code can ensure that data coding is independent of the encoding of previous data.
Two main baseband signals are given below.
(1) In the encoding of the level, the binary value corresponds to a high low voltage of the signal level, respectively. For example, the simplest code method is not zero-return code, its logic high binary value is 1, and the logic low level binary value is 0. In order to avoid a series of continuous The signals caused by 0 or 1 are not synchronized and difficult to reconstruct the time base. After each binary value, the signal will fall back to 0 volts, which encoded is a zero code (RTurn to Zero, Rz). The positive pulse is used for binary value 1, and its duration is equal to the half symbol time, and the signal is low logic in other cases. In addition, the data is encoded. (NRZ or RZ) has nothing to do with the previous data coding.
(2) In the encoding of signal hopping, the binary number corresponds to changes in signal two voltage levels; information is included in the hopping of the signal, and the encoding of the data is related to the previous data coding. The clock synchronization under this encoding type is easier than the level of encoding.
1 Manchester code is one of the signal frequency modified coding. He represents a binary value in a transient value along the half cycle in each bit, so the intermediate rising edge of the symbol time corresponds to the binary value of 0, and the falling edge corresponds to the binary value 1. The bit rate is equivalent to the communication bandwidth. The intermediate jump of the symbol time is very important when receiving the synchronization signal, especially when multiple cards are in the operating range of the RFID, the anti-conflict detection process is more important. However, Mancerz has a higher bandwidth than other coding methods. In addition, Manchester code can ensure that data encoding is independent of the encoding of previous data.
2 Miller code is another type of hopping code, encoding the binary value 1 using intermediate hoping using symbol time. If a continuous 0 bit occurs, a hop will be added at the beginning of the symbol time, which ensures a change in logic levels at least after two symbol time periods. Another kind is called "repair Miller code", which is a modification of Miller code, the same as Miller code principle, but by negative pulses to replace each jump, transmit such code, the bandwidth required for this class is higher than the previous one. The coding is wider. The encoding of the previous data must be considered in Miller coding.
3 The double phase interval is similar to the Miller code, which is phase reversal at the beginning of each symbol time period, indicating 1. If the level is in addition to a phase in the middle of the symbol time in addition to the period beginning in the cycle, it means 0. This encoding mechanism can better synchronize data when the signal is received compared to Miller code. FM0 encoding is similar to Miller code, but also the previous encoded data.
Other coding methods are transferred through pulse time modulation. In this coding, the encoding method of pulse width modulation, pulse position modulation, and pulse interval is usually used.
(1) Pulse Width Modulation (PWM). The pulse has a regular interval and other signals, and its length is proportional to the signal cycle. The PWM code links the binary value to the positive pulse length. At the end of the symbol time, the level is usually returned to a low potential, moved to a high point, and then start new encoding.
(2) Pulse position modulation (PPM). The information is encoded according to the position of the pulse. PPM code uses a negative pulse to encode logic 1. Unlike improved Miller code, continuous logic 0 is typically encoded with a constant high level. The PPM code for the corresponding step is N-bit logic word can be encoded. The position of the pulse during the time period determines the codeword. However, any word must be encoded by the position of the negative pulse: A constant level is not possible in the entire symbol time. The bandwidth of this coding method is relatively narrow than the Manchester Ma code, and it is easy to implement, but this encoding method is low.
(3) Pulse Interval Coding (PIE). It is a deformed of PPM modulation. In this encoding method, the reader generates two falling edges to determine the interval of pulses, this interval change is a function of binary counts 0 and 1.
The coding technology of RFID information must consider the following constraints:
1. The encoding must save energy transmission as long as possible.
2, the encoding does not consume too much bandwidth.
3. If there are several RFID tags in the operating range of the reader, the encoding should be able to have conflicts of conflicts.
Due to PPM, PIE and PWM encoding have relatively stable signals, so that the first two constraints can be satisfied. It should also be seen that Manchester code can be more easily inspected conflict, so this encoding method is usually used in the return link to the RFID tag to send data to the reader.
The NRZ code is the lowest bandwidth of Miller code, and their bandwidth is only half of the data bit rate bandwidth. Followed by the Manchester coding, FM0 code and RZ code, their bandwidth is the same as communication traffic.
Selecting the binary encoding needs to consider remote power supply problems, the carrier signal should be as long as possible to meet remote power requirements, at which time NRZ or Lee code can be used. Taking into account the data interaction between the IRFID tag and the reader, it is important to detect the response included in the feedback signal. Enables Mandsec code, etc., the code, which is hopped in the symbol time, can simplify the task.
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