The Manchester code is a type of signal frequency coding. He uses a random value of each bit at the half-period edge to represent a binary value. Thus the middle rising edge of the symbol time corresponds to a binary value of 0 and the falling edge corresponds to a binary value of one. The bit rate is equivalent to the communication bandwidth. The intermediate transition of the symbol time is very important when receiving the sync signal, especially when the multiple cards are within the operating range of the RFID reader, the anti-collision detection process is more important. However, the Manchester code has a higher bandwidth than other encoding methods. Furthermore, the Manchester code during encoding ensures that the data encoding is independent of the encoding of the previous data.
The encoding of the two main baseband signals is given below.
(1) In the encoding of the level, the binary values correspond to the high and low voltages of the signal level, respectively. For example, one of the simplest coding methods, non return to zero (NRZ), has a logic high level with a binary value of 1, and a logic low level with a binary value of 0. To avoid a series of consecutive Signals caused by 0 or 1 are out of sync and difficult to reconstruct the time reference. After each binary value, the signal will fall back to 0 volts. This code is return to zero (RZ). A positive pulse pair is used for a binary value of 1, whose duration is equal to half the symbol time, and in other cases the signal is low logic level. In addition, the encoding of the data. (NRZ or RZ) is independent of previous data encoding.
(2) In the coding of signal hopping, the binary number corresponds to the change of the two voltage levels of the signal; the information is included in the hopping of the signal, and the encoding of the data is related to the previous data encoding. Clock synchronization under this encoding type is easier than in the encoding type of the level.
1 Manchester code is a kind of signal frequency modulation code. He represents a binary value with the instantaneous value of each bit at the half-period edge, so the middle rising edge of the symbol time corresponds to a binary value of 0, and the falling edge corresponds to a binary value of 1. The bit rate is equivalent to the communication bandwidth. The intermediate transition of the symbol time is very important when receiving the sync signal, especially when the multiple cards are within the operating range of the RFID reader, the anti-collision detection process is more important. However, the Manchester code has a higher bandwidth than other encoding methods. Furthermore, the Manchester code during encoding ensures that the data encoding is independent of the encoding of the previous data.
The 2 Miller code is another type of hopping code that encodes the binary value 1 using the intermediate hop of the symbol time. If a consecutive 0 bit occurs, a transition is added at the beginning of the symbol time, which ensures that there is a change in logic level after at least two symbol time periods. The other is called “Muller Code”, which is a variant of the Miller code, which is the same as the Miller code principle, but with a negative pulse instead of every jump, the bandwidth required to transmit such code is higher than the previous one. The code should be wide. The encoding of the previous data must be considered in Miller coding.
The 3 biphasic interval code is similar to the Miller code, which performs phase reversal at the beginning of each symbol time period, indicating 1. If the level has a phase reversal in the middle of the symbol time in addition to the inversion at the beginning of the period, it represents 0. Compared with the Miller code, this encoding mechanism can better synchronize the encoded data
when receiving the signal. The FM0 code is similar to the Miller code, and the previous coded data is also considered.
Other coding methods transmit information by pulse time modulation. In this type of coding, pulse width modulation, pulse position modulation, and pulse interval coding are commonly used.
(1) Pulse width modulation (PWM). The pulses are regularly spaced equal amplitude signals and their length is proportional to the signal period. The PWM code associates the binary value with the positive pulse length. At the end of the symbol time, the level usually goes back to low and then to the high point before starting a new code.
(2) Pulse position modulation (PPM). The information is encoded according to the position of the pulse. The PPM code uses a negative pulse to encode a logic one. Unlike the modified Miller code, a continuous logic 0 is typically encoded with a constant high level. The n-bit logical word can be encoded by using a ppm code of order n accordingly. The position of the pulse in the time period determines the code word. However, any word must be encoded by the position of the negative pulse: it is impossible to find a constant level for the entire symbol time. Compared with the Manchester code, the coding mode has a relatively narrow bandwidth and is easy to implement, but the coding mode has a low data rate.
(3) Pulse interval coding (PIE). It is a variant of PPM modulation in which the reader generates two falling edges to determine the interval of the pulses, which is a function of the binary numbers 0 and 1.
The coding techniques for RFID information must consider the following constraints:
1. The code must preserve the energy transfer for as long as possible.
2. Encoding can’t consume too much bandwidth.
3. If there are several RFID tags in the operating range of the reader, the code should be able to detect conflicts of interest.
Since PPM, PIE and PWM coding have relatively stable signals, the first two constraints can be satisfied. However, it should also be noted that the Manchester code can be more easily detected as a collision, so this encoding is typically used in the return link where the RFID tag sends data to the reader.
The NRZ code and the Miller code encode the lowest bandwidth, and their bandwidth is only half of the data bit rate bandwidth. This is followed by Manchester code, FM0 code and RZ code.
Their bandwidth and communication traffic are the same.
Selecting the binary code requires consideration of the remote power supply problem. The carrier signal should be as long as possible to meet the remote power supply requirements. In this case, the NRZ or the code can be used. Considering the data interaction between the iRFID tag and the reader, it is important to detect the response contained in the feedback signal. Encoding that allows the Manchester code to be hopped during the symbol time period simplifies the task.