Manchester Code, is also called phase coding (PE), is a synchronous clock coding technology, which is used by the physical layer to encode a synchronous bitstock clock and data. Manchester Coding is used in the Ethernet system. Manchester Code provides a simple way to encode simple binch sequences without a long cycle without a conversion level, thereby preventing loss of clock synchronization, or an analog link bit error from low frequency displacement in poor compensation. Under this technique, the actual binary data is transmitted through this cable, not transmitted as a sequence of logic 1 or 0 (technique, reflective). (NRZ)). Conversely, these bits are converted into a slightly different format, which has many advantages through direct binary coding.
Manchester coding is often used in local area network transmission. In Manchester encoding, there is a hop in the middle of each bit, and the hopping between the bits is made both the clock signal, and the data signal is made; "1" from high to low hop, from low to high hopping representation "0" . There is also a differential Manchester code. Each intermediate trip is only a time clock, and there is no hop in each start to indicate "0" or "1", there is a hop to "0", no hopping "1".
The encoding rule encoded by the Manchester is: in the signal bits from low to high hopping representation 0, the level is high to low hop from high to low hopping in the signal bits.
Changing the signal polarity at the beginning of the signal bit, indicating the logic "0"; does not change the signal polarity at the beginning of the signal bit, indicating the logic "1". As shown in Figure 1.
where: a) is not zero code, b) is the Manchester code, also called digital double phase. c) For the differential Manchester code, the conditional dual phase (CDP code). It is an improved Manchester code, which is characterized by the middle of each cycle, the waveform varies, if there is no change in the two cycles, it means "1": change, indicating "0" (The signal is always inverted in the middle of the signal bits; the signal polarity is not changed at the beginning of the signal bit, indicating that the logic "1": changes the signal polarity when the signal bit is started, indicating the logic "0").
Identify differential Manchester coding: mainly look at two adjacent waveforms, if the latter waveform is the same as the previous waveform, then the latter waveform represents 0, and if the waveform is different, it means 1.
The difference between the two
Manchester and differential Manchester coding is the basic identical code, the latter is the former improvement. Their feature is a bit synchronization clock in each bit of transmission, so a transfer can be allowed to have a long data bit. Each bit bit of Mancanster only is only half in the clock cycle. When transmitting "1", the first half of the clock cycle is high, and the latter is low; while transmitting "0" on the contrary. In this way, there will be a hop during each clock cycle, which is a bit synchronization signal. Differently Manchester Coding is an improvement in Manchester coding. It has a hopping in the middle of each clock position, and "1" is still "0", which is to distinguish between the start of each time clock.
Differently Manchester Coding is less than the Mandester coding, and therefore is more suitable for transmitting high-speed information, which is widely used in broadband high-speed networks. However, since each clock bit must have a change, the efficiency of these two codes can only reach around 50%; the zero code digital signal can be directly used, and the so-called baseband refers to the basic frequency band. Baseband transmission is the electrical pulse of digital signals directly in the line. This is the simplest transmission method, and the local area network of near-distance communications is transmitted. When the baseband is transmitted, it is necessary to solve the digital signal representation of digital data and signal synchronization problems between the two ends. For transmitting digital signals, the simplest and most common method is to represent two binary numbers with different voltage levels, that is, the digital signal consists of a rectangular pulse. According to the digital coding method, it can be divided into a single pole code and a bipolar code, the single pole code uses the voltage of the positive (or negative); the bipolar code is a three-input code, 1 is inversion, 0 is Keep zero level. Depending on whether the signal is zero, it can also be divided into zero code and non-zero code, the signal returns to zero zero to zero in the middle of the zero code element, rather than zero the zero code 1 level flip, and zero.
In a super-visual communication system, the received signal level is present because the multipath transmission presents a large fading phenomenon, and when the party or both parties quickly move, There is also a larger Doppler frequency shift in the received signal, and the size of the Doppler frequency shift is proportional to the speed of motion. When the transmission rate of communication is relatively, due to the existence of Most Push Transmission, the extraction of the receiving end carrier and the bit synchronization will be greatly affected, and the demodulation performance will drop.
In order to solve this problem, the Doppler frequency can be estimated and compensated, and the spread spectrum communication or complex error correction coding can also be used to reduce most Push Transmission to communication performance. Influence. However, when the hardware platform resource and frequency band are limited, the above method is no longer applicable due to the calculation of complex hardware resources, or the desired frequency band is overwhelmed. In this paper, the engineering needs of the over-site transmission of 300 Hz Doppler frequency shift in the channel presence of a communication rate of 600 bps, and it is proposed a resistance to Davpler modification based on differential Manchester soft decoding. Method, this method is simple to calculate, by organically combining the difference Manchester coding, differential demodulation and diversity merge, while effective smooth level fading, ultra-visual low-speed transmission of the hardware platform resource and frequency band The ability to have anti-Dappler frequency shift in the system.
Due to the large Doppler frequency shift on the channel, the frequency and phase information of the coherent carrier cannot be extracted from the received signal, so that the signal is uniformly demodulated by the inverse operation; even if it is The differential demodulation of a larger frequency offset is also unable to implement the effective demodulation of the received signal when the communication rate is 600 bps, and the channel exists 300 Hz Doppler frequency shift. Under the conditions of the hardware platform resource and frequency band, a differential manchester coding and differential demodulation joint design method is proposed, using differentiation Manchester coding to increase the size of the chip rate, enhance the difference-of-demodulating anti-frequency bias, which in turn Reduce the impact of most Puller on the performance of the demodulator. However, the improvement of the chip rate will cause the demodulator to generate performance loss at the low signal-to-noise ratio, and the Manchester soft decoding method can be effectively merged by the Soft-to-noise ratio of Manchester coding, so demodulation performance loss Do not increase due to the improvement of the Diagram of Manchester coding multiple, and performance loss can be controlled within an acceptable range by effective digital processing. Since the communication system operates under the super-visual channel conditions, there is a large fading of the level of the received signal, and diversity measures must be taken to effectively smooth down, improve the smooth reception capacity of the signal, thereby realizing the low speed communication system Effective communication under the passage of most Puller channel conditions.