Common Code Types for Baseband Transmission
1) The AMI code
The full name of the AMI (Alternative Mark Inversion) code is the alternate mark inversion code. blank) remain unchanged. E.g:
Message code: 0 1 1 0 0 0 0 0 0 0 1 1 0 0 1 1…
AMI code: 0 -1 +1 0 0 0 0 0 0 0 -1 +1 0 0 -1 +1…
The waveform corresponding to the AMI code is a pulse sequence with positive, negative, and zero levels. It can be regarded as a deformation of the unipolar waveform, that is, “0″ still corresponds to the zero level, while “1″ corresponds to positive and negative levels alternately.
The advantage of AMI code is that there is no DC component, there are few high and low-frequency components, and the energy is concentrated at the frequency of 1/2 code speed.
(Fig. 6-4); The codec circuit is simple, and the code polarity can be used to observe the error situation; if it is an AMI-RZ waveform, it can be changed to unipolar as long as it is full-wave rectified after receiving. RZ waveform from which the bit timing components can be extracted. Because of the above advantages, the AMI code has become one of the more commonly used transmission code types.
The disadvantage of the AMI code: When the original code has a long series of “0″, the level of the signal does not jump for a long time, which makes it difficult to extract the timing signal. One of the effective ways to solve the problem of even “0″ code is to use HDB3 code.
(2) The HDB3 code
The full name of the HDB3 code is the third-order high-density bipolar code. It is an improved type of AMI code. The purpose of the improvement is to maintain the advantages of the AMI code and overcome its shortcomings so that the number of consecutive “0″s does not exceed three. Its encoding rules are as follows:
First check the number of consecutive “0″s in the message code. When the number of consecutive “0″s is less than or equal to 3, it is the same as the encoding rule of the AMI code. When the number of consecutive “0″s exceeds 3, each of the 4 consecutive “0″s will be converted into a section and replaced with “000V”. V (value +1 or -1) should have the same polarity as its immediately preceding adjacent non-”0″ pulse (because this breaks the polarity alternation rule, so V is called a destroying pulse). Adjacent V-code polarities must alternate. When the value of the V code can meet the requirements in (2) but cannot meet this requirement, then replace “0000″ with “B00V”. The value of B is consistent with the following V pulse to solve this problem. Therefore, B is called a modulation pulse. The polarity of the transmission number after the V code should also be alternated.
In addition to the advantages of the AMI code, the HDB3 code also limits the number of consecutive “0″ codes to less than 3, so that the extraction of timing information can be guaranteed during the reception. Therefore, the HDB3 code is the most widely used code type in my country and Europe, and the interface code types below the A-law PCM quaternary group are all HDB3 codes.
In the above-mentioned AMI code and HDB3 code, each binary code is converted into a code with a 1-bit three-level value (+1, 0, -1), so this kind of code is also called a 1B1T code. In addition, it is also possible to design an HDBn code in which the number of “0″s does not exceed n.
(3) The Biphase code
The Biphase code is also called as Manchester code. It uses a period of positive and negative symmetrical square waves to represent “0″ and its inverse waveform to represent “1″. One of the encoding rules is that the “0″ code is represented by a “01″ two-digit code, and the “1″ code is represented by a “10″ two-digit code. For example,
Message code: 1 1 0 0 1 0 1
Biphase code: 10 10 01 01 10 01 10
A biphasic code waveform is a bipolar NRZ waveform with only two levels of opposite polarity. It has level jumps at the centre point of each symbol interval, so it contains rich bit timing information. There is no DC component, and the encoding process is also simple. The disadvantage is that the occupied bandwidth is doubled, which reduces the utilization rate of the frequency band. The bi-phase code is good for sending data terminal equipment over short distances, and it is often used as the type of transmission code in a local area network.
(4) Bi-phase differential code
In order to solve the decoding error caused by the polarity reversal of bi-phase code, the concept of differential code can be used. Biphase code uses the level transition in the middle of the duration of each symbol for synchronization and signals code representation (the transition from negative to positive represents binary “0″, and the transition from positive to negative represents binary “1″). In differential biphase code coding, the level transition in the middle of each symbol is used for synchronization, and whether there is an additional transition at the beginning of each symbol is used to determine the signal code. If there is a transition, it means binary “1″, and if there is no transition, it means binary “0″. This code is often used in local area networks.
CMI code
CMI code is the abbreviation of “mark inversion code. Like the bi-phase code, it is also a bipolar two-level code. The coding rule is: “1″ code is alternately represented by “11″ and “00″ two-digit code; the “0″ code is fixedly represented by “01″, and its waveform is shown in Figure 6-5(c).
CMI codes are easy to implement and contain rich timing information. In addition, since 10 is a forbidden code group, there will be no more than three consecutive codes, and this rule can be used for macroscopic error detection. This code has been recommended by ITU-T as the interface code type of the PCM quartet and is sometimes used in optical cable transmission systems with a rate lower than 8.448Mb/s.
Block encoding
To improve line coding performance, some kind of redundancy is required to ensure pattern synchronization and error detection. The introduction of block coding can achieve both of these purposes to some extent. The form of block coding is nBmB code, nBmT code and so on.
nBmB code is a type of block coding, which divides the n-bit binary code of the original information stream into a group and replaces it with a new code group of m-bit binary code, where m>n. Since m>n, the new code group may be There are 2^m combinations, so there are more (2^m-2^n) combinations. Among the 2″ combinations, the favourable code group is selected in some way as the allowed code group, and the rest are used as the forbidden code group to obtain good coding performance. For example, in 4B5B coding, the 5-bit code is used instead of the 4-bit code. Coding, for 4-bit grouping, there are only 2^4=16 different combinations, and for 5-bit grouping, there are 2^5=32 different combinations. In order to achieve synchronization, we can follow no more than one leading “0″ and Two suffixes “0″ are used to select code groups, and the rest are disabled code groups. In this way, if a disabled code group appears at the receiving end, it means that there is an error in the transmission process, thereby improving the error detection capability of the system. Both bi-phase codes and CMI codes can be regarded as 1B2B codes.
In the optical fibre communication system, m=n+1 is often chosen, and 1B2B code, 2B3B code, 3B4B code and 5B6B code are taken. Among them, the 5B6B code pattern has been practically used as a line transmission code pattern for the third group and the fourth group or more.
The nBmB code provides good synchronization and error detection functions, but it also pays a certain price, that is, the required bandwidth increases accordingly.
The design idea of nBmT code is to transform n binary codes into a new code group of m ternary codes, and m<n< span=”">. For example, the 4B3T code, which transforms 4 binary codes into 3 ternary codes. Obviously, under the same code rate, the information capacity of the 4B3T code is greater than that of 1B1T, which can improve the frequency band utilization rate. 4B3T code, 8B6T code, etc. are suitable for higher rate data transmission systems, such as high-order coaxial cable transmission systems.</n<>
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