A Design of HDB3 CODEC Based on FPGA
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27-11-2010, 12:39 PM

A Design of HDB3 CODEC Based on FPGA

The basic principles and structure of HDB3 was briefly introduced in this paper, and the shortcomings of the existing HDB3 encoder and decoder was analyzed. Then a new design of HDB3 encoder and decoder based on FPGA was proposed, and the hardware design circuit and software simulation were introduced. The simulation was achieved through the VERILOG-HDL in EP2C35F672C8 chip of CycloneII series in the development environment of Quartus II 7.2. The results show that the design meets the requirements of HDB3 encoder and decoder, which has a simple hardware circuit and flexible software, and runs fast,and can be used in practical communication systems.
The research about coding in communication link was an important topic of modern digital communication technology.when the line pattern was selected rightly,it was good for improving the communication quality, ameliorating the transmission performance and extending the transmission distance.HDB3(High Density Bipolar Codes)was called high degree bipolar coding,and it was allowed even the "0" number up to no more than 3,which was conducive to the recovery of timing signal. Because of it using polarity reversal, baseband signal without direct current component, very small low frequency components,this manchester code had a certain error detection capabilities and was easy to be decoded.Now, the code was widely used in digital mobile communications,digital optical fiber communications and digital microwave communication systems,which was an integral part of the the modern digital communication systems. It was also a interface pattern between digital fiber optic communications equipment and PCM equipment that was PCM-based group, secondary group and three groups recommended by CCITT. As a result, the research about HDB3 CODEC was essential.
HDB3 code was one kind of bipolar NRZ that was improved on the based of AMl code. HDB3 code was AMI (Alternative Mark Inversion)code, when the data did not appear in more than 4 or 4 with "0" characters. Namely, alternating polarity signal.When the data appears in more than 4 or 4 with "0" characters, firstly characters was encoded according to the coding rules of AMI, and then these steps were follow. The chapter "0" 4th must be change to be non-"0" pulse and it was marked "+V" or "-V" which was also called "Damaging Pulse".As for the "+ V" or "-V",its positive and negative should be the same as the polarity of former non- "0" symbol.At the same time, the sign of adjacent "V" must be alternated polarity to ensure the code without DC component. And in this paper, string "000V" was called "Damaging Sequence".When the number of Non-"0"’s symbols between the signal "V" and another adjacent one was odd, the compiled code was HDB3 code.On the contrary, the first one "0" in " Damaging Sequence" was should be replaced to "+B" or "-B".As for the "+ B" or "-B",it’s positive and negative should be opposite whit the polarity of former non-"0" symbol. And the non-"0" symbol behind from the sign of "V" and then started alternating. Its conversion steps was showed in Figure 1. Figure1. The formation of HDB3 code HDB3 code decoding was the reverse process of encoding, which decoding was more easily achieved than the coding of that. It can be seen from the encoding rules,the symbols’ polarity of Damaging Pulse"V" was always the same as that of the former including the "B".Therefore, from the HDB3 code sequence received was easy to identify the sign of "V". At the same time,it can be concluded that the sign of "V" and three adjacent symbols front must be that even the 0 symbol. So they can be restored into four "0" code adjacent.Then all the "-1" was change into "+1",which be available to the original news source.
At present, many scholars had been designed and studied HDB3 codec.In the first palce, plugging "B" was completed after adding "V". In the second place, the variable signal bipolar or unipolar output was achieved in reference. When destroy symbols were inserted, the polarities of signals were set aside in the programming process. Then the variable signal bipolar or unipolar output was also worked out in reference[2]. First of all, four "0" code adjacent and adding "V" were detected. Furthermore, a Damaging Sequence decision circuit was designed.Finally,the HDB3 code was realized. HDB3 codecs based on CD22103 was used by other scholars also.Most of these design methods were evolved based on the AMI codec.The rules of HDB3 codec was carried out in full accordance step by step,which process was complex,lack of optimization. Insertting "V" and "B" had not been separated in the process,but the two steps were carried out together by setting the signal parameters in this paper. Having been inserted pulse"V" and "B"was replaced by"+1"or "-1"separately according to the actual needs.The character string "00"was used to indicate symbol "0", "01"was stand for symbol "0",and"11" was seen as "-1"in the system compilation process. It was different from polarity of signal being judged after adding pulse"V and inserting pulse "B"in other design system.

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