Does the Backtracking Length Affect the Performance of Convolutional Codes?

The concept of backtracking length was mentioned earlier! What is backtracking length? Does its size affect the performance of convolutional codes? Backtracking length is sometimes called backtracking depth. First of all, we should have the concept of backtracking! There is a backtracking decision unit in the decoder, which is the core unit to obtain the decoding information. The unit will obtain the decoding information by backtracking according to the minimum state label and corresponding decoding information obtained when adding, comparing and selecting unit (ACS). What are the specific engineering practices? When the path storage record length exceeds the backtracking path decoding length for the first time, reset the flag bit and continue the operation. When the path storage record length reaches the defined path storage capacity, select to start backtracking from any state. The decoding backtracking depth is generally 4-10 times the number of registers. Therefore, the backtracking length can be simply understood as the distance from the time when you want to start the judgment to the time when you first calculate the measurement! Of course, this distance is generally the bit length! Is that the longer the better? Of course not! How to verify? Simulation program can help you! The previous program has the following statement!

%%%Hard judgment

tblen = 32; % Traceback length is an important indicator of backtracking length!!!

%In cont mode, vittec decoding will have delay, and the length of delay is tblen,

data5 = vitdec(data4,trellis,tblen,'cont','hard'); % Hard decision

Constantly change the value of tblen and see the change of bit error rate! In this way, we can know the influence of this value on the performance of convolutional code! This is the combination of theory and practice!

How many units does the decoder usually have?

The decoder consists of five units: metric calculation (BMG), addition comparison selection (ACS), cumulative metric, survival information and backtracking decision. To understand the functions of these specific units, please see the subsequent special series article matlab implementation of Viterbi decoding!

Engineering knowledge!

0 is often used for closing in projects! How many zeros do you want? Can fill the register! The following two factors need to be considered before!

And now? The device capability is too powerful, which is no longer an obstacle! If the storage space is large enough, you don't care about the value of constraint length. Convolutional codes of (2, 1, 9) are also often used! After talking about these concepts, we enter the related knowledge of concatenated code! What is concatenated code? New concept!

In practice, the most commonly used concatenated code is the concatenated combination of RS code and convolutional code! When I was working in an enterprise, I developed a piggyback satellite earth station for a satellite communication system. The transmission system of the system contains cascade coding! It adopts the coding technology of concatenation of RS coding and convolutional code. Of course, it also contains interleaving content. Cascade coding can improve the reliability of information transmission, and different coding schemes are adopted in different services to meet the different requirements of different services for bit error rate and information rate. Adding concatenated coding can improve the bit error performance of transmission by several orders of magnitude and improve the reliability of transmission.

The channel coding theorem points out that with the increase of code length, the decoding error probability is exponentially close to zero. Therefore, long codes must be used for reliable communication, but with the increase of code length, the complexity and computation of decoder also increase, so that it is difficult to be applied in practical applications. In order to solve the contradiction between performance and implementation difficulty, Forney proposed the concept of concatenated code in 1966, that is, several shorter codes are connected in series to form a coding combination with longer code length, and the process of compiling long codes is completed in several levels, so as to reduce the complexity of decoding at all levels. Concatenated codes are usually divided into two levels.

Usually, when constructing concatenated codes, the internal and external codes choose complementary code types. For example, RS code is used as the external code in the system, while convolutional code is used as the internal code. The advantage of serial concatenated code of RS code and convolutional code is that it combines the ability of RS code to correct burst errors and convolutional code to correct random errors, and achieves better error correction performance under relatively low complexity. This concatenated code combination has been approved by the international Spatial Data System Advisory Committee (CCSDS) Recommended as a standard. The specific choice of block code or convolutional code as internal code depends on the actual application (channel condition and code rate). The theory is introduced here first. Next, it's still according to the tradition of my article. It's time to further digest the theoretical knowledge by looking at the program!

Original title: communication principle and MATLAB simulation V3 Chapter 8 coding and modulation together (6)

The source of the article: WeChat official account: Communication Engineer album, welcome to add attention! Please indicate the source of the article.

Does the Backtracking Length Affect the Performance of Convolutional Codes? 1

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