1. Performance comparison of several main decoding algorithms
Influence of decoding algorithm on Turbo code
MAP algorithm, Log-MAP algorithm, Max-Log-MAP algorithm and SOVA algorithm are simulated and compared in AWGN environment. The system adopts BPSK modulation, the interleaving length of Turbo code is 1024, the generating polynomial of RSC subcode is (372 1), and the code rate of the system is R = 65448. The simulation results show that among the four algorithms, MAP algorithm has the best performance, and Log-MAP algorithm is close to MAP algorithm in low SNR, but it is quite different in high SNR. The performance of Max-Log-MAP algorithm is very close to that of SOVA algorithm. Generally speaking, the performance of Max-Log-MAP algorithm is always slightly better than that of SOVA algorithm. Compared with MAP and Log-MAP, their performance is obviously reduced. In terms of algorithm complexity, MAP algorithm is the most complex, followed by Log-MAP, Maximum Log-MAP and SOVA algorithm is the simplest. It can be seen that the decoding algorithm of Turbo codes with excellent performance is very complicated. If we simplify the algorithm or adopt a simple algorithm to make the decoding easy to realize, it will often be at the expense of performance degradation.
Influence of different iterations on the performance of Turbo codes.
The left figure shows the relationship between BER and Eb/N0 of Turbo codes with code rate 1/ 2 under different decoding iterations. The interleaving length of Turbo code is 1024, the generating polynomial of RSC subcode is (37,21), and the system code rate is R= 1/2. As stated in the decoding principle of Turbo codes, two decoders exchange external information for iteration. It can be concluded that the decoding performance is improved with the increase of iterative decoding times. The error performance in the first iteration is poor because the information between the two component decoders is not well utilized. With the increase of iterations, the external information between the two component decoders is better utilized, the estimation of information bits is closer to the maximum likelihood ratio, and the correctness of the decision output is higher. When the number of iterations reaches a certain value, the decoding performance tends to be stable, and adding new iterations has little improvement on the performance. Iteration increases the decoding delay, especially in large frame coding. When the number of iterations in the simulation increases, the running time will increase significantly.
After reaching a certain number of iterations, the newly increased number of iterations will not greatly improve the performance, but iteration will greatly increase the decoding delay. Therefore, in the actual design of Turbo code system, it is necessary to select appropriate iterations to achieve the best decoding performance within the allowable decoding delay. This method of specifying the number of iterations in advance is one of the methods to terminate the number of decoding iterations. When the required signal-to-noise ratio is relatively large and the bit error rate is not too high, the correct decoding can often be achieved after several iterations. At this time, if the preset number of iterations is large, the decoder will continue decoding until the preset number of iterations. The next few iterations did not significantly improve the performance, which was completely unnecessary, and the extra substitute food brought extra delay to the decoding.
3. The influence of different coding constraints K on the performance of Turbo codes.
Influence of Different Constraints on the Performance of Turbo Codes
The performance of Turbo codes with different subcodes is also very different. In the design of Turbo codes, we should first choose a good RSC subcode. Here, only several commonly used and good Turbo codes with different constraint lengths as subcodes are simulated to analyze the influence of constraint length on the performance of Turbo codes. It can be seen that with the increase of constraint length K, more information bits are involved in the coded symbols, so the stronger the decoding and error correction ability, the smaller the bit error rate HER. When BER