Combined Demodulation And Decoding Based On Turbo Synchronization

The modern communication system in which LDPC codes and Turbo codes are applied can work at SNR region near the Shannon limit. However, the conclusion above is drawn under the assumption that the demodulator can calibrate the carrier frequency,phrase and timing perfectly. In low SNR region, the modern channel coding performance is limited to the combination of the traditional demodulator and decoder.The synchronization error will cause both Turbo codes and LDPC codes poor performance so that the whole system work in the region far away from the Shannon limit. Research shows that the technology based on Turbo principle to iteratively calibrate carrier phase and symbol timing recovery(which is also called Turbo synchronization) is an effective solution.On the basis of cooperative research project with CETC-54, we proposed a joint demodulation and decoding scheme based on Turbo synchronization technology with the structure of Shannon limit codes. As the result, a great improvement has made in performance of synchronization in low SNR region. The main progress in this paper include:1. We propose not only an iterative symbol timing and carrier synchronization algorithm in the communication system synchronized on codes, but also design a jointly demodulation and decoding scheme based on Turbo synchronization technology. The timing recovery and carrier synchronization are embedded into iterative decoding process. Finally, we simulate the whole process.2. For satellite communication system with unique word, we proposed both non-iterative symbol synchronization and carrier synchronization algorithms firstly.Secondly, we combine V&V(Viterbi A J & Viterbi A M) algorithm and window search method to recover the frequency offset. Finally, we embedded the frequency offset, primary phase estimation and channel decoding into a round of Turbo iteration and do the computer simulation.It shows that the performance of Turbo synchronization scheme we put forward is 0.5d B worse at most than of the perfect timing in which the carrier phase is known to the receiver. Thus, it also improves 1d B or more in performance in comparison with the scheme of decoding isolated from demodulator. So it helps to make full use of the Shannon limit codes such as LDPC codes and Turbo codes in the real system as the performance in the low SNR region has greatly improved.