Synchronization Mechanism For BICM System At Low SNR

The BICM system can operate close to Shannon limit at low SNR region very well. However, this conclusion is based on the assumption of correcting carrier frequency, phase, and timing perfectly. In fact, this assumption is only established approximately when demodulator operates at a higher SNR region. At low SNR region, the information of the demodulator transmitting to the decoder will include lots of timing error and phase offset, which will cause the decoding of the BICM system performing badly. Therefore, using conventional solution separating decoding from demodulation can’t take full advantage of BICM system, so the whole system is still in a farther region from Shannon limit.The author combines this paper with program from China Electronics Technology Corporation No.54(CETC54). Basing on understanding BICM (bit-interleaved coded) system, I get a further study of the principle of carrier synchronization and the symbol timing synchronization. By using the feedback information of the decoder, the performances of timing and carrier phase recovery are improved. Decoding scheme combined with timing error and the phase offset is constructed, making sure that iterative decoding can work well in a close distance to Shannon limit in the low SNR regions.The author completed the works are followed:1. The framework of the BICM system including the synchronization process is introduced. Based on the expressions of capacity, the simulation results are obtained. By contrasting with the CM system capacity, in the case of the determined modulation constellations, Gray mapping can maximize the capacity of the BICM system.2. Introduction of the carrier phase synchronization methods are as follow:carrier phase synchronization based on the of soft information of LDPC decoding, information maximum likelihood phase estimation under the use of soft, phase synchronization under the assistant of the unique words and carrier phase synchronization based on PLL (Phase Locked Loop). By comparing the results, the method that carries out the optimum performance is found:the maximum likelihood phase estimation method combined with decoding.3. The symbol timing synchronization framework, function and implementation of each structure are presented. For iterative timing error detecting method, M & M timing error detector and windowing iterative search algorithm are introduced. For non-iterative timing error detecting method, Gardner timing detector is introduced. The pictures of simulation performance of all methods are printed. On this basis, through the use of feedback information of the decoder to improving the performance of the timing and carrier phase recovery, the combined synchronization renderings is got.