LDPC(Low-Density Parity-Check)碼為下個世代的先進通訊標準所採用的錯誤更正碼，其優異的錯誤更正能力可以逼近Shannon的理論值，配合訊息傳遞(message passing, MP)演算法，可以快速得到傳送端所發出之訊息。但是MP法不保證得到為最佳的解碼結果，故在本論文裡，將把它和A*法結合作混和解碼，但同時也把它們的優缺點作比較，由結果發現，A*法的編碼增益較MP法高，但是解碼的複雜度隨碼長度呈指數性增加，而非如MP法的線性增加。 若採用混和式架構，則可以得到與A*相同的編碼增益，同時解碼複雜度只略大於MP法。當使用碼長96 bits、碼率為1/2的LDPC碼，並在BER = 10E-5時，本論文所提出之混和式架構比MP法提高1.4 dB的編碼增益，經實驗統計結果得知約只有1%的接收序列在第一階段MP法不成功時，要轉入第二階段的A*法，因為A*法保證得到最佳解。故混和式架構的解碼方式，可以在編碼增益(由A*法所提供)與解碼複雜度(只有約1%的序列要作A*法，剩餘的約99%序列由快速的MP法解出)之間取得一個平衡。 LDPC code used by the advanced communication standard of the next generation is an error control code. Its error correction ability may approach the Shannon’s theoretical value. With the MP algorithm, it can decode received samples in high speed from transmitter. However, the MP method is suboptimum and optimum solution is not guaranteed. In this thesis, a hybrid decoding method is formed by combing the MP and the A* methods. We first make comparison between the MP and A* method and then show their combined performance. From the results, the coding gain of A* model is higher than that of MP model. The decoding complexity increases exponentially with the codeword length for A* method, but the MP method assumes linear increase. Using the hybrid structure, its coding gain is the same as that of A* method. Moreover, decoding complexity is slightly greater than that of MP one. For codeword length 96 bits and coding rate 1/2 and BER = 10E-5, the hybrid structure outperforms 1.4 dB coding gain than the MP method. This improvement only required 1% received sequences to be sent to A* decoding block. Our newly designed hybrid structure can solve both of the high coding gain and low decoding complexity while it has the ability to yield the optimum solution.
