正交頻分復用（OFDM，Orthogonal Frequency Division Multiplexing）是當前一種非常熱門的通信技術。它既是一種調制技術，也是一種復用技術。OFDM技術能夠解決產生多個互相正交的子載波和從子載波中恢復原信號的問題,能很好的對抗頻率選擇性衰落和ISI干擾問題。
信道編碼是提高通信系統性能的重要方法之一。數字信號在傳輸過程中往往由于各種原因，使數據流產生一定的誤碼，從而使得通信系統的性能有所下降。通過對信道進行編碼，使通信系統具有一定的糾錯能力和抗干擾能力。
本文簡要介紹了OFDM系統的基本原理，以及信道編碼的基本概念、分類和發展后，詳細闡述了信道編碼中卷積碼的編碼和譯碼原理及算法，并對基于卷積碼的OFDM系統性能進行了分析。并且針對卷積碼的譯碼算法的軟判決和硬判決進行了分析、比較。最后，對基于卷積碼的OFDM系統和卷積碼譯碼軟，硬判決方法對系統性能的影響運用MATLAB軟件進行了仿真。仿真結果表明:經過卷積碼編碼的OFDM系統比之前不進行編碼的系統性能有了很好的提升，在誤碼率等于10^-2時，大約5db的性能提升，而卷積碼譯碼的軟硬判決的仿真結果得出，軟判決比硬判決性能好。
關鍵詞：OFDM系統，信道編碼，卷積碼，軟判決，硬判決，誤比特率
ABSTRACT
OFDM(Orthogonal Frequency Division Multiplexing) is currently a very popular communication technology,it can be seen as either a modulation technique or a multiplexing technique.The technology can solve the problem of producing multiple subcarriers orthogonal to each other and recovering the original from the sub-carrier signal,what's more,it can be a good fight against frequency selective fading and ISI interference.
Channel coding is an important methods to improve communication system performance.In digital signal transmission,the data stream usually have some error datas cause by various reasons.By channel coding,the system has some error correction capability and anti-jamming capability.
In this paper, the basic principles of orthogonal frequency division multiplexing (OFDM) technology has been introduced,and this article briefly introduced the basic concepts,classifications and development of channel coding.After that,the article elaborated on the principle of encoding and decoding of the convolutional code and convolutional code,detailedly analyzed and compared the methods of convolutional Code’sjudgment.And when the bit error rate is 10^-2,the system has a improvement about 5db.The results showed that: the system with convolutional coding is superior before,and the soft judgment is better than hard judgment.
