正交频分复用（OFDM，Orthogonal Frequency Division Multiplexing）是当前一种非常热门的通信技术。它既是一种调制技术，也是一种复用技术。OFDM技术能够解决产生多个互相正交的子载波和从子载波中恢复原信号的问题,能很好的对抗频率选择性衰落和ISI干扰问题。
信道编码是提高通信系统性能的重要方法之一。数字信号在传输过程中往往由于各种原因，使数据流产生一定的误码，从而使得通信系统的性能有所下降。通过对信道进行编码，使通信系统具有一定的纠错能力和抗干扰能力。
本文简要介绍了OFDM系统的基本原理，以及信道编码的基本概念、分类和发展后，详细阐述了信道编码中卷积码的编码和译码原理及算法，并对基于卷积码的OFDM系统性能进行了分析。并且针对卷积码的译码算法的软判决和硬判决进行了分析、比较。最后，对基于卷积码的OFDM系统和卷积码译码软，硬判决方法对系统性能的影响运用MATLAB软件进行了仿真。仿真结果表明:经过卷积码编码的OFDM系统比之前不进行编码的系统性能有了很好的提升，在误码率等于10^-2时，大约5db的性能提升，而卷积码译码的软硬判决的仿真结果得出，软判决比硬判决性能好。
关键词：OFDM系统，信道编码，卷积码，软判决，硬判决，误比特率
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.
