Communication And Power Control Based On The Raptor Codes Over AWGN Channel

With the rapid development of cognitive technology, more and more attention has been paid to the reliable and efficient data transmission technology. As communication environment is complex in many cases, it is difficult to pre-know the exact channel state information. Rateless Codes provide us a new method to solve the problem, as a kind of efficient and reliable way for data transmission, they attribute the character of automatic rate adaption, and don't require feedback during the transmission,each symbol is generated independently from some distribution. This article is based on a kind of Rateless Codes over AWGN channel called Raptor Codes, and gives some contributions to Rateless Codes over AWGN channel.Firstly, the basic concept of LDPC Codes is introduced, including Tanner graph,degree distribution,PEG alogrithm, which are also appropriate to Raptor Codes. Then considering the shortage of the existing degree distribution, we proposed a new distribution for moderate length Raptor Codes, our simulation showed that the new degree distribution could improve the performance apparently, also it could eliminate the error floor of Raptor Codes. At last,we use PEG algorithm to construct LT Tanner graph based on the new distribution, including PEG LT constructed independently and PEG LT constructed with LDPC Codes, these two methods could enlarge the cycles in Tanner graph, thereby the performance of Raptor Codes could be improved.Thereafter, we investigated power control of Raptor Codes at the transmitter, our result showed that when the whole energy is equal at the transmitter,the performance of Raptor Codes could be improved clearly via reducing transmission power and increasing code length,also we gave theoretical basis.Then we compared the performance of fixed-rate LDPC Codes with Raptor Codes,we got as the distribution of LDPC Codes can't adapt to arbitrary rate, so when the rate decreased together with the code length increased, the performance of LDPC Codes went bad.Finally.some methods of correcting error frame are investigated.The basic idea is to get posterior probability of a small part of information symbols after a certain iteration,then change the initial probability of those information symbols.and start a new round of BP decoding. In order to increase the probability of correcting the error frame,we should have many rounds of BP decoding. If the error frame still can't be corrected, we give a new hard decision method, and it can reduce the error probability statistically.