Research On Codec Technology And Applications Of Fountain Codes

Recently, the digital fountain scheme has drawn increasingly attention and research efforts as an ideal solution to broadcasting and multicasting applications. In this dissertation, we focus on the study of fountain codes for applications in multimedia and deep space communications. Emphasis is put upon designing of degree distributions of encoders, decoding algorithm and scheme for certain applications, while contributing a way from theory to practice for fountain codes. The main results of the research lie in the below.First, from the perspective of encoder designing, we study fountain codes with short code-length, which is likely to reduce the decoding latency and buffer requirement. Degree distribution used by encoder is optimized for better fountain codes by using more reasonable heuristic constrains, while keeping the encoding and decoding schemes unchanged. As a result, we obtain practical fountain codes with the code length at the scale of 103, which is applicable in systems with limited storages and real-time constraints.Second, from the perspective of decoder designs, we propose an improved BP decoding algorithm which continues the iterative decoding process when the conventional BP decoder fails. By utilizing the remained encoded packets, the proposed algorithm increases the probability of successful decoding remarkably. For example, using the proposed algorithm, the probability of successfully decoding the LT codes, with typical code length of 104 and overhead of 5%, can reach 98.2% and increase by 18.3% compared to the conventional methods. Using the framework of the conventional iterative decoding, this algorithm remains the favorable property of low complexity.Third, we propose a new structure of fountain code with the ability of unequal error protection, namely UEP fountain code. This code realizes the incremental fountain scheme with the help of layered streaming for applications in environment of high error rate, non-feedback and low overhead. The proposed scheme is decoupled from the specific structure of rateless code and provides unequal error protection property by scheduling strategy. To find the optimal scheduling strategy, two optimization models are developed, which take the user satisfaction degree and the overhead as the optimization object. Simulation results show that the user satisfaction degree is improved by 10%~15% with the optimal UEP fountain codes, compared with the conventional ones.Finally, aiming at certain special applications with constraints on power efficiency, such as in deep space communications, we study a selection strategy of fountain codes. Specifically, we find out conditions, in which introducing fountain codes as outer codes can bring us coding gains. Moreover, we introduce the concept of eigen-slope of fountain codes. By doing so, a selection method of fountain codes is contrived for arbitrary channel conditions. It is shown that, with a good selection of fountain codes, reasonable coding gain may be achieved while application requirement of DSC be met.