Transmission Technology On Deep Space Communication Based On Rateless Codes And The Design Of Its System

Since the deep space communication environment has long transmission delay and quite low signal to noise ratio (SNR), and its channel is always changing, people have been studying the technology to implement the efficient and reliable transmission on deep space. The present deep space communication systems mainly use the error correcting codes such as concatenated codes and Turbo codes to ensure the reliable transmission. However, these fixed-rate error cor-recting codes can’t keep efficient transmission on varying channels, and need lots of feedback signaling. Rateless codes, as a new kind of channel codes, can adaptively adjust its rate according to the channel condition and only need a little feedback signaling. These excellent characteris-tics can help rateless codes quite adapt the deep space communication environment. The paper mainly studys the design of deep space communication system based on rateless codes and its implementation on Sora platform, the contents are listed as follows:We propose a kind of adaptive symbols removal encoding algorithm which can improve the decoding performance of rateless codes, and design a method to efficiently implement the encoding and decoding of rateless codes. In the process of normal rateless encoding, we always choose the input symbols randomly. However, some input symbols may have been connected with enough output symbols and can gain enough information to decode successfully. Continu-ing to choose these input symbols in the latter encoding process has little help on the decoding performance of the rateless codes. So we design the adaptive symbols removal encoding (SRE) algorithm, whose main idea is to remove those input symbols which have been accumulated e-nough information to decode successfully, and thus make the information be distributed more properly. Simulation results show that the Raptor codes generated by SRE algorithm have better decoding performance than normal Raptor codes. The proposed method of efficiently imple-menting rateless encoding and decoding is that, when encoding the rateless codes, transform the generated matrix of rateless codes to the corresponding XOR expressions and encode several bits at the same time. When decoding the rateless codes, use the serial belief propagation iterative decoding algorithm, and make use of the multi-thread technique, the table looking-up technique, the functions based on single instruction multiple data and so on to lower the decoding delay as much as possible.We design a rate adaptive pipeline transmission system based on rateless codes. We firstly propose the data splitting and sub-frame encapsulation method, in which the sending side first encodes the original data and generates enough encoding symbols, and then splits the encod- ing symbols into several parts and encapsulates each part to a sub-frame. Then we design the pipeline transmission mechanism based on rateless codes and half-duplex system. In the mech-anism, the sending side sends the sub-frames of current data package and encodes the next data package at the same time; the receiving side receives the frames of current data packages and decodes the previous data package at the same time. If the receiving side decodes successfully, the sending side will receive the ACK frame timely, otherwise the sending side waits the ACK frame out of time and then sends an incremental sub-frame, and once the receiving side receives an incremental sub-frame, it will join the incremental sub-frame with the previous sub-frames to decode. At last we design a adaptive algorithm with linear weighted calculation, which is to adjust the number of sub-frames needed to send according to the previous decoding feedbacks.At last, we design and implement the first rateless encoding communication system at home based on Sora platform. Sora platform is a software radio system in commodity PC architecture and is developed by Microsoft Research Asia. Sora not only has powerful signal processing a-bility, but also is a high-performance fully programmable software radio. We design the system parameters based on Sora, and implement the rateless encoding communication system. To sim-ulate the deep space communication environment, we mix the Gaussian noise with the received signal and can adjust the receiving SNR. The performance of the system shows that our designed system has high channel efficiency, good throughput performance and bit SNR performance at low SNR, and can adaptively adjusts the rate to optimum rate as the channel varies.