| Mars is the main objective planet of deep space exploration. As an important support of Mars exploration, communication technologies developed rapidly along with the increasing Mars missions. The existing Mars transmission services include scientific data and high resolution images and some other kinds of information. The communication system based on point-to-point transmission can't satisfy the needs of large data volume, high transmissibility and reliability. While the system based on networked transmission which uses the orbiter as relay will be the basic scheme in future Mars exploration missions. On the one hand, it can resist the signal fading caused by long distance and elevate the link reliability. On the other hand, it can partially solve the problem of link disruption caused by planetary shields and increase the observation time. Meanwhile, as a new technology in deep space communication, the long erasure codes have forward erasure correcting ability to ensure the transmission reliability instead of Automatic Repeat Request. And it can decrease the delay and increase the efficiency. This paper establishes a Mars relay network including multiple Martian surface probes, relay track detectors and the earth station. And it designs a distributed systematic Raptor code scheme combined with network coding under the framework of delay/disruption tolerant network, which uses the forward erasure ability and the rate-less characteristic of the fountain codes. As a result, it can guarantee the reliability of data transmission, reduce the system overhead and provide efficient erasure transmission capability. Main contents of this paper are as follows:First, through the investigation and research on DTN network architecture of Mars communication, the network model of scene that multiple Martian surface probes send data to the earth through the same relay is determined, and the function of each node in the network is defined. Raptor codes in 3GPP multimedia broadcast/multicast service is proved to have good coding performance and is suitable to be used in sources. Meanwhile, through the usage of remaining storage and computing capacity, the relay can join the network encoding operation on the traditional routing function.Secondly, a distributed systematic Raptor erasure correcting coding scheme is proposed. Compared with the existing schemes as DLT and SLRC, it achieves high decoding success probability with lower decoding redundancy, further improving the data recovery efficiency and saving transmission time. Additionally, for the case that sources energy are limited, symbol volume sources need to encode is been calculated by predicted channel conditions. Using the encoding symbol in relay cache optionally to network encoding, the scheme can resist the link erasure from relay to the destination. In this paper, three methods are proposed, and simulations of their decoding performances are shown. Due to the joint of the relay network coding, the overhead of source is effectively reduced and the recovery success probability is ensured as well.Finally, the case is discussed of which unequal code lengths are used at the sources due to the restrictions of probes' hardware resources, differences of transmission service and link conditions. Information of two sources is distinguished as different degrees of importance. Through the cooperation of relay network coding and forwarding strategy and the decoding algorithm, the scheme improves the more important information's forwarding probability and its coverage rate in network encoding. Thus, different sources' information achieve unequal protections. When the packet loss rate was large or the sources encoding symbols was insufficient, we preferentially recover the more important information so as to improve the overall performance of data transmission. The range of application of distributed systematic Raptor code is been expanded in Mars relay network. |
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