A low-delay encoding algorithm of LT code for data loss recovery in telemetry systems

A serially concatenated code is developed to guarantee reliable transmission of telemetry signals. The inner error correcting code is responsible for removing bit errors while the outer LT code plays the role of recovering data loss. Therefore, the serially concatenated code is able to provide reliability even though there exist bit errors and frame loss in telemetry links. However, the application of LT code to a telemetry system will bring a large system delay since a conventional LT encoder will not start to work until all source symbols are available. Aiming at reducing the system delay, an improved encoding algorithm of LT code is proposed. Different from the conventional LT encoder, the proposed LT encoder begins to generate coded symbols when it collects only a subset of source symbols. The coded symbol is generated as the modulo-2 addition of several source symbols which are selected from the available source symbols. The novelty of the new algorithm consists in the issue that how many source symbols and which source symbols are chosen to generate the coded symbol when only partial source symbols are available. The strategies of determining the degree of the coded symbol and the related source symbols are elaborately designed to make sure that the actualized degree distribution of coded symbols is very close to the predefined one and the actualized frequencies of selecting each source symbol are almost the same at the end of one block encoding. In addition, a reasonable encoding delay is necessary to ensure that coded symbols are transmitted in even rate and all source symbols can be recovered even though the telemetry channel is a fading channel with the maximum tolerable fade duration. Finally, the simulation of the proposed algorithm is implemented to obtain the encoding delay and the necessary encoding redundancy at different symbol loss ratio. The results demonstrate that the proposed algorithm can significantly reduce the encoding delay of a telemetry system at the cost of a slightly increase of the signal bandwidth compared with the conventional algorithm.