Convolutional codes are used in digital communication systems tocorrect errors that occur during transmission. Their use is nowcommonplace, particularly in power-limited satellite and deep spacecommunication systems. Before the decoder can correctly decode thereceived continuous bit sequence, however, it must establish wherecodewords begin and end in this sequence. The process of determiningcodeword boundaries is called node synchronization. In this paper wedevelop a node synchronization algorithm based on metric differencevalues with the objectives of simple implementation and reliableperformance. Focusing on satellite applications, we consider theconstraint length 7, rate 1/2 convolutional code standardized by theNational Aeronautics and Space Administration (NASA) and the EuropeanSpace Agency (ESA); the approach we develop can be extended to otherconvolutional coding systems. Through simulation, we investigate thestatistics of the received sequence, and highlight correlation in thesequence of the decoder metric values. In the light of theseobservations, we propose a synchronization algorithm, and develop afirst-order analysis of this simple up/down counter framingtechnique. We recommend framing parameters based on both theanalytical results and the expected response of the framing algorithmto correlated metric values. Development of a simple nodesynchronization algorithm that accommodates correlated metric valueshas not been reported previously in the literature. Simulationresults confirm that reliable node synchronization is possible withthis very simple approach.
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