Metric based node synchronization of the Viterbi decoder

Convolutional codes are used in digital communication systems to correct errors that occur during transmission. Before the decoder can correctly decode the received continuous bit sequence however, it must establish where codewords begin and end in this sequence. The process of determining codeword boundaries is called node synchronization. In this paper we develop a node synchronization algorithm based on metric difference values with the objectives of simple implementation and reliable performance. We consider the constraint length 7, rate 1/2 convolutional code standardized by the European Space Agency. We develop analytical results for the up/down counter framing algorithm, and through simulation investigate correlation in the sequence of metric values. Recommendations for framing parameters are based on both the analytical results and the response of the framing algorithm to correlated metric values. Simulations confirm that this simple approach results in reliable node synchronization