An arrangement and method for decoding a convolutionally encoded codeword employs a window sliding over the codeword. Path metrics are computed simultaneously forwards and backwards in the sliding window. A decoding result is computed in a synthesis unit on the basis of the path metrics. The sliding window is divided into four parts. Path metrics are computed in the forward direction in the first part of the sliding window and stored in a four-part memory. Path metrics are computed in the backward direction in two other parts of the sliding window in such a way that the input of the computation units comes from the fourpart memory; the computed metrics are applied to the synthesis unit; and a decoding result is computed.