In a process for decoding of non-linear codes in digital data transmission, fulfilling the condition of rectangularity, the Viterbi algorithm is used in a trellis of the code with a minimal number of edges and vertices, thus decreasing the number of necessary operations. The process may be applied especially to synchronization of telecommunication systems, namely to UMTS cell search. A distance-dependent merging algorithm may efficiently generate a minimal trellis of a given code. A storage means (11) contains the minimal trellis of the code occurring at code group determination and frame synchronization allowing for applying the Viterbi-decoding algorithm using calculation means (10) and (13).