  Variable length codes exhibit de-synchronization problems when transmitted
over noisy channels. Trellis decoding techniques based on Maximum A Posteriori
(MAP) estimators are often used to minimize the error rate on the estimated
sequence. If the number of symbols and/or bits transmitted are known by the
decoder, termination constraints can be incorporated in the decoding process.
All the paths in the trellis which do not lead to a valid sequence length are
suppressed. This paper presents an analytic method to assess the expected error
resilience of a VLC when trellis decoding with a sequence length constraint is
used. The approach is based on the computation, for a given code, of the amount
of information brought by the constraint. It is then shown that this quantity
as well as the probability that the VLC decoder does not re-synchronize in a
strict sense, are not significantly altered by appropriate trellis states
aggregation. This proves that the performance obtained by running a
length-constrained Viterbi decoder on aggregated state models approaches the
one obtained with the bit/symbol trellis, with a significantly reduced
complexity. It is then shown that the complexity can be further decreased by
projecting the state model on two state models of reduced size.
