However, we found that the efficiency of the phrasebased SMT system described in the previous section would be limited for this task, mainly due to two reasons: the character-based phrase models due to possible unseen phrases in an evaluation corpus, and the character sequence model as all candidate transcriptions confidently belong to the target language.
to make the phrase-based SMT system robust against data sparseness for the ranking task, we also make use of the IBM Model 4 (Brown et al., 1993) in both directions.