In a Chinese couplet, there is generally a direct one-to-one mapping between words at same position in the FS and SS, respectively.
the i th character/phrase in F is exactly "translated" into the i th character/phrase in S. Based on this rule, the phrase translation probability can be estimated by relative frequency in a training corpus: where m is the number of distinct phrases that can be mapped to the phrase i The inverted phrase translation model has been proven useful in previous SMT research work (Och and Ney, 2002); so we also include it in our phrase-based SMT model.