Abstract: Neural machine translation (NMT) suffers a performance deficiency when a limited vocabulary fails to cover the source or target side adequately, which happens frequently when dealing with morphologically rich languages. To address this problem, previous work focused on adjusting translation granularity or expanding the vocabulary size. However, morphological information is relatively under-considered in NMT architectures, which may further improve translation quality. We propose a novel method, which can not only reduce data sparsity but also model morphology through a simple but effective mechanism. By predicting the stem and suffix separately during decoding, our system achieves an improvement of up to 1.98 BLEU compared with previous work on English to Russian translation. Our method is orthogonal to different NMT architectures and stably gains improvements on various domains.

Introduction: Neural machine translation (NMT) BIBREF0 has shown better performance compared with statistic machine translation BIBREF1 . Such methods encode a source sentence into hidden states and generate target words sequentially by calculating a probability distribution on the target-side vocabulary. Most NMT systems limit target side vocabulary to a fixed size, considering the limit of graphics memory size and high computing complexity when predicting a word over the whole target side vocabulary (e.g., 30K or 50K). In addition, a larger target-side vocabulary can also make the prediction task more difficult. Word-level NMT systems suffer the problem of out of vocabulary (OOV) words, particularly for morphologically rich languages. For example, English to Russian machine translation faces a big challenge due to rich morphology of Russian words, which leads to much more OOV words than some other languages. Typically a specific tag is used to represent all OOV words, which is then translated during a post process BIBREF2 . This can be harmful to the translation quality. There has been several methods to address this problem. Some focused on translation granularity ( BIBREF3 , BIBREF3 ; BIBREF4 , BIBREF4 ; BIBREF5 , BIBREF5 ), while others ( BIBREF6 , BIBREF6 ; BIBREF7 , BIBREF7 ) effectively expand target side vocabulary. However, though those methods can avoid OOV, none of them has explicitly modeled the target side morphology. When dealing with language pairs such as English-Russian, the number of different target side words is large due to the rich suffixes in Russian. The above methods are limited in distinguishing one suffix from another. Since the total number of different stems in a morphologically rich language is much less than the number of words, a natural perspective to make a better translation on a morphologically-rich target-side language is to model stems and suffixes separately. We design a simple method, which takes a two-step approach for the decoder. In particular, stem is first generated at each decoding step, before suffix is predicted. Two types of target side sequences are used during training, namely stem sequence and suffix sequence, which are extracted from the original target side word sequence, as shown in Figure FIGREF1 . Sparsity is relieved since the number of stem types is much smaller than word types, and suffix types can be as small as several hundreds. Another advantage of this structure is that during the prediction of suffix, the previously generated stem sequence can be considered, which can further improve the accuracy of suffix prediction. We empirically study this method and compare it with previous work on reducing OOV rates ( BIBREF3 , BIBREF3 ; BIBREF4 , BIBREF4 ). Results show that our method gives significant improvement on the English to Russian translation task on two different domains and two popular NMT architectures. We also verify our method on training data consisting of 50M bilingual sentences, which proves that this method works effectively on large-scale corpora.

Question: what is the previous work they are comparing to?
Answer: Subword based NMT, Character-based NMT