The necessity of using a fixed-size word vocabulary in order to control the
model complexity in state-of-the-art neural machine translation (NMT) systems
is an important bottleneck on performance, especially for morphologically rich
languages. Conventional methods that aim to overcome this problem by using
sub-word or character-level representations solely rely on statistics and
disregard the linguistic properties of words, which leads to interruptions in
the word structure and causes semantic and syntactic losses. In this paper, we
propose a new vocabulary reduction method for NMT, which can reduce the
vocabulary of a given input corpus at any rate while also considering the
morphological properties of the language. Our method is based on unsupervised
morphology learning and can be, in principle, used for pre-processing any
language pair. We also present an alternative word segmentation method based on
supervised morphological analysis, which aids us in measuring the accuracy of
our model. We evaluate our method in Turkish-to-English NMT task where the
input language is morphologically rich and agglutinative. We analyze different
representation methods in terms of translation accuracy as well as the semantic
and syntactic properties of the generated output. Our method obtains a
significant improvement of 2.3 BLEU points over the conventional vocabulary
reduction technique, showing that it can provide better accuracy in open
vocabulary translation of morphologically rich languages.

本文提出了一种新的神经机器翻译方法，可以考虑语言的词形属性，基于无监督的形态学习在任何速率下减少给定输入语料库的词汇量，并在土耳其语 - 英语 NMT 任务中通过比较生成输出的语义和句法特性来验证方法的效果，与传统词汇量缩减技术相比，显著提高了 2.3 BLEU 分数，表现更好。