Low-frequency word prediction remains a challenge in modern neural machine translation (NMT) systems. Recent adaptive training methods promote the output of infrequent words by emphasizing their weights in the overall training objectives. Despite the improved recall of low-frequency words, their prediction precision is unexpectedly hindered by the adaptive objectives. Inspired by the observation that low-frequency words form a more compact embedding space, we tackle this challenge from a representation learning perspective. Specifically, we propose a frequency-aware token-level contrastive learning method, in which the hidden state of each decoding step is pushed away from the counterparts of other target words, in a soft contrastive way based on the corresponding word frequencies. We conduct experiments on widely used NIST Chinese-English and WMT14 English-German translation tasks. Empirical results show that our proposed methods can not only significantly improve the translation quality but also enhance lexical diversity and optimize word representation space. Further investigation reveals that, comparing with related adaptive training strategies, the superiority of our method on low-frequency word prediction lies in the robustness of token-level recall across different frequencies without sacrificing precision.

本文提出一种基于词频感知的令牌级对比学习方法，旨在从表示学习角度解决现代神经机器翻译系统中低频词预测的挑战。经实验证明，所提出的方法不仅可以显著提高翻译质量，还可以增加词汇多样性并优化词表示空间。与相关的自适应训练策略相比，该方法在不牺牲精度的前提下提高了低频词汇量的召回率稳健性。

面向频率的对比学习在神经机器翻译中的应用