Adaptation of Language Models for SMT Using Neural Networks with Topic Information.

Neural network language models (LMs) are shown to be effective in improving the performance of statistical machine translation (SMT) systems. However, state-of-the-art neural network LMs usually use words before the current position as context and neglect global topic information, which can help machine translation (MT) systems to select better translation candidates from a higher perspective. In this work, we propose improvement of the state-of-the-art feedforward neural language model with topic information. Two main issues need to be tackled when adding topics into neural network LMs for SMT: one is how to incorporate topics to the neural network; the other is how to get target-side topic distribution before translation. We incorporate topics by appending topic distribution to the input layer of a feedforward LM. We adopt a multinomial logistic-regression (MLR) model to predict the target-side topic distribution based on source side information. Moreover, we propose a feedforward neural network model to learn joint representations on the source side for topic prediction. LM experiments demonstrate that the perplexity on validation set can be greatly reduced by the topic-enhanced feedforward LM, and the prediction of target-side topics can be improved dramatically with the MLR model equipped with the joint source representations. A final MT experiment, conducted on a large-scale Chinese--English dataset, shows that our feedforward LM with predicted topics improves the translation performance against a strong baseline.