Recurrent neural network language model with structured word embeddings for speech recognition

Due to effective word context encoding and long-term context preserving, recurrent neural network language model (RNNLM) has attracted great interest by showing better performance over back-off n-gram models and feed-forward neural network language models (FNNLM). However, it still has the difficulty of modelling words of very low frequency in training data. To address this issue, a new framework of structured word embedding is introduced to RNNLM, where both input and target word embeddings are factorized into weighted sum of the corresponding sub-word embeddings. The framework is instantiated for Chinese, where characters can be naturally used as the sub-word units. Experiments on a Chinese twitter LVCSR task showed that the proposed approach effectively outperformed the standard RNNLM, yielding a relative PPL improvement of 8:8% and an absolute 0:59% CER improvement in N-Best re-scoring.