Before revision:  This work presents the iterative self-training approach as an improvement over the self-learning approach to further enhance the performance of the backward model. Over several iterations, the synthetic datagenerated by the backward model is used to improve its performance through forward translation.
After revision:  This work presents the iterative self-training approach as an improvement over the self-learning and back-translation respectively. Experimental results have shown the synthetic datagenerated by the backward model is used to improve its performance through forward translation.