Simultaneous Translation Policies: From Fixed to Adaptive (2020)

Abstract: Adaptive policies are better than fixed policies for simultaneous translation, since they can flexibly balance the tradeoff between translation quality and latency based on the current context information. But previous methods on obtaining adaptive policies either rely on complicated training process, or underperform simple fixed policies. We design an algorithm to achieve adaptive policies via a simple heuristic composition of a set of fixed policies. Experiments on Chinese -> English and German -> English show that our adaptive policies can outperform fixed ones by up to 4 BLEU points for the same latency, and more surprisingly, it even surpasses the BLEU score of full-sentence translation in the greedy mode (and very close to beam mode), but with much lower latency.

## introduction simultaneous translation (st) aims to provide good translation quality while keeping the latency of translation process as low as possible. this is very important for the scenarios that require simultaneity, such as international summits and negotiations. for this, human interpreters usually start translation before the source sentence ends. however, this makes the translation process much more challenging than the full-sentence translation, because to balance the translation quality and latency, interpreters need to make decisions on when to continue translation and when to stop temporarily to wait for more source side information, which are difficult, especially for syntactically divergent language pairs, such as german and english. the above decisions can be considered as two actions: read (wait for a new source word) and write (emit a translated target word) @xcite . then we only need to decide which action to choose at each step, and the solution can be represented by a policy. earlier works (yarmohammadi et al., 2013; @xcite @xcite study policies as a part of speech-to-speech st system, where the policies usually try to separate the source sentence into several chunks that can be translated safely. recent works focus on obtaining policies for text-to-text st, which can be generally divided into two categories: fixed and adaptive. fixed policies @xcite usually follow some simple rules to choose actions. for example, the wait-k policy by @xcite first chooses k read actions, and then chooses write and read alternatively. this kind of policies do not utilize the context information and can be either too aggressive or too conservative in different cases. by contrast, adaptive policies try to make decisions on the fly using the currently available information. it is obvious that this kind of policies is more desirable for st than the fixed ones, and different methods are explored to achieve an adaptive policy. the majority of such methods @xcite @xcite @xcite are based on full-sentence translation models, which may be simple to use but cannot outperform fixed policies applied with "genuinely simultaneous" models trained for st @xcite . other meth-ods @xcite try to learn a policy together with the underlying translation model, but they rely on complicated and time-consuming training process. in this paper, we propose to achieve an adaptive policy via a much simpler heuristic composition of a set of wait-k policies (e.g., @xmath0. see fig. 1 for an example. to further improve the translation quality of our method, we apply ensemble of models trained with different wait-k policies. our experiments on @xmath1. more interestingly, compared with full-sentence translation, our method achieves higher bleu scores than greedy search but with much lower latency, and is close to the results from beam search. ## preliminaries full-sentence translation. neural machine translation (nmt) model usually consists of two components: an encoder, which encodes the source sentence @xmath2, . . . , x m ) into a sequence of hidden states, and a decoder, which sequentially predicts target tokens conditioned on those hidden states and previous predictions. the probability of the predicted target sequence @xmath3, . . . , y n ) will be simultaneous translation. @xcite propose a prefix-to-prefix framework to train models to make predictions conditioned on partial source sentences. in this way, the probability of predicted sequence y becomes intuitively, this policy first waits k source tokens and then outputs predicted tokens concurrently with the rest of source sentence. in this example, we will choose an action based on the top probability p top , and apply a new policy (the dotted arrows) after the chosen action. ## obtaining an adaptive policy we can obtain an adaptive policy, whose lag at each step is between k min and k max , meaning that at each step, the target sequence falls behind the source sequence at most k max tokens and at least k min tokens. at each step, there is a wait-k policy synchronizing the adaptive policy, meaning that they have the same lag at that step. specifically, at any step t, if the lag of the adaptive policy is k , then we apply the nmt model with the wait-k policy and force it to predict existing target tokens until step t, when the model will make a new prediction as the output of step t. however, the above method only shows how to simulate the adaptive policy to make a prediction at one step if we would like to write at that step, but it does not tell us at which steps we should write. we utilize the model confidence to make such a decision. specifically, we set a probability threshold ρ k for each wait-k policy. at each step, if the nmt model follows a wait-k policy, and predicts the most likely token with probability higher than the threshold ρ k , then we consider the model is confident on this prediction, and choose write action; otherwise, we choose read action. figure 2 gives an example for this process. we define the process of applying a wait-k model m k with a wait-k policy on a given sequence pair (x, y) by the following y top , p top ← p k (m k , x, y) which forces model m k to predict y, and returns the top token y top at the final step with the corresponding probability p top . the process of reading and returning a new source token is denoted by read(), and expression x • x represents to append an element x to the end of sequence x. we denote @xmath4. then algorithm 1 gives the pseudocode of the above method. algorithm 1 st decoding with an adaptive policy input: two integers k min and k max , a set of nmt models m k , and a sequence of read action while y |y| = </@xmath5, p top ← p kmax (m kmax , x, y) y ← y • y top write action return y 4 ensemble of wait-k models using the corresponding model m k with each waitk policies may not give us the best performance. if we have a set of models trained independently with different wait-k policies, then we can apply ensemble of those models @xcite to improve the translation quality, which is also used to improve the translation quality of full-sentence translation @xcite . however, there may be two issues to apply ensemble of all models: (1) the runtime for each prediction could be longer, resulting in higher latency; and (2) the translation accuracy may be worse, for the best model for one policy may give bad performance when doing inference with another policy. to avoid these, we propose to apply ensemble of the top-3 models for each policy. that is, we first generate distribution with the top-3 models independently with the same policy, and then take the arithmetic average of the three distributions as the final token distribution at that step. performance with different policies. we first evaluate the performance of each model with different policies, which helps us to choose models for different policies. specifically, we apply each model with ten different wait-k policies on dev set to compare the performance. fig. 3 shows the results of five models. we find the best model for one policy may not be the one trained with that policy. for example, on @xmath6, the best model for wait-1 policy is the one trained with wait-3 policy. further, there is no one model could achieve the best performance for all policies. comparing different methods. we compare our method with others from literature: wait-k method @xcite (train and test models with the same wait-k policy), test-time waitk method @xcite (apply full-sentence model with wait-k policies), wait-if-diff (cho and esipova, 2016) (start with s 0 source tokens, choose to read only if top token at t-th step diffs from that at (t -δ)-th step), and wait-if-worse (cho and esipova, 2016) (start with s 0 source tokens, choose to read only if the top probability at t-th step is smaller than that at (t -δ)-th step). for wait-if-diff we set for our method, we test three different cases: (1) single, where for each policy we apply the corresponding model that trained with the same policy; (2) ensemble top-3, where for each policy we apply the ensemble of 3 models that achieve the highest bleu scores with that policy on dev set; the comparison is provided in fig. 4 (the corresponding numeric scores are provided in appendix a). compared with wait-k method, our single method achieves improvement of up to 2 bleu point, and our ensemble top-3 achieves improvement up to 4 bleu points. compared with full-sentence translation, our ensemble top-3 surprisingly outperforms greedy search with much lower latency (@xmath7, and achieves bleu scores close to that from beam search (see efficiency. to evaluate the efficiency, we present in ## conclusions we have designed a simple heuristic algorithm to obtain an adaptive policy based on a set of wait-k policies, and applied ensemble in our method to improve the translation quality while maintaining low latency. experiments show that our method not only outperforms the original wait-k method with relatively large gap, but also surpasses greedy full-sentence translation with much lower latency.