Goal-conditioned hierarchical reinforcement learning (HRL) is a promising approach for scaling up reinforcement learning (RL) techniques. However, it often suffers from training inefficiency as the action space of the high-level, i.e., the goal space, is often large. Searching in a large goal space poses difficulties for both high-level subgoal generation and low-level policy learning. In this paper, we show that this problem can be effectively alleviated by restricting the high-level action space from the whole goal space to a $k$-step adjacency region centered by the current state using an adjacency constraint. We theoretically prove that the proposed adjacency constraint preserves the optimal hierarchical policy, and show that this constraint can be practically implemented by training an adjacency network that can discriminate between adjacent and non-adjacent subgoals. Experimental results on discrete and continuous control tasks show that our method outperforms the state-of-the-art HRL approaches.

本文研究了基于目标的层次强化学习（HRL）中高层次行动空间（即目标空间）的训练效率问题，提出了使用邻接约束把高层次动作空间限制为当前状态的$k$步领域，证明该约束在确定性MDPs下保留了最优层次策略，实现方法是训练一个区分相邻和非相邻子目标的邻接网络，在实验中验证了该约束在离散和连续控制任务中显著提高了HRL方法的性能。

层次强化学习中生成受邻近约束的子目标