Cooperative multi-agent reinforcement learning (MARL) requires agents to explore to learn to cooperate. Existing value-based MARL algorithms commonly rely on random exploration, such as $\epsilon$-greedy, which is inefficient in discovering multi-agent cooperation. Additionally, the environment in MARL appears non-stationary to any individual agent due to the simultaneous training of other agents, leading to highly variant and thus unstable optimisation signals. In this work, we propose ensemble value functions for multi-agent exploration (EMAX), a general framework to extend any value-based MARL algorithm. EMAX trains ensembles of value functions for each agent to address the key challenges of exploration and non-stationarity: (1) The uncertainty of value estimates across the ensemble is used in a UCB policy to guide the exploration of agents to parts of the environment which require cooperation. (2) Average value estimates across the ensemble serve as target values. These targets exhibit lower variance compared to commonly applied target networks and we show that they lead to more stable gradients during the optimisation. We instantiate three value-based MARL algorithms with EMAX, independent DQN, VDN and QMIX, and evaluate them in 21 tasks across four environments. Using ensembles of five value functions, EMAX improves sample efficiency and final evaluation returns of these algorithms by 54%, 55%, and 844%, respectively, averaged all 21 tasks.

翻译：合作式多智能体强化学习要求智能体通过探索来学习协作。现有的基于价值的MARL算法通常依赖随机探索（如ε-贪婪策略），这种策略在发现多智能体协作方面效率低下。此外，由于其他智能体的同步训练，MARL环境对任意单个智能体而言呈现非平稳性，导致优化信号高度变异且不稳定。本文提出用于多智能体探索的集成价值函数（EMAX）——一个可扩展任意基于价值的MARL算法的通用框架。EMAX为每个智能体训练价值函数集成，以应对探索与非平稳性的关键挑战：（1）利用集成中价值估计的不确定性UCB策略引导智能体探索需要协作的环境区域；（2）以集成中的平均价值估计作为目标值，这些目标值相较于常用目标网络具有更低的方差，且能保证优化过程中梯度的稳定性。我们将EMAX实例化为三种基于价值的MARL算法（独立DQN、VDN和QMIX），并在四个环境的21个任务中开展评估。采用五个价值函数的集成后，EMAX使这些算法在所有21个任务中的平均样本效率与最终评估回报分别提升54%、55%和844%。