Okay, we have established the motivation for building an internal model M^ of the environment, typically consisting of a learned transition function P^(s′∣s,a) and a learned reward function R^(s,a,s′). Having such a model, even an approximate one, opens the door to planning. Instead of solely relying on real interactions, the agent can now use its internal model to simulate potential future scenarios and learn from them. One straightforward and versatile way to leverage the learned model is through trajectory sampling, sometimes called simulation-based planning or model-based rollouts.
The core idea is simple: treat the learned model M^ as if it were the real environment and generate sequences of states, actions, and rewards by interacting with it. These simulated trajectories provide "synthetic" experience that can be used to update value functions or policies, often significantly improving sample efficiency compared to purely model-free methods.
Starting from a given state s (which could be the agent's current state in the real world, a state encountered previously, or even a hypothetical state), the planning process via trajectory sampling unfolds as follows:
This generates a simulated trajectory: (s0,a0,r0,s1,a1,r1,…,sH,aH,rH). Note that s0 is the starting state for the simulation.
A flowchart illustrating the trajectory sampling process using a learned environment model M^. Starting from a state s, an action a is chosen, and the model predicts the next state s′ and reward r. This loop continues for a planning horizon H, generating a simulated trajectory used for learning updates.
Once we have one or more simulated trajectories starting from a state s, how can we use them for planning? There are several common approaches:
Monte Carlo Evaluation (Value Estimation): Generate multiple trajectories starting from state s. The return G=∑t=0Hγtrt is calculated for each trajectory. The average of these returns provides an estimate of the state value V(s) under the policy used during simulation. Similarly, if we simulate trajectories starting from state s after taking a specific action a, the average return estimates Q(s,a). These estimates can then be used to update the agent's value function approximator (e.g., a neural network). This is precisely the mechanism used in the planning phase of Dyna-Q, where simulated experiences (s,a,r,s′) generated by the model are fed into standard Q-learning or SARSA updates.
Policy Improvement via Action Selection: For a given state s, simulate trajectories for each possible action a∈A. Estimate the Q-value Q(s,a) for each action using the method above. The action with the highest estimated Q-value is then chosen for execution in the real environment. This approach uses the model to "look ahead" and evaluate the consequences of potential actions before committing to one. This resembles Monte Carlo Control applied to the learned model.
Generating Data for Policy Optimization: The simulated trajectories (st,at,rt,st+1) can be collected into a buffer, similar to an experience replay buffer but populated with synthetic data. This buffer can then be used to train a policy network directly using policy gradient methods or actor-critic algorithms. This allows the policy to be improved based on imagined outcomes.
While powerful, planning with trajectory sampling involves several design choices and potential issues:
Trajectory sampling provides a flexible bridge between model learning and control. By simulating interactions with its learned world model, the agent can augment its real experience, potentially leading to faster learning and better decision-making compared to purely model-free approaches, especially when real-world interactions are costly or limited. The next sections will explore specific architectures like Dyna-Q and planning methods like MCTS that build upon these core ideas.
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