---
title: "MCVI"
type: concept
related: [Mcvi, Partially Observable Markov Decision Process]
source: https://www.jemoka.com/posts/kbhmcvi/
confidence: high
status: active
---

MCVI solves POMDPs with continuous state space, but with discrete observation and action spaces. It does this by formulating a POMDP as a graph.
Fast algorithms require discretized state spaces, which makes the problem much more difficult to model. MCVI makes continuous representations possible for complex domains.
MC Backup Normal POMDP Bellman Backup isn&rsquo;t going to work well with continuous state spaces.
Therefore, we reformulate our value backup as:
\begin{equation} V_{t+1}(b) = \max_{a \in A} \qty(\int_{s} R(s,a)b(s) \dd{s}) + \gamma \sum_{o \in O}^{} p(o|b,a) V_{t}(update(b,a,o)) \end{equation}
whereby, a continuous belief update:
\begin{equation} update(b,a,o) = \kappa O(o|s&rsquo;,a) \int_{s \in S} T(s&rsquo;|s,a) b(s) \dd{s} \end{equation}
where \(\kappa\) is a normalisation constant to keep the new belief a probability.
But! Instead of actually taking the integral, we simulate a series of trajectories and sum the toal reward
MC-Backup at Graph We construct at graph by sticking each best action determined by rolling out \(L\) steps and computing the reward.
Collecting the values given each observation, we create a new node for the best action; the best action per observation is connected as well.
This creates a new optimal policy graph from the rollouts.
MCVI initial each reward at action to \(0\) for each observation, initialize each observation, node as \(0\) Take monte-carlo samples across the actions and states to take integrals to obtain: \(HV_{g}(b) = \max_{a \in A} \qty(\int_{s \in S} R(s,a)b(s) \dd{s} + \sum_{o}^{} ???)\) each future observation is sampled using monte-carlo simulation each backup, you pick one new node to add.
