1. Twenty Second Conference on Artificial Intelligence AAAI 2007 Improved State Estimation in Multiagent Settings with Continuous or Large Discrete State Spaces Prashant Doshi Dept. of Computer Science University of Georgia Speaker Yifeng Zeng Aalborg University, Denmark

2. State Estimation Physical State (Loc, Orient,...) Physical State (Loc, Orient,...) Single agent setting

3. Interactive state State Estimation Physical State (Loc, Orient,...) Physical State (Loc, Orient,...) Multiagent setting (See AAMAS 05)

4. State Estimation in Multiagent Settings Ascribe intentional models (POMDPs) to other agents Update the other agents' beliefs Estimate the interactive state (See JAIR’05)

5. Previous Approach Interactive particle filter (I-PF; see AAMAS'05, AAAI'05)  Generalizes PF to multiagent settings  Approximate simulation of the state estimation Limitations of the I-PF  Large no. of particles needed even for small state spaces Distributes particles over the physical state and model spaces  Poor performance when the physical state space is large or continuous

6. Factoring the State Estimation Update the physical state space Update other agent's model

7. Factoring the State Estimation Sample particles from just the physical state space Substitute in state estimation Implement using PF Perform as exactly as possible Rao-Blackwellisation of the I-PF

8. Assumptions on Distributions Prior beliefs  Singly nested and conditional linear Gaussian (CLG) Transition functions  Deterministic or CLG Observation functions  Softmax or CLG Why these distributions?  Good statistical properties  Well-known methods for learning these distributions from data  Applications in target tracking, fault diagnosis

9. Belief Update over Models Step 1: Update other agent's level 0 beliefs Product of a Gaussian and Softmax  Use variational approximation of softmax (see Jordan '99)  Softmax Gaussian – tight lower bound Update is then analogous to the Kalman filter

10. Belief Update over Models Step 2: Update belief over other's beliefs Solve other's models – compute other's policy  Large variance – Listen Obtain piecewise distributions L Updated Gaussian if prior belief supports the action 0 otherwise Updated belief over other's belief = Approximate piecewise with Gaussian using ML

11. Belief Update over Models Step 3: Form a mixture of Gaussians Each Gaussian is for the optimal action and possible observation of the other agent Weight the Gaussian with the likelihood of receiving the observation Mixture components grow unbounded  components after one step  components after t steps

12. Comparative Performance Compare accuracy of state estimation with I-PF (L1 metric)  Continuous multi-agent tiger problem  Public good problem with punishment RB-IPF focuses particles on the large physical state space Updates beliefs over other's models more accurately (supporting plots in paper)

13. Comparative Performance Compare run times with I-PF ( Linux, Xeon 3.4GHz, 4GB RAM ) Sensitivity to Gaussian approximation of piecewise distribution

14. Discussion How restrictive are the assumptions on the distributions?  Can we generalize RB-IPF, like I-PF? Will RB-IPF scale to large number of update steps?  Closed form mixtures are needed Is RB-IPF applicable to multiply-nested beliefs  Recursive application may not improve performance over I- PF

15. Thank you Questions?