Unlabeled Multi-Robot Motion Planning with Improved Separation Trade-offs

TL;DR

This paper introduces new polynomial-time algorithms for unlabeled multi-robot motion planning that improve separation bounds and provide near-optimal solutions in more densely packed environments.

Contribution

It generalizes existing algorithms to achieve better separation trade-offs, enabling efficient planning in more challenging scenarios.

Findings

Achieves polynomial-time constant-approximation algorithms for specific separation bounds.

Provides a solution when robots are only 2 units apart with obstacles separated by 3 units.

Shows obstacles separation of at least 1.5 may be necessary without robots separation assumptions.

Abstract

We study unlabeled multi-robot motion planning for unit-disk robots in a polygonal environment. Although the problem is hard in general, polynomial-time solutions exist under appropriate separation assumptions on start and target positions. Banyassady et al. (SoCG'22) guarantee feasibility in simple polygons under start--start and target--target distances of at least $4$, and start--target distances of at least $3$, but without optimality guarantees. Solovey et al. (RSS'15) provide a near-optimal solution in general polygonal domains, under stricter conditions: start/target positions must have pairwise distance at least $4$, and at least $\sqrt{5}\approx2.236$ from obstacles. This raises the question of whether polynomial-time algorithms can be obtained in even more densely packed environments. In this paper we present a generalized algorithm that achieve different trade-offs on the robots-separation and obstacles-separation bounds, all significantly improving upon the state of the art. Specifically, we obtain polynomial-time constant-approximation algorithms to minimize the total path length when (i) the robots-separation is $2\tfrac{2}{3}$ and the obstacles-separation is $1\tfrac{2}{3}$, or (ii) the robots-separation is $\approx3.291$ and the obstacles-separation $\approx1.354$. Additionally, we introduce a different strategy yielding a polynomial-time solution when the robots-separation is only $2$, and the obstacles-separation is $3$. Finally, we show that without any robots-separation assumption, obstacles-separation of at least $1.5$ may be necessary for a solution to exist.