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Title: PLEX: PLanner and EXecutor for Embodied Learning in Navigation

Paper content:
We present a method for policy learning to navigate indoor environments.
We adopt a hierarchical policy approach, where two agents are trained to work in cohesion with one another to perform a complex navigation task.
A Planner agent operates at a higher level and proposes sub-goals for an Executor agent.
The Executor reports an embedding summary back to the Planner as additional side information at the end of its series of operations for the Planner's next sub-goal proposal.
The end goal is generated by the environment and exposed to the Planner which then decides which set of sub-goals to propose to the Executor.
We show that this Planner-Executor setup drastically increases the sample efficiency of our method over traditional single agent approaches, effectively mitigating the difficulty accompanying long series of actions with a sparse reward signal.
On the challenging Habitat environment which requires navigating various realistic indoor environments, we demonstrate that our approach offers a significant improvement over prior work for navigation.
The ability to model and understand the world at a high-level is crucial for performing complex tasks in real world environments.
Part of this high-level understanding involves the ability to divide and plan out tasks that are complicated and have long time horizons into more manageable subtasks.
For example, when navigating to a new location, we typically break the task down into a set of manageable directions (i.e. drive along a certain road until a familiar landmark before taking a turn).
Imbuing machines with this ability of creating abstractions for long and complex tasks is an active area of research known as hierarchical learning (Sutton et al., 1998; 1999) .
Research for navigation has recently seen a rejuvenation due to the advent of learning-based approaches Parisotto & Salakhutdinov, 2017; Henriques & Vedaldi, 2018) .
Embodied learning-based approaches have shown some appealing properties over classical approaches such as being able to operate in complex environments with limited sensor data (Savva et al., 2019; Mishkin et al., 2019) .
However, there is a need for the ability to plan across long time horizons with sparse reward signals.
This in effect, causes limitations such as the inability to overcome small obstacles when navigating towards a given goal and the requirement of invoking the environment a large number of times for any meaningful learning to occur (Le et al., 2018) .
Works which have combined hierarchical reinforcement learning with imitation learning have shown promising results (Das et al., 2018b; Le et al., 2018) , by leveraging expert trajectories with policy sketches (Andreas et al., 2017) , which are less expensive to obtain; however these sketches still require annotation of the environment.
In this work, we study such hierarchical control for the task of indoor navigation, whereby an embodied agent is randomly spawned within a novel and complex environment and must learn to navigate this environment through interaction (Das et al., 2018a) .
We address this challenging learning problem through a hierarchical policy approach, where two agents are cooperatively trained together.
Each agent performs a different role, where one agent acts as a Planner, learning how to propose good sub-goals to an Executor agent, which acts at the low level to achieve these sub-goals (Fig. 1) .
In contrast to existing hierarchical policy learning approaches, communication between our two agents is two-way, where the Executor provides the Planner with a summary of its series of actions and recent observations.
This aids the Planner in deciding the next sub-goal with additional side Figure 1 : Our PLEX framework adopts a hierarchical policy approach, where a Planner proposes sub-goals for an Executor to act upon within an environment.
The Planner receives an egocentric, top-down view with the target location and an embedding summary provided by the Executor.
The Executor receives visual sensory data (i.e. colour and depth) as its input and a sub-goal provided by the Planner.
Our method reduces the need for long-term planning and addresses the known sample inefficiency problem accompanying memory models within deep reinforcement learning approaches.
information provided by the Executor.
To this end, we propose PLEX, a planning and executing learning framework which offers the following contributions:
• A hierarchical reinforcement learning approach where two agents specialise on different tasks but are jointly trained by sharing information • We demonstrate both theoretically and empirically that our method benefits from significantly improved sample efficiency as the time horizon is distributed between the Planner and Executor • By extension, our approach mitigates problems prevalent in long-horizon planning, especially those adopting LSTM (Hochreiter & Schmidhuber, 1997) planning approaches
In this work, we present a hierarchical reinforcement learning approach for solving PointGoal navigation tasks.
Our proposed approach uses a cooperative learning strategy in which two agents, an Executor and a Planner are jointly learned to solve this task.
This is enabled through a two-way communication channel established between the two agents through the use of an Executor Latent Information vector provided by the Executor and sub-goals generated by the Planner.
We motivate the use of this hierarchical approach both theoretically, as well as through empirical experiments which demonstrate a significant improvement in sampling efficiency of our approach, allowing our structured approach to perform significantly better on increasingly harder tasks when compared to baseline approaches.

assistant: text: We present a hierarchical learning framework for navigation within an embodied learning setting