Torsten Andre

Application-driven design of aerial communication networks

Networks of micro aerial vehicles (MAVs) equipped with various sensors are increasingly used for civil applications, such as monitoring, surveillance, and disaster management. In this article, we discuss the communication requirements raised by applications in MAV networks. We propose a novel system representation that can be used to specify different application demands. To this end, we extract key functionalities expected in an MAV network. We map these functionalities into building blocks to characterize the expected communication needs. Based on insights from our own and related real-world experiments, we discuss the capabilities of existing communications technologies and their limitations to implement the proposed building blocks. Our findings indicate that while certain requirements of MAV applications are met with available technologies, further research and development is needed to address the scalability, heterogeneity, safety, quality of service, and security aspects of multi- MAV systems.

An Experimental Study of Selective Cooperative Relaying in Industrial Wireless Sensor Networks

Strict reliability and delay requirements of factory monitoring and control applications pose challenges for wireless communications in dynamic and cluttered industrial environments. To reduce outage in such fading-rich areas, cooperative relays can be used to overhear source-destination transmissions and forward data packets that a source fails to deliver. This paper presents the results of an experimental study of selective cooperative relaying protocols that are implemented in off-the-shelf IEEE 802.15.4-compatible devices and evaluated in an industrial production plant. Three practical relay update schemes, which define when a new relay selection is triggered, are investigated: 1) periodic; 2) adaptive; and 3) reactive relay selections. The results show that all relaying protocols outperform conventional time diversity retransmissions in delivery ratio and number of retransmissions for packet delivery. Reactive selection provides the best overall delivery ratio of nearly 99% over the tested network. There is a tradeoff, however, between achievable delivery ratio and required selection overhead. This tradeoff depends on protocol and network parameters, and is studied via protocol emulation using empirical channel values.

An Autonomous Multi-UAV System for Search and Rescue

This paper proposes and evaluates a modular architecture of an autonomous unmanned aerial vehicle (UAV) system for search and rescue missions. Multiple multicopters are coordinated using a distributed control system. The system is implemented in the Robot Operating System (ROS) and is capable of providing a real-time video stream from a UAV to one or more base stations using a wireless communications infrastructure. The system supports a heterogeneous set of UAVs and camera sensors. If necessary, an operator can interfere and reduce the autonomy. The system has been tested in an outdoor mission serving as a proof of concept. Some insights from these tests are described in the paper.

Measurement-based analysis of cooperative relaying in an industrial wireless sensor network

Cooperative relaying is a communication technique that has been shown to improve link reliability between communicating entities. Most results in this area are obtained either analytically or via simulations. Rather few real-world experiments are conducted to backup theoretical results. This paper intends to go a step in this direction of applied research. We study the performance of cooperative relaying for industrial applications based on real-world measurements. These link-level measurements are conducted employing low-cost, off-the-shelf IEEE 802.15.4 devices in a factory characterized by a harsh and cluttered environment. Using the measured data, we emulate a simple cooperative relaying protocol to investigate the performance in terms of outage and packet delivery ratio and study parameters suitable for relay selection.

Packet Delivery Performance of Simple Cooperative Relaying in Real-World Car-to-Car Communications

We evaluate the packet delivery performance of low-complex cooperative relaying in car-to-car communications by real-world measurements. The ratio and temporal correlation of packet delivery are evaluated for suburban and highway environments using three cars equipped with programmable radios and serving as sender, relay, and destination. We compare the relaying performance to that of pure time diversity and show how temporal autocorrelation of packet delivery is a key factor in whether or not relaying exhibits benefits. Results are relevant in the design of relay selection protocols, as they give guidelines for the affordable selection delay.

Collaboration in Multi-Robot Exploration: To Meet or not to Meet?

Work on coordinated multi-robot exploration often assumes that all areas to be explored are freely accessible. This common assumption does not always hold, especially not in search and rescue missions after a disaster. Doors may be closed or paths blocked detaining robots from continuing their exploration beyond these points and possibly requiring multiple robots to clear them. This paper addresses the issue how to coordinate a multi-robot system to clear blocked paths. We define local collaborations that require robots to collaboratively perform a physical action at a common position. A collaborating robot needs to interrupt its current exploration and move to a different location to collaboratively clear a blocked path. We raise the question when to collaborate and whom to collaborate with. We propose four strategies as to when to collaborate. Two obvious strategies are to collaborate immediately or to postpone any collaborations until only blocked paths are left. The other two strategies make use of heuristics based on building patterns. While no single strategy behaves optimal in all scenarios, we show that the heuristics decrease the time required to explore unknown environments considering blocked paths.

Coordinated multi-robot exploration: Out of the box packages for ROS

We present and evaluate new ROS packages for coordinated multi-robot exploration, namely communication, global map construction, and exploration. The packages allow completely distributed control and do not rely on (but allow) central controllers. Their integration including application layer protocols allows out of the box installation and execution. The communication package enables reliable ad hoc communication allowing to exchange local maps between robots which are merged to a global map. Exploration uses the global map to spatially spread robots and decrease exploration time. The intention of the implementation is to offer basic functionality for coordinated multi-robot systems and to enable other research groups to experimentally work on multi-robot systems. The packages are tested in real-world experiments using Turtlebot and Pioneer robots. Further, we analyze their performance using simulations and verify their correct working.

Experimental evaluation of multihop-aware cooperative relaying

Through real-world measurements we evaluate the impact of multihop-aware cooperative relaying on end-to-end route performance in wireless multi-hop networks. In an experiment with 50 devices in an industrial environment cooperative relaying increases the end-to-end transmission reliability for multi-hop links to nearly 100% while reducing the average packet delay. We suggest how to include relay selection in the route discovery process of ad hoc routing protocols such as Ad hoc On-Demand Distance Vector (AODV) and Dynamic Source Routing (DSR) and perform two experiments. In the first experiment we discuss route discovery strategies focusing on small delay benefiting from reliable data link transmissions using cooperative relays. In the second experiment we compare reliability and delay for five different transmission powers with and without cooperative relaying.

Assessing the value of coordination in mobile robot exploration using a discrete-time Markov process

With the introduction of multi-robot systems for exploration the question then arises, whether coordination among robots in such systems is required. We propose a model based on Markov processes to evaluate the need for coordination in multi-robot systems during the exploration of unknown environments and determine possible gains achievable through coordination. The model is illustrated by exploration of an indoor office environment. We qualitatively identify characteristics of environments which make coordination necessary and allow to quantitatively include them in the model. The expected gain through coordination highly depends on the environment. We further investigate the impact of team sizes. In favorable environments explicit coordination is not needed at the cost of increased team sizes. This helps to raise understanding of factors having an impact on coordination functions and making it possible to approximate a possible gain through coordination.

Multi-hop networks with cooperative relaying assisted links

We assess the impact of cooperative relaying assisted hops on a multi-hop network. On the example of the reactive Dynamic Source Routing (DSR) protocol, we utilize cooperative relays to improve the link delivery probabilities of the individual hops allowing to significantly improve the end-to-end delivery ratio for large networks. First, we discuss two types to trigger retransmissions by cooperative relays in case direct transmissions between nodes fail. Second, we show that cooperative relaying assisted links lead to improved end-to-end delivery probabilities for multi-hop networks and determine the associated costs in terms of delay and consumed energy. For multi-hop routes with a large number of hops, cooperative relaying can decrease the costs while improving the end-to-end delivery ratio significantly making reliable communication possible where unassisted multi-hop communication is functionally not feasible.