Günther Brandner

Temporal Correlation of Interference in Wireless Networks with Rayleigh Block Fading

The temporal correlation of interference is a key performance factor of several technologies and protocols for wireless communications. A comprehensive understanding of interference correlation is especially important in the design of diversity schemes, whose performance can severely degrade in case of highly correlated interference. Taking into account three sources of correlation-node locations, channel, and traffic-and using common modeling assumptions-random homogeneous node positions, Rayleigh block fading, and slotted ALOHA traffic-we derive closed-form expressions and calculation rules for the correlation coefficient of the overall interference power received at a certain point in space. Plots give an intuitive understanding as to how model parameters influence the interference correlation.

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.

Building blocks of cooperative relaying in wireless systems

SummaryThe concept of cooperative relaying promises gains in robustness and energy efficiency in wireless networks. This survey gives an introduction and overview of promising methods that can be used for cooperative relaying: relay selection protocols, cooperative channel and network coding, and physical layer aspects such as cooperative modulation. The particular methods can be seen as building blocks that can be configured and optionally used for designing a system. They offer a large number of design options for relaying systems. Based on available hardware features, expected environment, and required services an efficient system needs to be tailored using the right subset of available methods.ZusammenfassungDas Konzept des kooperativen Weiterleitens von Nachrichten ermöglicht eine Verbesserung der Robustheit und Energieeffizienz von drahtlosen Netzwerken. Dieser Artikel stellt einige vielversprechende Methoden für Netzwerke mit kooperativem Relaying vor. Im Speziellen werden Protokolle zur Relayauswahl, Kanalcodierung, Network Coding sowie kooperative Modulationstechniken behandelt. Die vorgestellten Methoden sind als Bausteine für ein Netzwerk mit kooperativem Relaying zu verstehen. Jede einzelne Methode kann optional und gemeinsam mit anderen verwendet werden und in den meisten Fällen zusätzlich noch dem Zweck entsprechend konfiguriert werden. Somit ergibt sich eine hohe Anzahl von Möglichkeiten, ein System mit kooperativem Relaying zu entwerfen. Das tatsächliche Design hängt letztendlich von Parametern wie der zur Verfügung stehenden Hardware, den Annahmen über die Zielumgebung sowie den Anforderungen der Applikation ab.

Cooperative Relaying Under Spatially and Temporally Correlated Interference

We analyze the performance of an interference-limited decode-and-forward cooperative relaying system that comprises a source, a destination, and N relays, arbitrarily placed on the plane and suffering from interference by a set of interferers placed according to a spatial Poisson process. In each transmission attempt, first, the transmitter sends a packet; subsequently, a single one of the relays that received the packet correctly, if such a relay exists, retransmits it. We consider both selection combining and maximal ratio combining at the destination, Rayleigh fading, and interferer mobility. We derive expressions for the probability that a single transmission attempt is successful, as well as for the distribution of the transmission attempts until a packet is successfully transmitted. Results provide design guidelines applicable to a wide range of systems. Overall, the temporal and spatial characteristics of the interference play a significant role in shaping the system performance. Maximal ratio combining is only helpful when relays are close to the destination; in harsh environments, having many relays is particularly helpful, and relay placement is critical; the performance improves when interferer mobility increases; and a tradeoff exists between energy efficiency and throughput.

How does interference dynamics influence packet delivery in cooperative relaying

We show by means of stochastic geometry that interference dynamics has a strong impact on the performance of cooperative relaying. For both conventional and multi-hop-aware cooperative relaying we show that the packet delivery probability significantly changes with the dependence of interference among the links. Depending on the scenario under consideration, this change could be either an increase or decrease of the packet delivery probability. Especially for multi-hop-aware cooperative relaying, the performance gain is heavily reduced when interference possesses high temporal and spatial dependence.

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.

Impact of Random Mobility on the Inhomogeneity of Spatial Distributions

Simulation results of wireless networks heavily depend on the spatial distribution of its nodes. Even though the initial distribution may match the expectations of the researcher, its properties may get lost due to the applied mobility model after a few seconds. This paper analyzes the effects of three well-known mobility models on the inhomogeneity of the spatial distribution. Furthermore, the effects of penetrable borders of the simulation area on the node distribution are analyzed. Due to the encountered deviations, we propose and analyze the inhomogeneous random waypoint (IRWP) model. It maintains a desired inhomogeneity level in the long run and generates random clusters with respect to shape, size, position, and member nodes.

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.

On colliding first messages in slotted ALOHA

Considering n nodes performing random access using ALOHA with s slots, we study the probability that there occurs a non-colliding message in the first non-empty slot. If each node transmits with probability p in each slot and the number of slots is sufficiently large, a non-colliding first message occurs with probability Phi = 1-np/2 for large n and small np. If the number of slots is limited, the probability Phi is lower but can be maximized choosing an optimal p. To maximize Phi further, nodes can apply a slot-dependent transmit probability pi with i = 1, ..., s. It is shown that a ldquoslow start strategy,rdquo in which pi is low for low i and increases with increasing i, is beneficial. Our main contribution is an equation for the pi values that maximize Phi. We analyze how a higher probability of a non-colliding first message comes at the price of an increased delay of such a message. Besides being of interest for the theory of random access, the results are practically applicable to node selection protocols, such as relay selection in cooperative wireless networks.

Firefly synchronization with phase rate equalization and its experimental analysis in wireless systems

The convergence and precision of synchronization algorithms based on the theory of pulse-coupled oscillators is evaluated on programmable radios. Measurements in different wireless topologies show that such algorithms reach precisions in the low microsecond range. Based on the observation that phase rate deviation among radios is a limiting factor for the achievable precision, we propose a distributed algorithm for automatic phase rate equalization and show by experiments that an improved precision below one microsecond is possible in the given setups. It is also experimentally demonstrated that the stochastic nature of coupling is a key ingredient for convergence to synchrony. The proposed scheme can be applied in wireless systems for distributed synchronization of transmission slots, or sleep cycles.