Christian Dombrowski

Channel Coding versus Cooperative ARQ: Reducing Outage Probability in Ultra-Low Latency Wireless Communications

Nowadays wireless communications still lack the ability to provide high reliability and low latency, although mission-critical applications, such as found in industrial automation, rely on both requirements. The main challenge is that an improved reliability often comes at the price of an increased latency. It has been shown that cooperative schemes can effectively increase the reliability by leveraging spatial diversity. However, an important question remains how to integrate cooperative schemes when dealing with very short latency bounds and especially how much time should be reserved for potential retransmissions. In this work, we propose and evaluate a centralized communication system that uses cooperative ARQ to achieve high reliability under the constraint of a strict latency bound of 1 ms. We evaluate this system analytically, using an outage-capacity model with average channel state information, by varying the reserved time for retransmissions, where a shorter time for retransmissions allows to apply stronger channel codes in the original transmission. As a baseline, we use a system without cooperation mechanism, thus applying the given time for stronger channel codes in the direct transmission of a message. In case of cooperation, a third station may act as a relay if the original transmission failed. Our results reveal that an optimal size of the reserved retransmission time exists around 15% to 30% of the total frame time, increasing the reliability by several orders of magnitude, even for a large number of transmissions within a communication cycle.

Model-Checking Assisted Protocol Design for Ultra-reliable Low-Latency Wireless Networks

Recently, the wireless networking community is getting more and more interested in novel protocol designs for safety-critical applications. These new applications come with unprecedented latency and reliability constraints which poses many open challenges. A particularly important one relates to the question how to develop such systems. Traditionally, development of wireless systems has mainly relied on simulations to identify viable architectures. However, in this case the drawbacks of simulations – in particular increasing run-times – rule out its application. Instead, in this paper we propose to use probabilistic model checking, a formal model-based verification technique, to evaluate different system variants during the design phase. Apart from allowing evaluations and therefore design iterations with much smaller periods, probabilistic model checking provides bounds on the reliability of the considered design choices. We demonstrate these salient features with respect to the novel EchoRing protocol, which is a token-based system designed for safety-critical industrial applications. Several mechanisms for dealing with a token loss are modeled and evaluated through probabilistic model checking, showing its potential as suitable evaluation tool for such novel wireless protocols. In particular, we show by probabilistic model checking that wireless token-passing systems can benefit tremendously from the considered fault-tolerant methods. The obtained performance guarantees for the different mechanisms even provide reasonable bounds for experimental results obtained from a real-world implementation.

Is High Quality Sensing Really Necessary for Opportunistic Spectrum Usage

The major requirement for Cognitive Radio (CR) based opportunistic spectrum re-usage is reliable protection of the primary communication. This calls for a reliable detection of the presence of the Primary Users (PUs) as well as for an immediate reconfiguration of the secondary communication: Each time the PU has been detected, the Secondary Users (SUs) have to vacate the respective part of the spectrum and continue their communication elsewhere. Unfortunately enough, also false positives in the sensing process trigger the same type of reconfiguration, leading to more reconfigurations than actually necessary. Therefore, the efficiency of this reconfiguration process is of high interest. For a frequently postulated OFDM based spectrum pooling SU system two basic questions are considered: (1) How should the parameters of the secondary communication link be selected in order to achieve a stable Quality of Service (QoS) in spite of reconfigurations, and (2) how strongly is the QoS of the SU influenced by reconfigurations caused by a non ideal sensing process, i. e. by an excessive number of false positives.

Performance analysis of cooperative ARQ systems for wireless industrial networks

The proliferation of wireless communications has lead to a high interest to establish this technology in industrial settings. The main arguments in favor of wireless are reduced costs in deployment and maintenance, as well as increased flexibility. In contrast to home and office environments, industrial settings include mission-critical machine-to-machine applications, demanding stringent requirements for reliability and latency in the area of 1-10-9 PDR and 1ms, respectively. One way to achieve both is cooperative Automatic Repeat reQuest (ARQ), which leverages spatial diversity. This paper presents a wireless multi-user Time Division Multiple Access system with cooperative ARQ for mission-critical communication. We evaluate two design options analytically, using an outage-capacity model, to investigate whether the relaying of messages should be performed centrally at a multi-antenna AP with perfect Channel State Information (CSI) or decentrally at simultaneously transmitting stations with average CSI. Results indicate that both options are able to achieve the targeted communication guarantees when a certain degree of diversity is implemented, showing a stable system performance even with an increasing number of stations.

WARPsim: A code-transparent network simulator for WARP devices

Analyzing a communication protocol by means of simulation and real-world experimentation requires careful protocol implementation in both domains. Differences in the implementation may lead to significantly diverging performance results, which may affect the protocol design process adversely. A code-transparent simulation and experimentation framework for Wireless Access Research Platform (WARP) devices is proposed, which is called WARPsim. By extending the simulation engine appropriately, the same application code that runs on WARP devices can be used for simulation. This work studies the implications of this approach using the example of implementing time-critical Medium Access Control Layer (MAC) protocols on WARP devices. In the demonstration, various MAC protocols will be simulated using WARPsim, while changing protocol parameters, but also crucial aspects of the emulated hardware. A graphical representation integrated into the framework allows for an intuitive examination of the protocol behavior.