Hermann S. Lichte

Simulating wireless and mobile networks in OMNeT++ the MiXiM vision

Wireless communication has attracted considerable interest in the research community, and many wireless networks are evaluated using discrete event simulators like OMNeT++. Although OMNeT++ provides a powerful and clear simulation framework, it lacks of direct support and a concise modeling chain for wireless communication. Both is provided by MiXiM. MiXiM joins and extends several existing simulation frameworks developed for wireless and mobile simulations in OMNeT++. It provides detailed models of the wireless channel (fading, etc.), wireless connectivity, mobility models, models for obstacles and many communication protocols especially at the Medium Access Control (MAC) level. Further, it provides a user-friendly graphical representation of wireless and mobile networks in OMNeT++, supporting debugging and defining even complex wireless scenarios. Though still in development, MiXiM already is a powerful tool for performance analysis of wireless networks. Its extensive functionality and clear concept may motivate researches to contribute to this open-source project [4].

Cooperative Wireless Networking Beyond Store-and-Forward

In future wireless networks devices may cooperate to form logical links. Each of these links may consist of several independent physical channels which are shared by the cooperating partners. Even without multiple antennas this cooperation provides diversity in time and space. This so-called cooperation diversity increases the robustness of the link vs. fading and interference. After surveying approaches in cooperation diversity we focus on optimizing its performance by combining several cooperation schemes and by integrating cooperation into space-time coding. For multiple scenarios, we further discuss the factors and benefits introduced by user cooperation and how cooperation-aware resource allocation can be employed to further increase the performance of cooperative networks. When it comes to implementation, the question arises how cooperation can be integrated efficiently into existing wireless networks. A case study for 802.11-based WLANs reveals the issues that need to be solved in order to deploy cooperative techniques. We provide an overview of the state of the art in implementing cooperation approaches, analyze how appropriate these approaches solve the issues, and, where appropriate, point out their deficiencies. We conclude with a road map for future research necessary to tackle these deficiencies for the practical implementation of cooperation in next generation mesh, WLAN, WMAN, and cellular standards.

Design and Evaluation of a Routing-Informed Cooperative MAC Protocol for Ad Hoc Networks

Cooperative relaying has been shown to provide diversity gains which can significantly improve the packet error rate (PER) in wireless transmissions. In ad hoc wireless routing where packets may travel over a number of hops before reaching the destination, hop-wise cooperative relaying may severely reduce network capacity. This approach was mainly addressed in literature so far. In this paper, we efficiently apply cooperative relaying along a complete path and over multiple hops at the same time. We use information from the routing layer to improve the medium access control (MAC) layers performance. Simulations and testbed implementation show appealing gains through diversity resulting in up to 66% better PER performance and up to 148% goodput increase compared to conventional approaches.

Mobile Cooperative WLANs - MAC and Transceiver Design, Prototyping, and Field Measurements

We propose a practical medium access control (MAC) protocol and transceiver design for mobile cooperative WLANs. Our MAC protocol integrates selection decode-and-forward (SDF) cooperative relaying into IEEE 802.11. Unlike previous approaches, its cooperative signaling cycle allows communication if the direct link fails even for small signaling frames. Further SDF functions are efficiently supported by our ready- to-use transceiver design. We implement this design, including our MAC protocol, on a software defined radio (SDR) resulting in a cooperative IEEE 802.11g prototype. Using this prototype, we validate feasibility and performance of our approaches by extensive field measurements in indoor and railroad scenarios with an actual train moving the cooperating terminals.

Integrating multiuser dynamic OFDMA into IEEE 802.11a and prototyping it on a real-time software-defined radio testbed

Multiuser dynamic orthogonal frequency division multiple access (OFDMA) can achieve high downlink capacities in future wireless networks by optimizing the subcarrier allocation for each user. When it comes to the integration into current wireless local area network (WLAN) standards, dynamic OFDMA raises several implementation issues which are neglected in theoretical papers. Putting this emerging approach into practice requires to treat these issues accordingly and to demonstrate the feasibility of the system design. In this paper, we propose a dynamic OFDMA integration for the physical layer of the widespread IEEE 802.11a standard. To test our implementation and demonstrate its practical relevance we use a pragmatic approach: We prototype multiuser dynamic OFDMA on a real-time software-defined radio testbed for WLANs. We discuss details of our implementation and provide measurements showing that it does not introduce significant overhead into the IEEE 802.11a system at high subcarrier allocation quality. We particularly focus on the problems of our integration as well as the concepts and limitations of the used testbed.

Fading-resistant low-latency broadcasts in wireless multihop networks: the probabilistic cooperation diversity approach

Present broadcast approaches for wireless multihop networks distribute packets quickly to all nodes (i.e., with low latency) by constructing small broadcast trees, thereby reducing the number of forwarding transmissions. While these trees are sufficient in non-fading environments, we show that they have a low delivery rate under fading. As a solution, we (1) incorporate the Rayleigh fading model directly into tree construction to re-obtain complete distribution with high probability. To still achieve low latency at the same time, we combine transmissions at individual nodes to exploit cooperation diversity. Since in broadcasts, a packet has to be retransmitted by nodes along the tree anyway, we do not have to pay the multiplexing loss which hampers cooperation diversity in the unicast case. Thus, we (2) additionally exploit cooperation diversity during tree construction to gain improved reliability while still keeping the size of the tree low. This enables us to significantly decrease the time for broadcasts while still distributing packets to all nodes under fading with high probability. To justify our heuristic approach, we (3) show that finding minimum latency cooperative broadcasts is NP-complete.

Implementing MAC protocols for cooperative relaying: a compiler-assisted approach

Evaluating the performance of a cooperative relaying protocol requires an implementation for simulators and/or software-defined radios (SDRs) with an appropriate model for error detection, combining, and Medium Access Control (MAC) automaton. Such implementations are essential for meaningful evaluation of practical systems since any protocol introduces overhead that constrains the theoretical performance in non-obvious ways. Unfortunately, protocols for cooperative relaying often yield complex implementations which are tedious to implement and debug. Therefore, we identify basic operations that are inherent to all cooperative relaying protocols, and we propose a new language for their specification. Then, we show how to construct a compiler for the proposed language that generates most of the required implementation (model and MAC automaton) automatically. This approach prevents subtle mistakes during implementation of the protocol, and can significantly increase development time. In addition, this paper discusses code generation exemplarily for OMNeT++/Mobility Framework, but the approach is not restricted to a specific simulator or SDR.

Expected interference in wireless networks with geometric path loss: a closed-form approximation

In this letter, we derive 2¿¿P/((¿ - 2)g¿ - 2) as a closed-form approximation of the expected interference around a receiver in wireless networks. We use a geometric path loss model, assume that nodes are randomly distributed, and that only nodes outside a guard zone around the receiver simultaneously transmit. The derived solution depends on the path loss exponent ¿, node density ¿, transmission power per node P, and the radius g of the guard zone. The simplicity of this solution makes it widely applicable in wireless network analysis.

Automated Development of Cooperative MAC Protocols

Letting users cooperate is a promising approach to improve reliability and throughput in wireless networks, but it has not yet made the transition into practice. Unlike conventional wireless communication, cooperation distributes each single transmission among multiple users and channels. Consequently, a Medium Access Control (MAC) protocol developer has to cope with various new, heavily distributed protocol functions that are tedious to implement and to debug. To untie this complex development process, we propose to automate its most error-prone parts: Implementation of MAC automata, analysis, and code generation. To do so, we formalize cooperative MAC protocols by a new, easy-to-use specification language and propose a compiler for which we construct various backends to automatically analyze validity and performance of the specification and to translate the specified protocols into program code for simulators and even Software-Defined Radio (SDR) prototypes. All this provides a lightweight, heavily automated development process that quickly turns a cooperative MAC protocol specification into a practical implementation.

Opportunistic relaying vs. selective cooperation: analyzing the occurrence-conditioned outage capacity

Opportunistic Relaying (OR) and Selection Decode-and-Forward (SDF) cooperation protocols can both substantially improve the performance of wireless networks but fundamentally differ in utilized redundancy, relays, and channel knowledge. To analyze when SDF or OR improves error and data rate, we (1) derive their general outage probability and capacity for arbitrary relay configurations, (2) systematically benchmark both approaches in two-hop configurations, (3) study how often beneficial configurations occur in large networks, and, finally, condition our capacity results by this occurrence probability. Our results clearly show that OR maximizes the outage capacity at high acceptable error rate while SDF succeeds if a low error rate is required. SDF performs best if even the relays can cooperate among themselves, supported frequently in networks with more than three neighbors. Consequently, cooperating relays, adapting between OR and SDF, and joining these two approaches should be the focus of future protocol design. To this end, our paper provides a theoretical basis, adaptation rules, and design guidelines.