Daniel Willkomm

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].

COGNITIVE RADIOS FOR DYNAMIC SPECTRUM ACCESS - Dynamic Frequency Hopping Communities for Efficient IEEE 802.22 Operation

One of the key challenges of the emerging cognitive radio-based IEEE 802.22 wireless regional area networks (WRANs) is to address two apparently conflicting requirements: ensuring QoS satisfaction for WRAN services while providing reliable spectrum sensing for guaranteeing licensed user protection. To perform reliable sensing, in the basic operation mode on a single frequency band (non-hopping mode), one must allocate quiet times, that is, periodically interrupt data transmission that could impair the QoS of WRAN. This critical issue can be addressed by an alternative operation mode proposed in 802.22 called dynamic frequency hopping (DFH), where WRAN data transmission is performed in parallel with spectrum sensing without interruptions. DFH community, as described in this article, is a mechanism that coordinates multiple WRAN cells operating in the DFH mode, such that efficient frequency usage and reliable channel sensing are achieved. The key idea of DFH community is that neighboring WRAN cells form cooperating communities that coordinate their DFH operations

Primary Users in Cellular Networks: A Large-Scale Measurement Study

Most existing studies of spectrum usage have been performed by actively sensing the energy levels in specific RF bands including cellular bands. In this paper, we provide a unique, complementary analysis of cellular primary usage by analyzing a dataset collected inside a cellular network operator. One of the key aspects of our dataset is its scale - it consists of data collected over three weeks at hundreds of base stations. We dissect this data along different dimensions to characterize and model primary usage as well as understand its temporal and spatial variations. Our analysis reveals several results that are relevant if dynamic spectrum access (DSA) approaches are to be deployed for cellular frequency bands. For instance, we find that call durations show significant deviations from the often- used exponential distribution, which makes call-based modeling more complicated. We also show that a random walk process, which does not use call durations, can often be used for modeling the aggregate cell capacity. Furthermore, we highlight some applications of our results to improve secondary usage of licensed spectrum.

Primary user behavior in cellular networks and implications for dynamic spectrum access

Dynamic spectrum access approaches, which propose to opportunistically use underutilized portions of licensed wireless spectrum such as cellular bands, are increasingly being seen as a way to alleviate spectrum scarcity. However, before DSA approaches can be enabled, it is important that we understand the dynamics of spectrum usage in licensed bands. Our focus in this article is the cellular band. Using a unique dataset collected inside a cellular network operator, we analyze the usage in cellular bands and discuss the implications of our results on enabling DSA in these bands. One of the key aspects of our dataset is its scale-it consists of data collected over three weeks at hundreds of base stations. We dissect this data along different dimensions to characterize if and when spectrum is available, develop models of primary usage, and understand the implications of these results on DSA techniques such as sensing.

Reliable link maintenance in cognitive radio systems

Recently, secondary usage of spectrum has been considered in order to better exploit spectral resources and overcome the under-utilization of licensed spectrum. Since the licensed user still keeps primary access rights to its spectrum in such a secondary usage scenario, potential secondary users (SUs) have to vacate the spectrum in case the licensed user claims it. In order to maintain the quality of the secondary communication nevertheless, efficient mechanisms for link maintenance are needed. In this paper we present a general model for link maintenance in secondary usage scenarios. We state that the traditional way of adding redundancy to improve the communication not necessarily works in secondary usage scenarios. Furthermore we present performance results of a link maintenance approach applied to a secondary usage system based on opportunistic spectrum sharing, which verifies our assumptions

MiXiM: the physical layer an architecture overview

Simulating the physical layer of wireless communication remains a challenge. Communication standards like OFDM or MIMO systems go beyond the simple single narrow frequency band, single antenna model used in popular simulators. Yet, these technologies gain popularity, since they provide researchers with a plethora of possibilities that can be explored to invent new protocols or improve existing ones. However, building a detailed and sufficiently accurate model for such complex systems is a tremendous task that takes a lot of time. In this paper we present the physical layer model of MiXiM, which tackles this task. It provides the researcher with an easy to use interface to the wireless transmission medium. It models the wireless medium in all three dimensions (time, space and frequency) supporting the implementation of future wireless communication standards, but at the same time also supports easy modeling and simulation of traditional single frequency systems.

Double Hopping: A new approach for Dynamic Frequency Hopping in Cognitive Radio networks

One of the major challenges in designing cellular cognitive radio (CR) networks is the avoidance of secondary user (SU) interference to so called primary users (PUs) operating in the licensed bands. Usually, SU operation has to be interrupted periodically in order to detect PU activity and avoid the respective frequencies. Recently, dynamic frequency hopping (DFH) mechanisms have been suggested to enable reliable PU detection and continuous SU operation at the same time. Applying DFH in a multi-cell environment adds the challenge of mitigating co-channel interference (CCI). In this paper, we introduce a new DFH approach for cellular CR networks to allow reliable PU detection and continuous SU operation while avoiding CCI: double hopping (DH). We present a distributed frequency assignment heuristic for DH and compare it to the optimal assignment. We show that the performance of the sub-optimal distributed assignment is only slightly worse than the optimal performance, and, thus, outperforms existing distributed approaches by far.

Efficient QoS support for secondary users in cognitive radio systems [Dynamic Spectrum Management]

The fundamental condition for the legal approval of dynamic spectrum access approaches is the protection of the primary user. However, for dynamic spectrum access to become an attractive service reality, it is crucial to also ensure some quality of service support for the secondary user communication. In this article we discuss sensing-based opportunistic spectrum access approaches, in which primary user protection is achieved by a properly organized sensing process and secondary user communication reconfiguration. While the required reliability of the sensing process can be expressed in terms of rarely enough overlooking the primary user, we assume that the proper QoS for the secondary user is given by maintaining - with a given confidence level - a minimum bandwidth availability for the secondary user in spite of primary user dynamics. In this article we present an overview of approaches that might be used to achieve these objectives. In addition, we point out that both the sensing process and the secondary link maintenance (necessary to keep the required bandwidth in spite of reconfiguration due to detected primary users) require significant spectrum overhead. We identify and elaborate a fundamental trade-off in using these overheads in either sensing or link maintenance, and present examples of its optimization.

Experimental assessment of tradeoffs among spectrumsensing platforms

This paper reports experimental results comparing the performance of four platforms employed in spectrum sensing and dynamic spectrum access research: a sensing engine developed at imec and built around a prototype RFIC; the Universal Software Radio Peripheral (USRP) with the Iris software defined radio (SDR) solution; the TelosB sensor network platform; and the Wi-Spy low cost spectrum sensor solution targeted at the ISM band. We use experimental data to derive the receiver operating characteristics (ROC) of each of the four platforms. We observe that for low signal powers, narrow bandwidth signals, high shadowing, or stringent probability of false alarm (PFA) requirements tradeoffs among the platforms tested are most pronounced, whereas for high signal powers, large bandwidths, stable environments, and more flexible PFA requirements less expensive, commercial-off-the-shelf equipment performs sufficiently well.

About the practicality of using partially overlapping channels in IEEE 802.11 b/g networks

IEEE 802.11 WLANs are currently one of the most popular wireless technologies, but their immediate success results in dense deployments and high demand of user traffic. This in turn leads to decrease in throughput and poor spectrum utilization. Especially in the 2.4 GHz ISM band, where the spectrum is a very scarce resource, all available WLAN channels should be exploited in the best possible way to achieve higher utilization. One way to reach this goal is the usage of partially overlapping channels (POC). Most of the previous work related to POC is based on two major studies addressing 802.11 b, but none of them evaluates the POC behavior in the 802.11 g networks. Moreover, most of the previous results are based on simulations. The main contribution of this work is an experimental evaluation of POC in 802.11g networks. In this paper we confirm quantitatively that 802.11b reacts as expected from the previous studies, while 802.11 g reacts entirely different to the presence of adjacent channel interference. That leads to the conclusion that the usage of POC for 802.11g is not recommended.