James Gross

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

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

Dynamic resource allocation in OFDM systems: an overview of cross-layer optimization principles and techniques

Recently, a lot of research effort has been spent on cross-layer system design. It has been shown that cross-layer mechanisms (i.e., policies) potentially provide significant performance gains for various systems. In this article we review several aspects of cross-layer system optimization regarding wireless OFDM systems. We discuss basic optimization models and present selected heuristic approaches realizing cross-layer policies by means of dynamic resource allocation. Two specific areas are treated separately: models and dynamic approaches for single transmitter/receiver pairs (i.e., a point-to-point communication scenario) as well as models and approaches for point-to-multipoint communication scenarios (e.g., the downlink of a wireless cell). This article provides basic knowledge in order to investigate future OFDM cross-layer-optimization issues

Modeling and Tools for Network Simulation

A crucial step during the design and engineering of communication systems is the estimation of their performance and behavior; especially for mathematically complex or highly dynamic systems network simulation is particularly useful. This book focuses on tools, modeling principles and state-of-the art models for discrete-event based network simulations, the standard method applied today in academia and industry for performance evaluation of new network designs and architectures. The focus of the tools part is on two distinct simulations engines: OmNet++ and ns-3, while it also deals with issues like parallelization, software integration and hardware simulations. The parts dealing with modeling and models for network simulations are split into a wireless section and a section dealing with higher layers. The wireless section covers all essential modeling principles for dealing with physical layer, link layer and wireless channel behavior. In addition, detailed models for prominent wireless systems like IEEE 802.11 and IEEE 802.16 are presented. In the part on higher layers, classical modeling approaches for the network layer, the transport layer and the application layer are presented in addition to modeling approaches for peer-to-peer networks and topologies of networks. The modeling parts are accompanied with catalogues of model implementations for a large set of different simulation engines. The book is aimed at master students and PhD students of computer science and electrical engineering as well as at researchers and practitioners from academia and industry that are dealing with network simulation at any layer of the protocol stack.

Performance analysis of dynamic OFDMA systems with inband signaling

Within the last decade, the orthogonal frequency- division multiplexing (OFDM) transmission scheme has become part of several standards for wireless systems. Today, OFDM is even a candidate for fourth-generation wireless systems. It is well known that dynamic OFDMA systems potentially increase the spectral efficiency. They exploit diversity effects in time, space, and frequency by assigning system resources periodically to terminals. Informing the terminals about new assignments creates a signaling overhead. Up to now, this overhead has not been taken into account in studies on dynamic orthogonal frequency-division multiplexing access (OFDMA) systems. Yet this is crucial for a realistic notion of the performance achieved by dynamic approaches. In this paper, we close this gap. We introduce two forms of representing the signaling information and discuss how these affect system performance. The study of the signaling impact on the performance is conducted for an exemplary dynamic approach. We find that the throughput behavior of dynamic OFDMA systems is significantly influenced by the signaling overhead. In many situations, neglecting the overhead leads to wrong performance conclusions. Also, the performance difference between dynamic and static schemes is now much more sensible to the specific parameter set of the transmission scenario (e.g., frame length, subcarrier number, etc.). This leads to the proposal of access points which should adapt certain system parameters in order to provide optimal performance.

Optimal power masking in soft frequency reuse based OFDMA networks

Soft frequency reuse is a strong tool for co-channel interference mitigation in cellular OFDMA/LTE networks. The performance of such networks significantly depends on the configuration of the power masks that implement the soft frequency reuse patterns. In this paper, we investigate the performance of different power mask configurations against the optimal case, in which a central entity optimally distributes power and resource blocks among the users of the network. It is shown that large differences exist between the performance of different mask types and the optimal case in both, the overall cell throughput, as well as the cell-edge user performance.1

Subcarrier allocation for variable bit rate video streams in wireless OFDM systems

Wireless OFDM systems have attractive means for adapting wireless transmission to a given situation: one possibility is to assign a varying number of subcarriers to wireless terminals for downlink communication. Deciding how many and which subcarriers to assign to a given terminal is a difficult problem. This paper concentrates on deciding how many: we use the relative length of a terminals queue in an access point to determine this number. Applying this scheme to the transmission of homogeneous MPEG-4 videos, we obtain a significant capacity increase compared to nonadaptive subcarrier allocation schemes.

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.

Robust Clustering of Ad-Hoc Cognitive Radio Networks under Opportunistic Spectrum Access

The time and space varying nature of channel availability among cognitive radio nodes challenges connectivity and robustness of ad-hoc cognitive radio networks. Clustering of neighbouring cognitive radio nodes is a suitable approach to address this challenge. A cluster utilizes the same channel for payload communication among the nodes. As a consequence, clustering enables cooperative spectrum sensing, supports a coordinated channel switching and simplifies routing in ad-hoc cognitive radio networks. However, the sudden appearance of primary nodes can lead to the loss of connectivity within a cluster or between clusters. This impact can be mitigated to some extent by the way clusters are formed. In this work we discuss a distributed, low-complexity clustering algorithm that emphasizes the robustness of clusters by improving inter- and intra-cluster connectivity. The algorithm is proven to converge fast while numerical evaluation shows a significant improvement of robustness compared to related work.

Throughput study for a dynamic OFDM-FDMA system with inband signaling

OFDM-FDMA systems provide the flexibility to support simultaneous downlink data transmissions to different terminals. By dynamically assigning different sets of subcarriers to different terminals they also have the potential to react to fast changing attenuation states of wireless channels. It has been shown that hence dynamic OFDM-FDMA systems can improve various transmission metrics such as throughput or required power. However, these dynamic systems require a signaling mechanism informing each terminal prior to the data transmission itself which subcarriers they have been assigned. We study the dependency between overhead caused by the signaling system and number of subcarriers used in the system for a varying number of terminals in the cell. We find that in terms of resulting throughput per terminal there exists an optimal number of subcarriers into which the bandwidth should be split. This optimal number depends on the setting (especially, terminal number) and provides, if used, a significant performance increase compared to using a fixed number of subcarriers.