Daniel Hollos

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.

Multi-rate Relaying for Performance Improvement in IEEE 802.11 WLANs

It is well known that the presence of nodes using a low data transmit rate has a disproportionate impact on the performance of an IEEE 802.11 WLAN. ORP is an opportunistic relay protocol that allows nodes to increase their effective transmit rate by replacing a low data rate transmission with a two-hop sequence of shorter range, higher data rate transmissions, using an intermediate node as a relay. ORP differs from existing protocols in discovering relays experimentally, by optimistically making frames available for relaying. Relays identify themselves as suitable relays by forwarding these frames. This approach has several advantages compared with previously proposed relay protocols: Most importantly, ORP does not rely on observations of received signal strength to infer the availability of relay nodes and transmit rates. We present analytic and simulation results showing that ORP improves the throughput by up to 40% in a saturated IEEE 802.11b network.

Applying Ad-Hoc Relaying to Improve Capacity, Energy Efficiency, and Immission in Infrastructure-Based WLANs

In classical infrastructure-based wireless systems such as access point-equipped wireless LANs, all mobile terminals communicate directly with the access point. In order to improve the capacity or energy efficiency of such systems, the use of mobile terminals as intermediate relays has been proposed. The rationale is that intermediate relays reduce the communication distance and hence the emitted power. Therefore, relaying could also reduce electromagnetic immission. To assess these potential benefits, we study the effectiveness of various relaying algorithms in a uniform, HiperLAN/2-based system model that has been amended by relaying functionality. These algorithms jointly select intermediate relay terminals and assign transmission power as well as modulation to mobile terminals. The energy efficiency of a point-to-point communication is indeed improved by relaying, however, this effect only marginally transfers to scenarios taking into account several terminals. Nevertheless, it is still possible to extend the lifetime of a network by taking into account available battery capacities.

Analysis of Load Dependency of Handover Strategies in Mobile Multiaccess Ambient Networks

Wireless operators incorporate multiradio access technologies aiming at expanding their customer base and benefiting from synergies of existing and planned infrastructure. The result is areas with overlapping radio access technologies, enabling users to select from a large pool of available connections, based on several criteria. In these increasingly complex scenarios the importance of handover decisions cannot be underestimated. We study three different handover decision strategies, focusing on the tradeoff between continuous connectivity, network utilization and performance, and the associated costs, and compare them to the optimal strategy. We motivate the need for a distributed algorithm, especially when considering the deployment of multimedia applications, and reflect on the effective network capacity as a function of the handover strategy employed and the permitted offline time for the active mobile nodes in the network.

A geometric derivation of the probability of finding a relay in multi-rate networks

Relaying can improve performance of a wireless network, especially when transmission modes with different distance/cost tradeoffs are available. Examples of such modes include data rates or transmission power. This paper geometrically analyzes the probability that a high-cost direct transmission can be replaced by a combination of low-cost relay transmissions. The main result of the analysis is a technology-agnostic characterization of a communication systems amenability to relaying strategies and some recommendations for how to structure such systems

A receiver based protecting protocol for wireless multi-hop networks

Nowadays most medium access protocols designed for wireless ad hoc networks are based on collision avoidance strategies like the CSMA/CA based IEEE 802.11 protocol. But these types of protocols are not designed for multi-hop scenarios -- the efficiency of the channel utilization is too low which results in, among others, large packet delays. One popular approach to increase the channel utilization is to reserve time slots along a transmission path, thus having a scheduled access. However, a major problem is interference from nearby nodes, although these nodes are not on the same route. This might lead to destruction of ongoing data receptions. In this paper we suggest a new reservation protocol, called JamTDMA. It offers protection against this effect by advertising the reservations in a larger neighborhood. We will show that this protocol allows to improve the rate of successfully received packets while assuring an upper bound for the end-to-end delay.

A Study of Handover Strategies for Mobile Multiaccess Ambient Networks

In a network environment where multi-radio access becomes the norm, choosing the most appropriate network interface and base station becomes essential for distributing radio resources optimally and fairly, and for allowing each node to take full advantage of its multiaccess capability. This paper discusses the issues arising when mobile nodes are equipped with three different radio access technologies and have contradicting goals to the network provider in a high density scenario. We simulated three different decision strategies aimed to resolve such contradicting goals and compared those results to the optimal solution attained from a linear programming model.

On centralized and distributed frequency assignment in cognitive radio based frequency hopping cellular networks

Frequency Assignment is an important approach to mitigate multi-cell interference in cellular systems. In this paper, we consider frequency hopping as one possible frequency assignment approach. In particular we focus our attention on cognitive radio cellular systems as one of the very promising future access technologies, taking IEEE 802.22 as an example. While the optimal frequency assignment for such a system is conceptually straightforward as well as computationally complex, we demonstrate that usage of distributed methods leads to a significant loss of assignment efficiency. In addition, we suggest means of mitigating this adverse effect.

Recognizing Moving Groups with Infrastructure Support

Detection of nearby nodes in wireless ad hoc networks requires periodic neighborhood scanning which is, in terms of energy, a rather expensive procedure: The nodes transmit the discovery packets and, furthermore, their reception circuits have to be active most of the time. Since distributed moving group detection algorithms rely on those nearby node-detection procedures, this kind of overhead seems unavoidable for them. However, in most of the scenarios some fixed infrastructure co-exist which may support the detection process. In this paper we propose a method that uses the infrastructure to gain hints about moving groups and triggers the distributed process only if there is some degree of confidence that such a group exists. This reduces the overhead of the neighborhood scans by approximately 90% translating e.g. to longer lifetime for the battery-driven nodes

Routing groups in ambient networking

In this paper we discuss aggregated mobility patterns and physical proximity of nodes within Ambient Networks, where an Ambient Network can be described as a network that integrates heterogeneous nodes and access techniques. We illustrate how awareness of node mobility patterns can be used to identify groups of nodes moving together, and how this grouping can be used to decrease signaling overhead, for example the signaling associated with a mobility event, and increase transmission efficiency. First, we describe the architectural and naming issues associated with this concept and discuss a number of mobility optimizations that can be applied to these moving networks. Further, we introduce algorithms that can recognize the presence of a routing group to enable the use of routing and mobility optimizations. Finally, we assess the performance and benefits of the routing group approach by means of simulation.