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  • 1.
    Abusubaih, Murad
    et al.
    Telecommun. Networks Group, Technische Univ. Berlin.
    Gross, James
    Telecommun. Networks Group, Technische Univ. Berlin.
    Wiethoelter, Sven
    Telecommun. Networks Group, Technische Univ. Berlin.
    Wolisz, Adam
    Telecommun. Networks Group, Technische Univ. Berlin.
    On Access Point Selection in IEEE 802.11 Wireless Local Area Networks2006In: 31st IEEE Conference on Local Computer Networks, Proceedings 2006, IEEE conference proceedings, 2006, 879-886 p.Conference paper (Refereed)
    Abstract [en]

    In wireless local area networks often a station can potentially associate with more than one access point. Therefore, a relevant question is which access point to select "best" from a list of candidate ones. In IEEE 802.11, the user simply associates to the access point with the strongest received signal strength. However, this may result in a significant load imbalance between several access points, as some accommodate a large number of stations while others are lightly loaded or even idle. Moreover, the multi-rate flexibility provided by several IEEE 802.11 variants can cause low bit rate stations to negatively affect high bit rate ones and consequently degrade the overall network throughput. This paper investigates the various aspects of "best" access point selection for IEEE 802.11 systems. In detail, we first derive a decision metric the selection can be based on. Using this metric we propose two new selection mechanisms which are decentralized in the sense that the decision is performed by each station, given appropriate status information of each access point. In fact, only few bytes of status information have to be added to the beacon and probe response frames which does not impose significant overhead. In addition, we show that our mechanism improves station quality of service and better utilizes network resources compared to the conventional one implemented today in IEEE 802.11 devices.

  • 2.
    Abusubaih, Murad
    et al.
    Technische Universit¨at Berlin.
    Gross, James
    Technische Universit¨at Berlin.
    Wolisz, Adam
    Technische Universit¨at Berlin.
    An Inter-Access Point Coordination Protocol for Dynamic Channel Selection in IEEE802.11 Wireless LANs2007Conference paper (Refereed)
    Abstract [en]

    This paper presents and evaluates an Inter-Access Point Coordination protocol for dynamic channel selection in IEEE 802.11 WLANs. It addresses an open issue for the implementation of many distributed and centralized dynamic channel selection policies proposed to mitigate interference problems in Wireless LANs (WLANs). The presented protocol provides services to a wide range of policies that require different levels of coordination among APs by enabling them to actively communicate and exchange information. An Intra-Cell protocol that enables interaction between the AP and its accommodated stations to handle channel switching within the same cell is also presented.

  • 3.
    Abusubaih, Murad
    et al.
    TU Berlin.
    Wiethoelter, Sven
    TU Berlin.
    Gross, James
    Wolisz, Adam
    TU Berlin.
    A New Access Point Selection Policy for Multi-Rate IEEE 802.11 WLANs2008In: International Journal of Parallel, Emergent and Distributed Systems, ISSN 1744-5760, E-ISSN 1744-5779, Vol. 23, no 4, 291-307 p.Article in journal (Refereed)
    Abstract [en]

    In wireless local area networks, often a station can potentially associate with more than one access point (AP). Therefore, a relevant question is which AP to select 'best' from a list of candidate ones. In IEEE 802.11, the user simply associates to the AP with the strongest received signal strength. However, this may result in a significant load imbalance between several APs. Moreover, the multi-rate flexibility provided by several IEEE 802.11 variants can cause low bit rate stations to negatively affect high bit rate ones and consequently degrade the overall network throughput. This paper investigates the various aspects of 'best' AP selection for IEEE 802.11 systems. In detail, we first derive a new decision metric which can be used for AP selection. Using this metric, we propose two new selection mechanisms which are decentralised in the sense that the decision is performed by each station, given appropriate status information of each AP. In fact, only few bytes of status information have to be added to the Beacon and Probe Response frames which does not impose significant overhead. We show that our mechanism improves mean quality of service of all stations and better utilises network resources compared to the conventional one implemented today in IEEE 802.11 devices. Also, the schemes are appealing in terms of stability and provide their performance improvement even for denser or lighter network configurations.

  • 4. Ashraf, Shehzad Ali
    et al.
    Wang, Y.-P. Eric
    Eldessoki, Sameh
    Holfeld, Bernd
    Parruca, Donald
    Serror, Martin
    Gross, James
    KTH, School of Electrical Engineering (EES), Information Science and Engineering.
    From Radio Design to System Evaluations for Ultra-Reliable and Low-Latency Communication2017In: European Wireless 2017 - 23rd European Wireless Conference, 2017, 8011336Conference paper (Refereed)
    Abstract [en]

    Ultra-reliable and low-latency communication is the enabler for many new use cases, including wireless industrial automation. Fulfilling varying requirements of these use cases demands a flexible radio design. To address this, a holistic approach needs to be adopted. Therefore, this paper presents the radio access concepts affecting the communication reliability and latency, and comprehensively evaluates link and system level considerations through simulations. In particular, we describe the choice of suitable modulation and coding schemes, and discuss the impact of different numerologies and waveform candidates. We also point out the key principles for radio frame design to reduce the end-to-end latency. The presented concepts are then used to evaluate the performance at system level for an industrial scenario. It is shown that by an appropriate design of the radio interface for 5G system, the required low-latency and high reliability for industrial applications and many other use cases can be achieved.

  • 5. Berger, Andre
    et al.
    Gross, James
    RWTH Aachen University, Germany.
    Harks, Tobias
    The k–Constrained Bipartite Matching Problem: Approximation Algorithms and Applications to Wireless Networks2010In: Proceedings IEEE INFOCOM, 2010, IEEE conference proceedings, 2010, 1-9 p.Conference paper (Refereed)
    Abstract [en]

    In communication networks, resource assignment problems appear in several different settings. These problems are often modeled by a maximum weight matching problem in bipartite graphs and efficient matching algorithms are well known. In several applications, the corresponding matching problem has to be solved many times in a row as the underlying system operates in a time-slotted fashion and the edge weights change over time. However, changing the assignments can come with a certain cost for reconfiguration that depends on the number of changed edges between subsequent assignments. In order to control the cost of reconfiguration, we propose the k-constrained bipartite matching problem for bipartite graphs, which seeks an optimal matching that realizes at most k changes from a previous matching. We provide fast approximation algorithms with provable guarantees for this problem. Furthermore, to cope with the sequential nature of assignment problems, we introduce an online variant of the k-constrained matching problem and derive online algorithms that are based on our approximation algorithms for the k-constrained bipartite matching problem. Finally, we establish the applicability of our model and our algorithms in the context of OFDMA wireless networks finding a significant performance improvement for the proposed algorithms.

  • 6.
    Berger, Andre
    et al.
    Maastricht University.
    Harks, Tobias
    Maastricht University.
    Gross, James
    KTH, School of Electrical Engineering (EES), Communication Theory.
    Tenbusch, Simon
    RWTH Aachen University.
    Constrained Resource Assignments: Fast Approximations and Applications in Wireless Networks2015In: Management science, ISSN 0025-1909, E-ISSN 1526-5501Article in journal (Other academic)
    Abstract [en]

    Resource assignment problems occur in a vast variety of applications, from scheduling problems over image recognition to communication networks. Often these problems can be modeled by a maximum weight matching problem in (bipartite) graphs or generalizations thereof, and efficient and practical algorithms are known for these problems. Although in some of the applications an assignment of the resources may be needed only once, in many of these applications, the assignment has to be computed more often for different scenarios. In that case it is often essential that the assignments can be computed very fast. Moreover, implementing different assignments in different scenarios may come with a certain cost for the reconfiguration of the system. In this paper, we consider the problem of determining optimal assignments sequentially over a given time horizon, where consecutive assignments are coupled by constraints that control the cost of reconfiguration. We develop fast approximation and online algorithms for this problem with provable approximation guarantees and competitive ratios. Moreover, we present an extensive computational study about the applicability of our model and our algorithms in the context of orthogonal frequency division multiple access (OFDMA) wireless networks, finding a significant performance improvement for the total bandwidth of the system using our algorithms. For this application (the downlink of an OFDMA wireless cell) , the run time of matching algorithms is extremely important, having an acceptable range of a few milliseconds only. For the considered realistic instances, our algorithms perform extremely well: the solution quality is, on average, within a factor of 0.8–0.9 of optimal off-line solutions, and the running times are at most 5 ms per phase even in the worst case. Thus, our algorithms are well suited to be applied in the context of OFDMA systems.

  • 7.
    Bohge, Mathias
    et al.
    TU Berlin.
    Gross, James
    Telecommunication Networks Group (Fachgebiet TKN), TU Berlin.
    Meyer, Michael
    Ericsson Research.
    Wolisz, Adam
    TU Berlin.
    A New Optimization Model for Dynamic Power and Sub-Carrier Allocations in Packet-Centric OFDMA Cells2007In: Frequenz, ISSN 0016-1136, Vol. 61, no 1-2, 35-38 p.Article in journal (Refereed)
    Abstract [en]

    It is well known that applying dynamic resource allocation to down-link transmissions of OFDMAsystems provides a significant performance increase, by taking advantage of diversity effects. In order toquantify the maximum possible gain achievable by applying dynamic mechanisms, several optimizationproblems have been suggested for studying a dynamic system's optimal behaviour. However, so far theseoptimization approaches do not take sophisticated system requirements into account, as there are differentpacket-arrival processes, buffering constraints, scheduling policies, as well as QoS requirements per serviceclass. In this paper we present a new optimization model that is based on a packet-centric system view andincludes the system requirements mentioned above. We compare the performance results of the conventionaland the new approach and study the impact of dynamic power allocation in both cases.

  • 8.
    Bohge, Mathias
    et al.
    TU Berlin.
    Gross, James
    Telecommunications Networks Group (TKN), Technical University Berlin.
    Meyer, Michael
    Ericsson Research.
    Wolisz, Adam
    TU Berlin.
    Dynamic Resource Allocation in OFDM Systems: An Overview of Cross-Layer Optimization Principles and Techniques2007In: IEEE Network, ISSN 0890-8044, Vol. 21, no 1, 53-59 p.Article in journal (Refereed)
    Abstract [en]

    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.

  • 9.
    Bohge, Mathias
    et al.
    TU Berlin.
    Gross, James
    RWTH Aachen University, UMIC Research Centre,.
    Wolisz, Adam
    TU Berlin.
    Optimal Soft Frequency Reuse and Dynamic Sub-carrier Assignments in Cellular OFDMA Networks2010In: European transactions on telecommunications, ISSN 1124-318X, E-ISSN 1541-8251, Vol. 21, no 8, 704-713 p.Article in journal (Refereed)
    Abstract [en]

    Soft frequency reuse (SFR) is a common technique for co-channel interference (CCI) mitigation in cellular OFDMA networks. The performance of such networks significantly depends on the configuration of the power profiles that implement the soft frequency reuse patterns. In this paper, we investigate the performance of static soft frequency reuse by comparing it against the optimal case, in which a central entity optimally distributes power among the users of the network. It is shown that there is a significant performance gap between both approaches, which needs to be filled by adaptive SFR mechanisms. Moreover, we show that the achievable gain of static SFR is small in a system that is able to optimally decide on terminal/sub-carrier assignments.

  • 10.
    Bohge, Mathias
    et al.
    Telecommunication Networks Group, TU Berlin.
    Gross, James
    Telecommunication Networks Group, TU Berlin.
    Wolisz, Adam
    Telecommunication Networks Group, TU Berlin.
    The Potential of Dynamic Power and Sub-carrier Assignments in Multi-User OFDM-FDMA Cell2005In: Global Telecommunications Conference, 2005. GLOBECOM '05. IEEE (Volume 5 ), IEEE conference proceedings, 2005, 2932-2936 p.Conference paper (Refereed)
    Abstract [en]

    OFDM systems are known to overcome the impairment of the wireless channel by splitting the given system bandwidth into parallel sub-carriers, on which data symbols can be transmitted simultaneously. This enables the possibility of enhancing the system's performance by deploying adaptive (dynamic) mechanisms, namely power and modulation adaption and dynamic sub-carrier assignments. In multi-user communication systems (OFDM-FDMA), these mechanisms can be used to achieve a level of system fairness ensuring that each terminal receives at least an environment-specific minimum amount of data per down-link phase. However, it has been doubted by multiple previous investigations that dynamic power adaption provides enough performance gain in order to be applied in such systems, as it increases the computational load significantly. In this study we discuss the performance gain due to the different approaches and show that in specific communication scenarios enabling a dynamic power distribution provides a significant performance increase compared to dynamic schemes without power adaption.

  • 11. Bohge, Mathias
    et al.
    Gross, James
    Wolisz, Mathias
    Optimal Power Masking in Soft Frequency Reuse based OFDMA Networks2009Conference paper (Refereed)
    Abstract [en]

    Soft frequency reuse is a strong tool for co-channel interference mitigation in cellular OFDMAILTE 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.

  • 12.
    Dombrowski, Christian
    et al.
    RWTH Aachen University.
    Gross, James
    KTH, School of Electrical Engineering (EES), Communication Theory.
    EchoRing: A Low-Latency, Reliable Token-passing MAC Protocol for Wireless Industrial Networks2015In: European Wireless 2015; 21th European Wireless Conference; Proceedings of, IEEE conference proceedings, 2015, 1-8 p.Conference paper (Refereed)
    Abstract [en]

    Spurred by recent industrial trends, such as factoryautomation or phase synchronization in the smart grid, thereis a significant interest for wireless industrial networks lately.In contrast to traditional applications, the focus is on carryingout communication at very short latencies together with highreliabilities. Meeting such extreme requirements with wirelessnetworks is challenging. A potential candidate for such a networkis a token-passing protocol, as it allows to bound latencies.However, it lacks mechanisms to cope with the dynamics ofwireless channels. In this paper, we present EchoRing, a novelwireless token-passing protocol. Cooperative communication andan improved fault tolerance allow this decentralized protocol tosupport industrial applications over wireless networks. Based onexperimental results, we demonstrate the suitability of EchoRingto support demands of industrial applications. EchoRing outper-forms other schemes by several orders of magnitude in terms ofreliability for latencies of and below10ms.

  • 13. Dombrowski, Christian
    et al.
    Gross, James
    KTH, School of Electrical Engineering (EES), Communication Theory.
    EchoRing: A low-latency, reliable token-passing MAC protocol for wireless industrial networks2015In: Proceedings of 21st European Wireless Conference, European Wireless 2015, Institute of Electrical and Electronics Engineers (IEEE), 2015Conference paper (Refereed)
    Abstract [en]

    Spurred by recent industrial trends, such as factory automation or phase synchronization in the smart grid, there is a significant interest for wireless industrial networks lately. In contrast to traditional applications, the focus is on carrying out communication at very short latencies together with high reliabilities. Meeting such extreme requirements with wireless networks is challenging. A potential candidate for such a network is a token-passing protocol, as it allows to bound latencies. However, it lacks mechanisms to cope with the dynamics of wireless channels. In this paper, we present EchoRing, a novel wireless token-passing protocol. Cooperative communication and an improved fault tolerance allow this decentralized protocol to support industrial applications over wireless networks. Based on experimental results, we demonstrate the suitability of EchoRing to support demands of industrial applications. EchoRing outperforms other schemes by several orders of magnitude in terms of reliability for latencies of and below 10 ms.

  • 14.
    Dombrowski, Christian
    et al.
    RWTH Aachen University.
    Gross, James
    KTH, School of Electrical Engineering (EES), Communication Theory.
    EchoRing- Meeting Hard Real-time Constraints by Decentralized Wireless Networks2015Conference paper (Refereed)
    Abstract [en]

    Given the rising demand for wireless solutions in the area of machine-to-machine communication, we present the novel EchoRing protocol. It is designed to serve the communication needs of industrial applications, while being optimized for the wireless channel specifically. Directly taking known principles of tethered communication to the wireless domain is likely to yield degraded performance results. Additional techniques have to be added to make known principles be able to master the challenges of wireless channel dynamics. On the other hand, the majority of currently existing wireless communication standards are developed to allow mobility on the last hop of a transmission path that originates in the Internet or a local home network. Hence, the focus is on supporting the best-effort paradigm of the data streams. However, in industrial environments this best-effort paradigm gets replaced by the need to steadily achieve very high reliabilities at very short deadlines.

    In this demonstration, we will show how industrial applications can be interconnected wirelessly despite the drawbacks of the wireless channel. The experimental setup allows to compare different medium access control protocols under varying conditions.

     

  • 15. Dombrowski, Christian
    et al.
    Gross, James
    KTH, School of Electrical Engineering (EES), Communication Theory.
    Reducing Outage Probability over Wireless Channels Under Hard Real-Time Constraints2011Conference paper (Refereed)
  • 16. Dombrowski, Christian
    et al.
    Junges, Sebastian
    Katoen, Joost Pieter
    Gross, James
    KTH, School of Electrical Engineering (EES), Communication Theory.
    Model-Checking Assisted Protocol Design for Ultra-Reliable Low-Latency Wireless Networks2016Conference paper (Refereed)
    Abstract [en]

    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 tokenpassing 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.

  • 17. Dombrowski, Christian
    et al.
    Petreska, Neda
    Görtzen, Simon
    Schmeink, Anke
    Gross, James
    KTH, School of Electrical Engineering (EES), Communication Theory.
    Energy-efficient multi-hop transmission for machine-to-machine communications2013In: 2013 11th International Symposium and Workshops on Modeling and Optimization in Mobile, Ad Hoc and Wireless Networks, WiOpt 2013, 2013, 341-348 p.Conference paper (Refereed)
    Abstract [en]

    Emerging machine-to-machine communication scenarios are envisioned to deal with more stringent quality-of-service demands. This relates mainly to outage and latency requirements, which are for example for safety-critical messages quite different than for traditional applications. On the other hand, it is widely accepted that machine-to-machine communication systems need to be energy-efficient because of the widespread use of battery-powered devices, but also due to their huge deployment numbers. In this paper, we address these issues with respect to multi-hop transmissions. Specifically, we deal with minimizing the consumed energy of transmitting a packet with end-to-end outage and latency requirements. We account for the cases in which the system can utilize solely average channel state information, or in addition obtain and profit from instantaneous channel state information. The developed solution is based on convex optimization. It is shown numerically that despite accounting for the energy consumption of acquiring instantaneous channel state information, especially as the outage and latency requirements become tough, it is by up to 100 times more energy efficient to convey a packet with instantaneous than with average channel state information.

  • 18. Eisenblaetter, Andreas
    et al.
    Geerdes, Hans-Florian
    Gross, James
    Punyal, Oscar
    Schweiger, Jonas
    A Two-Stage Approach to WLAN Planning: Detailed Performance Evaluation Along the Pareto Frontier2010In: Proceedings of the 8th International Symposium on Modeling and Optimization in Mobile, Ad Hoc and Wireless Networks (WiOpt), 2010, 2010, 227-236 p.Conference paper (Refereed)
    Abstract [en]

    This paper proposes an efficient solution to the open problem of network planning for large-scale WLAN deployments. WLAN performance is governed by the Csma-Ca protocol, whose dynamic effects are difficult to capture. Accurate performance evaluation depends on simulations and takes time. A detailed analysis of dozens candidate designs with varying Ap positions and channel assignments during network planning is therefore infeasible. In our solution, we first identify few good candidate designs using a multi-criteria optimization model, which features notions of cell overlap and station throughput. These candidate designs are taken from the corresponding Pareto frontier. In the second step, we evaluate the performance of the candidate designs by means of simulations. We apply our method to a realistic, large-scale planning scenario for an indoor office environment. The detailed simulations reveal important characteristics of the candidate designs that are not captured by the optimization model. The resulting performance differs significantly across the candidate designs. Hence, this approach successfully combines the benefits of mathematical optimization and simulations, yet avoiding their individual drawbacks.

  • 19.
    Forssell, Henrik
    et al.
    KTH, School of Electrical Engineering (EES), Communication Theory. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Thobaben, Ragnar
    KTH, School of Electrical Engineering (EES), Communication Theory. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Gross, James
    KTH, School of Electrical Engineering (EES), Communication Theory. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Skoglund, Mikael
    KTH, School of Electrical Engineering (EES), Communication Theory. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Feature-Based Multi-User Authentication for Parallel Uplink Transmissions2016In: 2016 9TH INTERNATIONAL SYMPOSIUM ON TURBO CODES AND ITERATIVE INFORMATION PROCESSING (ISTC), IEEE, 2016, 355-359 p.Conference paper (Refereed)
    Abstract [en]

    We study a multi-user up-link scenario where an attacker tries to impersonate the legitimate transmitters. We present a new framework for deriving a posteriori attack probabilities from the channel observations at the access point, which enables fast intrusion detection and authentication at the physical layer and can be exploited to reduce the security overhead by offtoading higher-layer authentication schemes. This is highly relevant for delay-sensitive applications that are targeted in 5G where the security overhead may limit the real-time performance. We take a factor-graph approach that can easily be extended to take into account other features, channel models, and radio access schemes. While related works only consider single-link scenarios, the multi-user approach in this paper allows us to exploit the cross-channel correlation of the large-scale fading parameters that is due to the propagation environment for improving the detection performance. As numerical results show, especially for slowly changing channels with high correlation our approach provides significant performance gains.

  • 20.
    Ghauch, Hadi
    et al.
    KTH, School of Electrical Engineering (EES), Communication Theory. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Ur Rahman, Mahboob
    KTH, School of Electrical Engineering (EES), Communication Theory. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Imtiaz, Sahar
    KTH, School of Electrical Engineering (EES), Communication Theory. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Gross, James
    KTH, School of Electrical Engineering (EES), Communication Theory. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Coordination and Antenna Domain Formation in Cloud-RAN Systems2016In: 2016 IEEE International Conference on Communications, ICC 2016, Institute of Electrical and Electronics Engineers (IEEE), 2016, 7511264Conference paper (Refereed)
    Abstract [en]

    We study here the problem of Antenna Domain Formation (ADF) in cloud RAN systems, whereby multiple remote radio-heads (RRHs) are each to be assigned to a set of antenna domains (ADs), such that the total interference between the ADs is minimized. We formulate the corresponding optimization problem, by introducing the concept of interference coupling coefficients among pairs of radio-heads. We then propose a low-overhead algorithm that allows the problem to be solved in a distributed fashion, among the aggregation nodes (ANs), and establish basic convergence results. Moreover, we also propose a simple relaxation to the problem, thus enabling us to characterize its maximum performance. We follow a layered coordination structure: after the ADs are formed, radio-heads are clustered to perform coordinated beamforming using the well known Weighted-MMSE algorithm. Finally, our simulations show that using the proposed ADF mechanism would significantly increase the sum-rate of the system (with respect to random assignment of radio-heads).

  • 21. Gross, James
    Admission Control based on OFDMA Channel Transformations2009In: World of Wireless, Mobile and Multimedia Networks & Workshops, 2009. WoWMoM 2009. IEEE International Symposium on a, IEEE conference proceedings, 2009, 1-11 p.Conference paper (Refereed)
    Abstract [en]

    It is well known that channel-dependent OFDMA resource assignment algorithms provide a significant performance improvement compared to static (i.e. channel-unaware) approaches. Such dynamic algorithms constantly adapt resource assignments to current channel states according to some objective function. Due to these dynamics, it is difficult to predict the resulting performance for such schemes given a certain scenario (characterized by the number of terminals in the cell and their average channel gains). Hence, previous work on admission control for OFDMA systems neglects the performance improvement from channel-dependent resource assignments and bases analysis on the average channel gains instead. In this paper we provide for the first time an analytical framework for admission control in OFDMA systems applying channel-dependent resource assignments. The framework is based on fundamental transformations of the channel gains caused by the channel-dependent assignment algorithms. We provide closed-form expressions for these transformations and derive from them probability functions for the rate achieved per terminal and frame. These functions can then be used for admission control as demonstrated in this paper for Voice-over-IP streams in IEEE 802.16e systems.

  • 22.
    Gross, James
    Mobile Network Performance Group.
    Scheduling with Outdated CSI: Effective Service Capacities of Optimistic vs. Pessimistic Policies2012In: 2012 IEEE 20th International Workshop on Quality of Service (IWQoS), IEEE conference proceedings, 2012, 1-9 p.Conference paper (Refereed)
    Abstract [en]

    The concept of the effective service capacity is an analytical framework for evaluating QoS-constrained queuing performance of communication systems. Recently, it has been applied to the analysis of different wireless systems like point-to-point systems or multi-user systems. In contrast to previous work, we consider in this work slot-based systems where a scheduler determines a packet size to be transmitted at the beginning of the slot. For this, the scheduler can utilize outdated channel state information. Based on a threshold error model, we derive the effective service capacity for different scheduling strategies that the scheduler might apply. We show that even slightly outdated channel state information leads to a significant loss in capacity in comparison to an ideal system with perfect channel state information available at the transmitter. This loss depends on the risk-level the scheduler is willing to take which is represented by an SNR margin. We show that for any QoS target and average link state there exists an optimal SNR margin improving the maximum sustainable rate. Typically, this SNR margin is around 3 dB but is sensible to the QoS target and average link quality. Finally, we can also show that adapting to the instantaneous channel state only pays off if the correlation between the channel estimate and the channel state is relatively high (with a coefficient above 0.9).

  • 23.
    Gross, James
    et al.
    KTH, School of Electrical Engineering (EES), Communication Theory.
    Alvarez, Pablo
    Wolisz, Adam
    The Signaling Overhead in Dynamic OFDMA Systems: Reduction by Exploiting Frequency Correlation2007In: Communications, 2007. ICC '07. IEEE International Conference on, IEEE conference proceedings, 2007, 5143-5148 p.Conference paper (Refereed)
    Abstract [en]

    Dynamic OFDMA systems provide a significant performance gain compared to static OFDM approaches. In reality this gain is reduced by an overhead due to signaling. Previous work has shown that this loss is particularly high if the number of sub-carriers is large. In this paper we present two approaches to reduce this overhead by exploiting the correlation in frequency. These two schemes are shown to provide a significant reduction of the signaling overhead.

  • 24.
    Gross, James
    et al.
    KTH, School of Electrical Engineering (EES), Communication Theory.
    Emmelmann, Marc
    Punyal, Oscar
    Wolisz, Adam
    Dynamic Single-User OFDM Adaptation for IEEE 802.11 Systems2007In: Proceedings of ACM/IEEE International Symposium on Modeling, Analysis and Simulation of Wireless and Mobile Systems 2007 (MSWIM 2007), 2007Conference paper (Refereed)
    Abstract [en]

    Earlier paper have demonstrated that the achievable throughput of OFDM systems can benefit significantly from individual modulation/transmit power selection on a per sub-carrier basis according to the actual gain of individual sub-carriers (so called dynamic OFDM scheme). Usage of such approach requires, however, providing support for additional functionality like: acquisition of the subcarrier gains, signaling of the used modulation types between the sender and receiver, etc. Therefore dynamic OFDM is actively pursued for future radio interfaces, rather then considered as extension of existing OFDM based standards. In this paper we present for the first time a proposal how the widely accepted IEEE 802.11a/g systems might be extended to support the dynamic OFDM in a singleuser (point-to-point) setting while assuring backward compatibility. We address these issues by a) presenting a set of protocol modifications; and b) a performance evaluation of the suggested extension (referred further on to as single-user 802.11 DYN mode) demonstrating the potential of performance improvement.

  • 25.
    Gross, James
    et al.
    Rhein Westfal TH Aachen, UMIC Res Ctr, Mobile Network Performance Grp.
    Emmelmann, Marc
    TU Berlin.
    Puñal, Oscar
    RWTH Aachen University.
    Wolisz, Adam
    TU Berlin.
    Enhancing IEEE 802.11a/n with Dynamic Single-User OFDM Adaptation2009In: Performance evaluation (Print), ISSN 0166-5316, Vol. 66, no 3-5, 240-257 p.Article in journal (Refereed)
    Abstract [en]

    Earlier paper have demonstrated that the achievable throughput of OFDM systems can benefit significantly from individual modulation/transmit power selection on a per sub-carrier basis according to the actual gain of individual sub-carriers (so called dynamic OFDM scheme). Usage of such an approach requires, however, providing support for additional functionalities such as: acquisition of the sub-carrier gains, signaling of the modulation types used between sender and receiver, etc. Therefore dynamic OFDM is actively pursued for future radio interfaces, rather than considered as extension of existing OFDM based standards. In this paper we introduce a proposal on how the widely accepted IEEE 802.11a/g systems as well as the emerging IEEE 802.11n system might be extended to support the dynamic OFDM in a single-user (point-to-point) setting. The presented approach guarantees backward compatibility to legacy devices. We address these issues by presenting (a) a set of protocol modifications required to incorporate dynamic OFDM in 802.11a/g/n; and (b) a performance evaluation of the suggested extension (referred to further on as single-user 802.11 DYN mode). Although 802.11n already includes advanced MAC and PHY features, i.e., frame aggregation and MIMO transmissions, our performance evaluation demonstrates that a further improvement is achievable by incorporating dynamic OFDM.

  • 26. Gross, James
    et al.
    Fitzek, Frank
    Wolisz, Adam
    Chen, Bing
    Gruenheid, Rainer
    Framework for Combined Optimization of DL and Physical Layer in Mobile OFDM Systems2001In: Proc. of the 6th International OFDM-Workshop (InOWo) 2001, Hamburg, 2001Conference paper (Refereed)
    Abstract [en]

    Wide-band mobile communication systems are based on the Orthogonal Frequency Division Multiplexing (OFDM) transmission technique for several reasons [1]. The objective of this paper is the design of flexible Data Link Control (DLC) protocols, which combine the information of the time variant radio channel (described by the channel transfer function) and the incoming data streams from different applications with both, constant and variable data rates. The goal is to fulfil the quality of service (QoS) requirements in terms of priority and throughput for all wireless terminals and to improve the average system throughput.

  • 27.
    Gross, James
    et al.
    IEEE, Germany .
    Geerdes, Hans-Florian
    TU Berlin.
    Karl, Holger
    Universitaet Paderborn.
    Wolisz, Adam
    TU Berlin.
    Performance Analysis of Dynamic OFDMA Systems with Inband Signaling2006In: IEEE Journal on Selected Areas in Communications, ISSN 0733-8716, E-ISSN 1558-0008, Vol. 24, no 3, 427-436 p.Article in journal (Refereed)
    Abstract [en]

    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.

  • 28.
    Gross, James
    et al.
    KTH, School of Electrical Engineering (EES), Communication Theory.
    Karl, Holger
    Fitzek, Frank
    Wolisz, Adam
    Comparison of Heuristic and Optimal Subcarrier Assignment Algorithms2003In: Proc. of the 2003 International Conference on Wireless Networks ICWN'03, June 2003, 2003, 249-255 p.Conference paper (Refereed)
    Abstract [en]

    Wireless multi carrier approaches can support the simultaneous transmissions to several wireless terminals,each one getting only some of the available subcarriers. Judiciously assigning subcarriers to terminals can be used to, e.g., increase the capacity of a wireless cell; however, the resulting assignment problem is difficult. The first subproblem is to decide the number of carriers assigned to each terminal. Then, carriers have to be assigned to individual terminals. For this assignment problem, we present in this paper an optimal but impractical algorithm as well as two new heuristic ones, which have a considerably lower complexity and can be used in real systems. We show that the performance of even the simple heuristic algorithm is comparable with that of the optimal algorithm. The advanced heuristic increases the performance further, at a modest increase in complexity. Both heuristics are at most 5% off from the optimal algorithm’s performance.

  • 29. Gross, James
    et al.
    Karl, Holger
    Wolisz, Adam
    On the Effect of Inband Signaling and Realistic Channel Knowledge on Dynamic OFDM-FDMA Systems2004In: Proc of European Wireless 2004, 2004Conference paper (Refereed)
    Abstract [en]

    Dynamically assigning subcarriers of OFDM systems to multiple different terminals in a cell has been shown to be beneficial in terms of different transmission metrics. However, the success of such a scheme depends on the ability of the access point to inform terminals of their newest subcarrier assignments as well as on the accuracy of the channel state information used to generate new assignments. It is not clear whether the overhead required to implement these two system abilities consumes all of the potential performance increase possible by dynamically assigning subcarriers.

    In this paper, a specific MAC structure is selected enabling the operation of a dynamic OFDM system. Then, we study the question of the required overhead. A static assignment variant serves as a comparison scheme. We investigate the performance difference of these two schemes for various scenarios where at first signaling and then realistic channel knowledge is added to the system model. The results in terms of average throughput and goodput per terminal are obtained for a varying number of terminals in the cell as well as for a varying transmit power. We find that the performance is not only decreased for the dynamic scheme but also for the static one, such that the overall ratio favors the dynamic rather than the static scheme especially in realistic system environments.

  • 30. Gross, James
    et al.
    Karl, Holger
    Wolisz, Adam
    Throughput Optimization of Dynamic OFDM-FDMA Systems with Inband Signaling2004In: Proc. of the 2nd International Symposium on Modeling and Optimization in Mobile, Ad Hoc, and Wireless Networks 2004 (WiOpt 2004), 2004Conference paper (Refereed)
    Abstract [en]

    Dynamically assigning subcarriers to terminals in an OFDM-FDMA system optimizes various performance metrics. However, it also requires that terminals are periodically informed about their assigned subcarriers. Hence, the signaling overhead has to be taken into account for both system design and performance evaluation. We investigate two possibilities how to design an inband signaling system, with either a fixed or variable overhead per MAC frame and characterize them as linear optimization problems. We provide performance results for both schemes under a varying number of subcarriers and different terminal speeds and show that the variable scheme outperforms the fixed one for low speeds, that the performance difference vanishes as terminal speeds go up, but also that the fixed scheme is more sensitive to variations in the number of subcarriers.

  • 31.
    Gross, James
    et al.
    Telecommunication Networks Group, Technical University of Berlin.
    Klaue, Jirka
    TU Berlin.
    Karl, Holger
    Universitaet Paderborn.
    Wolisz, Adam
    TU Berlin.
    Cross-Layer Optimization of OFDM Transmission Systems for MPEG-4 Video Streaming2004In: Computer Communications, ISSN 0140-3664, E-ISSN 1873-703X, Vol. 27, no 11, 1044-1055 p.Article in journal (Refereed)
    Abstract [en]

    In this paper, we study the performance of a combined link- and physical-layer approach for the downlink of a dynamic OFDM-frequency division multiple access transmission system delivering MPEG-4 video streams. The approach consists of three different mechanisms-queue management, resource allocation, and subcarrier assignment-which can be used separately as well as in combination. This crosslayer approach allows to utilize channel-related knowledge and semantic information of the streams packets. Consecutively adding these mechanisms to the studied system, we judge their respective impact on system performance. To do so, we present a new metric for quality assessment of long video transmissions (considerably longer than 10 s) and apply it to the simulation results. In particular, we introduce a local distortion threshold-based metric instead of an overall average judgment. We find that our combined system approach increases the number of supportable terminals per cell by up to 300%.

  • 32. Gross, James
    et al.
    Klaue, Jirka
    Karl, Holger
    Wolisz, Adam
    Subcarrier Allocation for Variable Bit Rate Video Streams in Wireless OFDM Systems2003In: Vehicular Technology Conference, 2003. VTC 2003-Fall. 2003 IEEE 58th (Volume:4 ), IEEE conference proceedings, 2003, 2481-2485 p.Conference paper (Refereed)
    Abstract [sv]

    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 terminal's 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.

  • 33. Gross, James
    et al.
    Paoluzzi, Irene
    Karl, Holger
    Wolisz, Adam
    Throughput Study for a Dynamic OFDM-FDMA System with Inband Signaling2004In: Vehicular Technology Conference, 2004. VTC 2004-Spring. 2004 IEEE 59th (Volume:3 ), IEEE conference proceedings, 2004, 1787-1791 p.Conference paper (Refereed)
    Abstract [en]

    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.

  • 34. Gross, James
    et al.
    Punyal, Oscar
    Emmelmann, Marc
    Multi-User OFDMA Frame Aggregation for Future Wireless Local Area Networking2009In: NETWORKING 2009: 8th International IFIP-TC 6 Networking Conference, Aachen, Germany, May 11-15, 2009. Proceedings, Springer, 2009, 220-233 p.Conference paper (Refereed)
    Abstract [en]

    State-of-the-art wireless local area networking enables frame aggregation as approach to increase MAC efficiency. However, frame aggregation is limited to the aggregation of packets destined for the same station. In order to serve different stations, the access point still has to contend for the channel multiple times. In this paper we propose and evaluate a novel approach that enables multi-user frame aggregation. We combine this concept with channel-dependent OFDMA resource assignments, yielding a higher PHY efficiency (by exploiting multi-user diversity and instantaneous channel state information) as well as a higher MAC efficiency. The downside to this approach is the increase in protocol overhead to enable such multi-user OFDMA frame aggregation. However, we show that the proposed approach outperforms state-of-the-art 802.11n for different packet sizes and stations to be served.

  • 35.
    Gross, James
    et al.
    KTH, School of Electrical Engineering (EES), Communication Theory.
    Punyal, Oscar
    RWTH Aachen University.
    Pereira, Carlos
    University of Porto.
    Aguiar, Ana
    University of Porto.
    Experimental Characterization and Modeling of RF Jamming Attacks on VANETs2015In: IEEE Transactions on Vehicular Technology, ISSN 0018-9545, E-ISSN 1939-9359, Vol. 64, no 2, 524-540 p.Article in journal (Refereed)
    Abstract [en]

    In this work, we evaluate the performance of 802.11p-based vehicular communications in the presence of RF jamming attacks. Specifically, we characterize the transmission success rate of a car-to-car link subject to constant, periodic, and reactive RF jamming. First, we conduct extensive measurements in an anechoic chamber, where we study the benefits of built-in techniques for interference mitigation. In addition, we identify that the periodic transmission of preamblelike jamming signals can hinder successful communication despite being up to five orders of magnitude weaker than the signal of interest. We further provide the rationale behind this remarkably high jammer eectiveness. Additionally, we quantify the impact of reaction delay and interference signal length on the eectiveness of the reactive jammer. Next, by means of outdoor measurements, we evaluate the suitability of the indoor measurements for being used as a model to characterize the performance of car-to-car communications in the presence of RF jamming. Finally, we conduct outdoor measurements emulating a vehicular platoon and study the threats that RF jamming poses to this VANET application. We observe that constant, periodic, but also reactive jammer can hinder communication over large propagation areas, which would threaten road safety.

  • 36. Gross, James
    et al.
    Radmacher, Frank
    Thomas, Wolfgang
    A Game-Theoretic Approach to Routing under Adversarial Conditions2010In: Theoretical Computer Science: 6th IFIP TC 1/WG 2.2 International Conference, TCS 2010, Held as Part of WCC 2010, Brisbane, Australia, September 20-23, 2010. Proceedings, Springer, 2010, 355-370 p.Conference paper (Refereed)
    Abstract [en]

    We present a game-theoretic framework for modeling and solving routing problems in dynamically changing networks. The model covers the aspects of reactivity and non-termination, and it is motivated by quality-of-service provisioning in cognitive radio networks where data transmissions are interfered by primary systems. More precisely, we propose an infinite two-player game where a routing agent has to deliver network packets to their destinations while an adversary produces demands by generating packets and blocking connections. We obtain results on the status of basic problems, by showing principal limitations to solvability of routing requirements and singling out cases with algorithmic solutions.

  • 37.
    Gross, James
    et al.
    UMIC Research Centre, RWTH Aachen University, Germany.
    Reyer, Michael
    Institute for Theoretical Information Technology, RWTH Aachen University, Germany.
    Performance Prediction for OFDMA Systems with Dynamic Power and Subcarrier Allocation2011In: Computer Communications, ISSN 0140-3664, E-ISSN 1873-703X, Vol. 34, no 8, 973-984 p.Article in journal (Refereed)
    Abstract [en]

    It is well known that channel-dependent OFDMA resource assignment algorithms provide a significant performance improvement compared to static (i.e., channel-unaware) approaches. Such dynamic algorithms constantly adapt resource assignments to current channel states according to some objective function. Due to these dynamics, it is difficult to predict the resulting performance for such schemes given a certain scenario (characterized by the number of terminals in the cell and their average channel gains). In this paper we provide a novel, analytical framework for performance prediction, which takes dynamic power and subcarrier allocation into account. The analysis is based on fundamental transformations of the channel gains caused by the dynamic subcarrier allocations. This insight allows for deriving probability functions of the achieved rate per subcarrier which ultimately yields expressions for the expected minimal rates as well as outage probabilities for certain rate demands. Hence, the methods presented in this paper for performance prediction can be employed for admission control in systems with dynamic resource allocation. We illustrate the applicability of our derivations with respect to the capacity of 802.16e systems for Voice-over-IP and video streams. The results demonstrate a significant improvement compared to state-of-the art approaches but also reveal room for improvement of this approach compared to the optimal system performance.

  • 38.
    Gross, James
    et al.
    TU Berlin.
    Valentin, Stefan
    Bell Labs.
    Karl, Holger
    Universitaet Paderborn.
    Wolisz, Adam
    TU Berlin.
    A Study of Impact of Inband Signalling and Realistic Channel Knowledge for an Example Dynamic OFDM-FDMA System2005In: European transactions on telecommunications, ISSN 1124-318X, E-ISSN 1541-8251, Vol. 16, no 1, 37-49 p.Article in journal (Refereed)
    Abstract [en]

    Dynamically assigning sub-carriers of orthogonal frequency division multiplexing (OFDM) systems to multiple different terminals in a cell has been shown to be beneficial in terms of different transmission metrics. The success of such a scheme, however depends on the ability of the access point to inform terminals of their newest sub-carrier assignments as well as on the accuracy of the channel state information used to generate new assignments. It is not clear whether the overhead required to implement these two functions consumes all of the potential performance increase possible by dynamically assigning subcarriers. In this paper, a specific MAC structure is selected enabling the operation of a dynamic OFDM system, incorporating a signalling scheme for dynamically assigned sub-carriers. Based on this structure, we study the overhead required for a dynamic sub-carrier scheme; a static variant that serves as a comparison case. We investigate the performance difference of these two schemes for various scenarios where at first signalling and then realistic channel knowledge is added to the system model. Average throughput and goodput per terminal serve as figures of merit; the number of terminals in the cell, the transmission power per sub-carrier, the delay spread and the movement speed of the terminals are varied. We find that a realistic overhead model decreases the performance of both static and dynamic schemes such that the overall ratio favours in all cases except for higher speeds the dynamic rather than the static scheme especially in realistic system environments.

  • 39.
    Gross, James
    et al.
    Rhein Westfal TH Aachen, Mobile Network Performance Grp.
    Weyres, Marco
    Wolisz, Adam
    Effective Service Capacity Analysis of Opportunistic Multi-Carrier OFDMA Systems2012In: Proceedings of the 8th ACM Symposium on QoS and Security for Wireless and Mobile Networks, Association for Computing Machinery (ACM), 2012, 17-26 p.Conference paper (Refereed)
    Abstract [en]

    Exact queuing-theoretic modeling of wireless systems is tough due to the complex service processes that arise from the interaction of the wireless channel with state-of-the-art signal processing algorithms. Nevertheless, with the rise of video applications like streaming and telephony in upcoming fourth generation cellular networks, such analysis is important for devising admission control strategies. In this paper we apply recent insights into approximation approaches for queuing systems to opportunistic OFDMA systems and study different approaches for applying the modulation type to OFDM subcarriers. In particular, we provide analytical closed-form expressions for the delay distribution of three opportunistically scheduling OFDMA systems (adaptive modulation, link adaptation per terminal, link adaptation over all subcarriers). After validating these expressions, we evaluate the different system designs numerically. We find that although adaptive modulation systems have a higher complexity, their queuing performance is only slightly superior to systems with link adaptation per terminal. Furthermore, the systems feature in general an optimal number of terminals that can be supported in the cell, leveraging multi-user diversity with limited available resources. Nevertheless, these optima change from scheme to scheme.

  • 40. Gross, James
    et al.
    Willig, Andreas
    Measurements of a Wireless Link in different RF-isolated Environments2002Conference paper (Refereed)
  • 41. Hithnawi, A.
    et al.
    Li, S.
    Shafagh, H.
    Duquennoy, S.
    Gross, James
    KTH, School of Electrical Engineering (EES), Communication Theory.
    Poster abstract: Cross-layer optimization for low-power wireless coexistence2015In: SenSys 2015 - Proceedings of the 13th ACM Conference on Embedded Networked Sensor Systems, ACM Digital Library, 2015, 443-444 p.Conference paper (Refereed)
    Abstract [en]

    We present a system that leverages physical layer features to combat Cross-Technology Interference (CTI) in low-power wireless networks. Our system incorporates: (i) a lightweight interference detection mechanism for low-power radios that recognizes the type of interference in the received signal, (ii) a lightweight error detection mechanism to estimate and characterize error patterns within interfered packets, and (iii) a CTI-aware protocol that dynamically adapts transmission and recovery mode to the current interference patterns. We implement a prototype of our system for the lowpower IEEE 802.15.4 in software defined radios (SDR). Our early results of the system components demonstrate that we can achieve a high accuracy in error detection and interference type identification. Moreover, we observed a significant performance improvement compared to the standard 802.15.4 systems without interference-awareness.

  • 42.
    Hithnawi, Anwar
    et al.
    ETH Zurich.
    Su, Li
    ETH Zurich.
    Shafagh, Hossein
    ETH Zurich.
    Gross, James
    KTH, School of Electrical Engineering (EES), Communication Theory.
    Duquennoy, Simon
    SICS.
    CrossZig: Combating Cros-Technology Interference in Low-Power Wireless Networks2016Conference paper (Refereed)
    Abstract [en]

    Low-power wireless devices suffer notoriously from Cross- Technology Interference (CTI). To enable co-existence, researchers have proposed a variety of interference mitigation strategies. Existing solutions, however, are designed to work with the limitations of currently available radio chips. In this paper, we investigate how to exploit physical layer properties of 802.15.4 signals to better address CTI. We present CrossZig, a cross-layer solution that takes advantage of physical layer information and processing to improve low-power communication under CTI. To this end, CrossZig utilizes physical layer information to detect presence of CTI in a corrupted packet and to apply an adaptive packet recovery which incorporates a novel cross-layer based packet merging and an adaptive FEC coding. We implement a prototype of CrossZig for the low-power IEEE 802.15.4 in a software-defined radio platform. We show the adaptability and the performance gain of CrossZig through experimental evaluation considering both micro-benchmarking and system performance under various interference patterns. Our results demonstrate that CrossZig can achieve a high accuracy in error localization (94.3% accuracy) and interference type identification (less than 5% error rate for SINR ranges below 3 dB). Moreover, our system shows consistent performance improvements under interference from various interfering technologies.

  • 43. Hu, Wendong
    et al.
    Willkomm, Daniel
    TU Berlin.
    Abusubaih, Murad
    TU Berlin.
    Gross, James
    Technical University Berlin.
    Vlantis, George
    Gerla, Mario
    UCLA.
    Wolisz, Adam
    TU Berlin.
    Dynamic Frequency Hopping Communities for Efficient IEEE 802.22 Operation2007In: IEEE Communications Magazine, ISSN 0163-6804, E-ISSN 1558-1896, Vol. 45, no 5, 80-87 p.Article in journal (Refereed)
    Abstract [en]

    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.

  • 44. Hu, Yulin
    et al.
    Gross, James
    KTH, School of Electrical Engineering (EES), Communication Theory.
    On the Outage Probability and Effective Capacity of Multiple Decode-and-forward Relay System2012In: Wireless Days (WD), 2012 IFIP, 2012, 1-8 p.Conference paper (Refereed)
    Abstract [en]

    This paper investigates the quality-of-service performance of multiple decode-and-forward relay system. In particular, we study the outage probability and effective capacity performance of the system based on average channel gain. First, a closed-form expression of outage probability is derived which is more concise than the previous work. Second, we introduce four retransmission strategies and derive the effective capacities of the multi-relay system under these strategies. By means of simulation, we provide the numerical verification of our formula derivation. The simulation results illustrate perfect agreement for our theoretical analysis. In addition, we evaluate the system queue behavior under the strategies. The simulation shows that relay can significantly improve the system queue behavior and that the combined signal processing only pays off under relay-driven retransmission strategies.

  • 45. Hu, Yulin
    et al.
    Gross, James
    Ding, Zhizhong
    The Outage Performance of Realtime Transmission in Multiple Asynchronous Relays Enhanced OFDM System2013In: Computing, Networking and Communications (ICNC), 2013 International Conference on, IEEE conference proceedings, 2013, 283-289 p.Conference paper (Refereed)
    Abstract [en]

    This paper1 considers the outage performance of multiple relays transmission in Orthogonal Frequency Division Multiplexing (OFDM) systems. Relaying has been suggested in general to improve the reliability of wireless systems significantly and is therefore a candidate for wireless industrial communication systems that need to meet tough latency and outage requirements. Different from previous works, this paper takes synchronization mismatch of hardware processing time at multiple relays into account and characterizes the performance impact of such asynchronous relays on the reliability of the system. We focus especially on the case where the transceivers do not have instantaneous channel state information but still need to fullfill tough reliability and latency constraints. We find that with respect to this scenario the synchronization mismatch can lead to a significant loss of reliability even if the individual mismatch of relays is relatively small compared to the cyclic prefix of the OFDM symbol. Consequently, this paper devises countermeasures to efficiently combat the synchronization issues either by introducing a longer cyclic prefix or by decreasing/adjusting the number of active relays. We find that all proposed countermeasures have their application domains which are determined by the average SNR and the severity of synchronization errors.

  • 46.
    Hu, Yulin
    et al.
    RWTH Aachen University.
    Gross, James
    KTH, School of Electrical Engineering (EES), Communication Theory.
    Schmeink, Anke
    RWTH Aachen University.
    On the Capacity of Relaying with Finite Blocklength2015In: IEEE Transactions on Vehicular Technology, ISSN 0018-9545, E-ISSN 1939-9359, no 99Article in journal (Refereed)
    Abstract [en]

    In this paper, the relaying performance is studied under the finite blocklength regime. The overall error probability of relaying is derived. Moreover, we investigate the Blocklength- Limited capacity (BL-capacity) of relaying. We prove that the BL-capacity of relaying is quasiconcave in the overall error probability. Therefore, the BL-capacity has a global maximum value which can be achieved by choosing an appropriate error probability. Through numerical investigations, we validate our analytical model and compare the performance of relaying under the finite blocklength regime versus the Shannon capacity regime.

  • 47.
    Hu, Yulin
    et al.
    RWTH Aachen University.
    Gross, James
    KTH, School of Electrical Engineering (EES), Communication Theory.
    Schmeink, Anke
    RWTH Aachen University.
    On the Performance Advantage of Relaying under the Finite Blocklength Regime2015In: IEEE Communications Letters, ISSN 1089-7798, Vol. 19, no 5, 779-782 p.Article in journal (Refereed)
    Abstract [en]

    We consider a two-phase relaying system with finite blocklengths. We study the performance difference of relaying under the finite blocklength regime as well as under the Shannon capacity regime. Most importantly, we are interested in the conditions that lead to a higher performance of relaying under the finite blocklength regime. We find that these situations are characterized by error probabilities of relaying, e.g., the overall error probability of relaying and the error probability of the bottleneck link of relaying. We identify scenarios where relaying outperforms direct transmission under the finite blocklength regime even if their performances are similar under the Shannon capacity regime. Moreover, we prove that under these scenarios the performance advantage of relaying is more significant with short blocklengths. Finally, numerical results are provided and discussed.

  • 48.
    Hu, Yulin
    et al.
    RWTH Aachen University.
    Gross, James
    KTH, School of Electrical Engineering (EES), Communication Theory.
    Schmeink, Anke
    RWTH Aachen University.
    QoS-Constrained Energy Efficiency of Cooperative ARQ in Multiple DF Relay Systems2015In: IEEE Transactions on Vehicular Technology, ISSN 0018-9545, E-ISSN 1939-9359, 848-859 p.Article in journal (Refereed)
    Abstract [en]

    In this paper, we study the higher-layer performance as well as the "throughput, delay, energy consumption" tradeoff problem for multi-relay assisted cooperative automatic repeat request (C-ARQ) protocols. We study a practical scenario where only the average channel state information is available at the source and relays. We consider four multi-relay C-ARQ protocols, and derive closed-form expressions for the transmission delay distribution, the energy consumption and the higherlayer queuing performance. Furthermore, we analyze the QoSconstrained energy efficiency performances of the protocols. Our analysis is validated by simulations. In addition, we evaluate the system performance under these C-ARQ protocols and for different topologies. We conclude several guidelines for the design of efficient C-ARQ protocols. Finally, a simple extension of the studied C-ARQ protocols is proposed, which improves the QoSconstrained energy efficiency by 4%.

  • 49. Hu, Yulin
    et al.
    Gross, James
    KTH, School of Electrical Engineering (EES), Communication Theory.
    Schmeink, Anke
    Wang, Tong
    Maximizing energy efficiency for multiple DF relay system with QoS constraint2013In: Proceedings of the Tenth International Symposium on Wireless Communication Systems (ISWCS 2013), IEEE conference proceedings, 2013, 419-423 p.Conference paper (Refereed)
    Abstract [en]

    Multiple relays assisted transmission is an effective way to enhance the reliability of wireless communication network. However multiple relays joining in the transmission costs more energy and may reduce the energy efficiency of the system. In this paper, we investigate the energy efficiency performance of multiple decode-and-forward relays system under a quality of service constraint. In particular, we study a feedback-limited scenario where only the knowledge of average channel gain is available at the source. First, we define a tradeoff factor and based on it design a tradeoff mechanism between energy efficiency and outage probability. Second, we prove that the energy efficiency of the system is a piecewise strictly monotonic function of our tradeoff factor and has only one extreme value which is the global maximum. Third, by means of simulation we show that the numerical results perfectly match our theoretical analysis. In addition, we show that energy efficiency can achieve the extreme value under the loose QoS constraint.

  • 50.
    Hu, Yulin
    et al.
    RWTH Aachen University.
    Schmeink, Anke
    RWTH Aachen University.
    Gross, James
    KTH, School of Electrical Engineering (EES), Communication Theory.
    Blocklength-Limited Performance of Relaying under Quasi-Static Rayleigh Channels2016In: IEEE Transactions on Wireless Communications, ISSN 1536-1276, E-ISSN 1558-2248, Vol. 15, no 7, 4548-4558 p., 7434063Article in journal (Refereed)
    Abstract [en]

    In this paper, the blocklength-limited performance of a relaying system is studied, where channels are assumed to experience quasi-static Rayleigh fading while at the same time only the average channel state information (CSI) is available at the source. Both the physical-layer performance (blocklength-limited throughput) and the link-layer performance (effective capacity) of the relaying system are investigated. We propose a simple system operation by introducing a factor based on which we weight the average CSI and let the source determine the coding rate accordingly. In particular, we show that both the blocklengthlimited throughput and the effective capacity are quasi-concave in the weight factor. Through numerical analysis, we investigate the relaying performance with average CSI while considering perfect CSI scenario and direct transmission as contrasts. We observe that relaying is more efficient than direct transmission in the finite blocklength regime. Moreover, this performance advantage of relaying under the average CSI scenario is more significant than under the perfect CSI scenario. Finally, the speed of convergence (between the blocklength-limited performance and the performance with infinite blocklengths) in relaying system is faster in comparison to the direct transmission under both the average CSI scenario and the perfect CSI scenario.

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