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  • 1. Brandt, Rasmus
    et al.
    Mochaourab, Rami
    KTH Royal Institute of Technology, Sweden.
    Bengtsson, Mats
    Distributed Long-Term Base Station Clustering in Cellular Networks using Coalition Formation2016Ingår i: IEEE Transactions on Signal and Information Processing over Networks, Vol. 2, nr 3, s. 362-375Artikel i tidskrift (Refereegranskat)
  • 2. Brandt, Rasmus
    et al.
    Mochaourab, Rami
    RISE Research Institutes of Sweden. KTH Royal Institute of Technology, Sweden.
    Bengtsson, Mats
    Globally Optimal Base Station Clustering in Interference Alignment-Based Multicell Networks2016Ingår i: IEEE Signal Processing Letters, ISSN 1070-9908, E-ISSN 1558-2361, Vol. 23, nr 4, s. 512-516Artikel i tidskrift (Refereegranskat)
  • 3.
    Brandt, Rasmus
    et al.
    KTH Royal Institute of Technology, Sweden.
    Mochaourab, Rami
    KTH Royal Institute of Technology, Sweden.
    Bengtsson, Mats
    KTH Royal Institute of Technology, Sweden.
    Interference Alignment-Aided Base Station Clustering using Coalition Formation2015Ingår i: Proceedings of the 49th Asilomar Conference on Signals, Systems and Computers, 2015Konferensbidrag (Refereegranskat)
    Abstract [en]

    Base station clustering is necessary in large interference networks, where the channel state information (CSI) acquisition overhead otherwise would be overwhelming. In this paper, we propose a novel long-term throughput model for the clustered users which addresses the balance between interference mitigation capability and CSI acquisition overhead. The model only depends on statistical CSI, thus enabling long-term clustering. Based on notions from coalitional game theory, we propose a low-complexity distributed clustering method. The algorithm converges in a couple of iterations, and only requires limited communication between base stations. Numerical simulations show the viability of the proposed approach.

  • 4.
    Chong, Z.
    et al.
    Dresden University of Technology, Germany.
    Mochaourab, Rami
    Dresden University of Technology, Germany.
    Jorswieck, Eduard
    Dresden University of Technology, Germany.
    Pricing in Noncooperative Interference Channels for Improved Energy Efficiency2010Ingår i: EURASIP Journal on Wireless Communications and Networking, Vol. 2010Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We consider noncooperative energy-efficient resource allocation in the interference channel. Energy efficiency is achieved when each system pays a price proportional to its allocated transmit power. In noncooperative game-theoretic notation, the power allocation chosen by the systems corresponds to the Nash equilibrium. We study the existence and characterize the uniqueness of this equilibrium. Afterwards, pricing to achieve energy-efficiency is examined. We introduce an arbitrator who determines the prices that satisfy minimum QoS requirements and minimize total power consumption. This energy-efficient assignment problem is formulated and solved. We compare our setting to that without pricing with regard to energy-efficiency by simulation. It is observed that pricing in this distributed setting achieves higher energy-efficiency indifferent interference regimes.

  • 5.
    Dargie, W.
    et al.
    Technical University of Dresden, Germany.
    Mochaourab, Rami
    Technical University of Dresden, Germany.
    Schill, A.
    Technical University of Dresden, Germany.
    Guan, L.
    Loughborough University, UK.
    A topology control protocol based on eligibility and efficiency metrics2011Ingår i: Journal of Systems and Software, Vol. 84, nr 1, s. 2-11Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The question of fairness in wireless sensor networks is not studied very well. It is not unusual to observe in the literature fairness traded for low latency or reliability. However, a disproportional use of some critical nodes as relaying nodes can cause premature network fragmentation. This paper investigates fairness in multi-hop wireless sensor networks and proposes a topology control protocol that enables nodes to exhaust their energy fairly. Moreover, it demonstrates that whereas the number of neighboring nodes with which a node should cooperate depends on the density of the network, increasing this number beyond a certain amount does not contribute to network connectivity.

  • 6.
    Dargie, W.
    et al.
    Technical University of Dresden, Germany.
    Schill, A.
    Technical University of Dresden, Germany.
    Mochaourab, Rami
    Technical University of Dresden, Germany.
    Guan, L.
    Loughborough University, UK.
    A Topology Control Protocol for 2D Poisson Distributed Wireless Sensor Networks2009Konferensbidrag (Refereegranskat)
    Abstract [en]

    Topology control in a wireless sensor network is useful for ensuring that the network remains connected in the presence of nodes that exhaust their energy or become altogether dysfunctional (for whatever reasons). It also ensures that all the link that can be established are energy-efficient links and the nodes utilize their energy fairly. In this paper, we propose a fair and energy efficient topology control protocol for a two-dimensional random sensor deployment in which the nodes can estimate the distances to their neighbors and vary their transmission power accordingly. The protocol applies a neighbor eligibility metric in order to ensure a fair distribution of energy in the network. We introduce the notion of weighted relaying regions defined over the plane of a searching node to drop out inefficient links. Unlike most topology control protocols that rely on nearest neighbor approaches, we use a distance measure that is radio characteristic and channel condition dependent. We verify the performance of the protocol through simulation results on network graph properties and energy consumption.

  • 7.
    Ghauch, Hadi
    et al.
    KTH Royal Institute of Technology, Sweden.
    Mochaourab, Rami
    KTH Royal Institute of Technology, Sweden.
    Bengtsson, Mats
    KTH Royal Institute of Technology, Sweden.
    Skoglund, Mikael
    KTH Royal Institute of Technology, Sweden.
    Distributed precoding and user selection in MIMO interfering networks2015Ingår i: 2015 IEEE 6th International Workshop on Computational Advances in Multi-Sensor Adaptive Processing, CAMSAP 2015, 2015, s. 461-464Konferensbidrag (Refereegranskat)
    Abstract [en]

    In this work we shed light on the problem of precoding and user selection in MIMO networks. We formulate the problem using the framework of stable matching, whereby a set of users wish to be matched to a set of serving base stations, such as to maximize the sum-rate performance of the system. Though the problem is NP-hard, we propose a suboptimal heuristic that tackles the problem in a distributed fashion: we apply a many-to-one stable matching algorithm to generate a sequence of matchings, and the Weighted MMSE algorithm to perform the precoding. We benchmark our algorithm againt the recently proposed Weighted MMSE with User Assignment algorithm.

  • 8.
    Ho, K. M. Z.
    et al.
    Eurecom Institute, France.
    Gesbert, D.
    Eurecom Institute, France.
    Jorswieck, E.
    Dresden University of Technology, Germany.
    Mochaourab, Rami
    Dresden University of Technology, Germany.
    Beamforming on the MISO interference channel with multi-user decoding capability2010Konferensbidrag (Refereegranskat)
    Abstract [en]

    This paper considers the multiple-input-single-output interference channel (MISO-IC) in which transmitters and receivers share the same time and frequency resources. We consider receivers with interference decoding capability (IDC) so that the interference signal can be decoded and subtracted from the received signal. On the MISO-IC with single user decoding, transmit beamforming vectors are designed to mitigate interference at the receivers. With IDC, receivers can potentially decode interference which yields a higher data rate. Yet, decoding interference pose a rate constraint on the interferer and in turn on the sum rate of the system. This brings some interesting questions: when should the Txs mitigate interference and when should Txs amplify interference? Under what situations should Txs change from mitigating interference to amplifying interference? We answer these questions in this paper.

  • 9.
    Holfeld, B.
    et al.
    Fraunhofer, Germany.
    Mochaourab, Rami
    Fraunhofer, Germany.
    Wirth, T.
    Fraunhofer, Germany.
    Stable Matching for Adaptive Cross-Layer Scheduling in LTE Downlink2013Ingår i: 2013 IEEE 77TH VEHICULAR TECHNOLOGY CONFERENCE (VTC SPRING), 2013Konferensbidrag (Refereegranskat)
    Abstract [en]

    Resource allocation from base stations to mobile users in realistic MIMO-OFDMA systems such as the 3GPP Long Term Evolution (LTE) downlink is based on limited and quantized channel feedback over a fine-granular resource grid of multiple dimensions. This allows for opportunistic scheduling but impedes application of enhanced cross-layer strategies due to the discrete and combinatorial problem space. Integer optimization for this allocation problem is strongly complex and prohibits use of efficient algorithms. Provided solutions in practice are given by sub-optimal greedy heuristics. In this paper, we apply two-sided stable matchings for adaptive multi-user scheduling. Our framework gives Pareto-efficient allocations and yields a tunable tradeoff between system throughput and user fairness. We form stable pairings of system resources and users based on queue-and channel-aware lists of preferred matches. The derived concept aims to find a stable matching state under presence of non-strict preference relations if such exist or redefines the allocation problem to a solvable strict problem instance. A performance evaluation for scheduling is done by system level simulations for high traffic loads in a realistically modeled LTE deployment.

  • 10.
    Jorswieck, E.
    et al.
    Dresden University of Technology, Germany.
    Mochaourab, Rami
    Dresden University of Technology, Germany.
    Beamforming in Underlay Cognitive Radio: Null-Shaping Constraints and Greedy User Selection2010Konferensbidrag (Refereegranskat)
    Abstract [en]

    In a multiuser cognitive radio setting, multiple secondary systems coexist with multiple primary systems. We assume the secondary systems consist of transmitter-receiver pairs, and each transmitter is equipped with multiple antennas while all receivers use a single antenna. In this setting, the secondary transmitters are to operate under the constraints of producing no interference at the primary users. Such constraints on the secondary systems are referred to as null-shaping constraints. Three results are derived in this work. First, the Pareto optimal operation points for the secondary systems under null-shaping constraints are characterized by real-valued parametrization of the transmission strategies. Second, we show that all points on the Pareto boundary of the secondary systems achievable rate region without interference constraints can be achieved as the outcome of a noncooperative game by imposing certain virtual null-shaping constraints. Third, motivated by the surprising result that all Pareto efficient points can be achieved as a Nash equilibrium, we consider the problem of selecting a subset of the noncooperative secondary users for operation under the objective of maximizing their achievable sum rate. A low complexity suboptimal greedy secondary user selection algorithm is proposed and its performance is illustrated by simulations

  • 11.
    Jorswieck, E.
    et al.
    Dresden University of Technology, Germany.
    Mochaourab, Rami
    Dresden University of Technology, Germany.
    Power Control Game in Protected and Shared Bands: Manipulability of Nash Equilibrium2009Konferensbidrag (Refereegranskat)
    Abstract [en]

    We consider a downlink resource allocation problem for two systems or cells each consisting of a base station and multiple mobile stations. Each cell operates on a protected band for exclusive use and a band shared with the other cell. On simultaneous transmission in the shared band, the two systems disturb one another with interference. The strategies of the systems are their choices of power allocations in their two bands with regard to individual sum power constraints. Here, the conflict present between the systems is best analyzed using game theory. In the first part, we study the static non-cooperative power control game with complete information. Existence and uniqueness of the Nash equilibrium are examined. Based on a suitable feedback model, we ask in the second part, whether the Nash equilibrium can be manipulated by reporting untruthful information within the system. The aim is to find suitable functions to suppress false feedback information from giving advantage to the dishonest system and degrading fairness between the systems. Here, we apply results from mechanism design to propose a modified expected payoff function at the mobile stations to force truthful feedback.

  • 12.
    Jorswieck, E.
    et al.
    Dresden University of Technology, Germany.
    Mochaourab, Rami
    Dresden University of Technology, Germany.
    Secrecy Rate Region of MISO Interference Channel: Pareto Boundary and Non-Cooperative Games2009Konferensbidrag (Refereegranskat)
    Abstract [en]

    Competition in interference channels is modeled using noncooperative game theory. The well known Nash equilibrium often leads to inefficient solutions. In the current work,we study the achievable secrecy rate of a multiple-antenna(MISO) interference channel (IFC). First, we characterizethe Pareto boundary of the secrecy rate region. Next, theparameterization is exploited to compute the Nash equilibrium. In contrast to the usual case in which the rate is considered as utility function, the Nash outcome is not maximum ratio transmission (MRT), it depends on the SNR andthe channel realizations and it can be shown to be almostefficient at all SNR values. All results are illustrated bysample region plots, reaction curves, and average price ofanarchy figures.

  • 13.
    Jorswieck, E.
    et al.
    TU Dresden, Germany.
    Mochaourab, Rami
    Fraunhofer, Germany.
    Shannon Meets Walras on Interference Networks2013Ingår i: 2013 Information Theory and Applications Workshop, ITA 2013 - Conference Proceedings, 2013Konferensbidrag (Refereegranskat)
    Abstract [en]

    In modern wireless communication networks, the layers of the protocol stack close ranks. Technology based layers like the PHY and MAC layer are developed considering assumptions and constraints on the service and application layers. The coexistence of several wireless transmission links operated by different users or operators requires interference coordination on the PHY and MAC. Bilateral agreements or policies consider business aspects and regulatory specifications. In this context, models from multiuser information theory are combined with microeconomic models. There are several connections between both areas: the utility functions and capacities or achievable rates, the strategy spaces and resources or coding schemes, the budget sets and the constraints on powers or rates. In this paper, we focus on one market equilibrium, the Walras equilibrium, and develop a distributed algorithm which finds an efficient operating point for three representative interference channel models: power allocation and single user decoding, beamforming and single user decoding, and rate splitting with successive decoding.

  • 14. Jorswieck, E.
    et al.
    Mochaourab, Rami
    Ho, K. M. Z.
    Game Theory for Physical Layer Security on Multi-Antenna Interference Channels2013Ingår i: Physical Layer Security in Wireless Communications / [ed] Xiangyun Zhou, Lingyang Song, Yan Zhang, Auerbach Publications, CRC Press , 2013Kapitel i bok, del av antologi (Övrigt vetenskapligt)
  • 15.
    Jorswieck, E.
    et al.
    Dresden University of Technology, Germany.
    Mochaourab, Rami
    Dresden University of Technology, Germany.
    Mittelbach, M.
    Dresden University of Technology, Germany.
    Effective Capacity Maximization in Multi-Antenna Channels with Covariance Feedback2009Ingår i: 2009 IEEE INTERNATIONAL CONFERENCE ON COMMUNICATIONS, VOLS 1-8, 2009Konferensbidrag (Refereegranskat)
    Abstract [en]

    The optimal transmit strategies of single-user multi-antenna systems with respect to average capacity maximization are well understood. However, the performance measure does neglect delay aspects which are important for higher layer design. Therefore, we consider the maximization of the effective capacity in a single-user multi-antenna system with covariance knowledge. The optimal transmit strategy is derived and the properties as a function of the decay-rate requirement of the buffer occupancy are analyzed. In particular, we show that the larger the decay-rate requirement, the smaller the beamforming optimality range, i.e., the more spatial eigenmodes are activated. This behavior is illustrated by numerical simulations and explained by the channel hardening effect.

  • 16. Jorswieck, Eduard
    et al.
    Mochaourab, Rami
    Walrasian Model for Resource Allocation and Transceiver Design in Interference Networks2013Ingår i: Mechanisms and Games for Dynamic Spectrum Allocation, Cambridge University Press , 2013, s. 75-107Kapitel i bok, del av antologi (Övrigt vetenskapligt)
    Abstract [en]

    The traditional way of handling spectrum for cellular wireless wide area networks and metropolitan area networks arose about 90 years ago based on the capabilities of radio transceivers and the regulatory requirements. Spectrum divided in chunks of certain bandwidth is exclusively licensed to operators by public auctions [4] for a decade or more duration. Furthermore, one radio access technology is assigned to the spectrum bands, e.g., global system for mobile communications, universal mobile telecommunications standard, long-term evolution (advanced), or high-speed packet access. Economists have long argued that market mechanisms should be applied to radio spectrum [20]. This trend to more flexible and market-oriented use of spectrum is supported by novel developments in radio technology, e.g., multiple antenna systems, multi-carrier techniques, and multi-user decoding. Following the development from intra-operator spectrum sharing to orthogonal interoperator spectrum sharing, the most promising yet most complex way of sharing spectrum is non-orthogonal inter-operator spectrum sharing in which several transmitters are active on the same frequency band and on the same time. On the physical layer, this leads to the classical interference channel model. Depending on the radio access technology and the transceiver structure, this type of sharing leads to a conflict situation between the transmitters (potentially belonging to different wireless network operators) because of interference on the same resource

  • 17.
    Jorswieck, Eduard
    et al.
    Dresden University of Technology, Germany.
    Mochaourab, Rami
    Dresden University of Technology, Germany.
    Mittelbach, M.
    Dresden University of Technology, Germany.
    Effective Capacity Maximization in Multi-Antenna Channels with Covariance Feedback2010Ingår i: IEEE Transactions on Wireless Communications, Vol. 9, nr 10, s. 2988-2993Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The tradeoff between average transmission rate and average delay is important for the system design of future wireless communication systems. In double-correlated multiple antenna channels, the spatial degrees of freedom allow to optimize the transmit strategy under throughput/delay priority. In this work, we maximize the effective capacity of a MIMO system with covariance feedback. Interestingly, the larger the delay requirement is, the more spatial degrees of freedom are used to avoid low instantaneous transmission rates. This fact is shown analytically by deriving a closed-form expression for the beamforming optimality range as a function of the spatial correlation, the SNR, and the QoS exponent. Numerical simulations illustrate the average effective capacity optimization and confirm the theoretical results.

  • 18.
    Kurras, M.
    et al.
    Heinrich Hertz Institute, Germany.
    Thiele, L.
    Heinrich Hertz Institute, Germany.
    Mochaourab, Rami
    Heinrich Hertz Institute, Germany.
    From Single- to Multi-User Scheduling in LTE-A Uplink Exploiting Virtual MIMO2012Ingår i: 2012 CONFERENCE RECORD OF THE FORTY SIXTH ASILOMAR CONFERENCE ON SIGNALS, SYSTEMS AND COMPUTERS (ASILOMAR), 2012, s. 1172-1176Konferensbidrag (Refereegranskat)
    Abstract [en]

    This paper focuses on frequency and spatial domain scheduling in a frequency division duplex (FDD) Long Term Evolution - Advanced (LTE-A) system for the uplink direction. First we present a single-user (SU) channel-dependent scheduling (CDS) algorithm for continuous bandwidth allocation. There, we introduce a parameter to adjust the distribution of the user throughput without any loss in system performance. This is done as a preparation for the main contribution of this paper where we take advantage of the fact that a base station (BS) in LTE-A will be equipped with two ore more receive antennas by exploiting virtual multiple-input multiple-output (MIMO) through multi-user (MU) scheduling. We take the output of the SU-algorithm and group additional user in a second spatial layer to increase the system throughput. Note that our proposed MU algorithm requires a SU scheduler considering the continuous bandwidth constraint for assigned resources per user, because we do not change scheduling decisions made by the SU algorithm. The evaluation in this paper is done by extensive system level simulations in a third generation partnership project (3GPP) conform multi-cell scenario, assuming both SU and full MU interference. Numerical results show that our MU-algorithm improves the system throughput by 25-50 % compared to SU scheduling in uplink.

  • 19.
    Larsson, Erik G.
    et al.
    Linköping University, Sweden.
    Jorswieck, Eduard A.
    Dresden University of Technology, Germany.
    Lindblom, Johannes
    Linköping University, Sweden.
    Mochaourab, Rami
    Dresden University of Technology, Germany.
    Game Theory and the Flat-Fading Gaussian Interference Channel: Analyzing Resource Conflicts in Wireless Networks2009Ingår i: IEEE signal processing magazine (Print), Vol. 26, nr 5, s. 18-27Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    In this article, we described some basic concepts from noncooperative and cooperative game theory and illustrated them by three examples using the interference channel model, namely, the power allocation game for SISO IFC, the beamforming game for MISO IFC, and the transmit covariance game for MIMO IFC. In noncooperative game theory, we restricted ourselves to discuss the NE and PoA and their interpretations in the context of our application. Extensions to other noncooperative approaches include Stackelberg equilibria and the corresponding question "Who will go first?" We also correlated equilibria where a certain type of common randomness can be exploited to increase the utility region. We leave the large area of coalitional game theory open.

  • 20.
    Luo, J.
    et al.
    Fraunhofer, Germany.
    Lindblom, J.
    Linköping University, Sweden.
    Li, J.
    Ilmenau University of Technology, Germany.
    Mochaourab, Rami
    Technische Universitat Dresden, Germany.
    Kortke, A.
    Fraunhofer, Germany.
    Karipidis, E.
    Linköping University, Sweden.
    Haardt, M.
    Ilmenau University of Technology, Germany.
    Jorswieck, E. A.
    Communications Laboratory, Germany.
    Larsson, E. G.
    Linköping University, Sweden.
    Transmit Beamforming for Inter-Operator Spectrum Sharing: From Theory to Practice2012Ingår i: Proceedings of the International Symposium on Wireless Communication Systems, 2012, s. 291-295Konferensbidrag (Refereegranskat)
    Abstract [en]

    In this paper, four transmit beamforming (BF) techniques are selected and compared to realize inter-operator spectrum sharing, which is a promising solution for the spectrum shortage problem. The BF techniques include two game-theoretic (GT) algorithms, zero-forcing (ZF) and minimum mean square error (MMSE). After a brief description of the BF techniques in a multiple-input single-output (MISO) system, their computational complexity is analyzed. The effectiveness of these techniques in real radio frequency (RF) signal transmission is verified by implementation on a flexible hardware-in-the-loop (HIL) testbed. First, several important aspects regarding practical implementation are discussed. Afterwards, the HIL measurement results are shown, where considerable sum rate gain can be observed due to spectrum sharing. Finally, the appropriate BF technique can be chosen based on a tradeoff between complexity and performance.

  • 21.
    Mochaourab, Rami
    et al.
    KTH Royal Institute of Technology, Sweden.
    Bengtsson, Mats
    KTH Royal Institute of Technology, Sweden.
    Linear transceiver optimization in multicell MIMO based on the generalized benders decomposition2015Ingår i: IEEE Workshop on Signal Processing Advances in Wireless Communications, SPAWC, 2015, s. 430-434Konferensbidrag (Refereegranskat)
    Abstract [en]

    We study the maximum sum rate optimization problem in the multiple-input multiple-output interfering broadcast channel. The multiple-antenna transmitters and receivers are assumed to have perfect channel state information. In this setting, finding the optimal linear transceiver design is an NP-hard problem. We show that a reformulation of the problem renders the application of generalized Benders decomposition suitable. The decomposition provides us with an optimization structure which we exploit to apply two different optimization approaches. While one approach is guaranteed to converge to a local optimum of the original problem, the other approach hinges on techniques which can be promising for devising a global optimization method.

  • 22.
    Mochaourab, Rami
    et al.
    KTH Royal Institute of Technology, Sweden.
    Bengtsson, Mats
    KTH Royal Institute of Technology, Sweden.
    Stable Matching with Externalities for Beamforming and User Assignment in Multi-cell MISO Systems2016Ingår i: Proc WSA 2016, 2016, s. 549-554Konferensbidrag (Refereegranskat)
    Abstract [en]

    We consider the problem of distributed joint user association and beamforming in multi-cell multiple-input single-output systems. Assuming perfect local channel state information, each base station applies a distributed beamforming scheme called WSLNR-MAX [1] which depends on the user association in the network. We determine the user association by a proposed stable matching with externalities algorithm which also takes the beamforming vectors at the base stations into account. The merit in the stable matching model is the distributed implementation aspects. Each user asks to be matched with a base station according to his preferences, and each base station decides independently which users to accept. Simulation results reveal efficient distributed operation of the system compared to matching without externalities.

  • 23.
    Mochaourab, Rami
    et al.
    KTH Royal Institute of Technology, Sweden.
    Bjornson, Emil
    Linköping University, Sweden.
    Bengtsson, Mats
    KTH Royal Institute of Technology, Sweden.
    Adaptive Pilot Clustering in Heterogeneous Massive MIMO Networks2016Ingår i: IEEE Transactions on Wireless Communications, Vol. 15, nr 8, s. 5555-5568Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We consider the uplink of a cellular massive multiple-input multiple-output network. Acquiring channel state information at the base stations (BSs) requires uplink pilot signaling. Since the number of orthogonal pilot sequences is limited by the channel coherence, pilot reuse across cells is necessary to achieve high spectral efficiency. However, finding efficient pilot reuse patterns is non-trivial, especially in practical asymmetric BS deployments. We approach this problem using the coalitional game theory. Each BS has a few unique pilots and can form coalitions with other BSs to gain access to more pilots. The BSs in a coalition, thus, benefit from serving more users in their cells at the expense of higher pilot contamination and interference. Given that a cell’s average spectral efficiency depends on the overall pilot reuse pattern, the suitable coalitional game model is in the partition form. We develop a low-complexity distributed coalition formation based on individual stability. By incorporating a BS intercommunication budget constraint, we are able to control the overhead in message exchange between the BSs and ensure the algorithm’s convergence to a solution of the game called individually stable coalition structure. Simulation results reveal fast algorithmic convergence and substantial performance gains over the baseline schemes with no pilot reuse, full pilot reuse, or random pilot reuse pattern.

  • 24.
    Mochaourab, Rami
    et al.
    KTH Royal Institute of Technology, Sweden.
    Björnson, Emil
    Linköping University, Sweden.
    Bengtsson, Mats
    KTH Royal Institute of Technology, Sweden.
    Pilot Clustering in Asymmetric Massive MIMO Networks2015Ingår i: 2015 IEEE 16th International Workshop on Signal Processing Advances in Wireless Communications (SPAWC), 2015, s. 231-235Konferensbidrag (Refereegranskat)
    Abstract [en]

    We consider the uplink of a cellular massive MIMO network. Since the spectral efficiency of these networks is limited by pilot contamination, the pilot allocation across cells is of paramount importance. However, finding efficient pilot reuse patterns is non-trivial especially in practical asymmetric base station deployments. In this paper, we approach this problem using coalitional game theory. Each cell has its own unique pilots and can form coalitions with other cells to gain access to more pilots. We develop a low-complexity distributed algorithm and prove convergence to an individually stable coalition structure. Simulations reveal fast algorithmic convergence and substantial performance gains over one-cell coalitions and full pilot reuse.

  • 25.
    Mochaourab, Rami
    et al.
    KTH Royal Institute of Technology, Sweden.
    Brandt, Rasmus
    KTH Royal Institute of Technology, Sweden.
    Ghauch, Hadi
    KTH Royal Institute of Technology, Sweden.
    Bengtsson, Mats
    KTH Royal Institute of Technology, Sweden.
    Overhead-Aware Distributed CSI Selection in the MIMO Interference Channel2015Ingår i: 2015 23rd European Signal Processing Conference, EUSIPCO 2015, Institute of Electrical and Electronics Engineers (IEEE), 2015, s. 1038-1042Konferensbidrag (Refereegranskat)
    Abstract [en]

    We consider a MEMO interference channel in which the transmitters and receivers operate in frequency-division duplex mode. In this setting, interference management through coordinated transceiver design necessitates channel state information at the transmitters (CSI-T). The acquisition of CSI T is done through feedback from the receivers, which entitles a loss in degrees of freedom, due to training and feedback. This loss increases with the amount of CSI-T. In this work, after formulating an overhead model for CSI acquisition at the transmitters, we propose a distributed mechanism to find for each transmitter a subset of the complete CSI, which is used to perform interference management. The mechanism is based on many-to-many stable matching. We prove the existence of a stable matching and exploit an algorithm to reach it. Simulation results show performance improvement compared to full and minimal CSI-T.

  • 26.
    Mochaourab, Rami
    et al.
    Fraunhofer, Germany.
    Cao, P.
    Technical University of Dresden, Germany.
    Jorswieck, E.
    Technical University of Dresden, Germany.
    Alternating Rate Profile Optimization in Single Stream MIMO Interference Channels2013Ingår i: ICASSP, IEEE International Conference on Acoustics, Speech and Signal Processing - Proceedings, 2013, 2013, s. 4834-4838Konferensbidrag (Refereegranskat)
    Abstract [en]

    We consider a set of transmitter-receiver pairs operating concurrently in the same spectral band. The transmitters and receivers are equipped with multiple antennas and are restricted to apply single stream beamforming. This setting corresponds to the single stream multiple-input multiple-output (MIMO) interference channel. We assume perfect channel state information at the transmitters and the single-user decoding receivers. Efficient operating points in this setting correspond to points on the Pareto boundary of the achievable rate region. Characterizing all Pareto optimal points in the MIMO interference channel is still an unsolved problem. An approach to attain different Pareto optimal points in the MIMO interference channel is rate profile optimization. Given the nonconvexity of the problem, we propose an alternating approach based on successive optimization of the transmit and receive beamforming vectors. For fixed receive beamforming vectors, a solution for the rate profile optimization exists and is solved by a set of convex feasibility problems. For fixed transmit beamforming vectors, we show that the rate profile optimization can be solved by a set of feasibility problems each corresponding to an inverse field of values problem. The convergence of the alternating algorithm is guaranteed to a stationary point of the original problem.

  • 27.
    Mochaourab, Rami
    et al.
    Fraunhofer, Germany.
    Cao, P.
    TU Dresden, Germany.
    Jorswieck, E.
    TU Dresden, Germany.
    Alternating Rate Profile Optimization in Single Stream MIMO Interference Channels2014Ingår i: IEEE Signal Processing Letters, Vol. 21, nr 2, s. 221-224Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The multiple-input multiple-output interference channel is considered with perfect channel information at the transmitters and single-user decoding receivers. With all transmissions restricted to single stream beamforming, we consider the problem of finding all Pareto optimal rate-tuples in the achievable rate region. The problem is cast as a rate profile optimization problem. Due to its nonconvexity, we resort to an alternating approach: For fixed receivers, optimal transmission is known. For fixed transmitters, we show that optimal receive beamforming is a solution to an inverse field of values problem. We prove the solution’s stationarity and compare it with existing approaches.

  • 28.
    Mochaourab, Rami
    et al.
    Dresden University of Technology, Germany.
    Chong, Z.
    Dresden University of Technology, Germany.
    Jorswieck, E.
    Dresden University of Technology, Germany.
    Pricing in Noncooperative Interference Channels for Improved Energy-Efficiency2010Ingår i: Proceedings of the 5th International Conference on Cognitive Radio Oriented Wireless Networks and Communications, CROWNCom 2010, 2010Konferensbidrag (Refereegranskat)
    Abstract [en]

    We consider noncooperative energy-efficient resource allocation in the interference channel. Energy-efficiency is achieved when each system pays a price proportional to its allocated transmit power. In noncooperative game-theoretic notation, the power allocation chosen by the systems corresponds to the Nash equilibrium. We study the existence and characterize the uniqueness of this equilibrium. Afterwards, pricing to achieve energy-efficiency is examined. We introduce an arbitrator who determines the prices that satisfy minimum QoS requirements and minimize total power consumption. This energy-efficient assignment problem is formulated and solved. Simulation results on energy-efficiency are then given where we compare our setting to the one without pricing. It is observed that pricing in this distributed setting achieves higher energy-efficiency in different interference regimes.

  • 29.
    Mochaourab, Rami
    et al.
    Dresden University of Technology, Germany.
    Dargie, W.
    Dresden University of Technology, Germany.
    A Fair and Energy-Efficient Topology Control Protocol for Wireless Sensor Networks2008Ingår i: Proceedings of the 2nd ACM International Conference on Context-awareness for Self-managing Systems, CASEMANS’08, 2008, s. 6-15Konferensbidrag (Refereegranskat)
    Abstract [en]

    In energy constrained wireless sensor networks, energy conservation techniques are to be applied in order to maximize the system lifetime. We tackle the problem of increasing network lifetime through the topology control assignment. In a two-dimensional random sensor deployment, the nodes can estimate the distances to their neighbors and can vary their transmission ranges accordingly. Supporting self-organization of the sensor nodes, each node locally selects its appropriate neighbors according to a neighbor eligibility metric. Here, we introduce the notion of weighted relaying regions defined over the plane of a searching node. This is aimed at dropping out inefficient links in the network in order to reduce the overall energy consumption. Contrary to most topology control protocols that rely on nearest neighbor approaches, we use a distance measure that is radio characteristic and channel condition dependent. This in turn, proves more adequate for energy conservation in dense network deployments. Considering network dynamics that might arise due to node mobility or node failures, our topology control protocol is to be run periodically. Fairness between the nodes can be increased in updating the topology considering the changing energy reserves of the nodes. We verify the performance of the protocol through simulation results on network graph properties and energy consumption.

  • 30.
    Mochaourab, Rami
    et al.
    RISE., Swedish ICT, Acreo.
    Holfeld, Bernd
    Fraunhofer, Germany.
    Wirth, Thomas
    Fraunhofer, Germany.
    Distributed Channel Assignment in Cognitive Radio Networks: Stable Matching and Walrasian Equilibrium2015Ingår i: IEEE Transactions on Wireless Communications, Vol. 14, nr 7, s. 3924-3936Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We consider a set of secondary transmitter-receiver pairs in a cognitive radio setting. Based on channel sensing and access performances, we consider the problem of assigning channels orthogonally to secondary users through distributed coordination and cooperation algorithms. Two economic models are applied for this purpose: matching markets and competitive markets. In the matching market model, secondary users and channels build two agent sets. We implement a stable matching algorithm in which each secondary user, based on his achievable rate, proposes to the coordinator to be matched with desirable channels. The coordinator accepts or rejects the proposals based on the channel preferences which depend on interference from the secondary user. The coordination algorithm is of low complexity and can adapt to network dynamics. In the competitive market model, channels are associated with prices and secondary users are endowed with monetary budget. Each secondary user, based on his utility function and current channel prices, demands a set of channels. A Walrasian equilibrium maximizes the sum utility and equates the channel demand to their supply. We prove the existence of Walrasian equilibrium and propose a cooperative mechanism to reach it. The performance and complexity of the proposed solutions are illustrated by numerical simulations.

  • 31.
    Mochaourab, Rami
    et al.
    Dresden University of Technology, Germany.
    Jorswieck, E.
    Dresden University of Technology, Germany.
    Beamforming in Interference Networks for Uniform Linear Arrays2011Ingår i: Proceedings of the 5th European Conference on Antennas and Propagation (EUCAP), 2011, s. 2445-2449Konferensbidrag (Refereegranskat)
    Abstract [en]

    We consider a multiple-input single-output interference channel where each transmitter is equipped with a uniform linear array. By controlling the geometry of the array, i.e. adapting the antenna spacing, the rotation of the array, and the number of antenna elements, we investigate whether the capacity of the channel can be achieved with single user decoding capabilities at the receivers. This objective is reached when it is possible for each transmitter to perform maximum ratio transmission to its intended receiver while simultaneously nulling the interference at all unintended receivers. We provide for the two and three user case the necessary antenna spacing and rotation angle of the array in closed form. For the four user case, an integer programming problem is formulated which additionally determines the required number of antennas.

  • 32.
    Mochaourab, Rami
    et al.
    Dresden University of Technology, Germany.
    Jorswieck, E.
    Dresden University of Technology, Germany.
    Coalition Formation in MISO Interference Channels2011Ingår i: 4th IEEE International Workshop on Computational Advances in Multi-Sensor Adaptive Processing, CAMSAP 2011, 2011, s. 237-240Konferensbidrag (Refereegranskat)
    Abstract [en]

    We consider a multi-link multiple-input single-output interference channel. A link in this setting is noncooperative if its transmission does not take into account the interference it generates at other links. Noncooperative operation of the links is generally not efficient. To improve this situation, we study link cooperation via coalitional games. In coalitional games, a player has an incentive to cooperate with other players if this improves his payoff. We model the setting as a game in coalitional form without transferable utility. The players (links) in a coalition either perform zero forcing transmission or Wiener filter precoding to each other. Necessary and sufficient conditions, in terms of a lower signal-to-noise ratio (SNR) threshold, are provided under which all players have the incentive to cooperate and form a grand coalition. Inaddition, we provide sufficient conditions under which all players have no incentive to cooperate. In this case, the SNR has to be below a specified SNR threshold. Hence, there exists an SNR range in which the links would profit in forming subcoalitions. Therefore, we turn our attention to coalition formation games between the links. We utilize a coalition formation algorithm, called merge-and-split, to determine stable user grouping. Numerical results show that while in the low SNR regime noncooperation is efficient with single-player coalitions, in the high SNR regime all users benefit in forming a grand coalition. Coalition formation shows its significance in the mid SNR regime where subset user cooperation provides joint performance gains.

  • 33.
    Mochaourab, Rami
    et al.
    Dresden University of Technology, Germany.
    Jorswieck, E.
    Dresden University of Technology, Germany.
    Resource Allocation in Protected and Shared Bands: Uniqueness and Efficiency of Nash Equilibria2009Konferensbidrag (Refereegranskat)
    Abstract [en]

    Motivated by the question, is non-cooperative spectrum sharing desirable or not, we consider a scenario utilizing protected and shared bands. In a static non-cooperative setting consisting of two communication system pairs, we study the existence, uniqueness and efficiency of a fixed point of the iterative water-filling algorithm which corresponds to the Nash equilibrium. There exist several sufficient conditions for the convergence of the algorithm in the literature mostly based on the contraction mapping theorem. We derive necessary and sufficient conditions for convergence by relating the game to supermodular games. There, the best response dynamics is globally convergent when a unique Nash equilibrium exists. In order to understand the loss in efficiency due to non-cooperation, we study the Price of Anarchy of the system. We show that the performance of the noncooperative system cannot fall below two third of that of the cooperative system in the high signal to noise ratio regime. Theoretical results are illustrated by numerical simulations for a simplified system scenario.

  • 34.
    Mochaourab, Rami
    et al.
    Dresden University of Technology, Germany.
    Jorswieck, E.
    Dresden University of Technology, Germany.
    Robust Pareto Optimal Beamforming in Two-User Multiple-Input Single-Output Interference Channel2011Ingår i: 19TH EUROPEAN SIGNAL PROCESSING CONFERENCE (EUSIPCO-2011), 2011Konferensbidrag (Refereegranskat)
    Abstract [en]

    We consider a two-user multiple-input single-output interference channel in which the receivers treat interference as additive noise. The transmitters are assumed to have imperfect channel state information to the receivers. The transmitters choose their beamforming vectors considering worst case power gains at the receivers. We provide a real-valued parametrization of the beamforming vectors that achieve the Pareto boundary of the rate region with channel uncertainties. Simulation results and high SNR analysis show that the gain in spectrum sharing under imperfect channel state information converges to the setting of time division multiple access. Moreover, we provide analytical results for the maximum sum rate in asymptotic cases of low and high signal to noise ratios.

  • 35.
    Mochaourab, Rami
    et al.
    Dresden University of Technology, Germany.
    Jorswieck, E.
    Dresden University of Technology, Germany.
    Walrasian Equilibrium in Two-User Multiple-Input Single-Output Interference Channel2011Ingår i: 2011 IEEE INTERNATIONAL CONFERENCE ON COMMUNICATIONS (ICC), 2011Konferensbidrag (Refereegranskat)
    Abstract [en]

    We propose a decentralized resource allocation scheme in the two-user multiple-input single-output interference channel. The mechanism is motivated by economic models which define equilibria in competitive settings. We model the situation between the links as a competitive market where the links are consumers, the transmission strategies are goods. In Walrasian equilibrium, the demand of each good equals the supply which constitutes an efficient operating point. For the two-user case, the Walrasian equilibrium and the corresponding prices can be computed in closed form. An arbitrator with perfect channel knowledge computes and distributes the Walrasian prices to the consumers (transmitters) which calculate in a decentralized manner their optimal demand (beamforming) of each good subject to their budget constraint (initial maximum ratio transmission solution). The Walrasian equilibrium is Pareto optimal and dominates the Nash equilibrium. Moreover, utilizing the conflict representation of the consumers in the Edgeworth box, we provide the closed-form solution to all Pareto optimal points for the two-user case.

  • 36.
    Mochaourab, Rami
    et al.
    Fraunhofer, Germany.
    Jorswieck, E.
    Dresden University of Technology, Germany.
    Walrasian Equilibrium Power Allocation in Protected and Shared Bands2012Ingår i: 2012 6TH INTERNATIONAL CONFERENCE ON NETWORK GAMES, CONTROL AND OPTIMIZATION (NETGCOOP), 2012Konferensbidrag (Refereegranskat)
    Abstract [en]

    We consider two transmitter-receiver pairs (links) operating on protected and shared frequency bands. Each link has a frequency band to use exclusively without any interference from the other link. A frequency band is shared between the two links. We study the power allocation problem of the two transmitters in the available bands using models from microeconomic theory. We model the setting as a competitive market in which the links are the consumers and transmission power are goods which can be bought by the consumers at specific prices. We assume there exists an authority, the arbitrator, which determines the prices of the goods and forwards them to the consumers. The Walrasian equilibrium in this model is Pareto optimal and corresponds to the prices that equate the demand to the supply of goods. We show that the Walrasian equilibrium always exists in our setting. We provide sufficient conditions for the uniqueness of the Walrasian equilibrium and also for the global stability of the price adjustment process to reach the equilibrium.

  • 37.
    Mochaourab, Rami
    et al.
    Dresden University of Technology, Germany.
    Jorswieck, E. A.
    Dresden University of Technology, Germany.
    Exchange economy in two-user multiple-input single-output interference channels2012Ingår i: IEEE Journal on Selected Topics in Signal Processing, Vol. 6, nr 2, s. 151-164Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We study the conflict between two links in a multiple- input single-output interference channel. This setting is strictly competitive and can be related to perfectly competitive market models. In such models, general equilibrium theory is used to determine equilibrium measures that are Pareto optimal. First, we consider the links to be consumers that can trade goods within themselves. The goods in our setting correspond to beamforming vectors. We utilize the conflict representation of the consumers in the Edgeworth box, a graphical tool that depicts the allocation of the goods for the two consumers, to provide closed-form solution to all Pareto optimal outcomes. Afterwards, we model the situation between the links as a competitive market which additionally defines prices for the goods. The equilibrium in this economy is called Walrasian and corresponds to the prices that equate the demand to the supply of goods. We calculate the unique Walrasian equilibrium and propose a coordination process that is realized by an arbitrator which distributes the Walrasian prices to the consumers. The consumers then calculate in a decentralized manner their optimal demand corresponding to beamforming vectors that achieve the Walrasian equilibrium. This outcome is Pareto optimal and dominates the noncooperative outcome of the systems. Thus, based on the game theoretic model and solution concept, an algorithm for a distributed implementation of the beamforming problem in multiple-input single-output interference channels is provided.

  • 38.
    Mochaourab, Rami
    et al.
    Dresden University of Technology, Germany.
    Jorswieck, E. A.
    Dresden University of Technology, Germany.
    Optimal Beamforming in Interference Networks with Perfect Local Channel Information2011Ingår i: IEEE Transactions on Signal Processing, Vol. 59, nr 3, s. 1128-1141Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We consider settings in which T multi-antenna transmitters and K single-antenna receivers concurrently utilize the available communication resources. Each transmitter sends useful information only to its intended receivers and can degrade the performance of unintended systems. Here, we assume the performance measures associated with each receiver are monotonic with the received power gains. In general, the joint performance of the systems is desired to be Pareto optimal. However, designing Pareto optimal resource allocation schemes is known to be difficult. In order to reduce the complexity of achieving efficient operating points, we show that it is sufficient to consider rank-1 transmit covariance matrices and propose a framework for determining the efficient beamforming vectors. These beamforming vectors are thereby also parameterized by T(K-1)real-valued parameters each between zero and one. The framework is based on analyzing each transmitter’s power gain-region which is composed of all jointly achievable power gains at the receivers. The efficient beamforming vectors are on a specific boundary section of the power gain-region, and in certain scenarios it is shown that it is necessary to perform additional power allocation on the beamforming vectors. Two examples which include broadcast and multicast data as well as a cognitive radio application scenario illustrate the results

  • 39.
    Mochaourab, Rami
    et al.
    Dresden University of Technology, Germany.
    Jorswieck, E. A.
    Dresden University of Technology, Germany.
    Robust Beamforming in Interference Channels with Imperfect Transmitter Channel Information2012Ingår i: Signal Processing, Vol. 92, nr 10, s. 2509-2518Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We consider K links operating concurrently in the same spectral band. Each transmitter has multiple antennas, while each receiver uses a single antenna. This setting corresponds to the multiple-input single-output interference channel. We assume perfect channel state information at the single-user decoding receivers whereas the transmitters only have estimates of the true channels. The channel estimation errors are assumed to be bounded in elliptical regions whose geometry is known at the transmitters. Robust beamforming optimizes worst-case received power gains, and a Pareto optimal point is a worst-case achievable rate tuple from which it is impossible to increase a link's performance without degrading the performance of another. We characterize the robust beamforming vectors necessary to operate at any Pareto optimal point. Moreover, these beamforming vectors are parameterized by real-valued parameters. We analyze the system's spectral efficiency at high and low signal-to-noise ratio (SNR). Zero forcing transmission achieves full multiplexing gain at high SNR only if the estimation errors scale linearly with inverse SNR. If the errors are SNR independent, then single-user transmission is optimal at high SNR. At low SNR, robust maximum ratio transmission optimizes the minimum energy per bit for reliable communication. Numerical simulations illustrate the gained theoretical results.

  • 40.
    Mochaourab, Rami
    et al.
    Dresden University of Technology, Germany.
    Jorswieck, E.
    Dresden University of Technology, Germany.
    Ho, K. M. Z.
    Eurecom Institute, France.
    Gesbert, D.
    Eurecom Institute, France.
    Bargaining and Beamforming in Interference Channels2010Ingår i: Conference Record - Asilomar Conference on Signals, Systems and Computers 2010, 2010, s. 272-276Konferensbidrag (Refereegranskat)
    Abstract [en]

    Utilizing the real-valued parametrization of each transmitter’s efficient beamforming vectors, we propose a decentralized resource allocation scheme in the multiple-input single-output interference channel. The scheme is motivated by bargaining concepts in game theory. The aim of these concepts is to improve the joint payoff of the users from the Nash equilibrium outcome. In each bargaining-step, each user proposes a strategy. A user accepts any proposal if it increases his payoff. Otherwise, new proposals are made. When all proposals are accepted, a new bargaining-stage begins. We prove the scheme’s convergence and demonstrate its performance by simulations. In comparison to previous approaches, our bargaining outcome is arbitrarily close to the Pareto boundary of the achievable single-user rate region. We further discuss the control overhead and complexity of this scheme.

  • 41.
    Mochaourab, Rami
    et al.
    KTH Royal Institute of Technology, Sweden.
    Jorswieck, Eduard A.
    TU Dresden, Germany.
    Coalitional Games in MISO Interference Channels: Epsilon-Core and Coalition Structure Stable Set2014Ingår i: IEEE Transactions on Signal Processing, Vol. 62, nr 24, s. 6507-6520Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The multiple-input single-output interference channel is considered. Each transmitter is assumed to know the channels between itself and all receivers perfectly and the receivers are assumed to treat interference as additive noise. In this setting, noncooperative transmission does not take into account the interference generated at other receivers which generally leads to inefficient performance of the links. To improve this situation, we study cooperation between the links using coalitional games. The players ( links) in a coalition either perform zero forcing transmission or Wiener filter precoding to each other. The epsilon-core is a solution concept for coalitional games that takes into account the overhead required in coalition deviation. We provide necessary and sufficient conditions for the strong and weak epsilon-core of our coalitional game not to be empty with zero forcing transmission. Since, the epsilon-core only considers the possibility of joint cooperation of all links, we study coalitional games in partition form in which several distinct coalitions can form. We propose a polynomial-time distributed coalition formation algorithm based on coalition merging and prove that its solution lies in the coalition structure stable set of our coalition formation game. Simulation results reveal the cooperation gains for different coalition formation complexities and deviation overhead models.

  • 42.
    Mochaourab, Rami
    et al.
    RISE - Research Institutes of Sweden, ICT, Acreo.
    Oechtering, Tobias J.
    KTH Royal Institute of Technology, Sweden.
    Private Filtering for Hidden Markov Models2018Ingår i: IEEE Signal Processing Letters, ISSN 1070-9908, E-ISSN 1558-2361, Vol. 25, nr 6Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Consider a hidden Markov model describing a system with two types of states: a monitored state and a private state. The two types of states are dependent and evolve jointly according to a Markov process with a stationary transition probability. It is desired to reveal the monitored states to a receiver but hide the private states. For this purpose, a privacy filter is necessary which suitably perturbs the monitored states before communication to the receiver. Our objective is to design the privacy filter to optimize the trade-off between monitoring accuracy and privacy, measured through a time-invariant distortion measure and Shannon's equivocation, respectively. As the optimal privacy filter is difficult to compute using dynamic programming, we adopt a suboptimal greedy approach through which the privacy filter can be computed efficiently. Here, the greedy approach has the additional advantage of not being restricted to finite time horizon setups. Simulations show the superiority of the approach compared to a privacy filter which only adds independent noise to the observations. 

  • 43.
    Mochaourab, Rami
    et al.
    RISE Research Institutes of Sweden, Digitala system, Industriella system.
    Sinha, Sugandh
    RISE Research Institutes of Sweden, Digitala system, Industriella system.
    Greenstein, Stanley
    Stockholm University, Sweden.
    Papapetrou, Panagiotis
    Stockholm University, Sweden.
    Demonstrator on Counterfactual Explanations for Differentially Private Support Vector Machines2022Ingår i: Proc of ECML PKDD 2022, 2022Konferensbidrag (Refereegranskat)
    Abstract [en]

    We demonstrate the construction of robust counterfactual explanations for support vector machines (SVM), where the privacy mechanism that publicly releases the classifier guarantees differential privacy. Privacy preservation is essential when dealing with sensitive data, such as in applications within the health domain. In addition, providing explanations for machine learning predictions is an important requirement within so-called high risk applications, as referred to in the EU AI Act. Thus, the innovative aspects of this work correspond to studying the interaction between three desired aspects: accuracy, privacy, and explainability. The SVM classification accuracy is affected by the privacy mechanism through the introduced perturbations in the classifier weights. Consequently, we need to consider a trade-off between accuracy and privacy. In addition, counterfactual explanations, which quantify the smallest changes to selected data instances in order to change their classification, may become not credible when we have data privacy guarantees. Hence, robustness for counterfactual explanations is needed in order to create confidence about the credibility of the explanations. Our demonstrator provides an interactive environment to show the interplay between the considered aspects of accuracy, privacy, and explainability.

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  • 44.
    Mochaourab, Rami
    et al.
    RISE Research Institutes of Sweden, Digitala system, Industriella system.
    Sinha, Sugandh
    RISE Research Institutes of Sweden, Digitala system, Industriella system.
    Greenstein, Stanley
    Stockholm University, Sweden.
    Papapetrou, Panagiotis
    Stockholm University, Sweden.
    Demonstrator on Counterfactual Explanations for Differentially Private Support Vector Machines2023Ingår i: Lecture Notes in Computer Science , Springer Science and Business Media Deutschland GmbH , 2023, Vol. 13718, s. 662-666Konferensbidrag (Refereegranskat)
    Abstract [en]

    We demonstrate the construction of robust counterfactual explanations for support vector machines (SVM), where the privacy mechanism that publicly releases the classifier guarantees differential privacy. Privacy preservation is essential when dealing with sensitive data, such as in applications within the health domain. In addition, providing explanations for machine learning predictions is an important requirement within so-called high risk applications, as referred to in the EU AI Act. Thus, the innovative aspects of this work correspond to studying the interaction between three desired aspects: accuracy, privacy, and explainability. The SVM classification accuracy is affected by the privacy mechanism through the introduced perturbations in the classifier weights. Consequently, we need to consider a trade-off between accuracy and privacy. In addition, counterfactual explanations, which quantify the smallest changes to selected data instances in order to change their classification, may become not credible when we have data privacy guarantees. Hence, robustness for counterfactual explanations is needed in order to create confidence about the credibility of the explanations. Our demonstrator provides an interactive environment to show the interplay between the considered aspects of accuracy, privacy, and explainability. 

  • 45.
    Mochaourab, Rami
    et al.
    RISE Research Institutes of Sweden, Digitala system, Industriella system.
    Sinha, Sugandh
    RISE Research Institutes of Sweden, Digitala system, Industriella system.
    Greenstein, Stanley
    Stockholm University, Sweden.
    Papapetrou, Panagiotis
    Stockholm University, Sweden.
    Robust Counterfactual Explanations for Privacy-Preserving SVM2021Ingår i: Proc of ICML 2021, 2021Konferensbidrag (Refereegranskat)
    Abstract [en]

    We consider counterfactual explanations for privacy-preserving support vector machines (SVM), where the privacy mechanism that publicly releases the classifier guarantees differential privacy. While privacy preservation is essential when dealing with sensitive data, there is a consequent degradation in the classification accuracy due to the introduced perturbations in the classifier weights. Therefore, counterfactual explanations need to be made robust against such perturbations in order to ensure, with high confidence, that the explanations are valid. In this work, we suitably model the uncertainties in the SVM weights and formulate the robust counterfactual explanation problem. Then, we study optimal and efficient suboptimal algorithms for its solution. Experimental results illustrate the connections between privacy levels, classifier accuracy, and the confidence levels that validate the counterfactual explanations.

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  • 46.
    Mochaourab, Rami
    et al.
    RISE Research Institutes of Sweden, Digitala system, Industriella system.
    Venkitaraman, Arun
    School of Electrical Engineering, Switzerland.
    Samsten, Isak
    Stockholm University, Sweden.
    Papapetrou, Panagiotis
    Stockholm University, Sweden.
    Rojas, Cristian
    KTH Royal Institute of Technology, Sweden.
    Post-hoc Explainability for Time Series Classification: Towards a Signal Processing Perspective2022Ingår i: IEEE signal processing magazine (Print), ISSN 1053-5888, E-ISSN 1558-0792, Vol. 39, nr 4, s. 119-129Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Time series data correspond to observations of phenomena that are recorded over time [1]. Such data are encountered regularly in a wide range of applications, such as speech and music recognition, monitoring health and medical diagnosis, financial analysis, motion tracking, and shape identification, to name a few. With such a diversity of applications and the large variations in their characteristics, time series classification is a complex and challenging task. One of the fundamental steps in the design of time series classifiers is that of defining or constructing the discriminant features that help differentiate between classes. This is typically achieved by designing novel representation techniques [2] that transform the raw time series data to a new data domain, where subsequently a classifier is trained on the transformed data, such as one-nearest neighbors [3] or random forests [4]. In recent time series classification approaches, deep neural network models have been employed that are able to jointly learn a representation of time series and perform classification [5]. In many of these sophisticated approaches, the discriminant features tend to be complicated to analyze and interpret, given the high degree of nonlinearity.

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  • 47.
    Mochaourab, Rami
    et al.
    Dresden University of Technology, Germany.
    Zorba, N.
    University of Jordan, Jordan.
    Jorswieck, E
    Dresden University of Technology, Germany.
    Nash Equilibrium in Multiple Antennas Protected and Shared Bands2012Ingår i: Proceedings of the International Symposium on Wireless Communication Systems 2012, 2012, s. 101-105Konferensbidrag (Refereegranskat)
    Abstract [en]

    Two non-cooperating cells are considered, each with its protected band to provide service to its high priority users. A shared band for the two cells is employed to deliver service to low priority users. We formulate the situation between the two cells as a non-cooperative game and study its Nash equilibrium. We prove that the game belongs to a class of games called supermodular games which have several interesting properties, such as the global stability of a unique Nash equilibrium. We provide a sufficient condition for the existence of a unique Nash equilibrium and study its efficiency by extensive simulations.

  • 48.
    Ok, Jungseul
    et al.
    KTH Royal Institute of Technology, Sweden.
    Se-Young, Yun
    Korea Advanced Institute of Science and Technology, South Korea.
    Proutiere, Alexandre
    KTH Royal Institute of Technology, Sweden.
    Mochaourab, Rami
    RISE Research Institutes of Sweden. KTH Royal Institute of Technology, Sweden.
    Collaborative Clustering: Sample Complexity and Efficient Algorithms2017Ingår i: Proceedings of Machine Learning Research 76:1–42, 2017, 2017, Vol. 76, s. 288-329Konferensbidrag (Refereegranskat)
  • 49.
    Wang, Zhendong
    et al.
    Stockholm University, Sweden.
    Samsten, Isak
    Stockholm University, Sweden.
    Miliou, Ioanna
    Stockholm University, Sweden.
    Mochaourab, Rami
    RISE Research Institutes of Sweden, Digitala system, Industriella system.
    Papapetrou, Panagiotis
    Stockholm University, Sweden.
    Glacier: guided locally constrained counterfactual explanations for time series classification2024Ingår i: Machine Learning, ISSN 0885-6125, E-ISSN 1573-0565Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    In machine learning applications, there is a need to obtain predictive models of high performance and, most importantly, to allow end-users and practitioners to understand and act on their predictions. One way to obtain such understanding is via counterfactuals, that provide sample-based explanations in the form of recommendations on which features need to be modified from a test example so that the classification outcome of a given classifier changes from an undesired outcome to a desired one. This paper focuses on the domain of time series classification, more specifically, on defining counterfactual explanations for univariate time series. We propose Glacier, a model-agnostic method for generating locally-constrained counterfactual explanations for time series classification using gradient search either on the original space or on a latent space that is learned through an auto-encoder. An additional flexibility of our method is the inclusion of constraints on the counterfactual generation process that favour applying changes to particular time series points or segments while discouraging changing others. The main purpose of these constraints is to ensure more reliable counterfactuals, while increasing the efficiency of the counterfactual generation process. Two particular types of constraints are considered, i.e., example-specific constraints and global constraints. We conduct extensive experiments on 40 datasets from the UCR archive, comparing different instantiations of Glacier against three competitors. Our findings suggest that Glacier outperforms the three competitors in terms of two common metrics for counterfactuals, i.e., proximity and compactness. Moreover, Glacier obtains comparable counterfactual validity compared to the best of the three competitors. Finally, when comparing the unconstrained variant of Glacier to the constraint-based variants, we conclude that the inclusion of example-specific and global constraints yields a good performance while demonstrating the trade-off between the different metrics. © The Author(s) 2024.

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  • 50.
    Wang, Zhendong
    et al.
    Stockholm University, Sweden.
    Samsten, Isak
    Stockholm University, Sweden.
    Mochaourab, Rami
    RISE Research Institutes of Sweden, Digitala system, Industriella system.
    Papapetrou, Panagiotis
    Stockholm University, Sweden.
    Learning Time Series Counterfactuals via Latent Space Representations2021Ingår i: Soares C., Torgo L. (eds) Discovery Science. DS 2021. Lecture Notes in Computer Science, vol 12986. Springer, Cham. https://doi.org/10.1007/978-3-030-88942-5_29, 2021, s. 369-384Konferensbidrag (Refereegranskat)
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