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  • 2551.
    Zetterberg, Per
    KTH, School of Electrical Engineering (EES), Signal Processing. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    INTERFERENCE ALIGNMENT (IA) AND COORDINATED MULTI-POINT (COMP) WITH IEEE802.11AC FEEDBACK COMPRESSION: TESTBED RESULTS2014In: Proceedings of the IEEE International Conference on Acoustics, Speech and Signal Processing, ISSN 1520-6149, p. 6854791-Article in journal (Refereed)
    Abstract [en]

    We have implemented interference alignment (IA) and joint transmission coordinated multipoint (CoMP) on a wireless testbed using the feedback compression scheme of the new 802.11ac standard. The performance as a function of the frequency domain granularity is assessed. Realistic throughput gains are obtained by probing each spatial modulation stream with ten different coding and modulation schemes. The gain of IA and CoMP over TDMA MIMO is found to be 26% and 71%, respectively under stationary conditions. In our dense indoor office deployment, the frequency domain granularity of the feedback can be reduced down to every 8th subcarrier (2.5MHz), without sacrificing performance.

  • 2552.
    Zetterberg, Per
    KTH, School of Electrical Engineering (EES), Signal Processing. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Method for Decreasing Fading in a Telecommunication System1998Patent (Other (popular science, discussion, etc.))
    Abstract [en]

    The present invention provides a method for decreasing fading and more particularly the down-link fading in a mobile telecommunication system having transmit lobe diversity. The telecommunication system comprises at least one base station having an antenna array providing lobe diversity, and communicating with mobile terminal units. According to the invention, the average path-loss between the base station and a mobile terminal unit is measured by the base station in order to select a set of alternative lobes. The instantaneous down-link path-loss of the alternative lobes between the base station and the mobile terminal unit is also measured by the mobile in order to select a communication lobe from said set of alternative lobes, such that the communication is effected in the communication lobe.

  • 2553.
    Zetterberg, Per
    KTH, School of Electrical Engineering (EES), Signal Processing. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Performance of Antenna Tilting and Beamforming in an Urban Macrocell2009In: 2009 1ST INTERNATIONAL CONFERENCE ON WIRELESS COMMUNICATION, VEHICULAR TECHNOLOGY, INFORMATION THEORY AND AEROSPACE & ELECTRONIC SYSTEMS TECHNOLOGY, VOLS 1 AND 2, NEW YORK: IEEE , 2009, p. 197-202Conference paper (Refereed)
    Abstract [en]

    In this paper we investigate the use of very high gain antenna columns (up to twelve stacked 8dBi elements) in an urban macro-cell. We also investigate electrical downtilt, and horizontal beamforming using an array of very high gain antenna columns. In particular we investigate simultaneous beamforming and electrical downtilt. The results show that the vertical gain is close to the theoretical frees-space gain, while electrical downtilt increases the effective path-loss slope. Interestingly, the electrical downtilt also increases the azimuth RMS-angle-spread and thereby decreases the horizontal gain achievable with beamforming.

  • 2554.
    Zetterberg, Per
    KTH, School of Electrical Engineering (EES), Signal Processing. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Simulation of Spatially Correlated Large-Scale Parameters and Obtaining Model Parameters from Measurements2008Report (Other academic)
  • 2555.
    Zetterberg, Per
    KTH, School of Electrical Engineering (EES), Signal Processing. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    WIreless DEvelopment LABoratory(WIDELAB) Equipment Base2013Report (Other academic)
    Abstract [en]

    This paper documents some of the hard- and soft-ware used by the signal- processing group at KTH, in it’s experimental work on wireless systems. We call these items collectively WIreless DEvelopment LABoratory (WIDELAB), and this paper aims to document the most important pieces and some of the knowledge needed to understand, operate and develop them further. To access the hard and software described contact perz@s3.kth.se

  • 2556.
    Zetterberg, Per
    et al.
    KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Fardi, Ramin
    Open Source SDR Frontend and Measurements for 60-GHz Wireless Experimentation2015In: IEEE Access, E-ISSN 2169-3536, Vol. 3, p. 445-456Article in journal (Refereed)
    Abstract [en]

    This paper aims to make 60-GHz experimentation possible for a wider range of research groups. We do this by describing a low-cost front-end that can be used in combination with any baseband processing platform. We provide detailed instructions and software for connection with the USRP N200/N210, including general classes for controlling the board and example single-input single-output and 2 x 2 Multiple-Input Multiple Output applications. In addition, we provide measurements to assess the impact of phase noise and other hardware impairments in low-cost millimeter-wave systems for hybrid measurement and simulation studies. Finally, we also perform performance measurements on the hardware. All our materials, such as the hardware design, the software, and the measurements, are freely available.

  • 2557.
    Zetterberg, Per
    et al.
    KTH, School of Electrical Engineering (EES), Signal Processing. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Jaldén, Niklas
    Ottersten, Björn
    KTH, School of Electrical Engineering (EES), Signal Processing. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Bengtsson, Mats
    KTH, School of Electrical Engineering (EES), Signal Processing. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Investigation of Simple Algorithms for Estimation of Delay-Spread and Angle-Spread2012Report (Other academic)
    Abstract [en]

    In this paper we describe two simple methods for estimation of delay-spread and angle-spread, re-spectively. The algorithms are simple in the sense that the transmitted signal may consist of only threesuperimposed CW tones -and the receiver need only two antennas. The algorithms are also simple in thesense that the computational cost is very low. We verify the algorithms by applying them to wideband andmulti-antenna measurement data, respectively.

  • 2558.
    Zetterberg, Per
    et al.
    KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Mavrokefalidis, C.
    Lalos, A. S.
    Matigakis, E.
    Experimental Investigation of Cooperative Schemes on a Real-Time DSP-Based Testbed2009In: EURASIP Journal on Wireless Communications and Networking, ISSN 1687-1472, E-ISSN 1687-1499Article in journal (Refereed)
    Abstract [en]

    Experimental results on the well-known cooperating relaying schemes, amplify-and-forward (AF), detect-and-forward (DF), cooperative maximum ratio combining (CMRC), and distributed space-time coding (DSTC), are presented in this paper. A novel relaying scheme named "selection relaying" (SR), in which one of two relays are selected base on path-loss, is also tested. For all schemes except AF receive antenna diversity is as an option which can be switched on or off. For DF and DSTC a feature "selective" where the relay only forwards frames with a receive SNR above 6 dB is introduced. In our measurements, all cooperative relaying schemes above increase the coverage area as compared with direct transmission. The features "antenna diversity" and "selective" improve the performance. Good performance is obtained with CMRC, DSTC, and SR.

  • 2559.
    Zetterberg, Per
    et al.
    KTH, School of Electrical Engineering (EES), Signal Processing. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Mavrokefalidis, Christos
    Lalos, Aris
    Emmanouil, Matigakis
    Implementation and Testing of Co-Operative Schemes on a Real-Time DSP-Based Testbed2009In: ICT-MobileSummit 2009 Conference Proceedings / [ed] Paul Cunningham and Miriam Cunningham, 2009Conference paper (Refereed)
    Abstract [en]

    Experimental and simulation results on well-known co-operating relayingschemes such as: amplify-and-forward (AF), detect-and-forward (DF), co-operativemaximum-ratio combining (CMRC), and distributed space-time coding (DSTC) arepresented in this paper. A novel relaying scheme named “selection relaying” (SR), inwhich one of two relays are selected base on path-loss, is also tested.We also implement the modes “Selective” and “Antenna Diversity”. The “Selective”option is designed to avoid forwarding of erroneous frames.In our measurements, all co-operative relaying schemes above increase the coveragearea as compared with direct transmission. The features “Antenna diversity” and “Selective”improve the performance. Good performance is obtained with CMRC, DSTCand SR.

  • 2560.
    Zetterberg, Per
    et al.
    KTH, School of Electrical Engineering (EES), Signal Processing. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Moghadam, Nima N.
    KTH, School of Electrical Engineering (EES), Signal Processing. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    An experimental investigation of SIMO, MIMO, interference-alignment (IA) and coordinated multi-point (CoMP)2012In: 2012 19th International Conference on Systems, Signals and Image Processing, IWSSIP 2012, 2012, p. 211-216Conference paper (Refereed)
    Abstract [en]

    In this paper we present experimental implementations of interference alignment (IA) and coordinated multi-point transmission (CoMP). We provide results for a system with three base-stations and three mobile-stations all having two antennas. We further employ OFDM modulation, with high-order constellations, and measure many positions both line-of-sight and non-line-of-sight under interference limited conditions. We find the CoMP system to perform better than IA at the cost of a higher back-haul capacity requirement. During the measurements we also logged the channel estimates for offline processing. We use these channel estimates to calculate the performance under ideal conditions. The performance estimates obtained this way is substantially higher than what is actually observed in the end-to-end transmissions - in particular in the CoMP case where the theoretical performance is very high. We find the reason for this discrepancy to be the impact of dirty-RF effects such as phase-noise and non-linearities. We are able to model the dirty-RF effects to some extent. These models can be used to simulate more complex systems and still account for the dirty-RF effects (e.g., systems with tens of mobiles and base-stations). Both IA and CoMP perform better than reference implementations of single-user SIMO and MIMO in our measurements.

  • 2561.
    Zhang, Bo
    KTH, School of Electrical Engineering (EES), Communication Theory. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Location Based Pre-caching and Network Coding in Smart Content Distribution2013Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis
    Abstract [en]

    Since the boom of the smart phones, there is a huge amount of applicationsthat deal with data located in the cloud. This fact can makethese applications unavailable when the network is inaccessible due tocoverage or congestion. A solution to these problems have been designedand developed for the Android OS in this master thesis. The approachis the application of location-based pre-caching, downloading the contentof an application before the user enters in the zone where the applicationmay use this content. Network coding has also been introduced in orderto reduce the amount of data sent over the wireless networks. A cachingscheme is introduced in binary network coding and applied to the problemof retransmission in wireless broadcast network. A binary network codingalgorithm is designed which could asymptotically approach the efficiency of linear network coding with a much lower decoding complexity. 

  • 2562. Zhang, Fan
    et al.
    Xia, Weiguo
    KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Trentelman, Harry L.
    Johansson, Karl H.
    KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Scherpen, Jacquelien M. A.
    Robust synchronization of directed Lur'e networks with incremental nonlinearities2014Conference paper (Refereed)
    Abstract [en]

    In this paper we deal with robust synchronization problems for directed Lur'e networks subject to incrementally passive nonlinearities and incrementally sector bounded non-linearities, respectively. By making use of general algebraic connectivities of strongly connected graphs and subgraphs, sufficient synchronization conditions are obtained for diffusively interconnected identical Lur'e systems on both the strongly connected interconnection topology and the topology containing a directed spanning tree. The static feedback gain matrices are determined by the matrices defining the individual agent dynamics and the general algebraic connectivities. The synchronization criteria obtained in the present paper extend those for undirected Lur'e networks in our previous work.

  • 2563.
    Zhang, Guoqiang
    et al.
    KTH, School of Electrical Engineering (EES), Sound and Image Processing. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Kleijn, W. Bastiaan
    KTH, School of Electrical Engineering (EES), Sound and Image Processing. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    BOUNDING THE RATE REGION OF THE TWO-TERMINAL VECTOR GAUSSIAN CEO PROBLEM2011In: 2011 DATA COMPRESSION CONFERENCE (DCC), 2011, p. 488-488Conference paper (Refereed)
    Abstract [en]

    The rate region of the two-terminal vector Gaussian CEO problem is studied. A lower bound on the rate region is derived. It is obtained by lower-bounding a weighted sum rate for each supporting hyperplane of the rate region. The bound is in the form of a closed-form expression rather than the form of an optimization problem.

  • 2564.
    Zhang, Guoqiang
    et al.
    KTH, School of Electrical Engineering (EES), Sound and Image Processing. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Kleijn, W. Bastiaan
    KTH, School of Electrical Engineering (EES), Sound and Image Processing. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Østergaard, Jan
    KTH, School of Electrical Engineering (EES), Sound and Image Processing.
    Bounding the Rate Region of Vector Gaussian Multiple Descriptions with Individual and Central Receivers2010In: Data Compression Conference Proceedings, 2010, p. 13-19Conference paper (Refereed)
    Abstract [en]

    In this work, we study the rate region of the vector Gaussian multipledescription problem with individual and central quadratic distortion con-straints. In particular, we derive an outer bound to the rate region of theL-description problem. The bound is obtained by lower bounding a weightedsum rate for each supporting hyperplane of the rate region. The key ideais to introduce at most L-1 auxiliary random variables and further imposeupon the variables a Markov structure according to the ordering of the de-scription weights. This makes it possible to greatly simplify the derivationof the outer bound. In the scalar Gaussian case, the complete rate regionis fully characterized by showing that the outer bound is tight. In this case,the optimal weighted sum rate for each supporting hyperplane is obtained bysolving a single maximization problem. This contrasts with existing results,which require solving a min-max optimization problem.

  • 2565.
    Zhang, Guoqiang
    et al.
    KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre. KTH, School of Electrical Engineering (EES), Sound and Image Processing.
    Klejsa, Janusz
    KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Kleijn, W. Bastiaan
    KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre. KTH, School of Electrical Engineering (EES), Sound and Image Processing.
    ANALYSIS OF K-CHANNEL MULTIPLE DESCRIPTION QUANTIZATION: 2009 DATA COMPRESSION CONFERENCE, PROCEEDINGS2008In: DCC 2009: 2009 DATA COMPRESSION CONFERENCE, PROCEEDINGS / [ed] Storer JA, Marcellin MW, LOS ALAMITOS, CA: IEEE COMPUTER SOC , 2008, p. 53-62Conference paper (Refereed)
    Abstract [en]

    This paper studies the tight rate-distortion bound for K-channel symmetric multiple-description coding for a memoryless Gaussian source. We find that the product of a function of the individual side distortions (for single received descriptions) and the central distortion (for K received descriptions) is asymptotically independent of the redundancy among the descriptions. Using this property, we analyze the asymptotic behaviors of two different practical multiple-description lattice vector quantizers (MDLVQ). Our analysis includes the treatment of a MDLVQ system from a new geometric viewpoint, which results in an expression for the side distortions using the normalized second moment of a sphere of higher dimensionality than the quantization space. The expression of the distortion product derived from the lower bound is then applied as a criterion to assess the performance losses of the considered MDLVQ systems.

  • 2566.
    Zhang, Guoqiang
    et al.
    KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Lundin, Henrik
    KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Kleijn, W. Bastiaan
    KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Band control policy of playout scheduling for voice over IP2008In: Proceedings European Signal Processing Conference, 2008Conference paper (Refereed)
    Abstract [en]

    We study adaptive-playout scheduling for VoIP using the framework of stochastic impulse control theory. A Wiener process is introduced to model the fluctuation of the buffer length in the absence of control. In this context, the control signal consists of length units that correspond to inserting or dropping a pitch cycle. We define an optimality criterion that has an adjustable trade-off between average buffing delay and average control length (the length of the pitch cycles added plus the length of the pitch cycles dropped). The clock-drift effect is treated in a unified manner within this framework. A band control policy is shown to be optimal. The algorithm does not require knowledge of the clock drift. It maintains the buffer length within a band region by imposing impulse control (inserted or dropped pitch cycles) whenever the bounds of the band are reached. Our experiments show that the proposed method outperforms a popular reference method.

  • 2567. Zhang, H.
    et al.
    Qi, Y.
    Wu, Junfeng
    KTH, School of Electrical Engineering (EES), Automatic Control. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Optimal jamming power allocation against remote state estimation2017In: American Control Conference (ACC), 2017, Institute of Electrical and Electronics Engineers (IEEE), 2017, p. 1660-1665, article id 7963191Conference paper (Refereed)
    Abstract [en]

    This paper investigates a remote state estimation problem, where a smart sensor observes the state of a physical plant, locally estimates the state with Kalman filter, and then sends its local estimation data to a remote estimator through a communication network. There is a jamming attacker in the communication network who intentionally blocks the network with the purpose of deteriorating estimation quality subject to the jamming energy constraint. From the viewpoint of jamming attacker, in order to maximize attack effect, we focus on the questions 'when to jam the communication network', and 'how much power the attacker should adopt to jam the network'. After theoretic analysis, we point out that any grouped together attack schedule is optimal for the given jamming power. We provide a close form of jamming power when a sufficient condition holds, and the corresponding jamming effect on remote state estimation is explicitly presented. For a general case, we design a jamming power allocation algorithm and show the computational complexity of the proposed algorithm is not worse than O(T), where T is the length of the time horizon considered.

  • 2568. Zhang, H.
    et al.
    Zhao, Z.
    Meng, Ziyang
    KTH, School of Electrical Engineering (EES), Automatic Control. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Lin, Z.
    Experimental verification of a multi-robot distributed control algorithm with containment and group dispersion behaviors: The case of dynamic leaders ?2013In: IFAC Proceedings Volumes (IFAC-PapersOnline), 2013, no PART 1, p. 153-158Conference paper (Refereed)
    Abstract [en]

    This paper studies the containment and group dispersion control for a multi-robot system in the presence of dynamic leaders. Each robot is represented by a double-integrator dynamic model and a distributed control algorithm is developed to drive the multi-robot system to follow a group of dynamic leaders with containment and group dispersion behaviors. The effectiveness of the algorithm is then verified on a multi-robot control platform.

  • 2569. Zhang, Hejin
    et al.
    Zhao, Zhiyun
    Meng, Ziyang
    KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Lin, Zongli
    Experimental verification of a multi-robot distributed control algorithm with containment and group dispersion behaviors: The case of dynamic leaders2014In: IEEE/CAA Journal of Automatica Sinica, ISSN 2329-9266, Vol. 1, no 1, p. 54-60, article id 7004620Article in journal (Refereed)
    Abstract [en]

    This paper studies the containment and group dispersion control for a multi-robot system in the presence of dynamic leaders. Each robot is represented by a doubleintegrator dynamic model and a distributed control algorithm is developed to drive the multi-robot system to follow a group of dynamic leaders with containment and group dispersion behaviors. The effectiveness of the algorithm is then verified on a multi-robot control platform.

  • 2570.
    Zhang, Kuize
    et al.
    KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Zamani, Majid
    Infinite-step opacity of nondeterministic finite transition systems: A bisimulation relation approach2017In: 2017 IEEE 56th Annual Conference on Decision and Control, CDC 2017, IEEE , 2017, p. 5615-5619Conference paper (Refereed)
    Abstract [en]

    It is known that the problem of verifying the infinite-step opacity of nondeterministic finite transition systems (NFTSs)is PSPACE-hard. In this paper, we investigate whether it is possible to use classical bisimulation relation to come up with abstract NFTSs and verify the infinite-step opacity of original NFTSs over their abstractions. First, we show that generally bisimulation relation does not preserve infinitestep opacity. Second, by adding some additional conditions to bisimulation relation, we prove that a stronger version of bisimulation relation, called here opacity-preserving bisimulation relation, preserves infinite-step opacity. Therefore, if one can find an abstract NFTS for a large NFTS under an opacity-preserving bisimulation relation, then the infinite-step opacity of the original NFTS can be verified by investigating that over the abstract NFTS. Finally, we show that under some mild assumptions, the quotient relation between an NFTS and its quotient system becomes opacity-preserving bisimulation relation which provides a scheme for constructing opacity-preserving abstractions of large- scale NFTSs. We show the effectiveness of the results using several examples throughout the paper.

  • 2571. Zhang, Lin
    et al.
    Liu, Jiaqi
    Xiao, Ming
    KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre. KTH, School of Electrical Engineering (EES), Information Science and Engineering.
    Wu, Gang
    Liang, Ying-Chang
    Li, Shaoqian
    Performance Analysis and Optimization in Downlink NOMA Systems With Cooperative Full-Duplex Relaying2017In: IEEE Journal on Selected Areas in Communications, ISSN 0733-8716, E-ISSN 1558-0008, Vol. 35, no 10, p. 2398-2412Article in journal (Refereed)
    Abstract [en]

    We study a downlink non-orthogonal multiple access system with cooperative full-duplex relaying, where the near user in terms of the base station (BS) is enabled to act as a full-duplex relay for the far user. In particular, we first derive the outage probability and ergodic sum rate with closed-form expressions when the power allocations at the BS and relay (or the near user) are fixed. Then, we analytically obtain the optimal power allocations with closed-form expressions at the BS and relay to minimize the outage probability. Furthermore, by taking the fairness between the near user and far user into account, we characterize the optimal power allocations with closed-form expressions at the BS and relay to maximize the minimum achievable rate of users. Simulation results validate the correctness of the theoretical analysis and demonstrate the advantages of the proposed algorithms over the state of the art.

  • 2572. Zhang, Q. L.
    et al.
    Khan, S. N.
    He, Sailing
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering. KTH, School of Electrical Engineering (EES), Centres, Alfvén Laboratory Centre for Space and Fusion Plasma Physics.
    REALIZATION OF LEFT HANDEDNESS THROUGH CSRRs AND SRRs IN MICROSTRIP LINE2009In: Microwave and optical technology letters (Print), ISSN 0895-2477, E-ISSN 1098-2760, Vol. 51, no 3, p. 757-760Article in journal (Refereed)
    Abstract [en]

    A new technique to realize left-handedness in microstrip tramsimission line is proposed by periodically loading combination of split-ring resonators (SRRs) and complementary split-ring resonators (CSRRs). Because of the negative permeability offered by SRRs and the negative permittivity of the CSRRs in the designed frequency (5 GHz), a pass-band is achieved. The design is compact in size, low profile and independent of substrate permittivity. It is also 50% smaller comparing with EBG structure. These features are valuable for future microwave devices like couplers, filters, etc. Comparisons between the simulated and measured results are discussed in details.

  • 2573. Zhang, X.
    et al.
    Matthaiou, M.
    Coldrey, M.
    Björnson, Emil
    KTH, School of Electrical Engineering (EES), Signal Processing. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre. SUPELEC, France .
    Impact of residual transmit RF impairments on training-based MIMO systems2014In: 2014 IEEE International Conference on Communications, ICC 2014, 2014, p. 4741-4746Conference paper (Refereed)
    Abstract [en]

    Radio-frequency (RF) impairments, that exist intimately in wireless communications systems, can severely degrade the performance of traditional multiple-input multiple-output (MIMO) systems. Although compensation schemes can cancel out part of these RF impairments, there still remains a certain amount of impairments. These residual impairments have fundamental impact on the MIMO system performance. However, most of the previous works have neglected this factor. In this paper, a training-based MIMO system with residual transmit RF impairments (RTRI) is considered. In particular, we derive a new channel estimator for the proposed model, and find that RTRI can create an irreducible estimation error floor. Moreover, we show that, in the presence of RTRI, the optimal training sequence length can be larger than the number of transmit antennas, especially in the low and high signal-to-noise ratio (SNR) regimes. An increase in the proposed approximated achievable rate is also observed by adopting the optimal training sequence length. When the training and data symbol powers are required to be equal, we demonstrate that, at high SNRs, systems with RTRI demand more training, whereas at low SNRs, such demands are nearly the same for all practical levels of RTRI.

  • 2574. Zhang, X.
    et al.
    Nakhai, M. R.
    Zheng, G.
    Lambotharan, S.
    Ottersten, Björn
    KTH, School of Electrical Engineering (EES), Signal Processing. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre. KTH, School of Electrical Engineering and Computer Science (EECS), Information Science and Engineering.
    Calibrated Learning for Online Distributed Power Allocation in Small-Cell Networks2019In: IEEE Transactions on Communications, ISSN 0090-6778, E-ISSN 1558-0857, p. 1-1Article in journal (Refereed)
  • 2575.
    Zhang, Xi
    et al.
    KTH, School of Electrical Engineering (EES), Signal Processing. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Jorswieck, Eduard A.
    Communications Laboratory, Faculty of Electrical Engineering and Information Technology,Dresden University of Technology (TUD), Germany.
    Ottersten, Björn
    KTH, School of Electrical Engineering (EES), Signal Processing. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Paulraj, Arogyasvsami
    Information Systems Laboratory, Stanford University, US.
    On the Asymptotic Optimality of Opportunistic Norm-Based User Selection with Hard SINR Constraint2009In: EURASIP Journal on Advances in Signal Processing, ISSN 1687-6172, E-ISSN 1687-6180, ISSN 1687-6172Article in journal (Refereed)
    Abstract [en]

    Recently, user selection algorithms in combination with linear precoding have been proposed that achieve the same scaling as the sum capacity of the MIMO broadcast channel. Robust opportunistic beamforming, which only requires partial channel state information for user selection, further reduces feedback requirements. In this work, we study the optimality of the opportunistic norm-based user selection system in conjunction with hard SINR requirements under max-min fair beamforming transmit power minimization. It is shown that opportunistic norm-based user selection is asymptotically optimal, as the number of transmit antennas goes to infinity when only two users are selected in high SNR regime. The asymptotic performance of opportunistic norm-based user selection is also studied when the number of users goes to infinity. When a limited number of transmit antennas and/or median range of users are available, only insignificant performance degradation is observed in simulations with an ideal channel model or based on measurement data.

  • 2576.
    Zhang, Xi
    et al.
    KTH, School of Electrical Engineering (EES), Signal Processing. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Jorswieck, Eduard A.
    KTH, School of Electrical Engineering (EES), Signal Processing. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Ottersten, Björn
    KTH, School of Electrical Engineering (EES), Signal Processing. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Paulraj, Arogyaswami
    Information Systems Laboratory, Stanford University, USA.
    User selection schemes in multiple antenna broadcast channels with guaranteed performance2007In: 2007 IEEE 8TH WORKSHOP ON SIGNAL PROCESSING ADVANCES IN WIRELESS COMMUNICATIONS, NEW YORK: IEEE , 2007, p. 1-5Conference paper (Refereed)
    Abstract [en]

    Recently, user selection algorithms in combination with linear precoding have been proposed that achieve the same scaling as the sum capacity of the MIMO BC. In this work, we study the properties of four user selection algorithms in conjunction with beamforming that guarantee certain SINR requirements under transmit power minimization. It is shown that for a large number of transmit antennas, the norm-based user selection performs close to the optimum. The results are illustrated by various numerical simulations.

  • 2577.
    Zhang, Xi
    et al.
    KTH, School of Electrical Engineering (EES), Signal Processing. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Palomar, Daniel P.
    Dept. of Electronic and Computer Engineering, Hong Kong University of Science and Technology, Kowloon, Hong Kong.
    Ottersten, Björn
    KTH, School of Electrical Engineering (EES), Signal Processing. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Robust MAC MIMO transceiver design with partial CSIT and CSIR2007In: CONFERENCE RECORD OF THE FORTY-FIRST ASILOMAR CONFERENCE ON SIGNALS, SYSTEMS & COMPUTERS, VOLS 1-5, NEW YORK: IEEE , 2007, p. 324-328Conference paper (Refereed)
    Abstract [en]

    In practical wireless systems, the design of linear MIMO transceivers should be robust to imperfect channel state information (CSI), especially in multiuser systems where accurate CSI is difficult to obtain. This paper considers the case in which the CSI is perturbed at both sides of the link. In such a case, it is possible to optimally design robust MIMO transceivers to minimize the average sum MSE of all the data streams and the users. For systems with a sum power constraint, our robust design can be shown to be a convex SDP problem; for systems with individual power constraints, our robust design is based on an underlying SDP problem.

  • 2578.
    Zhang, Xi
    et al.
    KTH, School of Electrical Engineering (EES), Signal Processing. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Palomar, Daniel P.
    Electrical Engineering, Princeton University, Princeton.
    Ottersten, Björn
    KTH, School of Electrical Engineering (EES), Signal Processing. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Statistically robust design of linear MIMO transceivers2008In: IEEE Transactions on Signal Processing, ISSN 1053-587X, E-ISSN 1941-0476, Vol. 56, no 8, p. 3678-3689Article in journal (Refereed)
    Abstract [en]

    The treatment of channel state information (CSI) is critical in the design of MIMO systems. Accurate CSI at the transmitter is often not possible or may require high feedback rates. Herein, we consider the robust design of linear MIMO transceivers with perfect CSI either at the transmitter or at both sides of the link. The framework considers the design problem where the imperfect CSI consists of the channel mean and covariance matrix or, equivalently, the channel estimate and the estimation error covariance matrix. The robust transceiver design is based on a general cost function of the average MSEs as well as a design with individual MSE based constraints. In particular, a lower bound of the average MSE matrix is explored for the design when only the CSI at the transmitter is imperfect. Under different CSI conditions, the proposed robust transceivers exhibit a similar structure to the transceiver designs for perfect CSI, but with a different equivalent channel and/or noise covariance matrix.

  • 2579.
    Zhang, Xinlin
    et al.
    Chalmers, Dept Signals & Syst, S-41296 Gothenburg, Sweden..
    Matthaiou, Michail
    Chalmers, Dept Signals & Syst, S-41296 Gothenburg, Sweden.;Queens Univ Belfast, Sch Elect Elect Engn & Comp Sci, Belfast, Antrim, North Ireland..
    Björnson, Emil
    KTH, School of Electrical Engineering (EES), Signal Processing. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Coldrey, Mikael
    Ericsson AB, Ericsson Res, Gothenburg, Sweden..
    Debbah, Merouane
    SUPELEC, Alcatel Lucent Chair Flexible Radio, Gif Sur Yvette, France..
    On the MIMO Capacity with Residual Transceiver Hardware Impairments2014In: 2014 IEEE INTERNATIONAL CONFERENCE ON COMMUNICATIONS (ICC) / [ed] Jamalipour, A Deng, DJ, IEEE , 2014, p. 5299-5305Conference paper (Refereed)
    Abstract [en]

    Radio-frequency (RF) impairments in the transceiver hardware of communication systems (e.g., phase noise (PN), high power amplifier (HPA) nonlinearities, or in-phase/quadrature-phase (I/Q) imbalance) can severely degrade the performance of traditional multiple-input multiple-output (MIMO) systems. Although calibration algorithms can partially compensate these impairments, the remaining distortion still has substantial impact. Despite this, most prior works have not analyzed this type of distortion. In this paper, we investigate the impact of residual transceiver hardware impairments on the MIMO system performance. In particular, we consider a transceiver impairment model, which has been experimentally validated, and derive analytical ergodic capacity expressions for both exact and high signal-to-noise ratios (SNRs). We demonstrate that the capacity saturates in the high-SNR regime, thereby creating a finite capacity ceiling. We also present a linear approximation for the ergodic capacity in the low-SNR regime, and show that impairments have only a second-order impact on the capacity. Furthermore, we analyze the effect of transceiver impairments on large-scale MIMO systems; interestingly, we prove that if one increases the number of antennas at one side only, the capacity behaves similar to the finite-dimensional case. On the contrary, if the number of antennas on both sides increases with a fixed ratio, the capacity ceiling vanishes; thus, impairments cause only a bounded offset in the capacity compared to the ideal transceiver hardware case.

  • 2580. Zhang, Yebin
    et al.
    Yan, Guofeng
    Zhou, Bin
    Lee, El-Hang
    He, Sailing
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering. KTH, School of Electrical Engineering (EES), Centres, Alfvén Laboratory Centre for Space and Fusion Plasma Physics.
    Reflective Optical Fiber Refractometer Based on Long-Period Grating Tailored Active Bragg Grating2015In: IEEE Photonics Technology Letters, ISSN 1041-1135, E-ISSN 1941-0174, Vol. 27, no 11, p. 1173-1176Article in journal (Refereed)
    Abstract [en]

    A reflective optical fiber refractometer based on a long-period grating (LPG) followed by an active fiber Bragg grating (AFBG) is proposed. The AFBG is inscribed in a short section of cobalt-doped optical fiber, which can be heated by pump laser and acts as a sensing signal indicator. The LPG functions as a bridge between the surrounding refractive index (RI) and the AFBG reflection, tailoring the power of pump laser for heating. Experimental results demonstrate that the wavelength response of this fiber grating-based reflective refractometer can be easily adjusted, and a significantly improved sensitivity within the RI range from 1.3403 to 1.4654 has been achieved.

  • 2581. Zhang, Zhengquan
    et al.
    Ma, Zheng
    Xiao, Ming
    KTH, School of Electrical Engineering (EES), Information Science and Engineering. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Ding, Zhiguo
    Fan, Pingzhi
    Full-Duplex Device-to-Device-Aided Cooperative Nonorthogonal Multiple Access2017In: IEEE Transactions on Vehicular Technology, ISSN 0018-9545, E-ISSN 1939-9359, Vol. 66, no 5, p. 4467-4471Article in journal (Refereed)
    Abstract [en]

    This paper presents a full-duplex device-to-device (D2D)-aided cooperative nonorthogonal multiple access (NOMA) scheme to improve the outage performance of the NOMA-weak user in a NOMA user pair, where the NOMA-weak user is helped by the NOMA-strong user with the capability of full-duplex D2D communications. The expressions for the outage probability are derived to characterize the performance of the proposed scheme. The results show that the proposed cooperative NOMA scheme can achieve superior outage performance compared to the conventional NOMA and orthogonal multiple access (OMA). In order to further improve the outage performance, an adaptive multiple access (AMA) scheme is also studied, which dynamically switches between the proposed cooperative NOMA, conventional NOMA, and OMA schemes, according to the level of residual self-interference and the quality of links. The results show that the AMAscheme outperforms the above multiple access schemes in terms of outage performance.

  • 2582. Zhang, Zhengquan
    et al.
    Ma, Zheng
    Xiao, Ming
    KTH, School of Electrical Engineering (EES), Communication Theory. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Karagiannidis, George K.
    Ding, Zhiguo
    Fan, Pingzhi
    Two-Timeslot Two-Way Full-Duplex Relaying for 5G Wireless Communication Networks2016In: IEEE Transactions on Communications, ISSN 0090-6778, E-ISSN 1558-0857, Vol. 64, no 7, p. 2873-2887Article in journal (Refereed)
    Abstract [en]

    We propose a novel two-timeslot two-way full-duplex (FD) relaying scheme, in which the access link and the backhaul link are divided in the time domain, and we study the average end-to-end rate and the outage performance. According to the user equipment capability and services, we investigate two scenarios: three-node I-and four-node Y-relaying channels. Among various relaying protocols, the well-known amplify-and-forward and decode-and-forward are considered. Closed-form expressions for the average end-to-end rate and the outage probability, under the effect of residual self-interference and inter-user interference, are presented. The results show that the proposed two-timeslot two-way FD relaying scheme can achieve higher rate and better outage performance than the half-duplex one, when residual self-interference is below a certain level. Therefore, this relaying scheme presents a reasonable tradeoff between performance and complexity, and so, it could be efficiently used in the fifth-generation wireless networks.

  • 2583. Zhang, Zhengquan
    et al.
    Ma, Zheng
    Xiao, Ming
    KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre. KTH, School of Electrical Engineering (EES), Information Science and Engineering.
    Liu, Gang
    Fan, Pingzhi
    Modeling and Analysis of Non-Orthogonal MBMS Transmission in Heterogeneous Networks2017In: IEEE Journal on Selected Areas in Communications, ISSN 0733-8716, E-ISSN 1558-0008, Vol. 35, no 10, p. 2221-2237Article in journal (Refereed)
    Abstract [en]

    Multimedia broadcast/multicast service (MBMS) transmission, which distributes the media content to multiple users on the same radio resources by using point-to-multipoint communications, is a highly spectrum efficient mechanism for multimedia communications. In this paper, we study the application of power domain non-orthogonal transmission to MBMS enhancements in a K-tier heterogeneous network, in order to satisfy the ever-increasing demands for emerging applications and performance requirements. Then, we present non-orthogonal multi-rate MBMS transmission (NOMRMT) and non-orthogonal multi-service MBMS transmission schemes and investigate their performance by using stochastic geometry. A tractable mode is developed to analyze the performance of asynchronous and synchronous non-orthogonal MBMS transmission. Based on this model, analytical expressions for the signal-to-interference-plusnoise ratio coverage probability, average number of served users, and sum rate are derived. The results demonstrate that non-orthogonal MBMS transmission can achieve better performance than the orthogonal one, while synchronous non-orthogonal MBMS transmission is superior to the asynchronous one.

  • 2584.
    Zhao, Wang
    et al.
    KTH, School of Electrical Engineering (EES), Communication Theory.
    Wang, Chao
    KTH, School of Electrical Engineering (EES), Communication Theory.
    Xiao, Ming
    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.
    On the achievable degrees of freedom of partially cooperative X networks with delayed CSIT2012In: 2012 IEEE Global Communications Conference (GLOBECOM), New York: IEEE , 2012, p. 2450-2454Conference paper (Refereed)
    Abstract [en]

    We investigate the achievable degrees of freedom (DoF) in K-user X networks (K × K X networks) with delayed channel state information at transmitters (CSIT), where partial cooperation (i.e. message sharing) is potentially allowed among transmitters. We consider two possible cooperation scenarios. In the first scenario one of the transmitters serves as a super node which can obtain the messages of the other transmitters. By proper interference alignment (IA) design, we prove that a DoF 2K over K+1 can be achieved almost surely. In the second scenario, there is no super node but each transmitter shares its message to its left-side neighbor. We show that when K = 3, DoF 7 over 5 is achievable. In both cases, the achieved DoF are shown to be improved compared with non-cooperative X networks. Moreover, we use a simple example to show that sharing a subset of messages may also improve DoF.

  • 2585.
    Zhao, Wang
    et al.
    KTH, School of Electrical Engineering (EES), Communication Theory.
    Wang, Chao
    KTH, School of Electrical Engineering (EES), Communication Theory.
    Xiao, Ming
    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.
    The two-hop MISO broadcast network with quantized delayed CSIT2013In: 2013 IEEE Wireless Communications and Networking Conference (WCNC), IEEE Communications Society, 2013, p. 3022-3027Conference paper (Refereed)
    Abstract [en]

    We consider a downlink two-hop MISO broadcast network with a 2-antenna source communicating to 2 singleantenna destinations, assisted by 2 single-antenna intermediate relays. We investigate spectrally efficient transmission schemes and their achieved sum degrees of freedom (DoF), with quantized delayed channel state information (CSI) feedback. Assuming Grassmannian vector quantization, we study two feedback scenarios according to the feedback range limit, namely global-range feedback, i.e., the source can receive the feedback signals from both the relays and the destinations, and one-hop-range feedback, i.e., each node can only attain the feedback information of its upcoming hop. We establish a sum DoF lower bound for each case. Our results reveal that when the quantization rate at relays BR= α1log2(SNR) and at destinations BD = α2log2(SNR) for min 1, α2} ≥ 1, the optimal sum DoF 4/3 can be achieved with finite-rate delayed feedback.

  • 2586.
    Zhao, Wang
    et al.
    KTH, School of Electrical Engineering (EES), Communication Theory.
    Xiao, Ming
    KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre. KTH, School of Electrical Engineering (EES), Communication Theory.
    Skoglund, Mikael
    KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre. KTH, School of Electrical Engineering (EES), Communication Theory.
    Poor, H. Vincent
    Princeton University.
    Secrecy degrees of freedom of wireless X networks using artificial noise alignmentManuscript (preprint) (Other academic)
    Abstract [en]

    The problem of transmitting confidential messages in $M \times K$ wireless X networks is considered, in which each transmitter intends to send one confidential message to every receiver. In particular, the secrecy degrees of freedom (SDOF) of the considered network achieved by an artificial noise alignment (ANA) approach, which integrates interference alignment and artificial noise transmission, are studied. At first, an SDOF upper bound is derived for the $M \times K$ X network with confidential messages (XNCM) to be $\frac{K(M-1)}{K+M-2}$. By proposing an ANA approach, it is shown that the SDOF upper bound is tight when either $K=2$ or $M=2$ for the considered XNCM with time/frequency varying channels. For $K,M \geq 3$, it is shown that an SDOF $\frac{K(M-1)}{K+M-1}$ can be achieved, even when an external eavesdropper appears. The key idea of the proposed scheme is to inject artificial noise to the network, which can be aligned in the interference space at receivers for confidentiality. Moreover, for the network with no channel state information at transmitters, a blind ANA scheme is proposed to achieve the SDOF $\frac{K(M-1)}{K+M-1}$ for $K,M \geq 2$, with reconfigurable antennas at receivers. The proposed method provides a linear approach to handle secrecy coding and interference alignment.

  • 2587.
    Zhao, Wang
    et al.
    KTH, School of Electrical Engineering (EES), Communication Theory.
    Xiao, Ming
    KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre. KTH, School of Electrical Engineering (EES), Communication Theory.
    Wang, Chao
    KTH, School of Electrical Engineering (EES), Communication Theory.
    Skoglund, Mikael
    KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre. KTH, School of Electrical Engineering (EES), Communication Theory.
    Degrees of freedom of two hop MISO broadcast network with mixed CSIT2014In: IEEE Transactions on Wireless Communications, ISSN 1536-1276, E-ISSN 1558-2248, Vol. 13, no 12, p. 6982-6995Article in journal (Refereed)
    Abstract [en]

    A downlink two-hop MISO broadcast network is considered, with a two-antenna source communicating to 2 single-antenna destinations, via multiple single-antenna relays in between. The sum degrees of freedom (DOF) of the network with mixed channel state information at the transmitter (CSIT) is investigated. The mixed CSIT consists of accurate delayed CSIT and inaccurate instantaneous CSIT, and its availability is limited within each hop, i.e. the source is oblivious to the channels of the second hop. Given a transmission power P and a real value α in [0,1], if the variance of the error for instantaneous CSIT decreases as O(P-α), it is shown that the sum optimal DOF of the considered network is d = when there exist at least 3 intermediate relays. The result can be extended to the MIMO and multiple-hop cases. The proposed achievable schemes essentially combine the concept of retrospective interference alignment based on delayed CSIT and linear beamforming based on inaccurate instantaneous CSIT into an integrated form. Our results show that, in multi-hop MISO broadcast networks, delayed CSIT and inaccurate instantaneous CSIT can be exploited simultaneously to benefit network DOF.

  • 2588. Zheng, G.
    et al.
    Li, J.
    Wong, K. -K
    Petropulu, A. P.
    Ottersten, Björn
    KTH, School of Electrical Engineering (EES), Signal Processing. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Using simple relays to improve physical-layer security2012In: Communications in China (ICCC), 2012 1st IEEE International Conference on, IEEE , 2012, p. 329-333Conference paper (Refereed)
    Abstract [en]

    This paper studies different uses of two cooperating relays to improve the secrecy rate of a wiretap channel. These two relays are assumed to perform only simple functions: either amplify-and-forward (AF) or jamming. Complex functions such as decode-and-forward (DF) are not considered. We study three modes of cooperation: i) cooperative jamming (CJ), ii) AF-aided beamforming and iii) mixed AF-aided beamforming and CJ, all with individual relay power constraints. While i) is known in the literature, our efforts are spent on ii) and iii). In particular, for iii), we assume that the jamming signals in two communication stages are correlated, giving rise to improved performances. We also propose a heuristic approach for selecting the appropriate cooperating mode. Simulation results illustrate the performance gain of each scheme under different channel conditions and the effectiveness of the proposed mode selection method.

  • 2589. Zheng, Gan
    et al.
    Arapoglou, Pantelis-Daniel
    Ottersten, Björn
    KTH, School of Electrical Engineering (EES), Signal Processing. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Physical Layer Security in Multibeam Satellite Systems2012In: IEEE Transactions on Wirelss Communications, ISSN 1536-1276, Vol. 11, p. 852-863Article in journal (Refereed)
    Abstract [en]

    Security threats introduced due to the vulnerability of the transmission medium may hinder the proliferation of Ka band multibeam satellite systems for civil and military data applications. This paper sets the analytical framework and then studies physical layer security techniques for fixed legitimate receivers dispersed throughout multiple beams, each possibly surrounded by multiple (passive) eavesdroppers. The design objective is to minimize via transmit beamforming the costly total transmit power on board the satellite, while satisfying individual intended users' secrecy rate constraints. Assuming state-of-the-art satellite channel models, when perfect channel state information (CSI) about the eavesdroppers is available at the satellite, a partial zero-forcing approach is proposed for obtaining a low-complexity sub-optimal solution. For the optimal solution, an iterative algorithm combining semi-definite programming relaxation and the gradient-based method is devised by studying the convexity of the problem. Furthermore, the use of artificial noise as an additional degree-of-freedom for protection against eavesdroppers is explored. When only partial CSI about the eavesdroppers is available, we study the problem of minimizing the eavesdroppers' received signal to interference-plus-noise ratios. Simulation results demonstrate substantial performance improvements over existing approaches.

  • 2590. Zheng, Gan
    et al.
    Chatzinotas, Symeon
    Ottersten, Björn
    KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre. KTH, School of Electrical Engineering (EES), Signal Processing.
    Generic Optimization of Linear Precoding in Multibeam Satellite Systems2012In: IEEE Transactions on Wireless Communications, ISSN 1536-1276, E-ISSN 1558-2248, Vol. 11, no 6, p. 2308-2320Article in journal (Refereed)
    Abstract [en]

    Multibeam satellite systems have been employed to provide interactive broadband services to geographical areas under-served by terrestrial infrastructure. In this context, this paper studies joint multiuser linear precoding design in the forward link of fixed multibeam satellite systems. We provide a generic optimization framework for linear precoding design to handle any objective functions of data rate with general linear and nonlinear power constraints. To achieve this, an iterative algorithm which optimizes the precoding vectors and power allocation alternatingly is proposed and most importantly, the proposed algorithm is proved to always converge. The proposed optimization algorithm is also applicable to nonlinear dirty paper coding. As a special case, a more efficient algorithm is devised to find the optimal solution to the problem of maximizing the proportional fairness among served users. In addition, the aforementioned problems and algorithms are extended to the case that each terminal has multiple co-polarization or dual-polarization antennas. Simulation results demonstrate substantial performance improvement of the proposed schemes over conventional multibeam satellite systems, zero-forcing and regularized zero-forcing precoding schemes in terms of meeting the traffic demand, e. g., using real beam patterns, over twice higher throughput can be achieved compared with the conventional scheme. The performance of the proposed linear precoding scheme is also shown to be very close to the dirty paper coding.

  • 2591. Zheng, Gan
    et al.
    Chatzinotas, Symeon
    Ottersten, Björn
    KTH, School of Electrical Engineering (EES), Signal Processing. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Multi-gateway cooperation in multibeam satellite systems2012In: Personal Indoor and Mobile Radio Communications (PIMRC), 2012 IEEE 23rd International Symposium on, IEEE , 2012, p. 1360-1364Conference paper (Refereed)
    Abstract [en]

    Multibeam systems with hundreds of beams have been recently deployed in order to provide higher capacities by employing fractional frequency reuse. Furthermore, employing full frequency reuse and precoding over multiple beams has shown great throughput potential in literature. However, feeding all this data from a single gateway is not feasible based on the current frequency allocations. In this context, we investigate a range of scenarios involving beam clusters where each cluster is managed by a single gateway. More specifically, the following cases are considered for handling intercluster interference: a) conventional frequency colouring, b) joint processing within cluster, c) partial CSI sharing among clusters, d) partial CSI and data sharing among clusters. CSI sharing does not provide considerable performance gains with respect to b) but combined with data sharing offers roughly a 40% improvement over a) and a 15% over b).

  • 2592. Zheng, Gan
    et al.
    Jorswieck, Eduard
    Ottersten, Björn
    KTH, School of Electrical Engineering (EES), Signal Processing. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Cooperative Communications against Jamming with Half-Duplex and Full-Duplex Relaying2013In: Proceedings of IEEE 77th Vehicular Technology Conference: VTC2013-Spring, IEEE , 2013, p. 6692490-Conference paper (Refereed)
    Abstract [en]

    This paper studies the impact of jamming on the design of three-node two-hop cooperative amplify-and-forward (AF) communications with both half-duplex and full-duplex relaying. For the half-duplex relaying, the jammer is smart such that it can optimally allocate jamming power between listening and forwarding phases. Given separate source and relay power constraints, we derive the optimal jamming power allocation; with a total source and relay power constraint, we model the interaction between the legitimate system and the jammer as a noncooperative game and prove the existence and uniqueness of the Nash Equilibrium (NE). It is found that due to the fact that the end performance is limited by the weaker phase, the legitimate systems tries to balance the performance of two phases while the jammer attacks the system by making the two hops imbalanced. While for the full-duplex relaying, we show that if the self-interference can be properly controlled, it can bring substantial performance gain. Simulation results verify our analysis.

  • 2593. Zheng, Gan
    et al.
    Krikidis, Ioannis
    Ottersten, Björn
    KTH, School of Electrical Engineering (EES), Signal Processing. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Full-Duplex Cooperative Cognitive Radio with Transmit Imperfections2013In: IEEE Transactions on Wireless Communications, ISSN 1536-1276, E-ISSN 1558-2248, Vol. 12, no 5, p. 2498-2511Article in journal (Refereed)
    Abstract [en]

    This paper studies the cooperation between a primary system and a cognitive system in a cellular network where the cognitive base station (CBS) relays the primary signal using amplify-and-forward or decode-and-forward protocols, and in return it can transmit its own cognitive signal. While the commonly used half-duplex (HD) assumption may render the cooperation less efficient due to the two orthogonal channel phases employed, we propose that the CBS can work in a full-duplex (FD) mode to improve the system rate region. The problem of interest is to find the achievable primary-cognitive rate region by studying the cognitive rate maximization problem. For both modes, we explicitly consider the CBS transmit imperfections, which lead to the residual self-interference associated with the FD operation mode. We propose closed-form solutions or efficient algorithms to solve the problem when the related residual interference power is non-scalable or scalable with the transmit power. Furthermore, we propose a simple hybrid scheme to select the HD or FD mode based on zero-forcing criterion, and provide insights on the impact of system parameters. Numerical results illustrate significant performance improvement by using the FD mode and the hybrid scheme.

  • 2594.
    Zheng, Gan
    et al.
    Department of Electrical and Electronic Engineering, University College London.
    Wong, Kai-Kit
    Department of Electrical and Electronic Engineering, University College London.
    Ottersten, Björn
    KTH, School of Electrical Engineering (EES), Signal Processing. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Robust Cognitive Beamforming With Bounded Channel Uncertainties2009In: IEEE Transactions on Signal Processing, ISSN 1053-587X, E-ISSN 1941-0476, Vol. 57, no 12, p. 4871-4881Article in journal (Refereed)
    Abstract [en]

    This paper studies the robust beamforming design for a multi-antenna cognitive radio (CR) network, which transmits to multiple secondary users (SUs) and coexists with a primary network of multiple users. We aim to maximize the minimum of the received signal-to-interference-plus-noise ratios (SINRs) of the SUs, subject to the constraints of the total SU transmit power and the received interference power at the primary users (PUs) by optimizing the beamforming vectors at the SU transmitter based on imperfect channel state information (CSI). To model the uncertainty in CSI, we consider a bounded region for both cases of channel matrices and channel covariance matrices. As such, the optimization is done while satisfying the interference constraints for all possible CSI error realizations. We shall first derive equivalent conditions for the interference constraints and then convert the problems into the form of semi-definite programming (SDP) with the aid of rank relaxation, which leads to iterative algorithms for obtaining the robust optimal beamforming solution. Results demonstrate the achieved robustness and the performance gain over conventional approaches and that the proposed algorithms can obtain the exact robust optimal solution with high probability.

  • 2595.
    Zheng, Gan
    et al.
    Department of Electrical and Electronic Engineering, University College London, UK.
    Wong, Kai-Kit
    Department of Electrical and Electronic Engineering, University College London, UK.
    Paulraj, Arogyasvsami
    Information Systems Laboratory, Stanford University, US.
    Ottersten, Björn
    KTH, School of Electrical Engineering (EES), Signal Processing. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Collaborative-Relay Beamforming with Perfect CSI: Optimum and Distributed Implementation2009In: IEEE Signal Processing Letters, ISSN 1070-9908, E-ISSN 1558-2361, Vol. 16, no 4, p. 257-260Article in journal (Refereed)
    Abstract [en]

    This letter studies the collaborative use of amplify-and-forward (AF) relays to form a virtual multiple-input single-output (MISO) beamforming system with the aid of perfect channel state information (CSI) in a flat-fading channel. In particular, we optimize the relay weights jointly to maximize the received signal-to-noise ratio (SNR) at the destination terminal with both individual and total power constraints at the relays. We show that the optimal collaborative-relay beamforming (CRB) solution achieves the full diversity of a MISO antenna system. Another main contribution of this letter is a distributed algorithm that allows each individual relay to learn its own weight, based on the Karush-Kuhn-Tucker (KKT) analysis.

  • 2596.
    Zheng, Gan
    et al.
    Department of Electrical and Electronic Engineering, University College London, London, WC1E 7JE, UK..
    Wong, Kai-Kit
    Department of Electrical and Electronic Engineering, University College London, London, WC1E 7JE, UK..
    Paulraj, Arogyasvsami
    Information Systems Laboratory, Stanford University, US.
    Ottersten, Björn
    KTH, School of Electrical Engineering (EES), Signal Processing. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Robust and distributed beamforming2009In: 2009 International Conference on Wireless Communications and Signal Processing, 2009, Vol. WCSP 2009, p. 1629-1633Conference paper (Refereed)
    Abstract [en]

    This paper studies the use of amplify-and-forward (AF) relays for robust beamforming, with the aid of imperfect channel state information (CSI) at the sender. In particular, the maximization of the worst-case signal-to-noise ratio (SNR) at the destination terminal is studied under a bounded spherical region for the norm of CSI error vector. We show that under a condition on the estimated CSI quality, the robust-optimal collaborative-relay beamforming (CRBF) can be obtained by S-Procedure and rank relaxation techniques. In addition, a distributed algorithm is proposed by examining the structure of the optimal CRBF.

  • 2597.
    Zheng, Gan
    et al.
    Department of Electrical and Electronic Engineering, University College London.
    Wong, Kai-Kit
    Department of Electrical and Electronic Engineering, University College London.
    Paulraj, Arogyasvsami
    Information Systems Laboratory, Stanford University, US.
    Ottersten, Björn
    KTH, School of Electrical Engineering (EES), Signal Processing. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Robust Collaborative-Relay Beamforming2009In: IEEE Transactions on Signal Processing, ISSN 1053-587X, E-ISSN 1941-0476, Vol. 57, no 8, p. 3130-3143Article in journal (Refereed)
    Abstract [en]

    Relay communications is a promising technique to extend the range of wireless communications by forwarding the message from the sender to the intended destination. While fixed or variable-power relays have been previously investigated, this paper addresses the collaborative use of variable-phase variable-power amplify-and-forward (AF) relays for robust beamforming, with the aid of imperfect channel state information (CSI) at the sender. In particular, the maximization of the worst-case signal-to-noise ratio (SNR) at the destination terminal is studied under a bounded spherical region for the norm of the CSI error vector from the relays to the destination. Our main contribution is that we prove, under a condition on the quality of the estimated CSI, the robust-optimal collaborative-relay beamforming (CRBF) can be obtained by S-Procedure and rank relaxation techniques. In addition, a distributed algorithm is developed by examining the structure of the optimal CRBF solution. Results demonstrate a significant gain of CRBF over non-robust approaches.

  • 2598. Zhi, Ruicong
    et al.
    Flierl, Markus
    KTH, School of Electrical Engineering (EES), Sound and Image Processing (Closed 130101).
    Ruan, Qiuqi
    Kleijn, W. Bastiaan
    KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre. KTH, School of Electrical Engineering (EES), Sound and Image Processing (Closed 130101).
    Facial expression recognition based on graph-preserving sparse non-negative matrix factorization2009Conference paper (Refereed)
    Abstract [en]

    In this paper, we present a novel algorithm for representing facial expressions. The algorithm is based on the non-negative matrix factorization (NMF) algorithm, which decomposes the original facial image matrix into two non-negative matrices, namely the coefficient matrix and the basis image matrix. We call the novel algorithm graph-preserving sparse non-negative matrix factorization (GSNMF). GSNMF utilizes both sparse and graph-preserving constraints to achieve a non-negative factorization. The graph-preserving criterion preserves the structure of the original facial images in the embedded sub-space while considering the class information of the facial images. Therefore, GSNMF has more discriminant power than NMF. GSNMF is applied to facial images for the recognition of six basic facial expressions. Our experiments show that GSNMF achieves on average a recognition rate of 93.5% compared to that of discriminant NMF with 91.6%.

  • 2599. Zhi, Ruicong
    et al.
    Flierl, Markus
    KTH, School of Electrical Engineering (EES), Sound and Image Processing. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Ruan, Qiuqi
    Kleijn, W. Bastiaan
    KTH, School of Electrical Engineering (EES), Sound and Image Processing. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Graph-Preserving Sparse Nonnegative Matrix Factorization With Application to Facial Expression Recognition2011In: IEEE transactions on systems, man and cybernetics. Part B. Cybernetics, ISSN 1083-4419, E-ISSN 1941-0492, Vol. 41, no 1, p. 38-52Article in journal (Refereed)
    Abstract [en]

    In this paper, a novel graph-preserving sparse nonnegative matrix factorization (GSNMF) algorithm is proposed for facial expression recognition. The GSNMF algorithm is derived from the original NMF algorithm by exploiting both sparse and graph-preserving properties. The latter may contain the class information of the samples. Therefore, GSNMF can be conducted as an unsupervised or a supervised dimension reduction method. A sparse representation of the facial images is obtained by minimizing the l(1)-norm of the basis images. Furthermore, according to the graph embedding theory, the neighborhood of the samples is preserved by retaining the graph structure in the mapped space. The GSNMF decomposition transforms the high-dimensional facial expression images into a locality-preserving subspace with sparse representation. To guarantee convergence, we use the projected gradient method to calculate the nonnegative solution of GSNMF. Experiments are conducted on the JAFFE database and the Cohn-Kanade database with unoccluded and partially occluded facial images. The results show that the GSNMF algorithm provides better facial representations and achieves higher recognition rates than nonnegative matrix factorization. Moreover, GSNMF is also more robust to partial occlusions than other tested methods.

  • 2600. Zhong, J.
    et al.
    Kleijn, W. B.
    Hu, Xiaoming
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Optimization and Systems Theory. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Video quality improvement for multi-camera systems using camera control2014In: Proceedings of the 33rd Chinese Control Conference, CCC 2014, 2014, p. 1924-1931Conference paper (Refereed)
    Abstract [en]

    Multiview video captured by a multi-camera system has been widely used in many applications such as environmental surveillance, industrial inspection and 3D television. A multiview video contains a vast amount of data required to be transmitted and/or stored, and therefore compression is essential. However, distortion or artifacts are usually witnessed in the reconstructed multiview video after a raw multiview video is compressed at a limited bit-rate. Previous results on video compression mainly focus on the refinements of compression algorithms to improve the quality of reconstructed multiview videos. This paper uses a camera control for video capture to improve the quality of reconstructed multiview videos at a limited bit-rate. Individual cameras are controlled to adjust their pan angles and focal lengths at the video acquisition stage to compensate for the difference in feature-point locations. The optimal pan angle and focal length are designed for each camera. Experimental results validate that the proposed method capturing with camera control outperforms the conventional method capturing without camera control and the heuristic method of 'keeping the object of interest in the image center'.

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