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  • 2551.
    Zhang, Xu
    et al.
    University of Bedfordshire.
    Zhou, DiBin
    University of Bedfordshire.
    Xiao, Zhu
    University of Bedfordshire.
    Liu, Enjie
    University of Bedfordshire.
    Zhang, Jie
    University of Bedfordshire.
    Alayon Glazunov, Andres
    University of Bedfordshire.
    Dynamic Group PCI Assignment Scheme2011Ingår i: The Seventh International Conference on Wireless and Mobile Communications (ICWMC 2011), 2011, s. 101-106Konferensbidrag (Refereegranskat)
    Abstract [en]

    The Physical Cell Identity (PCI) is used to identify femtocell in LTE femtocell as the physical layer identity. Due to the fact that a) the PCIs are normally allocated without planning; and b) the limitation of the number of PCI, the cross-tier PCI confusion problem has arisen. The current solution in 3GPP Release 9 is to use Cell Global Identity (CGI) associated with PCI to solve this problem of confusion. However, using CGI has many serious drawbacks. Especially CGI might cause the inbound-handover is failure. In order to mitigate the CGI problem, this research proposes a dynamic PCI assignment scheme of DG-PCI in the macrocell and femtocell environment. The proposed scheme is tested by using system-level simulation. The simulations showed that the approach offers an optimal PCI distribution between macrocell and femtocell. The test showed that the CGI problem can be solved, and hence increase the successful rate of inbound-handover.

  • 2552. Zhang, Zhengquan
    et al.
    Ma, Zheng
    Xiao, Ming
    KTH, Skolan för elektro- och systemteknik (EES), Kommunikationsteori. KTH, Skolan för elektro- och systemteknik (EES), Centra, ACCESS Linnaeus Centre.
    Karagiannidis, George K.
    Ding, Zhiguo
    Fan, Pingzhi
    Two-Timeslot Two-Way Full-Duplex Relaying for 5G Wireless Communication Networks2016Ingår i: IEEE Transactions on Communications, ISSN 0090-6778, E-ISSN 1558-0857, Vol. 64, nr 7, s. 2873-2887Artikel i tidskrift (Refereegranskat)
    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.

  • 2553. Zhang, Zhengquan
    et al.
    Ma, Zheng
    Xiao, Ming
    KTH, Skolan för elektro- och systemteknik (EES), Centra, ACCESS Linnaeus Centre. KTH, Skolan för elektro- och systemteknik (EES), Teknisk informationsvetenskap.
    Liu, Gang
    Fan, Pingzhi
    Modeling and Analysis of Non-Orthogonal MBMS Transmission in Heterogeneous Networks2017Ingår i: IEEE Journal on Selected Areas in Communications, ISSN 0733-8716, E-ISSN 1558-0008, Vol. 35, nr 10, s. 2221-2237Artikel i tidskrift (Refereegranskat)
    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.

  • 2554.
    Zhao, David Y
    et al.
    KTH, Tidigare Institutioner, Signaler, sensorer och system.
    Kleijn, W Bastiaan
    KTH, Tidigare Institutioner, Signaler, sensorer och system.
    Multiple-description vector quantization using translated lattices with local optimization2004Ingår i: GLOBECOM: IEEE Global Telecommunications Conference, NEW YORK: IEEE , 2004, s. 41-45Konferensbidrag (Refereegranskat)
    Abstract [en]

    Multiple-description coding is a joint source- and channel coding technique suitable for real-time multimedia transmission over erasure channels. This work improves the previous methods of multiple-description vector quantization using lattice structured codebooks by introducing translated lattices in the single-description codebooks. The quantizer can easily adapt to the current channel condition, using the locally optimized combined-description codebooks, assuming that channel statistics are available at the encoder. Compared to previous methods, the central distortion is greatly reduced for noisy channels, without a significant effect on complexity.

  • 2555.
    Zhao, David Y.
    et al.
    KTH, Skolan för elektro- och systemteknik (EES), Ljud- och bildbehandling.
    Samuelsson, Jonas
    Nilsson, Mattias
    KTH, Skolan för elektro- och systemteknik (EES), Ljud- och bildbehandling.
    Gmm-based entropy-constrained vector quantization2007Ingår i: 2007 IEEE International Conference on Acoustics, Speech, and Signal Processing, Vol IV, Pts 1-3, 2007, s. 1097-1100Konferensbidrag (Refereegranskat)
    Abstract [en]

    In this paper, we present a scalable entropy-constrained vector quantizer based on Gaussian mixture models (GMMs), lattice quantization, and arithmetic coding. We assume that the source has a probability density function of a GMM. The scheme is based on a mixture component classifier, the Karhunen Loeve transform of the component, followed by a lattice quantization. The scalar elements of the quantized vector are entropy coded using a specially designed arithmetic coder. The proposed scheme has a computational complexity that is independent of rate, and quadratic with respect to vector dimension. The design is flexible and allows for adjusting the desired target rate on-the-fly. We evaluated the performance of the proposed scheme on speech-derived source vectors. It was demonstrated that the proposed scheme outperforms a fixed-rate GMM based vector quantizer, and performs closely to the theoretical optimum.

  • 2556.
    Zhao, David Yuheng
    KTH, Skolan för elektro- och systemteknik (EES), Ljud- och bildbehandling.
    Model Based Speech Enhancement and Coding2007Doktorsavhandling, sammanläggning (Övrigt vetenskapligt)
    Abstract [en]

    In mobile speech communication, adverse conditions, such as noisy acoustic environments and unreliable network connections, may severely degrade the intelligibility and natural- ness of the received speech quality, and increase the listening effort. This thesis focuses on countermeasures based on statistical signal processing techniques. The main body of the thesis consists of three research articles, targeting two specific problems: speech enhancement for noise reduction and flexible source coder design for unreliable networks.

    Papers A and B consider speech enhancement for noise reduction. New schemes based on an extension to the auto-regressive (AR) hidden Markov model (HMM) for speech and noise are proposed. Stochastic models for speech and noise gains (excitation variance from an AR model) are integrated into the HMM framework in order to improve the modeling of energy variation. The extended model is referred to as a stochastic-gain hidden Markov model (SG-HMM). The speech gain describes the energy variations of the speech phones, typically due to differences in pronunciation and/or different vocalizations of individual speakers. The noise gain improves the tracking of the time-varying energy of non-stationary noise, e.g., due to movement of the noise source. In Paper A, it is assumed that prior knowledge on the noise environment is available, so that a pre-trained noise model is used. In Paper B, the noise model is adaptive and the model parameters are estimated on-line from the noisy observations using a recursive estimation algorithm. Based on the speech and noise models, a novel Bayesian estimator of the clean speech is developed in Paper A, and an estimator of the noise power spectral density (PSD) in Paper B. It is demonstrated that the proposed schemes achieve more accurate models of speech and noise than traditional techniques, and as part of a speech enhancement system provide improved speech quality, particularly for non-stationary noise sources.

    In Paper C, a flexible entropy-constrained vector quantization scheme based on Gaus- sian mixture model (GMM), lattice quantization, and arithmetic coding is proposed. The method allows for changing the average rate in real-time, and facilitates adaptation to the currently available bandwidth of the network. A practical solution to the classical issue of indexing and entropy-coding the quantized code vectors is given. The proposed scheme has a computational complexity that is independent of rate, and quadratic with respect to vector dimension. Hence, the scheme can be applied to the quantization of source vectors in a high dimensional space. The theoretical performance of the scheme is analyzed under a high-rate assumption. It is shown that, at high rate, the scheme approaches the theoretically optimal performance, if the mixture components are located far apart. The practical performance of the scheme is confirmed through simulations on both synthetic and speech-derived source vectors.

  • 2557.
    Zhao, David Yuheng
    et al.
    KTH, Skolan för elektro- och systemteknik (EES), Ljud- och bildbehandling.
    Kleijn, Bastiaan
    KTH, Skolan för elektro- och systemteknik (EES), Ljud- och bildbehandling.
    HMM-based gain-modeling for enhancement of speech in noise2007Ingår i: IEEE transactions on speech and audio processing, ISSN 1063-6676, E-ISSN 1558-2353, Vol. 15, nr 3, s. 882-892Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Accurate modeling and estimation of speech and noise gains facilitate good performance of speech. enhancement methods using data-driven prior models. In this paper, we propose a hidden Markov model (HMM)-based speech enhancement method using explicit gain modeling. Through the introduction of stochastic gain variables, energy variation in both speech and noise is explicitly modeled in a unified framework. The speech gain models the energy variations of the speech phones, typically due to differences in pronunciation and/or different vocalizations of individual speakers. The noise gain helps to improve the tracking of the time-varying energy of nonstationary noise. The expectationmaximization (EM) algorithm is used to perform offline estimation of the time-invariant model parameters. The time-varying model'parameters are estimated online using the recursive EM algorithm. The. proposed gain modeling techniques are applied to a novel Bayesian speech estimator, and the performance of the proposed enhancement method is evaluated through objective and subjective tests. The experimental results confirm the advantage of explicit gain modeling, particularly for nonstationary noise sources.

  • 2558.
    Zhao, David Yuheng
    et al.
    KTH, Skolan för elektro- och systemteknik (EES), Ljud- och bildbehandling.
    Kleijn, Bastiaan
    KTH, Skolan för elektro- och systemteknik (EES), Ljud- och bildbehandling.
    Ypma, Alexander
    GN ReSound AS, Algorithm R&D, Eindhoven.
    de Vries, Bert
    GN ReSound AS, Algorithm R&D, Eindhoven.
    Online noise estimation using stochastic-gain HMM for speech enhancement2008Ingår i: IEEE transactions on speech and audio processing, ISSN 1063-6676, E-ISSN 1558-2353, Vol. 16, nr 4, s. 835-846Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We propose a noise estimation algorithm for single-channel noise suppression in dynamic noisy environments. A stochastic-gain hidden Markov model (SG-HMM) is used to model the statistics of nonstationary noise with time-varying energy. The noise model is adaptive and the model parameters are estimated online from noisy observations using a recursive estimation algorithm. The parameter estimation is derived for the maximum-likelihood criterion and the algorithm is based on the recursive expectation maximization (EM) framework. The proposed method facilitates continuous adaptation to changes of both noise spectral shapes and noise energy levels, e.g., due to movement of the noise source. Using the estimated noise model, we also develop an estimator of the noise power spectral density (PSD) based on recursive averaging of estimated noise sample spectra. We demonstrate that the proposed scheme achieves more accurate estimates of the noise model and noise PSD, and as part of a speech enhancement system facilitates a lower level of residual noise.

  • 2559.
    Zhao, David Yuheng
    et al.
    KTH, Skolan för elektro- och systemteknik (EES), Ljud- och bildbehandling.
    Samuelsson, Jonas
    Dolby Labs, Stockholm.
    Nilsson, Mattias
    KTH, Skolan för elektro- och systemteknik (EES), Ljud- och bildbehandling.
    On entropy-constrained vector quantization using gaussian mixture models2008Ingår i: IEEE Transactions on Communications, ISSN 0090-6778, E-ISSN 1558-0857, Vol. 56, nr 12, s. 2094-2104Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    A flexible and low-complexity entropy-constrained vector quantizer (ECVQ) scheme based on Gaussian mixture models (GMMs), lattice quantization, and arithmetic coding is presented. The source is assumed to have a probability density function of a GMM. An input vector is first classified to one of the mixture components, and the Karhunen-Loeve transform of the selected mixture component is applied to the vector, followed by quantization using a lattice structured codebook. Finally, the scalar elements of the quantized vector are entropy coded sequentially using a specially designed arithmetic coder. The computational complexity of the proposed scheme is low, and independent of the coding rate in both the encoder and the decoder. Therefore, the proposed scheme serves as a lower complexity alternative to the GMM based ECVQ proposed by Gardner, Subramaniam and Rao [1]. The performance of the proposed scheme is analyzed under a high-rate assumption, and quantified for a given GMM. The practical performance of the scheme was evaluated through simulations on both synthetic and speech line spectral frequency (LSF) vectors. For LSF quantization, the proposed scheme has a comparable performance to [1] at rates relevant for speech coding (20-28 bits per vector) with lower computational complexity.

  • 2560.
    Zhao, Kun
    et al.
    KTH, Skolan för elektro- och systemteknik (EES), Elektroteknisk teori och konstruktion. Mobile Communications AB, Sweden.
    Bengtsson, E.
    Ying, Z.
    He, Sailing
    KTH, Skolan för elektro- och systemteknik (EES), Elektroteknisk teori och konstruktion.
    Multiplexing efficiency of high order MIMO in mobile terminal in different propagation scenarios2016Ingår i: 2016 10th European Conference on Antennas and Propagation, EuCAP 2016, Institute of Electrical and Electronics Engineers (IEEE), 2016, artikel-id 7481345Konferensbidrag (Refereegranskat)
    Abstract [en]

    The multiplexing efficiency of 4×4 higher order MIMO with different MIMO topologies under different propagation scenarios in a mobile terminal is presented in this study. The mean effective gain, correlation coefficient and multiplexing efficiency of different MIMO topologies are compared and the impact from the angular spread of the incoming wave is also studied. The results are instructive for the high order MIMO antenna design in mobile terminal.

  • 2561.
    Zhao, Kun
    et al.
    KTH, Skolan för elektro- och systemteknik (EES), Elektroteknisk teori och konstruktion.
    Zhang, Shuai
    KTH, Skolan för elektro- och systemteknik (EES), Elektroteknisk teori och konstruktion.
    Ying, Z.
    Bolin, T.
    He, Sailing
    KTH, Skolan för elektro- och systemteknik (EES), Elektroteknisk teori och konstruktion.
    SAR study of different MIMO antenna designs for LTE application in smart mobile handsets2013Ingår i: IEEE Transactions on Antennas and Propagation, ISSN 0018-926X, E-ISSN 1558-2221, Vol. 61, nr 6, s. 3270-3279Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    This paper mainly focuses on the specific absorption rate (SAR) of the dual-element LTE MIMO antenna in mobile phones. Four designs of dual-element MIMO antenna (namely, dual semi-ground-free planar inverted-F antenna (PIFA), co-located antenna, dual OG PIFA in parallel position, and dual OG PIFA in orthogonal position) are studied under four typical LTE frequency points (0.75, 0.85, 1.9, and 2.1/2.6 GHz) when the ground plane length varies from 90 to 150 mm. The SAR, when dual elements operate simultaneously, is also studied through the SAR to PEAK location spacing ratio (SPLSR) according to the FCC standard. The simulations are carried out on both an SAM head phantom and a flat phantom by CST 2011, and measurements on the flat phantom are made with iSAR and DASY4 to verify the accuracy of our simulations.

  • 2562.
    Zhao, Wang
    et al.
    KTH, Skolan för elektro- och systemteknik (EES), Kommunikationsteori.
    Xiao, Ming
    KTH, Skolan för elektro- och systemteknik (EES), Centra, ACCESS Linnaeus Centre. KTH, Skolan för elektro- och systemteknik (EES), Kommunikationsteori.
    Skoglund, Mikael
    KTH, Skolan för elektro- och systemteknik (EES), Centra, ACCESS Linnaeus Centre. KTH, Skolan för elektro- och systemteknik (EES), Kommunikationsteori.
    Poor, H. Vincent
    Princeton University.
    Secrecy degrees of freedom of wireless X networks using artificial noise alignmentManuskript (preprint) (Övrigt vetenskapligt)
    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.

  • 2563.
    Zhao, Wang
    et al.
    KTH, Skolan för elektro- och systemteknik (EES), Kommunikationsteori.
    Xiao, Ming
    KTH, Skolan för elektro- och systemteknik (EES), Centra, ACCESS Linnaeus Centre. KTH, Skolan för elektro- och systemteknik (EES), Kommunikationsteori.
    Wang, Chao
    KTH, Skolan för elektro- och systemteknik (EES), Kommunikationsteori.
    Skoglund, Mikael
    KTH, Skolan för elektro- och systemteknik (EES), Centra, ACCESS Linnaeus Centre. KTH, Skolan för elektro- och systemteknik (EES), Kommunikationsteori.
    Degrees of freedom of two hop MISO broadcast network with mixed CSIT2014Ingår i: IEEE Transactions on Wireless Communications, ISSN 1536-1276, E-ISSN 1558-2248, Vol. 13, nr 12, s. 6982-6995Artikel i tidskrift (Refereegranskat)
    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.

  • 2564. Zheng, Gan
    et al.
    Krikidis, Ioannis
    Ottersten, Björn
    KTH, Skolan för elektro- och systemteknik (EES), Signalbehandling. KTH, Skolan för elektro- och systemteknik (EES), Centra, ACCESS Linnaeus Centre.
    Full-Duplex Cooperative Cognitive Radio with Transmit Imperfections2013Ingår i: IEEE Transactions on Wireless Communications, ISSN 1536-1276, E-ISSN 1558-2248, Vol. 12, nr 5, s. 2498-2511Artikel i tidskrift (Refereegranskat)
    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.

  • 2565.
    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, Skolan för elektro- och systemteknik (EES), Signalbehandling. KTH, Skolan för elektro- och systemteknik (EES), Centra, ACCESS Linnaeus Centre.
    Robust Cognitive Beamforming With Bounded Channel Uncertainties2009Ingår i: IEEE Transactions on Signal Processing, ISSN 1053-587X, E-ISSN 1941-0476, Vol. 57, nr 12, s. 4871-4881Artikel i tidskrift (Refereegranskat)
    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.

  • 2566.
    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, Skolan för elektro- och systemteknik (EES), Signalbehandling. KTH, Skolan för elektro- och systemteknik (EES), Centra, ACCESS Linnaeus Centre.
    Robust and distributed beamforming2009Ingår i: 2009 International Conference on Wireless Communications and Signal Processing, 2009, Vol. WCSP 2009, s. 1629-1633Konferensbidrag (Refereegranskat)
    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.

  • 2567.
    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, Skolan för elektro- och systemteknik (EES), Signalbehandling. KTH, Skolan för elektro- och systemteknik (EES), Centra, ACCESS Linnaeus Centre.
    Robust Collaborative-Relay Beamforming2009Ingår i: IEEE Transactions on Signal Processing, ISSN 1053-587X, E-ISSN 1941-0476, Vol. 57, nr 8, s. 3130-3143Artikel i tidskrift (Refereegranskat)
    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.

  • 2568. Zheng, Xueyan
    et al.
    Mahgerefteh, D.
    Matsui, Y.
    Ye, Xiong
    Bu, V.
    McCallion, K.
    Xu, Haitao
    Deutsch, M.
    Ereifej, H.
    Lewén, R.
    Wesström, J. -O
    Schatz, Richard
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Fotonik och optik (Stängd 20120101), Fotonik (Stängd 20120101).
    Rigole, P. -J
    Generation of RZ-AMI using a widely tuneable modulated grating Y-branch chirp managed laser2010Ingår i: 2010 Conference on Optical Fiber Communication, Collocated National Fiber Optic Engineers Conference, OFC/NFOEC 2010, 2010, s. 5465413-Konferensbidrag (Refereegranskat)
    Abstract [en]

    A 10 Gb/s RZ-AMI format is generated using a 45 nm tunable Modulated-Grating Y-branch Chirp Managed Laser (CML) with an optical delay line interferometer (DLI), achieving +/- 800 ps/nm dispersion tolerance window.

  • 2569. Zhong, Yang
    et al.
    Tufvesson, Fredrik
    A Dual Input-channel Software Defined Receiver Platformfor GSM WCDMA and Wi-Fi2011Ingår i: Wireless Communications and Signal Processing (WCSP), 2011Konferensbidrag (Refereegranskat)
  • 2570. Zhou, X. P.
    et al.
    Soares, F. M.
    Fontaine, N. K.
    Baek, J. H.
    Cheung, S.
    Shearn, M.
    Scherer, A.
    Olsson, Fredrik
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Materialfysik (Stängd 20120101), Halvledarmaterial, HMA (Stängd 20120101).
    Lourdudoss, Sebastian
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Materialfysik (Stängd 20120101), Halvledarmaterial, HMA (Stängd 20120101).
    Liu, K. Y.
    Tsang, W. T.
    Yoo, S. J. B.
    16-channel × 100-GHz monolithically integrated O-CDMA transmitter with SPECTS encoder and seven 10-GHz mode-locked lasers2010Ingår i: 2010 Conference on Optical Fiber Communication, Collocated National Fiber Optic Engineers Conference, OFC/NFOEC 2010, New York: IEEE , 2010, s. 5465650-Konferensbidrag (Refereegranskat)
    Abstract [en]

    We demonstrate a fully-integrated O-CDMA transmitter by monolithically integrating 7 collidingpulse mode-locked lasers with two arrayed waveguide gratings and 16 phase modulators in InP technology.

  • 2571. Zhu, Kun
    et al.
    Ou, Haiyan
    Fu, Hongyan
    Remb, Edvin
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Centra, Zhejiang-KTH Joint Research Center of Photonics, JORCEP. KTH, Skolan för elektro- och systemteknik (EES), Elektroteknisk teori och konstruktion.
    He, Sailing
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Centra, Zhejiang-KTH Joint Research Center of Photonics, JORCEP. KTH, Skolan för elektro- och systemteknik (EES), Elektroteknisk teori och konstruktion.
    A Simple and Tunable Single-Bandpass Microwave Photonic Filter of Adjustable Shape2008Ingår i: IEEE Photonics Technology Letters, ISSN 1041-1135, E-ISSN 1941-0174, Vol. 20, nr 21-24, s. 1917-1919Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    A simple and tunable single-hand pass microwave photonic filter of adjustable shape is proposed and experimentally demonstrated. The filter is based on multiple sources (spectrum-sliced by a Mach-Zehnder interferometer) and two cascaded incoherent optical structures, namely, a dispersion medium of 50-km single-mode fiber and a fiber ring delay line. In the experimental range from 1.0 to 3.8 GHz, a discretely tunable single-hand pass filter with a high Q-factor or a flat-top response is achieved by carefully matching the transfer functions of the two individual optical structures.

  • 2572. Zhu, Kun
    et al.
    Ou, Haiyan
    Ye, Chenhui
    Fu, Hongyan
    He, Sailing
    KTH, Skolan för elektro- och systemteknik (EES), Elektroteknisk teori och konstruktion.
    All-optical coherence-free microwave filter with switchable passbands based on phase and intensity hybrid modulation2011Ingår i: Optics Communications, ISSN 0030-4018, E-ISSN 1873-0310, Vol. 284, nr 1, s. 140-143Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    A simple all-optical coherence-free microwave notch filter is proposed and experimentally demonstrated. The filter with switchable passbands is based on a single-wavelength optical source and phase-intensity hybrid modulation scheme. The DC bias voltage applied to the Mach-Zehnder intensity modulator can be adjusted to switch signs of the tap coefficients, which affect the baseband response. Both lowpass and bandpass filtering responses are observed in the experiment and agree well with the simulated results.

  • 2573.
    Zhu, Shanying
    et al.
    Shanghai Jiao Tong Univ, Dept Automat, Shanghai 200240, Peoples R China.;Minist Educ China, Key Lab Syst Control & Informat Proc, Shanghai 200240, Peoples R China..
    Chen, Cailian
    Shanghai Jiao Tong Univ, Dept Automat, Shanghai 200240, Peoples R China.;Minist Educ China, Key Lab Syst Control & Informat Proc, Shanghai 200240, Peoples R China..
    Xu, Jinming
    Arizona State Univ, Ira A Fulton Sch Engn, Tempe, AZ 85281 USA..
    Guan, Xinping
    Shanghai Jiao Tong Univ, Dept Automat, Shanghai 200240, Peoples R China.;Minist Educ China, Key Lab Syst Control & Informat Proc, Shanghai 200240, Peoples R China..
    Xie, Lihua
    Nanyang Technol Univ, Sch Elect & Elect Engn, Singapore 639798, Singapore..
    Johansson, Karl H.
    KTH, Skolan för elektroteknik och datavetenskap (EECS), Centra, ACCESS Linnaeus Centre.
    Mitigating Quantization Effects on Distributed Sensor Fusion: A Least Squares Approach2018Ingår i: IEEE Transactions on Signal Processing, ISSN 1053-587X, E-ISSN 1941-0476, Vol. 66, nr 13, s. 3459-3474Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    In this paper, we consider the problem of sensor fusion over networks with asymmetric links, where the common goal is linear parameter estimation. For the scenario of bandwidth-constrained networks, existing literature shows that nonvanishing errors always occur, which depend on the quantization scheme. To tackle this challenging issue, we introduce the notion of virtual measurements and propose a distributed solution LS-DSFS, which is a combination of a quantized consensus algorithm and the least squares approach. We provide detailed analysis of the LS-DSFS on its performance in terms of unbiasedness and mean square property. Analytical results show that the LS-DSFS is effective in smearing out the quantization errors, and achieving the minimum mean square error (MSE) among the existing centralized and distributed algorithms. Moreover, we characterize its rate of convergence in the mean square sense and that of the mean sequence. More importantly, we find that the LS-DSFS outperforms the centralized approaches within a moderate number of iterations in terms of MSE, and will always consume less energy and achieve more balanced energy expenditure as the number of nodes in the network grows. Simulation results are presented to validate theoretical findings and highlight the improvements over existing algorithms.

  • 2574. Zhu, Xuncheng
    et al.
    Ding, Zhizhong
    Punyal, Oscar
    Gross, James
    Design and Implementation of Baseband Algorithms for OFDM-based Wireless Communication Systems2011Ingår i: 7th International Conference on Wireless Communications, Networking and Mobile Computing (WiCOM), 2011, IEEE conference proceedings, 2011, s. 1-5Konferensbidrag (Refereegranskat)
    Abstract [en]

    Orthogonal frequency-division multiplexing (OFDM) is an appropriate solution to combat inter-symbol interference (ISI) caused by multi-path propagation in wireless communications. OFDM-based wireless communication systems become more and more popular and the base-band algorithm is one of the significant parts of the systems. In this paper, we present the design and implementation of base-band algorithms for OFDM-based wireless communication systems. The results show that the designed algorithms are effective for implementation of the system.

  • 2575.
    Zorbas, Dimitros
    et al.
    University of Piraeus.
    Douligeris, Christos
    University of Piraeus.
    Fodor, Viktoria
    KTH, Skolan för elektro- och systemteknik (EES), Kommunikationsnät. KTH, Skolan för elektro- och systemteknik (EES), Centra, ACCESS Linnaeus Centre.
    Target location based sink positioning in wireless sensor networks2011Ingår i: 2011 18th International Conference on Telecommunications, ICT 2011, 2011, s. 21-26Konferensbidrag (Refereegranskat)
    Abstract [en]

    One of the main challenges in wireless sensor networks is to prolong the network lifetime by efficiently handling the limited battery life of the nodes. This problem becomes harder in applications where the nodes are randomly dropped in the field. In this paper we deal with the problem of the sink placement and of the network longevity, assuming a number of points in the field with known positions which must be covered by the sensors. Unlike other approaches, we consider the more realistic scenario where the coordinates of the sensors are not assumed to be known in advance and, thus, they cannot be used for the computation of the positions of the sinks. We present two solutions for the above problem; one based on the distance between the points and the second on the probability that a sensor may cover many points. We evaluate our approaches and compare them to algorithms that use the knowledge of the positions of the sensors in order to compute likely sink locations. It is shown that both proposed approaches present similar or better performance concerning network lifetime, while at the same time they significantly decrease the algorithm complexity.

  • 2576.
    Zuxing, Li
    KTH, Skolan för elektro- och systemteknik (EES), Kommunikationsteori. KTH, Skolan för elektro- och systemteknik (EES), Centra, ACCESS Linnaeus Centre.
    Assessment of Privacy in Distributed Detection Problems2013Självständigt arbete på avancerad nivå (masterexamen), 20 poäng / 30 hpStudentuppsats (Examensarbete)
    Abstract [en]

    As a promising technology, wireless sensor networks have a wide range of applications. However, the development of wireless sensor networks is stillin face of multiple challenges. Among these challenges, the privacy issue is acritical parameter involved in providing secure and reliable services and attracts much attention from researchers and engineers. In the last decade, alarge number of privacy solutions for wireless sensor networks have been proposed. However, most of them are taken as additional secure functionality blocks rather than being integrated in the original sensor network designs.

    In this thesis project, we will focus on the privacy assessment of a parallel distributed detection network, which represents a simplied physical-layer of wireless sensor networks. The security threat is assumed to come from a passive eavesdropper. Four privacy leakage criteria are proposed to evaluate the privacy issue of the distributed detection network in dierent scenarios. As references, the privacy leakages are evaluated by dierent criteria when the distributed detection system is optimized in the perspectives of Bayesian detection theory and information theory without considering the presence of the eavesdropper. Then, we propose the corresponding privacy-concerned distributed detection systems. Comparisons to the optimal detection systems are performed and they reveal the trade-off between privacy leakage suppression and detection performance degradation.

  • 2577.
    Ängskog, Per
    et al.
    Center for RF Measurement Technology, University of Gävle, Gävle, Sweden.
    Karlsson, Carl
    Center for RF Measurement Technology, University of Gävle, Gävle, Sweden.
    Ferrer Coll, Javier
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Centra, KTH Center för Trådlösa System, Wireless@kth.
    Chilo, Jose
    Center for RF Measurement Technology, University of Gävle, Gävle, Sweden.
    Stenumgaard, Peter
    Center for RF Measurement Technology, University of Gävle, Gävle, Sweden.
    Sources of Disturbances on Wireless Communication in Industrial and Factory Environments2010Ingår i: 2010 Asia-Pacific Symposium on Electromagnetic Compatibility (APEMC), 2010, s. 281-284Konferensbidrag (Refereegranskat)
    Abstract [en]

     Electrical and mechanical machinery, highly reflective industrial facilities and co-existing communication systems are the major sources of disturbances in wireless industrial applications. Characterization of industrial environments is important for the development of standards, to assess current and future deployment of wireless technologies, and to provide systems integrators and end user with guidelines. In this paper some deductions from measurements carried out at three industrial environments using traditional electric field strength, amplitude probability distribution (APD) and multipath time dispersion measurements are presented. These measurements have given surprising and interesting results.

  • 2578. Öhlén, P.
    et al.
    Skubic, B.
    Rostami, A.
    Fiorani, Matteo
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Kommunikationssystem, CoS, Optical Network Laboratory (ON Lab).
    Monti, Paolo
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Kommunikationssystem, CoS, Optical Network Laboratory (ON Lab).
    Ghebretensaé, Z.
    Mårtensson, J.
    Wang, K.
    Wosinska, Lena
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Kommunikationssystem, CoS, Optical Network Laboratory (ON Lab).
    Data plane and control architectures for 5G transport networks2016Ingår i: Journal of Lightwave Technology, ISSN 0733-8724, E-ISSN 1558-2213, Vol. 34, nr 6, s. 1501-1508, artikel-id 7397818Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Next generation 5G mobile system will support the vision of connecting all devices that benefit from a connection, and support a wide range of services. Consequently, 5G transport networks need to provide the required capacity, latency, and flexibility in order to integrate the different technology domains of radio, transport, and cloud. This paper outlines the main challenges, which the 5G transport networks are facing and discusses in more detail data plane, control architectures, and the tradeoff between different network abstraction models.

  • 2579.
    Österberg, Patrik
    KTH, Tidigare Institutioner, Mikroelektronik och informationsteknik, IMIT.
    Efficient multicast video streaming over heterogeneous networks2004Licentiatavhandling, sammanläggning (Övrigt vetenskapligt)
  • 2580.
    Östman, Thomas
    et al.
    KTH, Tidigare Institutioner, Signaler, sensorer och system.
    Kristensson, Martin
    KTH, Tidigare Institutioner, Signaler, sensorer och system.
    Ottersten, Björn
    KTH, Tidigare Institutioner, Signaler, sensorer och system.
    Asynchronous DS-CDMA Detectors Robust to Timing Errors1997Ingår i: Proceedings of IEEE 47th Vehicular Technology Conference.  VTC97, IEEE , 1997, s. 1704-1708Konferensbidrag (Refereegranskat)
    Abstract [en]

    An asynchronous DS-CDMA system with timing errors is considered. Two different approaches for dealing with the problem are presented. The first method is based on robustifying the MMSE estimator against timing errors. The second approach models the timing errors as an extra noise term and formulates the best linear unbiased estimator of the data symbols. Simulations of the bit error rate indicate that the two approaches have similar performance. However, the difference to non-robustified algorithms is large.

  • 2581.
    östman, Thomas
    et al.
    KTH, Tidigare Institutioner, Signaler, sensorer och system.
    Milstein, Laurence B
    Wireless Communication, University of California, San Diego, La Jolla, CA 92093 USA.
    Ottersten, Björn
    KTH, Tidigare Institutioner, Signaler, sensorer och system.
    Estimation of Time Delays in Multirate Asynchronous DS-CDMA Systems1998Ingår i: Proceedings International Symposium on Spread-Spectrum Techniques and Applications, IEEE , 1998, s. 97-101Konferensbidrag (Refereegranskat)
    Abstract [en]

    This article treats the problem of time delay estimation in a multirate asynchronous direct-sequence code-division multiple access (DS-CDMA) system in a near-far scenario. A subspace based algorithm for the time delayestimation problem is proposed. The algorithm is a minor modified version of the MUSIC estimator proposed by Ostman, Parkvall and Ottersten (see Proceedings of the 31st Asilomar Conference on Signals, Systems and Computers, 1997). Through theoretical analysis and simulations it is shown that the subspace approach to timedelay estimation in a multirate DS-CDMA system is applicable when the algorithm is applied in a way such that the low-rank structure of the system is conserved.

  • 2582.
    Östman, Thomas
    et al.
    KTH, Tidigare Institutioner, Signaler, sensorer och system.
    Ottersten, Björn
    KTH, Tidigare Institutioner, Signaler, sensorer och system.
    Low Complexity Asynchronous DS-CDMA Detectors1996Ingår i: IEEE Transactions on Vehicular Technology ConferenceIEEE 46th Vehicular Technology Conference. 'Mobile Technology for the Human Race'., 1996, 1996, s. 559-563Konferensbidrag (Refereegranskat)
    Abstract [en]

    Near-far resistant multiuser-detectors for an asynchronous direct-sequence code-division multiple access (DS-CDMA) system, which do not need the code-matched filter outputs from all users are presented. Two complex-valued, discrete-time vector models for the system are formulated. Detectors for both models, that work on blocks of data of M symbols are considered. The problem of estimation of the bits is posed in a least squares sense and as a minimum mean squared error problem. The performance of the detection algorithms is investigated by computing the BER (bit error rate) on simulated data. Comparisons with traditional detectors is also carried out. It is shown that detectors based on the compact vector model performs slightly worse thandetectors based on the more complex vector model.

  • 2583.
    östman, Thomas
    et al.
    KTH, Tidigare Institutioner, Signaler, sensorer och system.
    Ottersten, Björn
    KTH, Tidigare Institutioner, Signaler, sensorer och system.
    Near Far Robust Time Delay Estimation for Asynchronous DS-CDMA Systems with Bandlimited Pulse Shapes1998Ingår i: IEEE Transactions on Vehicular Technology Conference, IEEE , 1998, s. 1650-1654Konferensbidrag (Refereegranskat)
    Abstract [en]

    An algorithm for time delay estimation in an asynchronous direct-sequence code-division multiple access (DS-CDMA) system with bandlimited signals in a near-far scenario (no tight power control) is presented. The algorithm, which is a modified MUSIC algorithm, is validated through simulations and asymptotic analysis (large number of vector samples) for a square-root raised cosine pulse. It is shown that the algorithm has performance very similar to the case with rectangular pulses and hence it is near-far resistant.

  • 2584.
    Östman, Thomas
    et al.
    KTH, Tidigare Institutioner, Signaler, sensorer och system.
    Parkvall, Stefan
    KTH, Tidigare Institutioner, Signaler, sensorer och system.
    Ottersten, Björn
    KTH, Tidigare Institutioner, Signaler, sensorer och system.
    An Improved MUSIC Algorithm for Estimation of Time Delays in Asynchronous DS-CDMA Systems1999Ingår i: IEEE Transactions on Communications, ISSN 0090-6778, Vol. 47, nr 11, s. 1628-1631Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The problem of near–far resistant time-delay estimation in an asynchronous direct-sequence code-division multipleaccess system is considered, and by better exploiting the structureof the problem, estimators superior to previously known techniques are obtained. For a typical numerical example, a gain insignal-to-noise ratio of approximately 3 dB is obtained.

  • 2585.
    östman, Thomas
    et al.
    KTH, Tidigare Institutioner, Signaler, sensorer och system.
    Parkvall, Stefan
    KTH, Tidigare Institutioner, Signaler, sensorer och system.
    Ottersten, Björn
    KTH, Tidigare Institutioner, Signaler, sensorer och system.
    Improved MUSIC Algorithm for Estimation of Time Delays in Asynchronous DS-CDMA Systems1997Ingår i: Proceedings of Asilomar Conference on Signals, Systems & Computers, IEEE , 1997, s. 838-842Konferensbidrag (Refereegranskat)
    Abstract [en]

    An algorithm for time delay estimation in an asynchronous direct-sequence code-division multiple access (DS-CDMA) system which exploits the structure of the system better than previously known algorithms, is presented. The different algorithms are compared through simulation and asymptotic analysis (large number ofvector samples). For a typical scenario it is shown that the proposed algorithm decreases the standard deviationof the time delay errors with a factor of approximately 2.

  • 2586.
    Özyagci, Ali
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Kommunikationssystem, CoS. KTH, Skolan för informations- och kommunikationsteknik (ICT), Centra, KTH Center för Trådlösa System, Wireless@kth.
    Selfish Dynamic Spectrum Access in Multichannel Wireless Networks: Complete and incomplete information analysis2011Licentiatavhandling, sammanläggning (Övrigt vetenskapligt)
    Abstract [en]

    The increasing popularity and widespread deployment of wireless data systems fuel the increasing demand for more spectrum. On the other hand, various studies measuring spectrum utilization show that there is a huge variation in spectrum utilization at different times and locations. In view of this, various dynamic spectrum access (DSA) methods have been proposed in order to achieve more efficient utilization of spectrum resources by virtue of exploiting the variations in spectrum demand over time and space. Implementing DSA systems in a centralized way can lead to complexity and scalability problems due to the extensive control signaling involved. Therefore distributed implementations of DSA systems in which the users can access the system resources at their own discretion have been proposed. These distributed mechanisms typically incorporate cognitive radio systems which act as agents on behalf of users to measure the radio environment and make decisions based on these measurements. On the other hand, the freedom of the users in distributed systems to form their actions can lead each user to try to maximize its benefit from the system without regard the overall performance of the DSA system. Therefore, selfish behavior can prevail in distributed systems, which is likely to degrade the system performance.In this thesis we investigate the implications of selfish decision making in dynamic spectrum access systems. To address this broad problem, we focus our analysis on a particular system which can represent the essential properties of DSA systems and thus can shed light on the performance of the broad class of DSA systems with selfish users. Specifically, we model a DSA system as a multichannel random access system which uses ALOHA for medium access, and we analyze the behavior of the selfish users by modeling the system as a non-cooperative game. In this analysis we incorporate the effect of channel state information on the decision making of the users; we consider both cases when the users act on global (complete) and on local (incomplete) information. We determine the behavior of the selfish users at the Nash equilibria of the non-cooperative game and measure the performance of the system in terms of sum and individual utilities for various user loads and amount of available resources. We try to identify how the performance of the DSA system with selfish users compares with its cooperative counterpart. By performing these analyses we provide insights into the broader question of whether selfish users can utilize spectrum resources in a DSA system as well as cooperative users.

  • 2587.
    Özyagci, Ali
    et al.
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Kommunikationssystem, CoS. KTH, Skolan för informations- och kommunikationsteknik (ICT), Centra, KTH Center för Trådlösa System, Wireless@kth.
    Ileri, Ömer
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Kommunikationssystem, CoS. KTH, Skolan för informations- och kommunikationsteknik (ICT), Centra, KTH Center för Trådlösa System, Wireless@kth.
    Zander, Jens
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Kommunikationssystem, CoS. KTH, Skolan för informations- och kommunikationsteknik (ICT), Centra, KTH Center för Trådlösa System, Wireless@kth.
    Distributed multichannel random access networks with selfish users2010Ingår i: Proceedings of the Fifth International Conference on Cognitive Radio Oriented Wireless Networks Communications (CROWNCOM), 2010, 2010, s. 5577687-Konferensbidrag (Refereegranskat)
    Abstract [en]

    Dynamic spectrum access (DSA) schemes allow the users to share spectrum resources by taking advantage of the variations in spectrum demand over time and space. Carrying out dynamic spectrum allocation centrally, however, can be a complex task. For this reason, distributed schemes in which users can access the available channels independently may be preferable to centralized DSA schemes. Cognitive radio systems, which enable user terminals to sense their environment and form their action accordingly, are particularly well-suited for distributed systems. On the other hand, the freedom in distributed schemes gives the users the option to act selfishly, which has decisive effects on system performance. In this paper we consider a distributed multichannel wireless random access system where users selfishly access the channels in the system. We analyze the behavior of the selfish users by modeling the system as a non-cooperative game and we identify all stable operating points (Nash equilibria) of this game. We then compare the performance of this system with a number of cooperative distributed DSA schemes in terms of user utilities. Our results show that the performance of the selfish multichannel random access system can be comparable to cooperative schemes.

  • 2588.
    Özyagci, Ali
    et al.
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Kommunikationssystem, CoS, Radio Systems Laboratory (RS Lab). KTH, Skolan för informations- och kommunikationsteknik (ICT), Centra, KTH Center för Trådlösa System, Wireless@kth.
    Sung, Ki Won
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Kommunikationssystem, CoS, Radio Systems Laboratory (RS Lab). KTH, Skolan för informations- och kommunikationsteknik (ICT), Centra, KTH Center för Trådlösa System, Wireless@kth.
    Zander, Jens
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Kommunikationssystem, CoS, Radio Systems Laboratory (RS Lab). KTH, Skolan för informations- och kommunikationsteknik (ICT), Centra, KTH Center för Trådlösa System, Wireless@kth.
    Effect of propagation environment on area throughput of dense WLAN deployments2013Ingår i: 9th IEEE Broadband wireless access workshop (BWA 2013), IEEE Computer Society, 2013, s. 333-338Konferensbidrag (Refereegranskat)
    Abstract [en]

    Indoor wireless LAN deployments have become ubiquitous. As WLAN deployments become increasingly dense, WLANs start to cause more and more contention and interference to each other, to the point that they cause significant throughput degradation to other WLANs. Since WLANs are one of the most commonplace solutions to provide indoor broadband data access, it is crucial to assess the throughput limits of WLANs in order to understand at what demand level novel broadband access mechanisms will be critically needed. The amount of contention and interference that coexisting WLANs create on each other is influenced by the indoor propagation environment such as existence of walls or clutter. Although the indoor propagation environment has a significant impact on the interaction between WLANs, and consequently on the area throughput, the relationship between the indoor propagation environment and achievable area throughput has not received much attention. In this paper, we investigate the area throughput of densely deployed WLANs in different indoor propagation environments by conducting detailed MAC layer simulations using OPNET. The results show that the propagation conditions have a profound impact on achievable area throughput; as much as several tens of times increase in highly cluttered environments compared to open areas.

  • 2589.
    Özyagci, Ali
    et al.
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Kommunikationssystem, CoS. KTH, Skolan för informations- och kommunikationsteknik (ICT), Centra, KTH Center för Trådlösa System, Wireless@kth.
    Zander, Jens
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Kommunikationssystem, CoS. KTH, Skolan för informations- och kommunikationsteknik (ICT), Centra, KTH Center för Trådlösa System, Wireless@kth.
    Distributed Dynamic Spectrum Access in Multichannel Random Access Networks with Selfish Users2010Ingår i: Wireless Communications and Networking Conference (WCNC), 2010 IEEE, IEEE , 2010, s. 1-6Konferensbidrag (Refereegranskat)
    Abstract [en]

    Dynamic spectrum allocation schemes enable users to share spectrum resources by exploiting the variations in spectrum demand over time and space. Performing dynamic spectrum allocation centrally can be prohibitively complex. Therefore distributed schemes in which users can access the available channels independently may be preferable to centralized allocation. However, in distributed dynamic spectrum access, the lack of central coordination makes it difficult to utilize the system resources efficiently. Furthermore, if some or all of the users decide to deviate selfishly from the commonly agreed access procedure, this may have a decisive effect on system performance. In this paper we investigate the effect of incomplete information and selfish behavior on system performance in wireless access systems. We extend previous work by studying a distributed multichannel wireless random access system. Using a game-theoretic approach, we analyze the behavior of users in the selfish system and derive the transmission strategies at the Nash equilibrium. Our results show that lack of information leads to substantial degredation in performance of cooperative systems. We also show that there is a large incentive for selfish behavior in such cooperative systems. Selfish behavior of all users, however, causes further performance degradation, particularly in high load settings.

  • 2590.
    Özyagci, Ali
    et al.
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Kommunikationssystem, CoS, Radio Systems Laboratory (RS Lab). KTH, Skolan för informations- och kommunikationsteknik (ICT), Centra, KTH Center för Trådlösa System, Wireless@kth.
    Zander, Jens
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Kommunikationssystem, CoS, Radio Systems Laboratory (RS Lab). KTH, Skolan för informations- och kommunikationsteknik (ICT), Centra, KTH Center för Trådlösa System, Wireless@kth.
    Selfish multi-channel random access in heterogeneous channelsManuskript (preprint) (Övrigt vetenskapligt)
    Abstract [en]

    We analyze the behavior of selfish users in a multichannel random access system in which the propagation characteristics of the available channels in the system exhibit different statistics. We formulate the behavior of the selfish users as a Bayesian game and identify the transmission strategies at the Nash equilibria. Following this, we propose a simple iterative algorithm to obtain the transmission probabilities of the selfish uses at the Nash equilibria and investigate the convergence properties of this algorithm. Using the transmission probabilities of the selfish users at the Nash equilibria, we analyze the performance of the MRA system with selfish users in terms of sum and per-user utilities and compare this system with its cooperative and scheduling system counterparts. We find that selfish behavior results in significant performance loss compared to scheduling and cooperative systems, which increases as the system load increases.

  • 2591.
    Čelik, Haris
    et al.
    KTH, Skolan för elektroteknik och datavetenskap (EECS), Centra, KTH Center för trådlösa system, Wireless@kth.
    Sung, Ki Won
    KTH, Skolan för elektroteknik och datavetenskap (EECS), Centra, KTH Center för trådlösa system, Wireless@kth.
    Scalable Resource Allocation for Dynamic TDD with Traffic and Propagation Awareness2018Ingår i: 2018 IEEE WIRELESS COMMUNICATIONS AND NETWORKING CONFERENCE (WCNC), IEEE , 2018Konferensbidrag (Refereegranskat)
    Abstract [en]

    This paper presents a scalable scheduler for dynamic TDD systems in ultra-dense indoor deployments applicable to diverse traffic and radio propagation conditions. To minimize signalling for the channel estimation and feedback, the proposed scheduler employs offline BS-to-BS measurements to approximate the real interference for different traffic conditions. Thus, no CSI is required once the network is online. Furthermore, we show that signalling can be further reduced by considering only meaningful interferers above a received power threshold for the traffic information exchange. To perform the scheduling, a function that maps interference into individual BS activation probabilities is also introduced. Results show that the proposed scheme performs reasonably well in high interference situations compared to comparable scalable schedulers, and optimally when interference is low, both in average and 5th percentile sense.

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