Results 541-560 of 720.
Bookmark and Share    
Full Text
Peer Reviewed
See detailSpace-Frequency Coding for Dual Polarized Hybrid Mobile Satellite Systems
Shankar, Bhavani UL; Arapoglou, Pantelis-Daniel UL; Ottersten, Björn UL

in IEEE Transactions on Wireless Communications (2012), 11(8), 2806-2814

An increasing number of hybrid mobile systems comprising a satellite and a terrestrial component are becoming standardized and realized. The next generation of these systems will employ higher dimensions ... [more ▼]

An increasing number of hybrid mobile systems comprising a satellite and a terrestrial component are becoming standardized and realized. The next generation of these systems will employ higher dimensions adopting multiple-input multiple-output orthogonal frequency division multiplexing (MIMO-OFDM) techniques. In this work, we build upon recent studies of dual polarization MIMO for each component and propose the use of full-rate full-diversity (FRFD) codes adopting a space-frequency paradigm. We also propose a scheme taking advantage of the separation between the subcarriers to enhance the coding gain. By critically assessing the different options for the 4 transmit, 2 receive hybrid scenario taking into account system and channel particularities, we demonstrate that the proposed scheme is a solution for enhancing the performance of next generation hybrid mobile satellite systems. [less ▲]

Detailed reference viewed: 183 (3 UL)
Full Text
Peer Reviewed
See detailIterative Precoder Design and User Scheduling for Block-Diagonalized Systems
Tran, Le-Nam; Bengtsson, Mats; Ottersten, Björn UL

in IEEE Transactions on Signal Processing (2012), 60(7), 3726-3739

The block diagonalization (BD) scheme is a low-complexity suboptimal precoding technique for multiuser multiple input-multiple output (MIMO) downlink channels, which completely precancels the multiuser ... [more ▼]

The block diagonalization (BD) scheme is a low-complexity suboptimal precoding technique for multiuser multiple input-multiple output (MIMO) downlink channels, which completely precancels the multiuser interference. Accordingly, the precoder of each user lies in the null space of other users' channel matrices. In this paper, we propose an iterative algorithm using QR decompositions (QRDs) to compute the precoders. Specifically, to avoid dealing with a large concatenated matrix, we apply the QRD to a sequence of matrices of lower dimensions. One problem of BD schemes is that the number of users that can be simultaneously supported is limited due to zero interference constraints. When the number of users is large, a set of users must be selected, and selection algorithms should be designed to exploit the multiuser diversity gain. Finding the optimal set of users requires an exhaustive search, which has too high computational complexity to be practically useful. Based on the iterative precoder design, this paper proposes a low-complexity user selection algorithm using a greedy method, in which the precoders of selected users are recursively updated after each selection step. The selection metric of the proposed scheduling algorithm relies on the product of the squared row norms of the effective channel matrices, which is related to the eigenvalues by the Hadamard and Schur inequalities. An asymptotic analysis is provided to show that the proposed algorithm can achieve the optimal sum rate scaling of the MIMO broadcast channel. The numerical results show that the proposed algorithm achieves a good trade-off between sum rate performance and computational complexity. When users suffer different channel conditions, providing fairness among users is of critical importance. To address this problem, we also propose two fair scheduling (FS) algorithms, one imposing fairness in the approximation of the data rate, and another directly imposing fairness in the product of the sq- ared row norms of the effective channel matrices. [less ▲]

Detailed reference viewed: 135 (3 UL)
Full Text
Peer Reviewed
See detailDistributed Multicell Beamforming Design Approaching Pareto Boundary with Max-Min Fairness
Huang, Yongming; Zheng, Gan UL; Bengtsson, Mats et al

in IEEE Transactions on Wireless Communications (2012), 11(8), 2921-2933

This paper addresses coordinated downlink beamforming optimization in multicell time division duplex (TDD) systems where a small number of parameters are exchanged between cells but with no data sharing ... [more ▼]

This paper addresses coordinated downlink beamforming optimization in multicell time division duplex (TDD) systems where a small number of parameters are exchanged between cells but with no data sharing. With the goal to reach the point on the Pareto boundary with max-min rate fairness, we first develop a two-step centralized optimization algorithm to design the joint beamforming vectors. This algorithm can achieve a further sum-rate improvement over the max-min optimal performance, and is shown to guarantee max-min Pareto optimality for scenarios with two base stations (BSs) each serving a single user. To realize a distributed solution with limited intercell communication, we then propose an iterative algorithm by exploiting an approximate uplink-downlink duality, in which only a small number of positive scalars are shared between cells in each iteration. Simulation results show that the proposed distributed solution achieves a fairness rate performance close to the centralized algorithm while it has a better sum-rate performance, and demonstrates a better tradeoff between sum-rate and fairness than the Nash Bargaining solution especially at high signal-to-noise ratio. [less ▲]

Detailed reference viewed: 157 (1 UL)
Full Text
Peer Reviewed
See detailExploiting Polarization for Spectrum Sensing in Cognitive SatComs
Sharma, Shree Krishna UL; Chatzinotas, Symeon UL; Ottersten, Björn UL

in Proceedings of 7th International Conference on Cognitive Radio Oriented Wireless Networks (2012, June)

Exploring new techniques for spectrum sensing (SS), which can detect the weak primary signals instantly, has been an important research challenge. In this paper, the problem of enhancing SS efficiency in ... [more ▼]

Exploring new techniques for spectrum sensing (SS), which can detect the weak primary signals instantly, has been an important research challenge. In this paper, the problem of enhancing SS efficiency in cognitive SatComs has been considered. The analysis of different combining techniques has been carried out for SS using dual polarized antenna. Furthermore, polarization states of received signals are exploited and based on obtained polarization states, Optimum Polarization Based Combining (OPBC) technique has been used for carrying out SS in the satellite terminal. The sensing performance of OPBC technique has been compared to selection combining (SC), equal gain combining (EGC) and maximum ratio combining (MRC) techniques. The simulation results show that OPBC technique achieves a great improvement in sensing efficiency over other considered techniques at the expense of complexity in a dual polarized AWGN channel. [less ▲]

Detailed reference viewed: 172 (7 UL)
Full Text
Peer Reviewed
See detailPareto Characterization of the Multicell MIMO Performance Region With Simple Receivers
Björnson, Emil; Bengtsson, Mats; Ottersten, Björn UL

in IEEE Transactions on Signal Processing (2012), 60(8), 4464-4469

We study the performance region of a general multicell downlink scenario with multiantenna transmitters, hardware impairments, and low-complexity receivers that treat interference as noise. The Pareto ... [more ▼]

We study the performance region of a general multicell downlink scenario with multiantenna transmitters, hardware impairments, and low-complexity receivers that treat interference as noise. The Pareto boundary of this region describes all efficient resource allocations, but is generally hard to compute. We propose a novel explicit characterization that gives Pareto optimal transmit strategies using a set of positive parameters-fewer than in prior work. We also propose an implicit characterization that requires even fewer parameters and guarantees to find the Pareto boundary for every choice of parameters, but at the expense of solving quasi-convex optimization problems. The merits of the two characterizations are illustrated for interference channels and ideal network multiple-input multiple-output (MIMO). [less ▲]

Detailed reference viewed: 140 (0 UL)
Full Text
Peer Reviewed
See detailPhysical Layer Security in Multibeam Satellite Systems
Zheng, Gan UL; Arapoglou, Pantelis-Daniel UL; Ottersten, Björn UL

in IEEE Transactions on Wireless Communications (2012), 11(2), 852-863

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 ... [more ▼]

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. [less ▲]

Detailed reference viewed: 276 (0 UL)
Full Text
Peer Reviewed
See detailCognitive interference alignment between small cells and a macrocell
Chatzinotas, Symeon UL; Ottersten, Björn UL

in 19th International Conference on Telecommunications (ICT) 2012 (2012)

Detailed reference viewed: 151 (1 UL)
Full Text
Peer Reviewed
See detailCapacity Analysis of Dual-Hop Amplify-and-Forward MIMO Multiple-Access Channels
Chatzinotas, Symeon UL; Ottersten, Björn UL

in International Conference on Wireless Communications and Signal Processing, WCSP 2012 (2012)

Relay channels have been heavily studied during the last decades as a means of improving spectral efficiency, availability and coverage in combination with multiple antenna transceivers. Relay channels ... [more ▼]

Relay channels have been heavily studied during the last decades as a means of improving spectral efficiency, availability and coverage in combination with multiple antenna transceivers. Relay channels can comprise many hops but the most practical approach at the time being would be a dual-hop system. In addition, the simplest method of relaying in terms of complexity is amplify and forward. In this direction, we investigate the ergodic capacity of a dual-hop amplify-and-forward MIMO MAC and we derive asymptotic closed-form expressions based on the principles of free probability theory. We extend the current literature by proposing a analytical model which can accommodate variance-profiled Gaussian matrices. Numerical results are utilized to verify the accuracy of the derived closed-form expressions and evaluate the effect of the channel parameters. [less ▲]

Detailed reference viewed: 149 (1 UL)
Full Text
Peer Reviewed
See detailDetection of sparse random signals using compressive measurements
Shankar, Bhavani UL; Chatterjee, Saikat; Ottersten, Björn UL

in Acoustics, Speech and Signal Processing (ICASSP), 2012 IEEE International Conference on (2012)

We consider the problem of detecting a sparse random signal from the compressive measurements without reconstructing the signal. Using a subspace model for the sparse signal where the signal parameters ... [more ▼]

We consider the problem of detecting a sparse random signal from the compressive measurements without reconstructing the signal. Using a subspace model for the sparse signal where the signal parameters are drawn according to Gaussian law, we obtain the detector based on Neyman-Pearson criterion and analytically determine its operating characteristics when the signal covariance is known. These results are extended to situations where the covariance cannot be estimated. The results can be used to determine the number of measurements needed for a particular detector performance and also illustrate the presence of an optimal support for a given number of measurements. [less ▲]

Detailed reference viewed: 146 (5 UL)
Full Text
Peer Reviewed
See detailGeneric Optimization of Linear Precoding in Multibeam Satellite Systems
Zheng, Gan UL; Chatzinotas, Symeon UL; Ottersten, Björn UL

in IEEE Transactions on Wireless Communications (2012), 11(6), 2308-2320

Detailed reference viewed: 223 (18 UL)
Full Text
Peer Reviewed
See detailMulti-gateway cooperation in multibeam satellite systems
Zheng, Gan UL; Chatzinotas, Symeon UL; Ottersten, Björn UL

in IEEE 23rd International Symposium on Personal Indoor and Mobile Radio Communications (PIMRC) (2012)

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 ... [more ▼]

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). [less ▲]

Detailed reference viewed: 168 (9 UL)
Full Text
Peer Reviewed
See detailSpatio-Temporal ToF Data Enhancement by Fusion
Garcia Becerro, Frederic UL; Aouada, Djamila UL; Mirbach, Bruno et al

in 19th IEEE International Conference on Image Processing (2012)

Detailed reference viewed: 159 (5 UL)
Full Text
Peer Reviewed
See detailReal-Time Distance-Dependent Mapping for a Hybrid ToF Multi-Camera Rig
Garcia Becerro, Frederic UL; Aouada, Djamila UL; Mirbach, Bruno et al

in IEEE Journal of Selected Topics in Signal Processing (2012), 6(5), 1-12

Detailed reference viewed: 209 (6 UL)
Full Text
Peer Reviewed
See detailBilateral Filter Evaluation Based on Exponential Kernels
Al Ismaeil, Kassem UL; Aouada, Djamila UL; Mirbach, Bruno et al

in Pattern Recognition (ICPR), 2012 21st International Conference on (2012)

The well-known bilateral filter is used to smooth noisy images while keeping their edges. This filter is commonly used with Gaussian kernel functions without real justification. The choice of the kernel ... [more ▼]

The well-known bilateral filter is used to smooth noisy images while keeping their edges. This filter is commonly used with Gaussian kernel functions without real justification. The choice of the kernel functions has a major effect on the filter behavior. We propose to use exponential kernels with L1 distances instead of Gaussian ones. We derive Stein's Unbiased Risk Estimate to find the optimal parameters of the new filter and compare its performance with the conventional one. We show that this new choice of the kernels has a comparable smoothing effect but with sharper edges due to the faster, smoothly decaying kernels. [less ▲]

Detailed reference viewed: 138 (11 UL)
Full Text
Peer Reviewed
See detailFixed Full Duplex Relaying for Wireless Broadband Communication
Meng, Qingmin; Feng, Wei; Zheng, Gan UL et al

in Wireless Communications & Signal Processing (WCSP), 2012 International Conference on (2012)

We compare full duplex (FD) and half duplex (HD) relaying in terms of coverage range. We consider a practical FD relay model where both the dynamic ranges of transmitter and receiver and the level of the ... [more ▼]

We compare full duplex (FD) and half duplex (HD) relaying in terms of coverage range. We consider a practical FD relay model where both the dynamic ranges of transmitter and receiver and the level of the residual loop interference between transmitted and received signals are taken into account. We numerically evaluate the rate performance of the full duplex multi-hop (FD-MH) scheme and the half duplex decode-and-forward (HD-DF) scheme, where the former requires transmit power allocation under the dynamic range constraint. Results show that there is a tradeoff between these two relaying modes. [less ▲]

Detailed reference viewed: 187 (0 UL)
Full Text
Peer Reviewed
See detailNon-regenerative Two-Hop Wiretap Channels using Interference Neutralization
Gerbracht, Sabrina; Jorswieck, Eduard A.; Zheng, Gan UL et al

in Information Forensics and Security (WIFS), 2012 IEEE International Workshop on (2012)

In this paper, we analyze the achievable secrecy rates in the two-hop wiretap channel with four nodes, where the transmitter and the receiver have multiple antennas while the relay and the eavesdropper ... [more ▼]

In this paper, we analyze the achievable secrecy rates in the two-hop wiretap channel with four nodes, where the transmitter and the receiver have multiple antennas while the relay and the eavesdropper have only a single antenna each. The relay is operating in amplify-and-forward mode and all the channels between the nodes are known perfectly by the transmitter. We discuss different transmission and protection schemes like artificial noise (AN). Furthermore, we introduce interference neutralization (IN) as a new protection scheme. We compare the different schemes regarding the high-SNR slope and the high-SNR power offset and illustrate the performance by simulation results. It is shown analytically as well as by numerical simulations that the high SNR performance of the proposed IN scheme is better than the one of AN. [less ▲]

Detailed reference viewed: 132 (0 UL)
Full Text
Peer Reviewed
See detailLinear and nonlinear techniques for multibeam joint processing in satellite communications
Christopoulos, Dimitrios UL; Chatzinotas, Symeon UL; Zheng, Gan UL et al

in EURASIP Journal on Wireless Communications and Networking (2012), 2012(162),

Existing satellite communication standards such as DVB-S2 operate under highly-efficient adaptive coding and modulation schemes thus making significant progress in improving the spectral efficiencies of ... [more ▼]

Existing satellite communication standards such as DVB-S2 operate under highly-efficient adaptive coding and modulation schemes thus making significant progress in improving the spectral efficiencies of digital satellite broadcast systems. However, the constantly increasing demand for broadband and interactive satellite links emanates the need to apply novel interference mitigation techniques, striving towards Terabit throughput. In this direction, the objective of the present contribution is to investigate joint multiuser processing techniques for multibeam satellite systems. In the forward link, the performance of linear precoding is investigated with optimal non-linear precoding (i.e. Dirty Paper Coding) acting as the upper performance limit. To this end, the resulting power and precoder design problems are approached through optimization methods. Similarly, in the return link the concept of linear filtering (i.e. Linear Minimum Mean Square Error) is studied with the optimal successive interference cancellation acting as the performance limit. The derived capacity curves for both scenarios are compared to conventional satellite systems where beams are processed independently and interbeam interference is mitigated through a four color frequency reuse scheme, in order to quantify the potential gain of the proposed techniques. [less ▲]

Detailed reference viewed: 294 (28 UL)
Full Text
See detailInvestigation of Simple Algorithms for Estimation of Delay-Spread and Angle-Spread
Zetterberg, Per; Jaldén, Niklas; Ottersten, Björn UL et al

Scientific journal (2012)

In this paper we describe two simple methods for estimation of delay-spread and angle-spread, respectively. The algorithms are simple in the sense that the transmitted signal may consist of only three ... [more ▼]

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

Detailed reference viewed: 73 (0 UL)
Full Text
Peer Reviewed
See detailRobust Monotonic Optimization Framework for Multicell MISO Systems
Björnson, Emil; Zheng, Gan UL; Bengtsson, Mats et al

in IEEE Transactions on Signal Processing (2012), 60(5), 2508-2523

The performance of multiuser systems is both difficult to measure fairly and to optimize. Most resource allocation problems are nonconvex and NP-hard, even under simplifying assumptions such as perfect ... [more ▼]

The performance of multiuser systems is both difficult to measure fairly and to optimize. Most resource allocation problems are nonconvex and NP-hard, even under simplifying assumptions such as perfect channel knowledge, homogeneous channel properties among users, and simple power constraints. We establish a general optimization framework that systematically solves these problems to global optimality. The proposed branch-reduce-and-bound (BRB) algorithm handles general multicell downlink systems with single-antenna users, multiantenna transmitters, arbitrary quadratic power constraints, and robust- ness to channel uncertainty. A robust fairness-profile optimization (RFO) problem is solved at each iteration, which is a quasiconvex problem and a novel generalization of max-min fairness. The BRB algorithm is computationally costly, but it shows better convergence than the previously proposed outer polyblock approximation algorithm. Our framework is suitable for computing benchmarks in general multicell systems with or without channel uncertainty. We illustrate this by deriving and evaluating a zero-forcing solution to the general problem. [less ▲]

Detailed reference viewed: 154 (2 UL)