| 1. |
- Brandt, Rasmus, 1985-
(author)
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Coordinated Precoding for Multicell MIMO Networks
- 2014
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Licentiate thesis (other academic/artistic)abstract
- Enabling multiple base stations to utilize the spatial dimension in a coordinated manner has been shown to be a fruitful technique for improving the spectral efficiency in wireless interference networks. This thesis considers multicell systems where base stations and mobile stations are equipped with multiple antennas. The base stations coordinate their spatial precoding, but individually serve their mobile stations with data. For such coordinated precoding systems, interference alignment (IA) is a useful theoretical tool, due to its ability to serve the maximum number of interference-free data streams. Three topics related to interference alignment and coordinated precoding are studied.First, the feasibility of IA over a joint space-frequency signal space is studied. A necessary condition for space-frequency IA feasibility is derived, and the possible gain over space-only IA is analyzed. An upper bound on the degree of freedom gain is shown to increase in the number of subcarriers, but decrease in the number of antennas. Numerical studies, using synthetically generated channels and real-world channels obtained from indoors and outdoors channel measurements, are used for sum rate performance evaluation. The results show that although a degree of freedom gain is noticeable due to the space-frequency precoding, the sum rate of the system is mainly improved due to a power gain.Second, distributed channel state information (CSI) acquisition techniques are proposed, which provide estimates of the information necessary to perform distributed coordinated precoding. The methods are based on pilot-assisted channel estimation in the uplink and downlink, and correspond to different tradeoffs between feedback and signaling, backhaul use, and computational complexity. Naively applying the existing WMMSE algorithm for distributed coordinated precoding together with the estimated CSI however results in poor performance. A robustification of the algorithm is therefore proposed, relying on the well known diagonal loading technique. An inherent property of the WMMSE solutions is derived and, when enforced onto solutions with imperfect CSI, results in diagonally loaded receive filters. Numerical simulations show the effectiveness of the proposed robustification. Further, the proposed robustified and distributed WMMSE algorithm performs well compared to existing state-of-the-art robust WMMSE algorithms. In contrast to our approach, the existing methods however rely on centralized CSI acquisition.Third, coordinated precoding systems with hardware impairments are studied. Assuming that impairment compensation techniques have been applied, a model is used to describe the aggregate effect of the residual hardware impairments. An iterative resource allocation method accounting for the residual hardware impairments is derived, based on an existing resource allocation framework. Numerical simulations show that the proposed method outperforms all benchmarks. In particular, the gain over impairments-aware time-division multiple access is substantial.
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| 2. |
- Chenouard, Nicolas, et al.
(author)
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Objective comparison of particle tracking methods
- 2014
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In: Nature Methods. - : Springer Science and Business Media LLC. - 1548-7091 .- 1548-7105. ; 11:3, s. 281-U247
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Journal article (peer-reviewed)abstract
- Particle tracking is of key importance for quantitative analysis of intracellular dynamic processes from time-lapse microscopy image data. Because manually detecting and following large numbers of individual particles is not feasible, automated computational methods have been developed for these tasks by many groups. Aiming to perform an objective comparison of methods, we gathered the community and organized an open competition in which participating teams applied their own methods independently to a commonly defined data set including diverse scenarios. Performance was assessed using commonly defined measures. Although no single method performed best across all scenarios, the results revealed clear differences between the various approaches, leading to notable practical conclusions for users and developers.
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| 3. |
- Čirkić, Mirsad, 1984-
(author)
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Optimization of Computational Resources for MIMO Detection
- 2011
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Licentiate thesis (other academic/artistic)abstract
- For the past decades, the demand in transferring large amounts of data rapidly and reliably has been increasing drastically. One of the more promising techniques that can provide the desired performance is the multiple-input multiple-output (MIMO) technology where multiple antennas are placed at both the transmitting and receiving side of the communication link. One major implementation difficulty of the MIMO technology is the signal separation (detection) problem at the receiving side of the MIMO link. This is due to the fact that the transmitted signals interfere with each other and that separating them can be very difficult if the MIMO channel conditions are not beneficial, i.e., the channel is not well-conditioned.For well-conditioned channels, low-complexity detection methods are often sufficiently accurate. In such cases, performing computationally very expensive optimal detection would be a waste of computational power. This said, for MIMO detection in a coded system, there is always a trade-off between performance and complexity. The fundamental question is, can we save computational resources by performing optimal detection only when it is needed, and something simpler when it is not? This is the question that this thesis aims to answer. In doing so, we present a general framework for adaptively allocating computational resources to different (“simple” and“difficult”) detection problems. This general framework is applicable to any MIMO detector and scenario of choice, and it is exemplified using one particular detection method for which specific allocation techniques are developed and evaluated.
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| 4. |
- Dumard, Charlotte, et al.
(author)
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Multi-User MIMO Receiver Processing for Time-Varying Channels
- 2011
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In: Wireless Communications Over Rapidly Time-Varying Channels. - : Elsevier Academic Press. - 9780123744838 ; , s. 337-374
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Book chapter (other academic/artistic)abstract
- Wireless broadband communications for mobile users at vehicular speed is the cornerstone of future fourth-generation systems. This chapter deals with joint iterative channel estimation and multiuser detection for the uplink of a multicarrier (MC) code division multiple access (CDMA) system. MCCDMA is based on orthogonal frequency division multiplexing (OFDM) and employs spreading sequences in the frequency domain. Several complexity reduction methods are discussed enabling a real-world low-complexity implementation, such as an iterative approximation of the maximum a posteriori (MAP) detector in combination with a reduced-rank model for the time-varying channel. This reduced-rank channel model projects the time-varying channel onto a subspace spanned by band-limited and time-concentrated prolate spheroidal sequences. Two different multiuser detection methods are investigated: First, the Krylov subspace method is used to reduce the complexity of multiuser detection using LMMSE filtering. Second, sphere decoding is investigated and a sphere decoder is developed that exploits the reduced-rank channel model for complexity reduction.
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| 5. |
- Elia, Petros, et al.
(author)
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Fundamental rate-reliability-complexity limits in outage limited MIMO communications
- 2010
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In: IEEE International Symposium on Information Theory - Proceedings. - 9781424469604 ; , s. 2203-2207
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Conference paper (peer-reviewed)abstract
- The work establishes fundamental limits between rate, reliability and computational complexity, for the general setting of outage-limited MIMO communications. In the high-SNR regime, the limits are optimized over all encoders, all decoders, and all complexity regulating policies. The work then proceeds to explicitly identify encoder-decoder designs and policies, that meet this optimal tradeoff. In practice, the limits aim to meaningfully quantify different pertinent and interrelated measures, such as the optimal rate-reliability capabilities per unit complexity and power, the optimal diversity gains per complexity costs, or the optimal goodput per flop. Finally the tradeoff's simple nature, renders it useful for insightful comparison of the rate-reliability-complexity capabilities for different encoders-decoders.
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| 6. |
- Elia, Petros, et al.
(author)
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General DMT optimality of LR-aided linear MIMO-MAC transceivers with worst-case complexity at most linear in sum-rate
- 2010
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In: IEEE Information Theory Workshop 2010. - 9781424463725 ; , s. 5503161-
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Conference paper (peer-reviewed)abstract
- In the setting of multiple-access MIMO channels, the work establishes the DMT optimality of lattice-reduction (LR)-aided regularized linear decoders. This is achieved irrespective of the lattice design applied by each user. The decoding algorithms employ efficient solutions to the Nearby Vector Problem with Preprocessing in the presence of a regularized non-Euclidean metric, and in the presence of time-outs. The decoders' optimality induces a worst-case computational complexity that is at most linear in the users' sum-rate. This constitutes a substantial improvement over the state of art of DMT optimal decoding, including ML decoders with complexity that is exponential in the sum-rate, or lattice decoders based on solutions to the NP-hard closest vector problem (CVP). The optimality of the efficient decoders is established for all channel statistics, for all channel dimensions, for any number of users, and irrespective of the different rates. The findings directly apply to different computationally intensive multi-user settings such as multi-user MIMO, multi-user cooperative communications, and multi-user MIMO-OFDM.
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| 7. |
- Fertl, Peter, et al.
(author)
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Performance Assessment of MIMO-BICM Demodulators Based on Mutual Information
- 2012
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In: IEEE Transactions on Signal Processing. - 1053-587X .- 1941-0476. ; 60:3, s. 1366-1382
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Journal article (peer-reviewed)abstract
- We provide a comprehensive performance comparison of soft-output and hard-output demodulators in the context of non-iterative multiple-input multiple-output bit-interleaved coded modulation (MIMO-BICM). Coded bit error rate (BER), widely used in literature for demodulator comparison, has the draw-back of depending strongly on the error correcting code being used. This motivates us to propose the mutual information of the equivalent modulation channel (comprising modulator, wireless channel, and demodulator) as a code-independent performance measure. We present extensive numerical results for spatially independent identically distributed (i.i.d.) ergodic and quasi-static fading channels under perfect and imperfect channel state information. These results reveal that the performance ranking of MIMO demodulators is rate-dependent and provide new insights regarding MIMO-BICM system design, i.e., the choice of antenna configuration, symbol constellation, and demodulator for a given target rate.
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| 8. |
- Flåm, John, et al.
(author)
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Gaussian mixture modeling for source localization
- 2011
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In: ICASSP, IEEE International Conference on Acoustics, Speech and Signal Processing - Proceedings. - 9781457705380 - 9781457705373 ; , s. 2604-2607
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Conference paper (peer-reviewed)abstract
- Exploiting prior knowledge, we use Bayesian estimation to localize a source heard by a fixed sensor network. The method has two main aspects: Firstly, the probability density function (PDF) of a function of the source location is approximated by a Gaussian mixture model (GMM). This approximation can theoretically be made arbitrarily accurate, and allows a closed form minimum mean square error (MMSE) estimator for that function. Secondly, the source location is retrieved by minimizing the Euclidean distance between the function and its MMSE estimate using a gradient method. Our method avoids the issues of a numerical MMSE estimator but shows comparable accuracy.
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| 9. |
- Jaldén, Joakim, 1976-, et al.
(author)
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Distributed Bayesian detection for the butterfly network
- 2013
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In: 2013 IEEE 14th Workshop on Signal Processing Advances in Wireless Communications (SPAWC). - : IEEE. - 9781467355773 ; , s. 61-65
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Conference paper (peer-reviewed)abstract
- We consider a distributed detection problem where two nodes, or decision makers, observe a common source and aim to decide on one of several hypotheses. Before making their individual decisions, the nodes are allowed to communicate over rate-constrained links, through a bidirectional relay. We show that if the rate of the common relay-to-node link is greater than or equal to the rate of the individual node-to-relay links, and the individual decisions are not coupled by the cost metric, then network coding at the relay allows the overall problem to decouple into two separate two-node distributed detection problems over serial networks; and the two serial networks can be designed independently. However, if the rate of the relay-to-node link is strictly less than the node-to-relay links, no such decoupling can be assumed in general, and the overall detection network needs to be jointly designed.
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| 10. |
- Jaldén, Joakim, 1976-, et al.
(author)
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DMT Optimality of LR-Aided Linear Decoders for a General Class of Channels, Lattice Designs, and System Models
- 2010
-
In: IEEE Transactions on Information Theory. - 0018-9448 .- 1557-9654. ; 56:10, s. 4765-4780
-
Journal article (peer-reviewed)abstract
- This paper identifies the first general, explicit, and nonrandom MIMO encoder-decoder structures that guarantee optimality with respect to the diversity-multiplexing tradeoff (DMT), without employing a computationally expensive maximum-likelihood (ML) receiver. Specifically, the work establishes the DMT optimality of a class of regularized lattice decoders, and more importantly the DMT optimality of their lattice-reduction (LR)-aided linear counterparts. The results hold for all channel statistics, for all channel dimensions, and most interestingly, irrespective of the particular lattice-code applied. As a special case, it is established that the LLL-based LR-aided linear implementation of the MMSE-GDFE lattice decoder facilitates DMT optimal decoding of any lattice code at a worst-case complexity that grows at most linearly in the data rate. This represents a fundamental reduction in the decoding complexity when compared to ML decoding whose complexity is generally exponential in the rate. The results' generality lends them applicable to a plethora of pertinent communication scenarios such as quasi-static MIMO, MIMO-OFDM, ISI, cooperative-relaying, and MIMO-ARQ channels, in all of which the DMT optimality of the LR-aided linear decoder is guaranteed. The adopted approach yields insight, and motivates further study, into joint transceiver designs with an improved SNR gap to ML decoding.
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