Robust Multiuser Beamformers in MIMO-OFDM Systems
01 Oct 2006-pp 1-5
TL;DR: This paper addresses the problem of multi-user multiplexing using spatial diversity techniques for a MU-MIMO OFDM system so that a base station could serve multiple users in the same frequency band enabling a substantial saving in bandwidth utilization.
Abstract: Multiple antennas at the transmitter and the receiver have the potential to either increase the data rate through spatial multiplexing or enhance the quality of transmission through exploitation of diversity. In this paper, we address the problem of multi-user multiplexing using spatial diversity techniques for a MU-MIMO OFDM system so that a base station could serve multiple users in the same frequency band enabling a substantial saving in bandwidth utilization. Existing techniques require nearly perfect knowledge of the channel state information at the transmitter, which is typically not available in practice. In this paper, we design robust transmitter to account for the errors in channel state estimates, which in this case arises due to mean CSI feedback
Citations
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06 Oct 2009
TL;DR: A probabilistic-constrained beamforming based on signal-to-leakage ratio (SLR) criterion under consideration of inaccurate channel information is proposed and results show that the proposed beamformer achieves the lowest bit error rate (BER) and leaks the least transmit power from the desired user to all other users among the state-of-art transmit beamformers.
Abstract: Multi-user multiple-input and multiple-output (MU-MIMO) wireless systems have the potential to provide a substantial gain by using transmit beamforming to allow multi-user communication in the same frequency and time slots. The main challenge for transmit beamforming design is to suppress the co-channel interference (CCI) from other users. In order to completely cancel the CCI at each user, perfect channel state information (CSI) is required at base station, which is generally not available in practice. To overcome the performance degradation caused by the imperfections, the most common approach is the worst-case method, which leads to conservative result as the extreme (but rare) conditions may occur at a very low probability. In this work, we propose a probabilistic-constrained beamforming based on signal-to-leakage ratio (SLR) criterion under consideration of inaccurate channel information. The simulation results show that the proposed beamformer achieves the lowest bit error rate (BER) and leaks the least transmit power from the desired user to all other users among the state-of-art transmit beamformers.
8 citations
Cites background from "Robust Multiuser Beamformers in MIM..."
...Moreover, two criteria work as performance measurement of robust transmit beamformer, that is, signal-to-noise ratio (SINR) [2] [9], and signal-to-leakage ratio (SLR) [1] [3] [10]....
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11 May 2008
TL;DR: This paper proposes robust spatial multiplexing schemes based on a worst case performance optimization by incorporating imperfect channel state information in a multiuser MIMO system and examines the performance for errors introduced due to a partial channel state Information feedback scheme.
Abstract: We address the problem of transmit beamforming under channel uncertainties for a multiuser MIMO system, where both the transmitter and the receiver are equipped with multiple antennas. In transmit beamforming multi-user multiplexing is performed using spatial diversity techniques so that a base station could serve multiple users in the same frequency band enabling a substantial saving in bandwidth utilization. However, such techniques require nearly perfect knowledge of the channel state information at the transmitter, which is generally not available in practise. In this paper, we propose robust spatial multiplexing schemes based on a worst case performance optimization by incorporating imperfect channel state information. In the simulation, we have examined two scenarios. In the first the channel state information is assumed to have Gaussian distribution errors. In the second scenario, we analyze the performance for errors introduced due to a partial channel state information feedback scheme. In both scenarios, the proposed robust scheme outperforms the conventional scheme.
7 citations
Cites background from "Robust Multiuser Beamformers in MIM..."
...Some good examples are the recent advances in robust beamforming techniques [10]–[14]....
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TL;DR: An efficient radio resource allocation scheme based on PHY/MAC cross layer design and QoS-guaranteed scheduling for multi-user (MU), multi-service (MS),multi-input multi-output (MIMO) concept, orthogonal frequency division multiple access (OFDMA) systems is presented.
Abstract: Cross-layer strategies for resource allocation in wireless networks are essential to guaranty an efficient utilization of the scarce resource. In this paper, we present an efficient radio resource allocation scheme based on PHY/MAC cross layer design and QoS-guaranteed scheduling for multi-user (MU), multi-service (MS), multi-input multi-output (MIMO) concept, orthogonal frequency division multiple access (OFDMA) systems. It is about a downlink multimedia transmission chain in which the available resources as power and bandwidth, are dynamically allocated according to the system parameters. Among these parameters, we can mention the physical link elements such as channel state information, spectral efficiency and error code corrector rate, and MAC link variables, which correspond to the users QoS requirements and the queue status. Primarily, we use a jointly method which parametrizes these system parameters, according to the total power, and the bit error rate constraints. Secondly, we propose a QoS-guaranteed scheduling that shares the sub-carriers to the users. These users request several type of traffic under throughput threshold constraints. The main objective in this work is to adjust the average throughput per service of each user, according to their needs and likewise to satisfy a great number of connexions. Subsequently, we consider a model of moderated compartmentalization between various classes of services by partitioning the total bandwidth into several parts. Each class of service will occupy a part of the bandwidth and will be transmitted over a maximum number of sub-carriers. The simulation results show that the proposed strategy provides a more interesting performance improvement (in terms of average data rate and user satisfaction) than other existing resource allocation schemes, such as nonadaptive resource allocation strategy. The performances are also analyzed and compared for the two multi-service multi-user MIMO---OFDMA systems; with sub-carriers partitioning and without sub-carriers partitioning.
5 citations
Cites methods from "Robust Multiuser Beamformers in MIM..."
...Otherwise, the adaptive strategies with respect to power allocation [6, 8], sub-carriers distribution [13, 35], modulation and coding [21, 37], and beam-forming [30, 34] can be used in order to improve the performances....
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01 Jan 2008
TL;DR: A robust transmit beamformer which can successfully tolerate CSI imperfection is proposed which is application of the robust beamforming in the context of downlink MU-SC transmission.
Abstract: We address the problem of transmit beamforming under channel uncertainty for a multiuser (MU), single carrier frequency domain equalization multiple input multiple output (SC-FDE-MIMO) system. SC-FDE-MIMO scheme is effective solution with relative low complexity to combat inter -symbol interference (ISI) whilst exploiting multi antennas diversity gain. In our system, the signal to leakage ratio (SLR) criterion is used to design the transmit beamformer and the minimum mean square error (MMSE) criterion is employed for the receiver equalization and combining. Non-robust beamforming techniques require perfect channel state information (CSI) knowledge which is not available in practice. In this paper, we propose a robust transmit beamformer which can successfully tolerate CSI imperfection. Diagonal loading is employed to introduce robustness to channel state information errors. The novelty of this work, however, is the application of the robust beamforming in the context of downlink MU-SC transmission. Average bit error (BER) simulations are presented to verify the efficiency of the proposed method.
Cites background from "Robust Multiuser Beamformers in MIM..."
...In practice channel state information (CSI) will be always in error due to imperfections such as time variations of the channel, feedback delay, or quantization of the CSI [8] [9]....
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Dissertation•
01 Jan 2010
TL;DR: The probabilistic approach provides an attractive alternative to existing robust techniques under imperfect channel information at transmitter and provides a tight BER bound for adaptive modulation scheme, so that the maximum transmission rate can be achieved by taking advantage of transmit beamforming.
Abstract: Transmit beamforming (precoding) is a powerful technique for enhancing the channel capacity and reliability of multiple-input and multiple-output (MIMO) wireless systems. The optimum exploitation of the benefits provided by MIMO systems can be achieved when a perfect channel state information at transmitter (CSIT) is available. In practices, however, the channel knowledge is generally imperfect at transmitter because of the inevitable errors induced by finite feedback channel capacity, quantization and other physical constraints. Such errors degrade the system performance severely. Hence, robustness has become a crucial issue. Current robust designs address the channel imperfections with the worst-case and stochastic approaches. In worst-case analysis, the channel uncertainties are considered as deterministic and norm-bounded, and the resulting design is a conservative optimization that guarantees a certain quality of service (QoS) for every allowable perturbation. The latter approach focuses on the average performance under the assumption of channel statistics, such as mean and covariance. The system performance could break down when persistent extreme errors occur. Thus, an outage probability-based approach is developed by keeping a low probability that channel condition falls below an acceptable level. Compared to the aforementioned methods, this approach can optimize the average performance as well as consider the extreme scenarios proportionally. This thesis implements the outage-probability specification into transmit beamforming design for three scenarios: the single-user MIMO system and the corresponding adaptive modulation scheme as well as the multi-user MIMO system. In a single-user MIMO system, the transmit beamformer provides the maximum average received SNR and ensures the robustness to the CSIT errors by introducing probabilistic constraint on the instantaneous SNR. Beside the robustness against channel imperfections, the outage probability-based approach also provides a tight BER bound for adaptive modulation scheme, so that the maximum transmission rate can be achieved by taking advantage of transmit beamforming. Moreover, in multi-user MIMO (MU-MIMO) systems, the leakage power is accounted by probability measurement. The resulting transmit beamformer is designed based on signal-to-leakage-plus-noise ratio (SLNR) criteria, which maximizes the average received SNR and guarantees the least leakage energy from the desired user. In such a setting, an outstanding BER performance can be achieved as well as high reliability of signal-to-interference-plus-noise ratio (SINR). Given the superior overall performances and significantly improved robustness, the probabilistic approach provides an attractive alternative to existing robust techniques under imperfect channel information at transmitter. Declaration of originality I hereby declare that the research recorded in this thesis and the thesis itself was composed and originated entirely by myself in the School of Engineering at The University of Edinburgh as a candidate for the Doctorate of Philosophy. I has not been submitted for any other degree or award in any other university or educational institution. List your exceptions here and sign before your printed name. Huiqin Du School of Engineering University of Edinburgh, UK 2010
Cites background or methods from "Robust Multiuser Beamformers in MIM..."
...The optimal transmit directions are just the right singular vector of the nominal channel H [12, 32–44]....
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...Moreover, in multi-user MIMO systems, the worst-case approach is implemented to minimize the total transmit power [37], and to optimize the QoS requirements, including minimizing MSE [34], maximizing SINR [38–41], maximizing SLNR [12, 42–44]....
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...A common assumption is that the error is bounded in a spherical region [12, 32–44], ||E||F ≤ ξ , (2....
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References
More filters
TL;DR: While the proposed algorithms are suboptimal, they lead to simpler transmitter and receiver structures and allow for a reasonable tradeoff between performance and complexity.
Abstract: The use of space-division multiple access (SDMA) in the downlink of a multiuser multiple-input, multiple-output (MIMO) wireless communications network can provide a substantial gain in system throughput. The challenge in such multiuser systems is designing transmit vectors while considering the co-channel interference of other users. Typical optimization problems of interest include the capacity problem - maximizing the sum information rate subject to a power constraint-or the power control problem-minimizing transmitted power such that a certain quality-of-service metric for each user is met. Neither of these problems possess closed-form solutions for the general multiuser MIMO channel, but the imposition of certain constraints can lead to closed-form solutions. This paper presents two such constrained solutions. The first, referred to as "block-diagonalization," is a generalization of channel inversion when there are multiple antennas at each receiver. It is easily adapted to optimize for either maximum transmission rate or minimum power and approaches the optimal solution at high SNR. The second, known as "successive optimization," is an alternative method for solving the power minimization problem one user at a time, and it yields superior results in some (e.g., low SNR) situations. Both of these algorithms are limited to cases where the transmitter has more antennas than all receive antennas combined. In order to accommodate more general scenarios, we also propose a framework for coordinated transmitter-receiver processing that generalizes the two algorithms to cases involving more receive than transmit antennas. While the proposed algorithms are suboptimal, they lead to simpler transmitter and receiver structures and allow for a reasonable tradeoff between performance and complexity.
3,291 citations
TL;DR: It is shown that a simple scaling of the projection of tentative weights, in the subspace orthogonal to the linear constraints, can be used to satisfy the quadratic inequality constraint.
Abstract: Adaptive beamforming algorithms can be extremely sensitive to slight errors in array characteristics. Errors which are uncorrelated from sensor to sensor pass through the beamformer like uncorrelated or spatially white noise. Hence, gain against white noise is a measure of robustness. A new algorithm is presented which includes a quadratic inequality constraint on the array gain against uncorrelated noise, while minimizing output power subject to multiple linear equality constraints. It is shown that a simple scaling of the projection of tentative weights, in the subspace orthogonal to the linear constraints, can be used to satisfy the quadratic inequality constraint. Moreover, this scaling is equivalent to a projection onto the quadratic constraint boundary so that the usual favorable properties of projection algorithms apply. This leads to a simple, effective, robust adaptive beamforming algorithm in which all constraints are satisfied exactly at each step and roundoff errors do not accumulate. The algorithm is then extended to the case of a more general quadratic constraint.
1,851 citations
TL;DR: It is shown that a natural extension of the Capon beamformer to the case of uncertain steering vectors also belongs to the class of diagonal loading approaches, but the amount of diagonalloading can be precisely calculated based on the uncertainty set of the steering vector.
Abstract: The Capon (1969) beamformer has better resolution and much better interference rejection capability than the standard (data-independent) beamformer, provided that the array steering vector corresponding to the signal of interest (SOI) is accurately known. However, whenever the knowledge of the SOI steering vector is imprecise (as is often the case in practice), the performance of the Capon beamformer may become worse than that of the standard beamformer. Diagonal loading (including its extended versions) has been a popular approach to improve the robustness of the Capon beamformer. We show that a natural extension of the Capon beamformer to the case of uncertain steering vectors also belongs to the class of diagonal loading approaches, but the amount of diagonal loading can be precisely calculated based on the uncertainty set of the steering vector. The proposed robust Capon beamformer can be efficiently computed at a comparable cost with that of the standard Capon beamformer. Its excellent performance for SOI power estimation is demonstrated via a number of numerical examples.
1,113 citations
Book•
06 Jul 2007
948 citations
"Robust Multiuser Beamformers in MIM..." refers background in this paper
...(15) The solution to the above equation is given by the Rayleigh-Ritz quotient result [13] w u = P n (H̃u H̃u) (Hu Hu) o , (16) where P{·} is the principal eigenvector of the matrix, that is the eigenvector corresponding to its maximal eigenvalue....
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