-
[show abstract]
[hide abstract]
ABSTRACT: This paper investigates multiple input multiple output (MIMO) transmission techniques based on realistic assumptions on feedback of channel state information. We consider three conventional techniques as the baseline: 3GPP long-term evolution (LTE) single user MIMO (SU-MIMO) based on implicit channel feedback, zero-forcing multiuser MIMO (ZF MU-MIMO) based on explicit channel feedback, and ZF MU-MIMO based on implicit channel feedback. SU-MIMO may not be able to exploit the full spatial dimension of the downlink MIMO channel. ZF MU-MIMO has the potential to improve the spectral efficiency, but the explicit channel feedback is not compatible with implicit feedback whereas implicit based ZF MU-MIMO is limited by performance and also the commonly assumed rank restriction makes it impossible to perform dynamic switching of SU/MU MIMO transmission. We propose a new hybrid scheme which enables such dynamic switching of SU/MU MIMO transmission by allowing UE to feed back the implicit channel information without any rank restriction. Computer simulation results show the benefits of the new hybrid scheme, which can properly switch to the better transmission mode in various correlation scenarios.
Communications (ICC), 2011 IEEE International Conference on; 07/2011
-
[show abstract]
[hide abstract]
ABSTRACT: An interference management method for coordinating downlink transmission in a Multiple-Input Multiple-Output (MIMO) cellular network is proposed. The problem is to efficiently manage inter-cell interference in a multi-cell environment, by so-called transmitter cooperation, in order to reduce the diminishing effects of interference on the networks performance. To allow for application in deployable networks, an utmost concern of the presented algorithm is to provide a low-complexity solution avoiding costly combinatorial or non-convex optimization problems. The problem is solved by a network wide successive allocation of data streams and choosing linear transmit and receive filters for each data stream, such that interference is completely avoided. By embedding our new algorithm in a more general framework for interference coordination, we can show relevant gains for network performance where especially the cell-edge users profit.
Wireless Communications and Networking Conference (WCNC), 2011 IEEE; 05/2011
-
[show abstract]
[hide abstract]
ABSTRACT: The major problem of multiuser MIMO precoding and scheduling is the required Channel State Information (CSI) at the base station which needs to be provided to the base station via a feedback channel in case of frequency division duplex. Each user quantizes the channel direction information and computes a Channel Quality Indicator (CQI), e.g., based on the Signal-to-Interference-and-Noise Ratio (SINR), both fed back as CSI to the base station. Since the precoder is not known when the feedback information is computed, the SINR needs to be approximated where the quality of the approximation depends strongly on the number of finally scheduled data streams. In this paper, we propose limited feedback schemes which improve the performance of the scheduler by exploiting this dependency and adapting the feedback information accordingly. Three types of feedback schemes are presented: first, feeding back more than one approximation such that the base station can choose the more accurate one depending on the number of finally scheduled data streams, second, a time multiplexing of these approximations in order to decrease the feedback overhead, and third, feeding back the channel magnitude and estimating the approximations via additional statistical measures. Simulation results show that compared to state-of-the-art techniques, the proposed feedback schemes perform well for a wide range of correlation scenarios.
Global Telecommunications Conference (GLOBECOM 2010), 2010 IEEE; 01/2011
-
[show abstract]
[hide abstract]
ABSTRACT: Solving Quality of Service constrained optimization problems in the Multiple-Input Multiple-Output (MIMO) broadcast channel usually requires the iterative solution of a weighted sum rate maximization, which exhibits a considerable amount of numerical complexity itself, especially in Orthogonal Frequency Multiplexing (OFDM) systems. In this paper a low complexity framework for these problems is presented, where the allocation of data streams to users and carriers is done successively. An efficient rule for choosing the user and carrier to be allocated in each step is presented for both linear and non-linear precoding leading to small performance losses compared to the optimum.
Acoustics Speech and Signal Processing (ICASSP), 2010 IEEE International Conference on; 04/2010 · 4.63 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: We regard a MIMO network with two transmitters and two receivers, in which each transmitter sends information to both of the receivers, a scenario known as the MIMO X channel. For such a system, the recently proposed interference alignment technique is proven to achieve the maximum degrees of freedom, which cannot be reached by conventional zero-forcing. In contrast to other scenarios, for the MIMO X channel algebraic expressions to obtain interference alignment can be easily found. Additionally, the set of parameters for possible alignments is a continuous manifold rather than a discrete set, which directly raises the question of how to find the best alignment when aiming at maximizing a utility of the transmission rates. Due to the non-convexity of the problem, finding the global optimally solution is numerically exhaustive and we are willing to accept a locally optimal solution. In this work we show an efficient parametrization of the problem which allows to apply a projected gradient approach that guarantees an aligned solution. In numerical simulations we show the superiority of our method compared to existing algorithms.
Smart Antennas (WSA), 2010 International ITG Workshop on; 03/2010
-
[show abstract]
[hide abstract]
ABSTRACT: For downlink transmission in a multiuser multiple-input multiple-output (MIMO) communication system, quantized Channel State Information (CSI) is fed back to the base station in an uplink channel of finite rate. The quantized CSI is obtained via Channel Vector Quantization (CVQ) of the so-called composite channel vector, i.e., the product of the channel matrix and an estimation of the receive filter, which cannot be computed exactly at the stage of quantization because of its dependency on the finally chosen precoder. Here, the state-of-the-art approach estimates the receive filter and quantize the composite channel vector such that its Euclidean distance to the estimated composite channel vector is minimized. In this paper, we propose an alternative CVQ method which determines the estimated receive filter vector and the quantized composite channel vector such that the resulting Signal-to-Interference-and-Noise Ratio (SINR), or an approximation thereof, is maximized. Since the SINR is related to the individual user rates, and therefore related to the sum rate of the system, the presented solution aims at maximizing the system sum rate. Simulation results of a multiuser MIMO system with linear zero-forcing preceding show that the proposed schemes achieve significant performance improvements compared to the state-of-the-art method, especially in the low signal-to-noise ratio region.
Global Telecommunications Conference, 2009. GLOBECOM 2009. IEEE; 01/2010
-
[show abstract]
[hide abstract]
ABSTRACT: Maximizing the sum capacity in the multiple-input multiple-output (MIMO) broadcast channel requires the use of dirty paper coding (DPC). However, practical implementations of DPC which are nearly optimum exhibit high computational complexity. As an alternative to DPC linear zero-forcing can be used where the multiuser interference is completely canceled by linear beamforming. Determining the optimum user allocation, transmit and receive filters thereby constitutes a combinatorial and nonconvex optimization problem. To circumvent its direct solution and therefore reduce complexity, we propose an algorithm that successively allocates data streams to users and, in contrast to state-of-the-art approaches, includes the receive filters into the optimization. We then show several steps that reduce the complexity of the algorithm at marginal performance losses. Thus, performance of state-of-the-art approaches can be maintained while the computational complexity is reduced considerably, as it is shown by a detailed complexity analysis and simulation results.
IEEE Journal of Selected Topics in Signal Processing 01/2010; · 2.88 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Achieving the capacity region in the MIMO broadcast channel requires the use of dirty paper coding (DPC). When it cannot be afforded to satisfy the requirements of all users by DPC based approaches, it is necessary to identify the users which exhibit the highest performance gain compared to simpler approaches. In this paper we present a user grouping method that aims to identify those users with highest weighted sum rate gain. First some users are excluded by a simple criterion leading to a reduced user set, from which the final user group is selected by a more sophisticated criterion.
Communications, 2009. ICC '09. IEEE International Conference on; 07/2009
-
[show abstract]
[hide abstract]
ABSTRACT: To avoid the complexity of nearly optimum implementations for dirty paper coding (DPC) linear zero-forcing approaches are often preferred. Finding the optimum user allocation, transmit and receive filters for maximizing sum rate in the multiple-input multiple-output (MIMO) broadcast channel constitutes a non-convex combinatorial optimization problem. Here we present an efficient linear zero-forcing algorithm for this problem, which is derived from maximizing a lower bound for the sum rate. It successively allocates data streams to users and in contrast to state of the art approaches thereby also determines the corresponding receive filters. Compared to existing approaches drastic complexity reductions can be achieved even with slight performance gains.
Signals, Systems and Computers, 2008 42nd Asilomar Conference on; 11/2008
-
[show abstract]
[hide abstract]
ABSTRACT: Finding the matrix with the maximum singular value amongst a set of matrices is a common problem occurring in transmit signal processing algorithms for multiuser multiple-input multiple-output (MIMO) systems. However, computing the principal singular value of a matrix is a rather numerically complex task. Furthermore, in many practical scenarios, the number of users is large and for each user this task has to be conducted. In this paper we therefore propose a novel user preselection method which reduces the computational complexity at no performance loss. This is achieved by deselecting some users based on a simple criterion and thus avoiding explicit computations of the singular values of those users. This criterion is based on easily computable bounds for the principal singular values. Finally, a statistical analysis is provided and the application to the Successive Encoding Successive Allocation Method (SESAM) is shown.
Vehicular Technology Conference, 2008. VTC 2008-Fall. IEEE 68th; 10/2008
-
[show abstract]
[hide abstract]
ABSTRACT: Full channel state information at the transmitter leads to large performance gains in multiuser wireless communication systems. However, in practice this assumption cannot be justified, especially when the transmitter cannot directly estimate the channel. For this reason we consider finite rate feedback schemes in this paper, where the channel state information is fed back to the transmitter after quantization. We show performance in terms of sum rate in the MIMO OFDM broadcast channel and present a new combination of lossless and lossy compression of channel state information and subsequent quantization at the receivers.
Smart Antennas, 2008. WSA 2008. International ITG Workshop on; 03/2008
-
[show abstract]
[hide abstract]
ABSTRACT: Interleave division multiple access (IDMA) recently attracted many research activities because of its excellent performance despite its reasonable low complexity. The low complexity is usually realized by the multiuser detector that applies an approximation similar to the rake receiver for CDMA systems. So far, this type of detector has been most frequently considered in IDMA literature. In this paper we investigate the performance of IDMA based on linear minimum mean square error (MMSE) detection. The MMSE detector is more complex than the rake-like approximation. At the price of the complexity, however, it is shown that the MMSE detector brings several advantages over the rake-like approach such as the superior performance on channels with spectrally poor characteristics, effective iterative processing for lower SNR values, faster convergence and therefore shorter decoding delays, and better performance for short block length. We also confirm that the complexity can be drastically reduced by the low rank approximation of the MMSE filter by its multistage representation without compromising on the performance.
Communications, 2007. ICC '07. IEEE International Conference on; 07/2007
-
[show abstract]
[hide abstract]
ABSTRACT: Reduced-rank approximations of finite impulse response equalizers in Krylov subspaces, e.g., the conjugate gradient algorithm, can be used to decrease computational complexity involved in calculating the filter coefficients. However, an alternative approach would be to reduce the order of the corresponding full-rank filter or to even combine rank and order reduction. In this paper, we compare both reduction methods based on (G, D)-charts where we analyze the mean square error of the reduced-rank equalizers on complexity isosets, i.e., for tuples of the filter length G and its rank D resulting in a certain number of floating point operations. The application of (G, D)-charts to a coded system with an iterative receiver (turbo equalization) reveals the superiority of rank reduction, especially, if one is interested in low-complexity implementations
Signal Processing Advances in Wireless Communications, 2006. SPAWC '06. IEEE 7th Workshop on; 08/2006
-
[show abstract]
[hide abstract]
ABSTRACT: In this paper, we present two extensions of the block conjugate gradient (BCG) algorithm, a method which exploits the concept of block Krylov subspaces. First, we extend the BCG algorithm such that it is more flexible concerning the dimension of the block Krylov subspace. Second, a computationally efficient preconditioned BCG (PBCG) algorithm is introduced which turns out to outperform the standard BCG algorithm concerning the complexity-performance ratio. Hence, we provide a powerful implementation for reduced-rank signal processing in the minimum mean square error (MMSE) sense. Simulation results show the gain in rank-flexibility and convergence speed
Acoustics, Speech and Signal Processing, 2006. ICASSP 2006 Proceedings. 2006 IEEE International Conference on; 06/2006 · 4.63 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: In this paper, we consider the block conjugate gradient (BCG) algorithm as a robust and reduced-rank implementation of the matrix Wiener filter (MWF). Stopping the BCG iterations before the algorithm converges, reduces the computational complexity as well as the performance loss due to channel estimation errors which is especially a problem in low sample support scenarios. We investigate the inherent robustness of the BCG method by deriving the corresponding filter factor matrix which is a well-known tool in the theory of ill-posed problems to describe and analyze regularizing effects. Moreover, we present a robust BCG algorithm where we applied the regularization method of diagonal loading in order to increase both robustness against estimation errors and flexibility in choosing the number of BCG iterations without decreasing performance. Finally, we use quasi-optimal loading instead of common heuristic choices. Simulation results of a frequency-selective multi-user single-input multiple-output (MU-SIMO) system show the improved performance of the robust BCG filter compared to the conventional MWF despite of the enormously reduced computational complexity.
Signal Processing Advances in Wireless Communications, 2005 IEEE 6th Workshop on; 07/2005
-
[show abstract]
[hide abstract]
ABSTRACT: We derive a hybrid of beam-forming (BF) and space-time block coding (STBC), where the space-time code is transmitted over the beams generated by the steering vectors corresponding to the channel path directions. This is for the practical case where the transmit array may have adequate information on the departure angles of the dominant paths between transmitter and receiver, but unreliable information on the associated complex path gains. We compute analytically the signal-to-noise ratio (SNR) of the proposed hybrid for the specific case of a two-path channel model and using the orthogonal Alamouti code, and compare the result to the SNR of optimal linear precoding (LP) and the theoretically possible SNR of orthogonal STBC (OSTBC). Simulation results show that the performance of the BF/STBC hybrid can be very close to LP <sup>n</sup>der certain conditions - or even better in the practical case where there are phase estimation errors in the path gain estimates employed at the transmitter.
Acoustics, Speech, and Signal Processing, 2005. Proceedings. (ICASSP '05). IEEE International Conference on; 04/2005 · 4.63 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Space-time transmit structures for multi-antenna systems have received considerable interest. Circulant structures were among the first space-time coding techniques ever used for multiple-input multiple-output (MIMO) systems due to their simplicity and full rate. The fact that a circulant matrix is diagonalized by the discrete Fourier transformation matrix suggests that the circulant structure can be combined with an inverse fast Fourier transform (IFFT) at the transmitter and a fast Fourier transform (FFT) at the receiver. Using this method, the spatial mixing effect of the MIMO channel is decoupled but the diversity gain is lost. To recover the diversity advantage, we propose to spread the transmitted symbols over the diagonalized channel using the constellation rotation matrix for signal diversity designs. After spreading, every symbol experiences all the components of the frequency counterpart of the channel vector which makes our scheme provide full diversity. The proposed scheme is full rate and can be easily applied to any number of transmit antennas. Our simulation results show that the performance of our scheme is close to the performance of the ideal orthogonal space-time code and much better than the conventional circulant space-time code.
Acoustics, Speech, and Signal Processing, 2005. Proceedings. (ICASSP '05). IEEE International Conference on; 04/2005 · 4.63 Impact Factor
-
Signal Processing and Its Applications, 2005. Proceedings of the Eighth International Symposium on; 02/2005
-
[show abstract]
[hide abstract]
ABSTRACT: Focusing on systems with simple non-cooperating receivers, we present an approach to reduce the complexity of multiuser precoding schemes for frequency-selective MIMO channels. The transmit Wiener filter (TxWF) and Wiener Tomlinson-Harashima precoding (THP) both provide attractive performances, but the complexity involved in their computation may be prohibitive. We develop a reduced-complexity multiuser multistage TxWF (MSTxWF) by applying the multistage decomposition known from receive processing to the TxWF. We show that the block-Lanczos algorithm can he used for efficiently computing the reduced rank MSTxWF. Moreover, we extend the MSTxWF approach to Wiener temporal THP, resulting in a reduced-complexity nonlinear precoding scheme. Simulation results demonstrate that both schemes can provide close to optimum performance at significantly reduced complexity.
Vehicular Technology Conference, 2004. VTC2004-Fall. 2004 IEEE 60th; 10/2004
-
[show abstract]
[hide abstract]
ABSTRACT: In this paper, we consider an iterative or turbo receiver with linear detection using the Wiener filter (WF), i.e. the optimal linear filter based on the mean square error (MSE) criterion. Multiple antennas at the receiver increase the dimension of the observation vector which results in computationally intense detectors. We extend an optimal but computationally efficient algorithm, originally derived for a single receive antenna, to single-input multiple-output (SIMO) channels. To further reduce computational complexity, we apply the suboptimal low-rank multi-stage Wiener filter (MSWF) and approximate additionally second order statistics of non-stationary random processes by their time-invariant averages. Complexity investigations reveal the enormous capability of the proposed algorithms to decrease computational effort. Moreover, simulation results show that the reduced-rank MSWF behaves near optimum although the rank is drastically reduced to two or even one.
Vehicular Technology Conference, 2004. VTC2004-Fall. 2004 IEEE 60th; 10/2004
