Bell Laboratories Layered Space-Time Research Articles

Reduced-dimension MAP turbo-BLAST detection

The Bell Labs layered space-time (BLAST) architecture is a simple and efficient multiantenna coding structure that can achieve high spectral efficiency. Many BLAST detectors require more receiver antennas than transmitter antennas. We propose two novel turbo-processing BLAST detectors that can operate in systems with fewer receiver antennas than transmitter antennas. Both detectors are based on the group-detection strategy. The first proposed detector, the reduced-dimension maximum a posteriori (RDMAP) detector uses a dynamically formed group for each bit decision, while the second proposed detector, the group maximum a posteriori (GMAP) uses a static grouping. For both detectors, a maximum a posteriori (MAP) decision is made using a group of transmitted symbols, and the remaining signal contribution is treated as interference. The interference is characterized as nonzero mean colored-noise source that is whitened before a decision is made. Both proposed detectors are generalizations of the MAP detector and the turbo-processing minimum mean-squared error (MMSE) detector in Sellathurai and Haykin, and Abe and Matsumoto. An uncoded bit-error rate analysis for an independent Rayleigh fading environment is also presented. Simulated results are presented which show that both the RDMAP and GMAP detectors have a performance improvement over the MMSE detector, especially in systems having an excess number of transmitter antennas.

Blind multiuser detection for V-BLAST coded multicarrier CDMA system

Focusing on the space-time coded multiuser mobile communication systems in the frequency-selective fading environment, this paper proposes a Vertical Bell labs LAyered Space-Time (V-BLAST) coded Multicarrier Code-Division Multiple-Access (MC-CDMA) scheme and its blind channel identification algorithm. This algorithm employs an ESPRIT-like method and the singular value decomposition, and the channels between every transmit antenna of every user and every receive antenna of the base station are blindly estimated with a closed-form solution. Based on it, an equivalent Minimum Mean-Squared Error (MMSE) time-domain multiuser detector is derived. Moreover, the proposed scheme exploits the precoding in the transmitter in order to eliminate the constraint of more receive antennas than transmit ones, required by most conventional V-BLAST codec schemes. Computer simulation results demonstrate the validity of this proposed scheme.

Adaptive MIMO decision feedback equalization for receivers with time-varying channels

In an attempt to reduce the computational complexity of vertical Bell Labs layered space time (V-BLAST) processing with time-varying channels, an efficient adaptive receiver is developed based on the generalized decision feedback equalizer (GDFE) architecture. The proposed receiver updates the filter weight vectors and detection order using a recursive least squares (RLS)-based time- and order-update algorithm. The convergence of the algorithm is examined by analysis and simulation, and it is shown that the proposed adaptive technique is considerably simpler to implement than a V-BLAST processor with channel tracking, yet the performances are almost comparable.

An Iterative Extension of BLAST Decoding Algorithm for Layered Space–Time Signals

We propose an iterative extension of the Bell Laboratory Layered Space-Time (BLAST) algorithm and its variant, vertical BLAST (VBLAST). A characteristic feature of the BLAST-type algorithm is that symbol decisions with low reliability are fed back to decode other symbols. Both performance analysis based on Gaussian approximation of residual interference, and simulation results demonstrate that error propagation due to unreliable decision feedback can severely limit system performance. The extended algorithm exploits inherent signal diversity in BLAST to mitigate residual interference, thus overcoming the performance bottleneck due to error propagation. It yields an impressive performance gain over BLAST. In particular, the extension of BLAST with zero-forcing interference nulling admits a simple QR implementation and exhibits excellent performance with low complexity.

Diversity Combining With Imperfect Channel Estimation

We propose an iterative extension of the Bell Laboratory Layered Space-Time (BLAST) algorithm and its variant VBLAST. A characteristic feature of the BLAST-type algorithm is that symbol de- cisions with low reliability are fed back to decode other symbols. Both performance analysis based on Gaussian approximation of residual inter- ference and simulation results demonstrate that error propagation due to unreliable decision feedback can severely limit system performance. The extended algorithm exploits inherent signal diversity in BLAST to mitigate residual interference, thus overcoming the performance bottleneck due to error propagation. It yields an impressive performance gain over BLAST. In particular, the extension of BLAST with zero-forcing interference nulling (EXT-ZF-BLAST) admits a simple QR implementation and exhibits excellent performance with low complexity. Abstract—An optimal diversity-combining technique is investigated for a multipath Rayleigh fading channel with imperfect channel state informa- tion at the receiver. Applying minimum mean-square error channel esti- mation, the channel state can be decomposed into the channel estimator spanned by channel observation and the estimation error orthogonal to channel observation. The optimal combining weight is obtained from first principle of maximum a posteriori detection, taking into consideration the imperfect channel estimation. The bit-error performance using the optimal diversity combining is derived and compared with that of the suboptimal application of maximal ratio combining. Numerical results are presented for specific channel models and estimation methods to illustrate the com- bined effect of channel estimation and detection on bit-error rate perfor- mance.

Optimum power allocation for a V-BLAST system with two antennas at the transmitter

In this letter we propose a power optimization scheme for 2/spl times/N (where N is the number of receive antennas) Vertical Bell Laboratories Layered Space-Time (V-BLAST) systems. The proposed scheme minimizes the uncoded probability of vector error for each channel realization, considering error propagation. Simulation results show that this approach can yield SNR gain of up to about 3 dB for a 2/spl times/2 system at probability of vector error 10/sup -3/ and can partially compensate for the performance loss due to error propagation.

Optimal diagonal precoder for multiantenna communication systems

In this paper, we examine a multiantenna single-user wireless communication system fitted with a QR-based successive cancellation receiver (QR receiver). Initially, our consideration is confined to uncoded binary phase shift keying (BPSK) signals transmitted through independent and identically distributed (IID) Rayleigh fading channels and to the design of an optimum precoder for the transmitter. For minimum feeding back of the channel state information (CSI) to the transmitter from the receiver, we stipulate the precoder to be in the form of a power loading square diagonal matrix. We proceed to develop the theory for the design of this diagonal matrix based on the minimization of the lower bound of the average bit error rate (BER) of transmission. The design obtained provides substantially lower error rates than most of other existing schemes under the same environment. The corresponding gain in signal-to-noise ratio (SNR) can be several decibels. To further improve the performance, we extend the design to include an optimal detection order of the received bits using an iterative approach. This iterative process proves to have fast convergence and results in a design providing significant SNR gain. We also propose a subchannel dropping scheme for cases in which SNR is low, and when the minimum BER precoder is equipped with this scheme, its average performance can be substantially superior to the Vertical Bell Laboratories Layered Space-Time (V-BLAST) detection. We extend our design of the optimum precoder to quadrature amplitude modulation (QAM) modulation scheme and similar performance gain has been observed.

Noise-predictive decision-feedback detection for multiple-input multiple-output channels

The decision-feedback (DF) detector is a nonlinear detection strategy for multiple-input multiple-output (MIMO) channels that can significantly outperform a linear detector, especially when the order in which the inputs are detected is optimized according to the so-called Bell Labs Layered Space-Time (BLAST) ordering. The DF detector may be implemented as the cascade of a linear detector, which mitigates interference at the expense of correlating the noise, followed by a noise predictor, which exploits the correlation in the noise to reduce its variance. With this architecture, existing linear detectors can be easily upgraded to DF detectors. We propose a low-complexity algorithm for determining the BLAST ordering that is facilitated by the noise-predictive architecture. The resulting ordered noise-predictive DF detector requires fewer computations than previously reported ordered-DF algorithms. We also propose and derive the ordered noise-predictive minimum-mean-squared-error DF detector and show how to determine its BLAST ordering with low complexity.

An MMSE-Nulling Partial-PIC Receiver for Multiuser Downlink MIMO MC-CDMA Systems

SUMMARY A minimum mean square error (MMSE) nulling partialparallel interference cancellation (PPIC) receiver for downlink multiple-input multiple-output (MIMO) multicarrier (MC)-CDMA systems is pro-posed. Our analysis shows that, for multiuser MIMO MC-CDMA systems,interference due to frequency selectivity in multipath fading channel causesdetrimental effects on cancelling, thus V-BLASTreceiver shows severe per-formance degradation. The proposed receiver with multistage processing does not produce an error floor in frequency selective fading channel en-vironments and achieves substantial performance gains. The system per-formance of the proposed receiver was evaluated through computer sim-ulations. The simulation results show that, with two stage PPIC process-ing, the proposed receiver achieves performance gains of 2.5–4dB at targetBER of 10 −3 over the linear MMSE receiver. key words: MIMO, MC-CDMA, V-BLAST, PIC 1. Introduction The development of wireless communication systems forhigh data rate transmission and high system flexibility isone of the main targets in next generation wireless com-munications research. MIMO MC-CDMA systems haveattracted significant research interest due to its high spec-tral efficiency, large system capacity and high flexibility fordata rate, and it has been proposed as one of the promissingcandidates for 4th generation wireless communication sys-tems [1]. MIMO MC-CDMA systems are combinations ofMIMO transmission, orthogonal frequency division multi-plexing (OFDM) signaling and CDMA schemes. MC mod-ulation, realized via OFDM, is well suited to high data rateapplications such as multimedia packet transmission andmobile internet in frequency selective fading channels.MIMO systems can achieve very high spectral effi-ciency without additional power or bandwidth in a rich mul-tipath environment by exploiting the extra space dimension.Vertical Bell Labs lAyered Space Time (V-BLAST) is a pop-ular single-carrier single-user MIMO detection algorithm,which is based on the ordered successive interference can-cellation (OSIC) [2]. The principle behind OSIC is that atthe beginning of each stage, the substream with the high-

Bayesian detection for BLAST

This work demonstrates the use of the Bayesian methodology for detection in Bell Laboratories Layered Space-Time (BLAST) systems. First, we introduce a procedure for constructing prior distributions and propose the use of two types of prior distributions for the problem. From the corresponding posterior distributions, we obtain the Bayesian linear and decision-feedback detectors and show their equivalence to the popular zero forcing and minimum mean square error (MMSE)-based detectors. Then, we establish an equivalent whitening filter output system model whose unique structure lends itself to constructing a dynamic state space model (DSSM) for BLAST systems, which evolves in space. This DSSM allows for the application of sequential Monte Carlo sampling, or particle filtering (PF), for detection in BLAST systems. We introduce two different particle filtering detectors: the generic particle filtering detector and the stochastic M algorithm. The stochastic M algorithm exploits the discrete nature of the problem in the implementation and, therefore, is much more efficient. Overall, a distinct advantage of the PF detectors is that they can greatly reduce error propagation and thereby achieve near optimum performance. In addition, since they aim at the approximation of the posterior distribution using weighted samples, they can provide soft (probabilistic) information about the unknowns.

MIMO-DFE based space-time receiver over frequency selective channels with limited error propagation

MIMO-DFE(Multiple-Input-Multiple-Output Decision Feedback Equalizer) based receiver architectures are researched recently to detect signals in BLAST(Bell laboratories LAyered Space-Time) over frequency-selective channels. Due to their recursive structure, these receivers may suffer from error propagation which results in an overall mean square error degradation. An MIMO-DFE based BLAST receiver with limited error propagation to combat frequency-selective channel is proposed, which employs both norm constraint on feedback filter taps and soft decision device. Simulation results show that the proposed receiver outperforms conventional ones in various frequency selective channels.

Error Propagation Analysis of V-BLAST With Channel-Estimation Errors

In this letter, expressions are given for the symbol-error rate (SER) of the Vertical Bell Laboratories Layered Space-Time (V-BLAST) system, taking into account error propagation due to channel-estimation errors. In addition to error propagation, suboptimal substream ordering due to imperfect channel estimates is accounted for. First, the conditional SER is determined using the distribution of the signal-to-interference-plus-noise ratio of each substream, conditioned on the channel estimate. Then, the average SER as a function of the channel estimation error-to-signal ratio (ESR) is upper bounded by averaging over the distribution of the channel estimates. The upper bound on the SER is tighter than previous bounds in the literature. Comparisons with exact simulations demonstrate the accuracy of the SER expressions for a large range of ESRs.

Adaptive antennas and mimo systems for wireless communications: part II

he first part of this special issue contained eight articles that presented overviews, descriptions of different methods, performance analyses, and new ideas, all from a theoretical/simulation point of view. Part II includes articles that focus more on practical and experimental issues related to the presented topic. The first article of this issue discusses the advantages of reconfigurable antennas and the requirements for radio frequency microelectromechanical systems (RF MEMS). A simplified prototype of the reconfigurable antenna is also presented along with simulation and experimental results for impedance and radiation performance. The second article discusses a classification of multiantenna testbeds for training and research purposes. The authors present the characteristics, capabilities and cost of the testbeds, highlight their role in an educational environment and provide experimental results. The following article presents a multiple-input multipleoutput orthogonal frequency-division multiplexed (MIMOOFDM) prototype for next-generation wide area wireless networks. The uplink is based on an enhanced Universal Mobile Telecommunications System (UMTS) physical layer, the downlink on shared access MIMO-OFDM with adaptive modulation and coding. Measurements for different operational scenarios demonstrate the capabilities of the system. The fourth article describes the implementation issues related to the development of a 3 × 3 OFDM Bell Labs Layered Space-Time (BLAST) testbed for wireless metropolitan area networks. It then presents initial results for the performance and complexity of three BLAST detection techniques using experimental data from an indoor environment. The fifth article first surveys the principal space-time coding/decoding technologies for MIMO wireless LAN systems. Then it describes indoor channel sounding trials and employs the results to scope the expected performance of space-time codes in realistic deployments. The next article discusses the effects of antenna mutual coupling on correlation, radiation efficiency, diversity gain, and MIMO capacity. Measurements in a reverberation chamber and simulations are also provided to support the analysis. The last article of this issue presents a discussion of the classes of distortion that affect the performance of adaptive antennas. An eight-element circular array adaptive antenna testbed is employed to illustrate the impact of temperature on performance, while design techniques for calibration are also described. I would like to thank again all the authors who submitted their articles to this feature topic and also the many reviewers who took time out of their busy schedules and did an excellent job by providing detailed reviews.

Pilot Symbol Assisted Hybrid Detection for OFDM-Based Spatial Multiplexing Systems

In this paper, we provide a new detection scheme for a pilot symbol assisted interference nulling and cancellation operation to reduce unexpected effects owing to parallel transmission in orthogonal frequency division multiplexing (OFDM)-based spatial multiplexing systems. We have shown that the investigated OFDM vertical Bell laboratories layered space time (VBLAST) detection based on hybrid processing performs better than ordinary OFDM-VBLAST detections based on serial processing and parallel processing, respectively.

A bi-directional zero-forcing BLAST receiver

Recently, the Bell Laboratories layered space time (BLAST) architecture with multiple transmit and multiple receive (MTMR) antennas has been investigated to realize high data rates in wireless communications. It is understood that the ordering is crucial for good performance to alleviate the effect of error propagation. However, the optimal ordering requires a higher computational complexity. To avoid this difficulty, we propose a bidirectional BLAST receiver that can mitigate the error propagation problem without the optimal ordering. It is shown that the bidirectional BLAST receiver can perform better with a lower computational complexity.

An Improved Square-Root Algorithm for BLAST

In this letter, an improved square-root algorithm for Bell Labs Layered Space-Time (BLAST) system is proposed. It speeds up the original square-root algorithm by 36% in terms of the number of multiplications and additions. Compared with the recursive algorithm, it requires only a 13.6% higher number of multiplications and a 38.9% higher number of additions. Since it uses unitary transformations to avoid the computation of any matrix inversion or "squaring" operation, it maintains the advantages of the square-root based algorithms that are numerically stable, robust, and hardware friendly.

Generalized Layered Space–Time Codes for High Data Rate Wireless Communications

We present the architecture of generalized layered space-time codes (GLST) as a combination of Bell Labs layered space-time (BLAST) architecture and space-time coding (STC) in multiple-antenna wireless communication systems. This approach provides both spectral and power efficiency with moderate complexity. The framework is to partition all the available transmit antennas into groups and apply STC on each group as component codes. Based on the mappings from coded symbols to transmit antenna groups, we can construct different GLST systems. Particularly, horizontal mapping and diagonal mapping are introduced and referred to as HGLST and DGLST respectively. The basic decoding of GLST, under quasi-static flat Rayleigh fading environments and assuming perfectly known channel state information (CSI) at the receiver, combines group interference suppression and group interference cancellation techniques. As a result, the individual STC on each group is decoded serially. To improve the overall system performance, we derive the optimal power allocation among all space-time codewords without requiring the knowledge of CSI at the transmitter and suitable for all GLST systems. We also derive the optimal serial decoding order based on the channel realizations at the receiver for HGLST systems without power allocation. Simulation results show that both can provide much improvement. To further enhance the system performance, we propose a low complexity hard-decision iterative decoding method. This method efficiently exploits full receive antenna diversity and, hence, dramatically improves the system performance which is confirmed by simulation.

Transmit Power Allocation for a Modified V-BLAST System

In this letter, we present a modification of Vertical Bell Laboratories Layered Space-Time (V-BLAST), and propose an effective transmit power allocation (TPA) scheme for the modified system. The proposed TPA scheme minimizes the uncoded bit-error rate (BER) averaged over all detection stages, and requires small feedback overhead. Simulation results show that the modified V-BLAST system with the proposed TPA scheme provides a significant reduction in the uncoded BER compared with the conventional V-BLAST system. When the minimum mean-square error nulling is adopted, the modified V-BLAST system is found to achieve the uncoded BER performance comparable to that of the maximum-likelihood detection for the conventional V-BLAST architecture.

Channel Estimation and Data Detection for MIMO Systems under Spatially and Temporally Colored Interference

The impact of interference on multiple-input multiple-output (MIMO) systems has recently attracted interest. Most studies of channel estimation and data detection for MIMO systems consider spatially and temporally white interference at the receiver. In this paper, we address channel estimation, interference correlation estimation, and data detection for MIMO systems under both spatially and temporally colored interference. We examine the case of one dominant interferer in which the data rate of the desired user could be the same as or a multiple of that of the interferer. Assuming known temporal interference correlation as a benchmark, we derive maximum likelihood (ML) estimates of the channel matrix and spatial interference correlation matrix, and apply these estimates to a generalized version of the Bell Labs Layered Space-Time (BLAST) ordered data detection algorithm. We then investigate the performance loss by not exploiting interference correlation. For a (5,5) MIMO system undergoing independent Rayleigh fading, we observe that exploiting both spatial and temporal interference correlation in channel estimation and data detection results in potential gains of 1.5 dB and 4 dB for an interferer operating at the same data rate and at half the data rate, respectively. Ignoring temporal correlation, it is found that spatial correlation accounts for about 1 dB of this gain.

D-BLAST OFDM with Channel Estimation

Multiple-input and multiple-output (MIMO) systems formed by multiple transmit and receive antennas can improve performance and increase capacity of wireless communication systems. Diagonal Bell Laboratories Layered Space-Time (D-BLAST) structure offers a low-complexity solution for realizing the attractive capacity of MIMO systems. However, for broadband wireless communications, channel is frequency-selective and orthogonal frequency division multiplexing (OFDM) has to be used with MIMO techniques to reduce system complexity. In this paper, we investigate D-BLAST for MIMO-OFDM systems. We develop a layerwise channel estimation algorithm which is robust to channel variation by exploiting the characteristic of the D-BLAST structure. Further improvement is made by subspace tracking to considerably reduce the error floor. Simulation results show that the layerwise estimators require 1 dB less signal-to-noise ratio (SNR) than the traditional blockwise estimator for a word error rate (WER) of when Doppler frequency is 40 Hz. Among the layerwise estimators, the subspace-tracking estimator provides a 0.8 dB gain for WER with 200 Hz Doppler frequency compared with the DFT-based estimator.