On channel estimation in cell-free massive MIMO for spatially correlated channels with correlated shadowing under Rician fading

This work addresses channel estimation (CE) in the uplink phase for a cell-free massive multiple-input multiple-output system operating under the time division duplex protocol. Considering that, channels are spatially correlated under the Rician fading model, where the investigated model is composed of two components: deterministic and nondeterministic, with the deterministic component describing the line-of-sight paths and the nondeterministic component describing the non-line-of-sight paths. Additionally, we dealt with correlated shadow fading that represents the most realistic situation. On the other hand, this work introduces a dynamic cooperation cluster framework in which the user is not served with the whole network ( i.e., all access points [APs]) but only the APs that present the best channel conditions regarding that user. In other words, this work proposes partial CE for each user because only APs with the best channel conditions are allowed to compute channel estimates. Consequently, we proposed partial channel estimators that perform the CE process with low complexity, namely, a partial minimum mean square error estimator and a partial element-wise minimum mean square error estimator. In addition, a simple pilot assignment technique is proposed in order to reduce interference signals so that each user experiences low interference from other users. Furthermore, the computational complexity required by each estimator is derived, where it is represented by the number of complex multiplications that each estimator requires in each consistency block. Theoretical and simulated results are provided, where the performance of each estimator is evaluated and analyzed using the normalized mean-square error metric.     

Joint channel estimation and symbol detection in Rayleighflat-fading channels with impulsive noise

A new technique for joint channel estimation and symbol detection in the Rayleigh flat-fading channels with impulsive noise is developed. This technique involves an approximation to the likelihood statistics for such channels, which in turn is based on the Masreliez approximation of nonlinear filtering. It is seen that the proposed detector outperforms the detector based on the Kalman filter     

Low-complexity joint channel estimation and decoding for pilotsymbol-assisted modulation and multiple differential detection systemswith correlated Rayleigh fading

Joint channel estimation and decoding in a time-varying Rayleigh fading channel is considered. Knowing that the optimal solution or even the truncated near-optimal solution using iterative processing has an exponential complexity which hinders the practicability, a reduced complexity approach is proposed. This approach keeps the existing channel estimation and decoding schemes almost intact, while applying iterative processing to effectively exchange information between them. Thus, the complexity is rendered linear, and estimator adaptability can be easily established. We apply this approach to pilot symbol-assisted modulation (PSAM) and differentially modulated systems. It turns out that the performance is improved and the robustness to fading parameters is enhanced. Through simulations, we also show that the proposed method performs almost as well as the near-optimal design     

Maximum-likelihood bearing estimation with partly calibrated arraysin spatially correlated noise fields

The problem of using a partly calibrated array for maximum likelihood (ML) bearing estimation of possibly coherent signals buried in unknown correlated noise fields is shown to admit a neat solution under fairly general conditions. More exactly, this paper assumes that the array contains some calibrated sensors, whose number is only required to be larger than the number of signals impinging on the array, and also that the noise in the calibrated sensors is uncorrelated with the noise in the other sensors. These two noise vectors, however, may have arbitrary spatial autocovariance matrices. Under these assumptions the many nuisance parameters (viz., the elements of the signal and noise covariance matrices and the transfer and location characteristics of the uncalibrated sensors) can be eliminated from the likelihood function, leaving a significantly simplified concentrated likelihood whose maximum yields the ML bearing estimates. The ML estimator introduced in this paper, and referred to as MLE, is shown to be asymptotically equivalent to a recently proposed subspace-based bearing estimator called UNCLE and rederived herein by a much simpler approach than in the original work. A statistical analysis derives the asymptotic distribution of the MLE and UNCLE estimates, and proves that they are asymptotically equivalent and statistically efficient. In a simulation study, the MLE and UNCLE methods are found to possess very similar finite-sample properties as well. As UNCLE is computationally more efficient, it may be the preferred technique in a given application     

Space-time modulation and coding over transmit correlated fading channels

Multi-dimensional space-time modulation schemes can be classified by the manner in which signal dimensions are shared among transmit antennas. In aggregate transmit antenna (ATA) systems, a generalization of Tarokh, Seshadri, Calderbank's (TSC) approach, there is total sharing of dimensions. In orthogonal transmit antenna (OTA) systems, a generalization of many traditional diversity schemes, there is no sharing of dimensions. In partially orthogonal transmit antennas (POTA), a combination of ATA with OTA, subsets of available dimensions are shared by subsets of transmit antennas. This letter considers such coded schemes for spatially correlated fading channels. Over strongly transmit correlated channels, in addition to coding and diversity gains, ATA and POTA can harvest a transmit beamforming-like gain. This letter presents a scheme, POTARep, designed to provide beamforming-like gain as well as diversity and coding gains, yielding improved performance over a highly transmit correlated channel.     

Joint symbol detection and channel parameter estimation inasynchronous DS-CDMA systems

A method for jointly estimating the time delay and complex gain parameters, as well as detecting the transmitted symbols in an asynchronous multipath DS-CDMA system, is presented. A short training sequence is used to obtain a coarse estimate of the channel parameters, which is consequently used to detect the symbols. By exploiting structure in the digitally modulated signals, the method iterates to (i) improve the estimate of the channel parameters and (ii) reduce the probability of incorrect detection. The method's efficacy is demonstrated by numerical simulations     

Space-time coding ambiguities in joint adaptive channel estimation and detection

This paper studies the error propagation effect that is caused by certain ambiguities in joint data detection-channel tracking algorithms for transmission diversity schemes. Here, we use a space-time (ST) receiver based on the maximum a posteriori (MAP) method that takes into account the channel estimation error assuming the unknown channel to have a given complex multivariate Gaussian probability density function (pdf) (i.e., a Ricean channel). The decision criterion that is expressed in quadratic form represents either a linear detector or a noncoherent-nonlinear detector in extreme cases. Then, the channel pdf for the next iteration is updated by estimates of the second-order statistics of the channel coefficients, and a very simple decision-directed adaptive algorithm is derived for adaptive channel estimation. The adaptive algorithm can efficiently track a fast Rayleigh fading channel and, as a result, achieves robust performance. However, the occurrence of two types of ambiguities initiated in deep fades result in error propagation. Some remedies called space-time ambiguity remedies (STARs) are proposed to prevent error propagation. A new time-varying space-time coding (TVST) scheme is suggested as a bandwidth-efficient method to combat the permutation ambiguity impairment. This coding scheme, in conjunction with a differential detector, can resolve the ambiguity problem.     

Space-time coding over correlated fading channels with antenna selection

In a previous paper by Bahceci et al., antenna selection ' for multiple-antenna transmission systems under the assumption that the subchannels between antenna pairs fade independently was studied. In this paper, the performance of such systems when the subchannels experience correlated fading is considered. It is assumed that the channel-state information (CSI) is available only at the receiver, the antenna selection is performed only at the receiver, and the selection is based on the instantaneous received signal power. The effects of channel correlations on the diversity and coding gain when the receiver system is a subset of the antennas are quantified. Theoretical results indicate that the correlations in the channel do not degrade the diversity order, provided that the channel is full rank. However, it does result in some performance loss in the coding gain.     

On noncoherent receivers for DSTM in spatially correlated fading

In this letter, we derive a multiple-symbol differential detection (MSDD) and a novel MSDD-based decision-feedback differential detection (MS-DFDD) receiver for differential space-time modulation transmitted over spatially correlated multiple-input multiple-output fading channels. We show that MS-DFDD outperforms previously proposed DFDD schemes that are based on scalar and vector prediction (SP-DFDD and VP-DFDD). In addition, we prove that at high signal-to-noise ratio (SNR) VP-DFDD is equivalent to SP-DFDD and thus fails to properly exploit the spatial fading correlations.     

A spatially and temporally correlated fading model for arrayantenna applications

In this paper, a new channel model is proposed. Since it is spatially and temporally correlated simultaneously, this new model is well consistent with the real environment of array antenna applications. The widely used sum of scattered waves and the measurement-based model have a common drawback of imperfect statistical properties, reducing the reliability of simulation results. The new model (spatially and temporally correlated fading model [STCFM]) is derived rigorously in the spatiotemporal domain so that it can provide high accuracy for the evaluation of the array antenna system. Simulation results show that direction of arrival (DOA) and angle of spread (AOS) are well defined in STCFM, BPSK with two-branch maximal ratio combining and DBPSK with differential detection are considered to verify the spatial and temporal correlation robustness of the new model, respectively. As is shown, the simulation results agree well with the theory     

OFDM系统的迭代联合信道估计与符号检测算法

提出了通过降低导频功率来提高OFDM传输效率,同时采用迭代的联合信道估计和符号检测算法来保证较好误码性能的接收方案。理论分析与仿真结果表明,当数据信息与导频信息的功率比不高于l0dB时,本文提出的算法能够有效地降低系统误码率,同时该方法还具有算法简单,收敛速度快的特点。     

一种空间相关高斯噪声背景下的时变时延估计算法

在空间相关高斯噪声的背景下,基于二阶统计量的时延估计方法会失效,该文提出了一种基于三阶统计量的自适应时变时延估计算法,并分析了算法的收敛性,最后的仿真结果表明该算法可以有效地抑制相关高斯噪声。     

Adaptive joint multiuser detection and channel estimation in multipath fading CDMA channels

The problem of joint multiuser detection and channel estimation in frequency-selective Rayleigh fading CDMA channels is considered. First the optimal multiuser detector for such channels is derived, which is seen to have a computational complexity exponential in the product of the number of users and the length of the transmitted data sequence. Two suboptimal detectors are then developed and analyzed, both of which employ decorrelating filters at the front-ends to eliminate the multiple-access interference and the multipath interference. The symbol-by-symbol detector uses a Kalman filter and decision feedback to track the fading channel for diversity combining. The per-survivor sequence detector is in the form of the Viterbi algorithm with the trellis updates being computed by a bank of Kalman filters in the per-survivor fashion. Both suboptimal detectors require the knowledge of all waveforms of all users in the channel and the channel fading model parameters. Adaptive versions of these suboptimal detectors that require only the knowledge of the waveform of the user of interest are then developed. The adaptive receivers employ recursive-least-squares (RLS) minimum-mean-square-error (MMSE) filters at the front-end to mitigate the interference, and use a bank of linear predictors to track the fading channels. It is shown that the front-end RLS-MMSE filters can be implemented using systolic arrays to exploit massively parallel signal processing computation, and to achieve energy efficiency. Finally, the performance of the suboptimal detectors and their adaptive versions are assessed by simulations. This revised version was published online in July 2006 with corrections to the Cover Date.     

DS-CDMA system with joint channel estimation and MAP detection intime-selective fading channels

In this paper, maximum a posteriori (MAP) detection is applied to a direct-sequence code-division multiple-access (DS-CDMA) system jointly with identification and estimation of time-selective fading channels. By sampling the outputs of the matched filter and combining antenna array elements, strong and time-varying multiple-access interference (MAI) is characterized and suppressed instantaneously. The decision statistics for MAP detection are obtained from the conditional probability density function of the prediction error. The prediction is accomplished by approximating the fading channel with a constrained nonlinear state model. Unknown parameters such as auto-regressive (AR) process coefficients, noise covariance matrices, and the antenna array vector are estimated based on received sample vectors only. Also, differential modulation is applied to eliminate the need for pilot insertion. Through computer simulations, near-optimum bit error rates (BERs) are found     

UN-MUSIC and UN-CLE: an application of generalized correlationanalysis to the estimation of the direction of arrival of signals inunknown correlated noise

A new approach is proposed for the consistent estimation of the directions of arrival (DOA) of signals in an unknown spatially-correlated noise environment. The signal and noise model used is based on the assumption that the data are received by two arrays well separated so that their noise outputs are uncorrelated. The generalized correlation decomposition of the cross-correlation matrix between the two arrays is then introduced. Of particular interest is the canonical correlation decomposition. The analysis of the generalized correlation leads to various interesting geometric and asymptotic properties of the eigenspace structure. Two algorithms, UN-MUSIC and UN-CLE, are developed to estimate the DOA of signals in unknown spatially correlated noise based on the utilization of these properties. Computer simulations show that these methods are superior in performance compared to conventional methods. Furthermore, it is demonstrated that the new methods are equally effective even when only one sensor array is employed     

基于分形的多径衰落信道估计

该文研究了多径衰落的分形性质,提出了一种利用分形维数和小波重构来改进最小均方误差方法的新型多径衰落信道估计算法。该算法改进了衰落信道参数估计的准确度,并消除了判决方法的错误传递性。仿真结果表明该算法能较准确地估计出多径衰落信道的参数,显著提高快衰落条件下接收机的误码性能。     

A deterministic channel simulation model for spatially correlatedRayleigh fading

We propose a deterministic vector channel simulation model for generating the fading waveforms that satisfy not only rigorous temporal correlation but also arbitrary spatial correlation by means of Doppler phase difference sampling. The proposed method is more efficient than the conventional pseudonoise (PN) filtered Gaussian model with coloring process in evaluating the laboratory level performance of the mobile communication systems employing adaptive arrays or space diversity     

Effective capacity of a correlated Rayleigh fading channel

The next generation wireless networks call for quality of service (QoS) support. The effective capacity (EC) proposed by Wu and Negi provides a powerful tool for the design of QoS provisioning mechanisms. In their previous work, Wu and Negi derived a formula for effective capacity of a Rayleigh fading channel with arbitrary Doppler spectrum. However, their paper did not provide simulation results to verify the accuracy of the EC formula derived in their paper. This is due to difficulty in simulating a Rayleigh fading channel with a Doppler spectrum of continuous frequency, required by the EC formula. To address this difficulty, we develop a verification methodology based on a new discrete‐frequency EC formula; different from the EC formula developed by Wu and Negi, our new discrete‐frequency EC formula can be used in practice. Through simulation, we verify that the EC formula developed by Wu and Negi is accurate. Furthermore, to facilitate the application of the EC theory to the design of practical QoS provisioning mechanisms in wireless networks, we propose a spectral‐estimation‐based algorithm to estimate the EC function, given channel measurements; we also analyze the effect of spectral estimation error on the accuracy of EC estimation. Simulation results show that our proposed spectral‐estimation‐based EC estimation algorithm is accurate, indicating the excellent practicality of our algorithm. Copyright © 2010 John Wiley & Sons, Ltd.     

Joint channel and symbol estimation by oblique projections

The problem of simultaneous blind channel and symbol estimation of a single-input multiple-output (SIMO) communication channel is considered. It is shown that the outer product of the channel vector and the channel input sequence can be obtained by a linear estimator that has the finite sample convergence property. Furthermore, this estimator can be obtained by the use of oblique projections. An order detection algorithm that avoids the use of subjective thresholding is also proposed. Applications to multiuser detection are also considered