Dynamic resource allocation with beamforming for MIMO OFDM systems: performance and effects of imperfect CSI

We propose three different dynamic resource allocation algorithms using adaptive beamforming for multiple-input multiple-output (MIMO) OFDM systems, and investigate their performance over multipath fading channels under perfect and imperfect channel state information (CSI). These approaches involve the use of adaptive modulation, adaptive frequency-domain power allocation, and/or adaptive sub-channel allocation. By employing the proposed approaches in MIMO/OFDM systems, significant performance improvement can be achieved compared to the conventional adaptive antenna array based OFDM. The investigation of the effects of imperfect CSI reveals that the adaptive-modulation based approach is too sensitive to channel estimation errors, and that its performance is worse than the adaptive frequency-domain power allocation and/or adaptive sub-channel allocation approaches. The performance analysis also shows that combining adaptive power allocation with sub-channel allocation yields the best performance under imperfect CSI while being robust to channel estimation errors.     

Frequency-selective multiple-input multiple-output channel estimation with transmitter non-linearities

The authors propose an algorithm based on the knowledge of training sequences to obtain an asymptotically unbiased estimator of non-linear multiple-input multiple-output (MIMO) channels, which involves the radio frequency front-end non-linearity and linear frequency selective MIMO channels. Although the impact of non-linearity in the transmitter side has been widely studied, most work on the channel estimation assumes linear channel models and ignores the non-linear effects. In this study, we develop a nonlinear channel estimator that can simultaneously estimate the linear MIMO channel model and non-linearity of the transmitter is developed. With these two sets of parameters, the non-linear channel model can be fully described. This channel estimation algorithm is implemented over an empirical MIMO channel model using an orthogonal frequency division multiplexing system.     

一种移动终端多天线方向图重构快速选择算法与基于滑动时间窗的加速实现

 多输入多输出(MIMO)系统中,天线选择技术可以在降低复杂度的同时,有效地提高系统的性能.但对于天线安装空间受限的小型终端,天线的数目将受到很大限制,本文结合方向图可重构技术,考虑基于线性接收机的空间复用系统,通过在已选择的发射天线上采用等功率分配等增益传输以减少反馈信息量.在此条件下,推导出空间相关衰落信道下自适应发射天线选择的统计容量公式下限,然后,在此基础上提出基于方向图重构的发射天线快速选择方法以最大化该容量值.此外,在实现中提出了一种基于滑动时间窗的梯度更新估计模型,缩短了算法所需时间.     

Space-time channel estimation and performance analysis for wireless MIMO-OFDM systems with spatial correlation

This paper treats channel estimation in multiple-input multiple-output (MIMO) orthogonal frequency division multiplexing (OFDM) systems with correlation at the receive antenna array. A two-step channel estimation algorithm is proposed. Firstly, the iterative quadrature maximum likelihood based time delay and spatial signature estimation is presented by utilizing special training signals with a cyclic structure. The receive spatial correlation matrix of the vector valued channel impulse response is formulated as a function of the spatial signature, the time delay, and the pulse shaping filter. The joint spatio-temporal (JST) filtering based minimum mean squared error channel estimator is derived by virtue of the spatial correlation. In addition, the effect of channel estimation errors on the bit error probability performance of the space-time block coded OFDM system over correlated MIMO channels is derived. The Cramer-Rao lower bound on the time delay estimate is provided for a benchmark of the performance comparison. The performance of proposed algorithms is illustrated based on analysis and computer simulations. The JST channel estimator achieves significant gains in the mean squared error compared to the temporal filtering. It also enables remarkable savings in the pilot symbol power level.     

MIMO-OFDM快衰落信道的稀疏自适应感知估计

在多输入多输出（MIMO）-正交频分复用（OFDM） 系统中,怎样在较高频谱利用率的情况下对快时变信道进行较为准确的估计是一个具有挑战性的课题。该文在利用压缩感知理论可提高系统频谱利用率的基础上,提出了一种适合于快时变环境下MIMO-OFDM 系统的稀疏自适应信道估计方法。该方法不再受到奈奎斯特采样频率条件约束,避免了传统导频辅助信道估计方法频谱利用率低的缺点。该文方法通过构建多天线群时频结构特征稀疏基,利用多天线间和群时变OFDM符号内信道冲激响应具有更强稀疏性的特点,对MIMO-OFDM快衰落信道进行稀疏变换。由于实际MIMO-OFDM快衰落信道往往处于频率选择性、时变性和多种干扰并存的复杂环境,受到干扰的信道参数对系统而言是未知,采用该方法克服了现有基于压缩感知理论的信道估计方法需要预先知道信道冲激响应稀疏度才能重构信道参数的不足,在信道稀疏度未知道的情况下,运用稀疏自适应的方法来对不同时频结构特征的信道参数进行估计。仿真结果表明所提估计方法具有对快时变信道参数估计的鲁棒性和较高频谱利用率,且均方误差小。      

一种多输入多输出系统中有限反馈预编码的自适应跟踪测度

该文提出一种新的适用于无线相关信道中有限反馈预编码多输入多输出系统的预编码矩阵自适应跟踪测度。由于该测度在统计意义下对信道的相关性与预编码矩阵变化的关系具有更优化的描述,所以基于该测度的方案在保持较低反馈信息量的前提下,能够进一步改善预编码系统性能。文中给出优化自适应跟踪测度的理论推导。系统仿真分别针对无线信道的频域相关和时域相关,并结合空间分集和空间复用两类系统结构予以讨论,并验证了理论分析结果。另外,由于该测度方案算法复杂度更低,所以更具工程实用价值。     

TD-LTE-Advanced系统SRS信道估计误差性能研究

在实际TD-LTE-Advanced系统中,针对非理想SRS信道,结合SRS信道估计误差的理论,建立SRS信道估计误差的模型,深入研究SRS误差对MIMO的性能影响。仿真分析显示,存在SRS误差的实际信道对协作多点传输（CoMP）有较大影响,但是相对其他基于波束赋形的MIMO模式仍能保证较高的增益。文章的研究工作分析非理想信道估计对系统性能的影响,并评估非理想SRS信道下多种MIMO模式的系统性能。     

Effects of Carrier Frequency Offset and Channel Estimation Errors on the Performance of MIMO-OFDM Systems in Spatially Correlated Channels

As an effective technique for combating multipath fading and for high-bit-rate transmission over wireless channels, orthogonal frequency division multiplexing (OFDM) is extensively used in high-rate wireless communication systems, such as, the wireless local area network (WLAN) and the digital television terrestrial broadcasting (DTTB) systems, to support high performance bandwidth-efficient multimedia services. Multiple antennas and transmit or receive diversity, multiple-input multiple-output orthogonal frequency division multiplexing (MIMO-OFDM), can be used to improve error performance and capacity of wireless systems. In this paper, we consider the effects of carrier frequency offset and channel estimation errors on the performance of MIMO-OFDM systems in spatially correlated channels. Theoretical calculations and computer simulations are done to analyze the performance degradation of MIMO-OFDM systems in spatially correlated channels due to carrier frequency offset and channel estimation errors, and the theoretical and simulated results match well.     

Joint pre-processing co-channel interference cancellation for single user MIMO

In general, multiplexing and diversity gains of single user MIMO systems are restricted by min(M,N) where M, N denote the number of antenna elements at a transmitter and receiver, respectively. In order to increase the multiplexing/diversity gains and improve the performance of single user MIMO systems, a joint pre-processing co-channel interference cancellation (JPCIC) method is proposed. The JPCIC is analyzed in both the perfect and the imperfect channel state information. The dependence of channel capacity on the number of antenna elements in every subset, the number of subsets, transmit powers and channel estimation errors is discussed. As theoretical calculation result, the channel capacity increases when the multiplexing/diversity gains and/or the transmit power increase in a certain channel model whether the channel estimation error is absent or present. Compared to the conventional zero-forcing method, the channel capacity of JPCIC is considerably higher because of higher multiplexing/diversity gains, however, it is less robust and decreased more rapidly due to incomplete cancellation of interference terms when the channel estimation error increases. There is a trade-off between the channel capacity and the complexity of system, however, according to quick development in circuit techniques and miniaturization of devices, the JPCIC is expected to be an attractive technology for MIMO system.     

信道反馈延迟时MIMO中继系统的线性预编码算法

针对采用放大转发(Amplify and Forward, AF)中继技术的多入多出(Multiple Input Multiple Output, MIMO)系统, 考虑信道估计误差及反馈存在延迟的情况, 提出一种基于最小均方误差(Minimum Mean Squared Error, MMSE)准则的预编码设计方案.假设基站-中继端及中继端-终端的信道均存在估计误差与反馈延迟, 在基站和中继端功率都受限条件下, 以MMSE为准则, 推导得到了基站预编码矩阵、中继转发矩阵和终端解码矩阵的闭式解.数值仿真结果表明, 该方案所提出的预编码算法能有效地改善系统的误比特率与均方误差.     

Feedback Constraints for Adaptive Transmission

In this article, the feedback constraints that limit the potential of AMC and MIMO systems for FDD systems are presented. It has been shown that while adaptive transmission efficiently utilizes the available bandwidth and transmit power, feedback constraints could be a significant impediment. Channel prediction can be employed to compensate the impact of feedback delay but its effectiveness would depend on the accuracy of the channel model. It is also showed that feedback errors could cause outage regions where AMC is not feasible and could also impact the benefits of multiuser scheduling. Limited feedback is a major issue for MIMO-AMC systems because channel state information grows with the rank of the MIMO system, the number of transmission modes, and the number of users for broadcast systems. Since feedback transmission consumes resources that could otherwise be used for data transmission on the reverse channel, feedback design is extremely important especially for power-limited devices such as mobile stations. Finally, some open problems are outlined that are of interest to adaptive transmission systems     

基于矢量线性运算的自适应球帽差分反馈算法

信道时间相关多输入多输出(MIMO)系统中一种重要有限反馈策略为差分反馈,该文提出一种适用于单用户多输入单输出(SU-MISO)系统的自适应球帽差分反馈算法。首先,运用相邻时刻信道间的扰动规律设计自适应差分反馈策略;其次,不同于现有差分码本的非线性构造方法,提出一种基于矢量伸缩和合成的球帽差分码本算法;最后,联合球帽码本空间分布规律和时间相关信道的统计规律推导出球帽半径计算方法。仿真结果说明,该算法能准确跟踪信道并较现有算法有一定性能提高。     

MIMO—OFDM低延迟信道估计改进算法

为了克服多输入多输出正交频分复用（MIMO—OFDM）移动系统中采用空间复用技术的下行链路性能出现衰退现象,提出一种信道估计改进算法。它根据信道状态并通过一个决策指导方案为4×4 MIMO系统提供大约4bit／s／Hz的数据。通过使用LDPC短码,在LDPC译码器输出端采用MIMO—OFDM符号重建方法,达到提高估计精度的目的。在3GPP MIMO空间信道模型下进行了仿真验证,结果表明指定速率下的估计精度得到改善。     

Environmental issues for MIMO capacity

Wireless communication using multiple-input multiple-output (MIMO) systems enables increased spectral efficiency for a given total transmit power. Increased capacity is achieved by introducing additional spatial channels that are exploited using space-time coding. In this paper, the environmental factors that affect MIMO capacity are surveyed. These factors include channel complexity, external interference, and channel estimation error. The maximum spectral efficiency of MIMO systems in which both transmitter and receiver know the channel (using channel estimate feedback) is compared with MIMO systems in which only the receiver knows the channel. Channel complexity is studied using both simple stochastic physical scattering and asymptotic large random matrix models. Both uncooperative (worst-case) and cooperative (amenable to multiuser detection) interference are considered. An analysis for capacity loss associated with channel estimation error at the transmitter is introduced.     

Equalization and semi-blind channel estimation for space-time block coded signals over a frequency-selective fading channel

In this paper, we investigate the equalization and channel identification for space-time block coded signals over a frequency-selective multiple-input multiple-output (MIMO) channel. The equalization has been considered by taking into account the cyclostationarity of space-time block coded signals. The minimum mean square error (MMSE) solutions have been derived for the linear and decision feedback (DF) equalizers. The channel estimation is required for the equalization. With known symbols (as pilot symbols), MIMO channels can be estimated. In addition, due to the redundancy induced by space-time block code, it is possible to identify MIMO channels blindly using the subspace method. We consider both blind and semi-blind channel estimation for MIMO channels. It is shown that the semi-blind channel estimate has fewer estimation errors, and it results in less (bit error rate) performance degradation of the MMSE linear and DF equalizers.     

Intercarrier interference in MIMO OFDM

In this paper, we examine multicarrier transmission over time-varying channels. We first develop a model for such a transmission scheme and focus particularly on multiple-input multiple output (MIMO) orthogonal frequency division multiplexing (OFDM). Using this method, we analyze the impact of time variation within a transmission block (time variation could arise both from Doppler spread of the channel and from synchronization errors). To mitigate the effects of such time variations, we propose a time-domain approach. We design ICI-mitigating block linear filters, and we examine how they are modified in the context of space-time block-coded transmissions. Our approach reduces to the familiar single-tap frequency-domain equalizer when the channel is block time invariant. Channel estimation in rapidly time-varying scenarios becomes critical, and we propose a scheme for estimating channel parameters varying within a transmission block. Along with the channel estimation scheme, we also examine the issue of pilot tone placement and show that in time-varying channels, it may be better to group pilot tones together into clumps that are equispaced onto the FFT grid; this placement technique is in contrast to the common wisdom for time-invariant channels. Finally, we provide numerical results illustrating the performance of these schemes, both for uncoded and space-time block-coded systems.     

Optimization of Switched MIMO Systems Over Rayleigh Fading Channels

This paper proposes a new switched MIMO system where the MIMO includes both diversity-based MIMO and spatial multiplexing MIMO (SM-MIMO). The proposed scheme is a closed-loop system where the feedback bits are sent back to the transmitter to indicate the channel quality. The switching operation is employed among groups of a certain number of transmit antennas. For diversity-based MIMO systems, the switching operation aims to improve the link performance, while the goal of the switching for the SM-MIMO systems is to increase the data rate. The authors analyze and optimize the proposed systems and show that there is considerable benefit in terms of performance and practical use     

MIMO-FSK non-coherent detection with spatial multiplexing infast-fading environment

Accurate estimationand real-time compensation for phase offset and Doppler shift are essential for coherent multi-input multi-output (MIMO) systems. Here, a spatial multiplexing MIMO scheme with non-coherent frequency-shiftkeying (FSK) detection is proposed. It is immune to random phase interference and Doppler shift while increasingcapacity. It is valuable that the proposed spatial multiplexing MIMO based on energy detection (ED) is equivalentto a linear system, and there is no mutual interference caused by the product of simultaneous signals in square-lawprocessing. The equivalent MIMO channel model is derived as a real matrix, which remains maximal multiplexingcapacity and reduces the channel estimation complexity. Simulation results show that the proposed scheme hasoutstanding performance over Rician flat fading channel, and experimental system obtains four times the capacitythrough 4 antennas on both transmitter and receiver.     

Spatio-temporal channel characterization: theoretical framework and applications in MIMO system design

The utilization of multiple antennas and space–time codes in multiple-input and multiple-output (MIMO) communication systems significantly improves the transmission channel capacity without using additional bandwidth and power. The improvement is achieved by decomposing the spatial structure of transmission channels and performing appropriate temporal and spatial multiplexing. In this paper, we propose a novel theoretical framework for MIMO channel modeling and characterization in order to facilitate the MIMO system design and performance evaluation. The channels are represented in space, time, wave vector, and frequency domains while the space–time and wave vector–frequency interdependences are considered. A realization of the theoretical framework, in a form of a practical framework, is also proposed to address the channel modeling and characterization at both transmitter and receiver sides. The utilization of the practical framework in MIMO communication system design is discussed to illustrate its applications in realistic scenarios. The angle of arrival estimation based on the proposed practical framework using field test measurement data is also presented as illustrative examples.     

Symbol-level adaptive modulation for coded OFDM on block fading channels

It has recently been recognized by many researchers that adaptive modulation is most effective when the channel diversity order is small. In this letter, we propose a simple adaptive modulation scheme for orthogonal frequency division multiplexing (OFDM) systems on channels that provide a small order of diversity. The proposed adaptation algorithm is based on a novel and very simple analytical formula we derive for the performance of BICM on block fading channels. Simulation results show that the derived analytical formula is very tight when the targeted bit error probability is small. OFDM systems on indoor channels provide small orders of diversity and form an ideal scenario for adaptation. In order to keep system complexity and feedback requirements at a minimum, we will consider symbol-level adaptive modulation wherein all the subcarriers in an OFDM symbol use the same modulation.