Optimizing MIMO antenna systems with channel covariance feedback

We consider a narrowband point-to-point communication system with n/sub T/ transmitters and n/sub R/ receivers. We assume the receiver has perfect knowledge of the channel, while the transmitter has no channel knowledge. We consider the case where the receiving antenna array has uncorrelated elements, while the elements of the transmitting array are arbitrarily correlated. Focusing on the case where n/sub T/=2, we derive simple analytic expressions for the ergodic average and the cumulative distribution function of the mutual information for arbitrary input (transmission) signal covariance. We then determine the ergodic and outage capacities and the associated optimal input signal covariances. We thus show how a transmitter with covariance knowledge should correlate its transmissions to maximize throughput. These results allow us to derive an exact condition (both necessary and sufficient) that determines when beamforming is optimal for systems with arbitrary number of transmitters and receivers.     

Optimization of antenna configuration for MIMO systems

In this letter, we investigate the issue of selecting the number of antennas at the base station and at the mobile to optimize ergodic capacity of a multiple-input/multiple-output system, when the costs of implementing antennas at the base station and at the mobile are unequal. Total system capacity, defined as a linear combination of the uplink and downlink ergodic capacity, is used as the objective function to be maximized. The asymptotic expression for the ergodic capacity is used as an approximation. The limiting case gives some insight on how the ratio of the number of antennas at the base station to the number of antennas at the mobile may change with signal-to-noise ratio and cost ratio when the total system capacity is maximized.     

On the information throughput and optimized power allocation for MIMO wireless systems with imperfect channel estimation

In this paper, we focus on the throughput analysis, outage evaluation and optimized power allocation for Multiple-Input Multiple-Output (MIMO) pilot-based wireless systems subject to short-term constraints on the radiated power and equipped with a feedback-path for communicating back to the transmitter the imperfect MIMO channel estimates available at the receiver. The case of the ergodic throughput for Gaussian distributed input signals is analyzed, and the conditions for the (asymptotical) achievement of the Shannon capacity are pointed out. The main contributions of this work may be so summarized. First, we develop closed-form analytical expressions for the computation of the ergodic information throughput conveyed by the considered MIMO system for the case of ideal feedback link. Second, we present an iterative algorithm for the optimized power allocation over the transmit antennas that explicitly accounts for the imperfect MIMO channel estimates available at the receiver. Third, after relaxing the assumption of ideal feedback link, we test the sensitivity of the proposed power allocation algorithm on errors possibly introduced by the feedback channel, and then, we numerically evaluate the resulting throughput loss. Finally, we develop closed-form upper and lower bounds on the outage probability that are asymptotically tight.     

非完美信道估计下大规模天线系统设计和研究

为满足用户对无线网络容量和覆盖范围日益提升的需求,大规模天线技术得到了广泛的研究。然而,在非完美信道估计情况下,大规模天线系统的天线数应该如何配置仍是一个亟需解决的问题。现考虑了一个发射端配备大规模天线阵列,且上行信道估计存在误差的系统模型,并以能量效率为优化准则,使用两种算法推导出了最优天线配置策略的闭式解,并通过仿真验证了该策略的优异性能。     

循环前缀块传输系统中的一种MIMO盲信道估计算法

信道估计一直是无线通信领域的研究热点之一,信道参数估计的好坏对系统的整体性能有着至关重要的影响。针对采用循环前缀的多输入多输出(MIMO-CP)系统,本文提出了一种基于子空间的盲信道估计方法,该算法利用了循环前缀所引起的冗余信息。基于子空间的盲信道估计算法都是通过对接收块的自相关矩阵进行奇异值分解(SVD)来实现信道估计的,因此需要利用尽可能多的接收块来得到准确的自相关矩阵的估计值,这就意味着会产生过久的判决延迟以及不能准确对快变信道进行跟踪。利用MIMO-CP系统中系统矩阵特有的块循环特性,对于连续的两个接收数据块以及对应的循环前缀部分组成的向量,可以重新构造一组新的向量而不改变系统的信道矩阵,因此可以通过较少的接收块来得到准确的自相关矩阵的估计值,该方法十分适用于对快变信道的盲估计。文章通过仿真分析了在不同的重复系数以及不同的接收块下该算法的性能并且比较了该算法与现有的“预编码”、“虚拟子载波”等盲信道估计算法的性能。仿真结果表明,提出的算法利用较少的数据块个数就得到了一个可靠的信道估计值和很好的误码率性能。      

Improving power allocation to MIMO eigenbeams under imperfect channel estimation

Eigenbeamforming exploits the spatial diversity of the MIMO channel by multiplexing data substreams along orthogonal modes. Estimation errors result in self-interference, which must be taken into account in the selection of modes for transmission and the allocation of transmitter power. Using perturbation analysis to evaluate the self-interference, a technique is proposed for determining the power allocation which achieves a specified performance, based only on the information available at the receiver.     

Robust channel estimation for the OFDM-based WLAN systems with imperfect synchronization

As an effective technique for combating multipath fading and for high data rate transmission over wireless channels, orthogonal frequ- ency division multiplexing (OFDM) is extensively used in wireless local area network (WLAN) systems to support high-performance bandwidth- efficient multimedia services. In this paper, a robust channel estimation scheme is proposed for the OFDM-based WLAN systems with imperfect synchronization. The frame structure information, the preamble information, the pilot information are efficiently utilized in the proposed channel estimation scheme. Simulation results are used to illustrate the performance of the proposed scheme.     

一种改进的MIMO OFDM系统信道估计算法

本文对MIMO OFDM系统中基于训练序列的信道估计问题进行了研究，针对信道冲击响应的最大抽头数大于每个OFDM符号中导频数的情况，提出一种有效的结合前后若干训练序列进行信道估计的算法和结合方式。仿真结果表明，在基于无线局域网（WLAN）中打包传送的MIMO OFDM系统里，本文的方法比采用块状训练序列的估计算法有着更小的归一化均方误差。     

大规模MIMO系统中低复杂度的稀疏信道估计

针对大规模多输入多输出（MIMO）系统信道估计算法复杂度高的缺陷,结合无线通信信道固有的稀疏性提出了一种低复杂度的稀疏信道估计算法。该算法是在传统的离散傅里叶变换（DFT）信道估计的基础上利用分离算法将信道抽头与噪声空间分离开来,使得信道估计时只需要计算信道抽头的部分,因此算法的计算复杂度被大大降低。仿真结果表明,该算法在保持低复杂度的同时,可获得接近最小均方误差（MMSE）性能。     

Robust frequency offset estimation for pilot symbol assisted packetCDMA with MIMO antenna systems

We propose frequency offset estimation and combining techniques for pilot symbol assisted (PSA) packet downlink code-division multiple access (CDMA) with multiple-input and multiple-output (MIMO) antenna systems. Orthogonal Walsh codes are used for dedicated pilot symbols at the transmit antennas. The discrete Fourier transform (DFT)-based frequency offset estimation is used for simple implementation. In addition, simple interpolation is also used for resolution improvement of the DFT. When identical frequency offset is assumed, the frequency offset estimates for each single-input single-output (SISO) sub-stream (i.e., transmit/receive antenna pair) can be combined at the receiver. Simulation results show that the estimation and combining techniques proposed deliver improvement in the frequency offset estimation performance     

Diversity combining with imperfect channel estimation

The optimal diversity-combining technique is investigated for a multipath Rayleigh fading channel with imperfect channel state information at the receiver. Applying minimum mean-square error channel estimation, 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 the 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 combined effect of channel estimation and detection on bit-error rate performance.     

非理想信道估计下迫零接收的空间相关多输入多输出空分复用系统的误码率分析

分析了非理想信道估计和发射空间相关信道下,采用迫零接收的多输入多输出空分复用无线通信系统的误码率性能.基于随机矩阵理论,导出了闭合形式的M-PSK和M-QAM误码率近似解析表达式,并通过仿真验证了表达式的有效性.所获得的表达式形式简单、便于数值计算,可以用于系统性能评估.     

非理想信道估计下的放大转发协同通信系统性能分析

针对两跳多中继放大转发协同通信系统,分析了信道估计误差对系统性能的影响。考虑机会中继和全参与中继两种中继策略,分别推导了系统误码率的闭式表达式,并分析了不同信道估计质量阶数下的系统分集增益。平均信噪比损失率分析表明机会中继较全参与中继对信道估计误差更为敏感。仿真结果验证了理论分析。     

Training-based and semiblind channel estimation for MIMO systems with maximum ratio transmission

This paper is a comparative study of training-based and semiblind multiple-input multiple-output (MIMO) flat-fading channel estimation schemes when the transmitter employs maximum ratio transmission (MRT). We present two competing schemes for estimating the transmit and receive beamforming vectors of the channel matrix: a training-based conventional least-squares estimation (CLSE) scheme and a closed-form semiblind (CFSB) scheme that employs training followed by information-bearing spectrally white data symbols. Employing matrix perturbation theory, we develop expressions for the mean-square error (MSE) in the beamforming vector, the average received signal-to-noise ratio (SNR) and the symbol error rate (SER) performance of both the semiblind and the conventional schemes. Finally, we describe a weighted linear combiner of the CFSB and CLSE estimates for additional improvement in performance. The analytical results are verified through Monte Carlo simulations.     

Efficient MIMO channel estimation for linear precoded OFDMA uplink systems

A novel and simple uplink MIMO channel estimation technique based on spread pilots well suited to LP OFDMA schemes is proposed. The combination with Alamouti code is studied and two different approaches are compared. The performance evaluated over a realistic MIMO channel shows the efficiency of this novel scheme, which appears to be a promising channel estimation for the uplink of future wideband wireless networks.     

A new family of optimized orthogonal Space-Times codes for PPM-based MIMO systems with imperfect channel estimates

In this contribution, we develop a single Multiple-Input Multiple Output (MIMO) transceiver for Orthogonal PPM (OPPM) data transmitted over (baseband) faded MIMO channels with a priori unknown path-gains. The signaling-scheme we adopt allows to equip the Maximum-Likelihood receiver with reliable estimates of the (possibly time-varying) MIMO channel, without reducing the conveyed information throughput. Hence, after evaluating the performance of the proposed transceiver via a suitable version of the Union-Chernoff Bound, we introduce a novel family of unitary orthogonal Space-Times Block Codes (e.g., the Space-Time OPPM codes), that are able to attain both maximum diversity and coding gains. Afterwards, we present closed-form formulas for evaluating the SNR loss induced by mistiming effects possibly impairing the received signals. Lastly, we report several numerical results supporting both the medium/long coverage ranges attained by the proposed transceiver in outdoor applications and its performance robustness against correlated channel fading, mistiming effects and degradation induced by dense-multipath fading. This work is partially supported by Italian National Project Women 2005093248     

Power-allocation policy and optimized design of multiple-antenna systems with imperfect channel estimation

We here focus on the optimized design and performance of Rayleigh-faded multiple-antenna multiple-input-multiple-output systems when both transmitter and receiver share imperfect (i.e., error impaired) channel estimates computed via training sequences. In particular, according to the emerging principle of the so-called synchronized detection, for this operating scenario we propose a simple water-filling (WF)-like strategy for allocating power over transmit antennas and test its actual performance. Afterward, we introduce and evaluate some related figures of merit summarizing the overall system's performance and use these last for noting effective system's design guidelines. In particular, some optimized solutions for power-versus-bandwidth efficiency tradeoff are presented and their validity limits are debated. All the developed results explicitly take into account the actual reliability of the achievable channel estimates.     

Optimum antenna configuration in MIMO systems: a differential evolution based approach

In this paper, we address the problem of determining the optimum antenna configuration for a multi‐input multi‐output (MIMO) system at any given signal‐to‐noise ratio (SNR). We used two‐level differential evolution (DE) algorithm that finds both an appropriate expression among a set of candidate expressions within the list of the optimization software used, and the parameter values (coefficients) belonging to the selected expression. The results of the proposed expression are compared with the results of high SNR approximation, asymptotic approach and optimum antenna number ratios. It is shown that the numerical outcomes produced by the new expression exhibit very good agreement with the optimum antenna number ratios, and this agreement is almost independent of the specific value of SNR. Copyright © 2010 John Wiley & Sons, Ltd.     

Optimal pilot sequence design for channel estimation in MIMO OFDM systems

In orthogonal frequency division multiplexing (OFDM) systems, since virtual subcarriers are not used for transmission, approach of conventional uniformly placed pilot tones is not applicable in some situations. In this letter, based on nonuniform pilot tone placement, we derive optimal pilot sequences, which can achieve the minimum mean square error of the least squares estimate for multiple-input multiple-output (MIMO) OFDM systems. Simulation results demonstrate the effectiveness of the proposed approach.