频率选择性快衰落信道中的MIMO—OFDM系统检测算法研究

当MIMO—OFDM（Multiple Input Multiple Output-Orthogonal Frequency Division Multiplexing）系统工作于频率选择性快衰落信道时，子载波正交性会受到破坏从而引入子载波间干扰（Inter-Cartier Interference：ICI）．ICI的存在将严重降低那些传统的用于检测准静止频率选择性衰落信道下MIMO—OFDM的检测算法的性能．本文将Schniter针对SISO（Sinsle Inputsingle Output）OFDM系统提出的最优线性预处理扩展到MIMO—OFDM系统，基于这个信号模型推广了基于最小均方误差滤波的迭代软判决干扰抵消（Minimum Mean Square Errorfiltering based Iterative Soft Decision Interference Cancellation：MMSE—ISDIC）逐符号检测算法，同时提出一种基于准最大后验概率准则的迭代软判决干扰抵消（quasi—maximum A postefiofi probability based ISDIC：quasi—MAP-ISDIC）联合检测算法．仿真结果表明在本文考虑的系统参数设定下这两种检测算法的性能均优于文献[8]中算法的性能，其中quasi．MAP-ISDIC检测算法能够获得接近基于理想ICI抵消的MAP检测算法的性能。     

基于训练序列的发射分集OFDM系统的空时子空间幅度跟踪信道估计

该文将基于训练序列的发射分集正交频分复用(OFDM)系统视为一个多时隙系统,利用空时信道空间(波达方向)子空间和时延子空间慢变的特点,将空时信道多时隙估计技术应用到发射分集OFDM系统中,并根据衰落幅度快变的特点提出了使用自适应滤波技术的空时子空间幅度跟踪信道估计算法,明显提高了系统的信道估计精度。该文对各种算法的跟踪性能进行了仿真和分析比较。     

一种新OFDM系统及其信道盲估计性能的研究

提出了一种采用不等长分组新的OFDM系统。采用不等长分组的好处是不需要加循环前缀，就能在发射序列中引入周期平稳性，因此采用子空间算法可实现信道的盲辨识。还对信道盲估计性能进行了分析。     

无线OFDM系统频率选择性衰落信道下的SOVA译码算法

本文对正交频分复用(OFDM)系统频率选择性衰落信道中,信道状态信息(channel state information 简称CSI)确知,以及CSI不能或不必精确估计时的SOVA译码算法进行了研究,给出了OFDM系统频率选择性信道BPSK调制时SOVA译码算法的数学描述.在此基础上,我们提出了两种SOVA译码算法:一种是基于盲信道估计的SOVA算法;另一种是不必训练和信道估计的差分SOVA算法,该算法可以在CSI完全未知的情况下进行可靠的SOVA译码.仿真结果说明该算法具有良好的性能.     

一种针对双衰落信道OFDM的新均衡算法

在OFDM系统中，子载波间的正交性是保证OFDM性能的重要保障。针对双选择性衰落信道下的OFDM系统，该文在分析载波间干扰（ICI）的基础上，提出了一种采用频域迭代消除ICI的均衡算法。分析和仿真结果表明此方法能有效地保证载波间的正交性和改善了OFDM系统的误码率（BER）性能。     

OFDM信道估计中基于随机导频的OMP算法研究

信道估计是正交频分复用（OFDM）系统中非常重要的一个环节,准确的信道状态信息是数据可靠接收的重要保证.文中具体采用基于随机导频的正交匹配追踪算法（OMP）对OFDM系统进行信道估计,通过与最小二乘（LS）算法性能对比,验证了OMP算法可以使用更少的导频获得较好的估计性能.并仿真分析了OMP算法对先验信道稀疏度的依赖程度,为实际应用提供了一定参考意义.     

一种新的自适应OFDM算法的探讨

在具有多径干扰的无线信道中仅应用传统OFDM既影响通信速率,也浪费资源.由于信道的频率选择性衰落,OFDM系统的每个子信道具有不同的传输能力.文中提出一种计算量较小、简单实用的自适应OFDM算法,在保证通信质量所需BER的前提下,按照各个子信道衰落程度动态的分配比特和功率,使所需要的发射总功率最小.仿真结果表明,采用该自适应OFDM算法可以提高OFDM系统的性能.     

一种衰落信道下OFDM信号的半盲检测算法

本文提出了一种衰落信道下OFDM信号的半盲检测算法。采用文献[1]为时变OFDM信道建立的2维非线性递归模型,本文利用稀疏的导频符号估计信道响应,并运用最短路径搜索原理寻找原始的发送序列。计算机仿真结果证明,该算法对OFDM信号进行了快速、准确的半盲检测。     

基于梳状导频的OFDM分组自适应比特装载与功率控制

无线通信环境中信道信息(ChannelStateInformation,CSI)随时间变化明显,需要不断地更新自适应比特装载和功率控制的策略。现有的OFDM自适应比特装载方法需要大量的信令来传递当前比特装载的策略,不适用于无线通信的应用环境。为此,本文针对OFDM系统的特征提出了一种利用插入梳状导频的信道估计得到的带有误差的信道信息,来完成自适应比特装载的算法。首先把OFDM子载波进行分组,然后利用基于梳状导频的信道估计对信道时变进行跟踪,使用即时获得的CS完成自适应比特装载算法。仿真实验证明,在保证算法简单、实用及保证要求的误码率(BER)性能的情况下,信道信令的开销低于传统算法的10%。     

有限比率反馈对OFDM系统性能的影响

讨论了多发射天线OFDM系统中有限比率反馈对通信系统误码率的影响。通过发射端的波束成形将多天线OFDM无线信道等价为独立的并行子信道，并结合有限比率反馈下的波束成形矢量集的最优设计，推导了多发射天线下OFDM系统中反馈比特数对误码率影响的表达式，同时给出了反馈比特数对误码率的相对误差的影响。数值仿真表明,当发射天线数为2且10dB≤Es/N0≤20dB时，为使误码率的相对误差小于20%，每一子信道需3～5的反馈比特数，当发射天线数为3时则需7～8的反馈比特数,可为带反馈信道的多天线OFDM系统设计提供参考。     

Limited Feedback Beamforming Over Temporally-Correlated Channels

Feedback of quantized channel state information (CSI), called limited feedback, enables transmit beamforming in multiple-input-multiple-output (MIMO) wireless systems with a small amount of overhead. Due to its efficiency, beamforming with limited feedback has been adopted in several wireless communication standards. Prior work on limited feedback commonly adopts the block fading channel model where temporal correlation in wireless channels is neglected. In this paper, we consider temporally correlated channels and design single-user transmit beamforming with limited feedback. Analytical results concerning CSI feedback are derived by modeling quantized CSI as a first-order finite-state Markov chain. These results include the information rate of the CSI quantizer output, the bit rate a CSI feedback channel is required to support, and the effect of feedback delay on throughput. In particular, based on the theory of Markov chain convergence rate, feedback delay is proved to reduce the throughput gain due to CSI feedback at least exponentially. Furthermore, an algorithm is proposed for CSI feedback compression in time. Combining the results in this work leads to a new method for designing limited feedback beamforming as demonstrated by a design example.     

无线多跳网络下基于过时信道状态信息的跨层资源分配

对于无线多跳网络跨层资源分配算法的研究大多是建立在假定每个节点能获得网络中其他节点的完美的信道状态信息(CSI)的基础上。但是由于信道的时变特性和CSI的反馈延时,在动态变化较快的无线网络中,节点所获得的CSI很可能是过时或者部分过时的。基于这个前提,该文首次在动态无线多跳网络跨层资源优化分配算法中考虑了CSI这种变化的影响,并提出了一种相应的分布式联合拥塞控制和功率分配算法。仿真结果证明该算法能够极大地提高网络效用和能量效用。     

Optimal transmission strategies and impact of correlation in multiantenna systems with different types of channel state information

We study the optimal transmission strategy of a multiple-input single-output (MISO) wireless communication link. The receiver has perfect channel state information (CSI), while the transmitter has different types of CSI, i.e., either perfect CSI, or no CSI, or long-term knowledge of the channel covariance matrix. For the case in which the transmitter knows the channel covariance matrix, it was recently shown that the optimal eigenvectors of the transmit covariance matrix correspond with the eigenvectors of the channel covariance matrix. However, the optimal eigenvalues are difficult to compute. We derive a characterization of the optimum power allocation. Furthermore, we apply this result to provide an efficient algorithm which computes the optimum power allocation. In addition to this, we analyze the impact of correlation on the ergodic capacity of the MISO system with different CSI schemes. At first, we justify the belief that equal power allocation is optimal if the transmitter is uninformed and the transmit antennas are correlated. Next, we show that the ergodic capacity with perfect CSI and without CSI at the transmitter is Schur-concave, i.e., the more correlated the transmit antennas are, the less capacity is achievable. In addition, we show that the ergodic capacity with covariance knowledge at the transmitter is Schur-convex with respect to the correlation properties. These results completely characterize the impact of correlation on the ergodic capacity in MISO systems. Furthermore, the capacity loss or gain due to correlation is quantified. For no CSI and perfect CSI at the transmitter, the capacity loss due to correlation is bounded by some small constant, whereas the capacity gain due to correlation grows unbounded with the number of transmit antennas in the case in which transmitter knows the channel covariance matrix. Finally, we illustrate all theoretical results by numerical simulations.     

Adaptive Signaling Based on Statistical Characterizations of Outdated Feedback in Wireless Communications

Wireless links form a critical component of communication systems that aim to provide ubiquitous access to information. However, the time-varying characteristics (or "state") of wireless channels caused by the mobility of transmitters, receivers, and objects in the environment make it difficult to achieve reliable communication. Adaptive signaling exploits any channel state information (CSI) available at the transmitter to provide the potential to significantly increase the throughput of wireless links and/or greatly reduce the receiver complexity. As such, adaptive signaling has attracted significant research interest in the last decade and has found application in numerous commercial wireless systems, ranging from cellular data systems to wireless local area networks (WLANs). However, one of the great challenges of wireless communications is that it is difficult to obtain perfect CSI due to the inherently noisy and outdated nature of CSI available at the transmitter. Over the last decade, we have championed the idea of choosing the appropriate transmitted signal based on statistical models for the current channel state conditioned on the channel measurements. In this semi-tutorial paper, we first review how this class of methods has been developed for single-antenna systems, and then present novel recent designs for multiple-antenna systems. Key to the development in each case is the development of the error characterization given the outdated estimates and the use of such to allocate data rate and power over time and possibly space. In general, the focus is on rate allocation, while power allocation is done through a pruning method. Numerical results will demonstrate in both the single-antenna and multiple-antenna cases that such an approach provides a robust method for improving system data rate versus the standard practice of employing link margin to compensate for such uncertainties.     

Robust transmit eigen beamforming based on imperfect channel state information

Transmit beamforming is a powerful technique for enhancing the performance and increasing the throughput of wireless communication systems that employ multiple antennas at the transmitter. A major drawback of most existing transmit beamforming techniques is that they require nearly perfect knowledge of the channel at the transmitter, which is typically not available in practice. Transmitter designs that address the imperfect channel state information (CSI) problem commonly use statistical models for the channel and/or mismatch between the presumed and actual transmitter CSI. Since these approaches are model based, they can suffer from mismodeling. In this paper, a more robust framework is proposed in which no statistical assumptions are made about the CSI mismatch or the channel. The goal is to design a transmitter that has the best performance under the worst-case CSI mismatch. The transmitter designed herein achieves this goal for all CSI mismatches below a certain threshold level. The proposed design combines beamforming along the eigenvectors of the (deterministic) autocorrelation of the channel matrix perceived by the transmitter and power loading across those beams. While the power-loading algorithm resembles conventional water-filling to some degree, it explicitly incorporates robustness to the CSI mismatch, and the water level can be determined in a simple systematic way.     

Fading Channel Prediction for Mobile Radio Adaptive Transmission Systems

Adaptive transmission methods can potentially aid the achievement of high data rates required for mobile radio multimedia services. To realize this potential, the transmitter needs accurate channel state information (CSI) for the upcoming transmission frame. In most mobile radio systems, the CSI is estimated at the receiver and fed back to the transmitter. However, unless the mobile speed is very low, the estimated CSI cannot be used directly to select the parameters of adaptive transmission systems, since it quickly becomes outdated due to the rapid channel variation caused by multipath fading. To enable adaptive transmission for mobile radio systems, prediction of future fading channel samples is required. Several fundamental issues arise in the design and testing of fading prediction algorithms for adaptive transmission systems. These include complexity, robustness, choice of an appropriate channel model for algorithm validation, channel estimation and noise reduction required for reliable prediction, and design and analysis of adaptive transmission methods aided by fading prediction algorithms. We use these criteria in the review of recent advances in the area of fading channel prediction. We also demonstrate that reliable fading prediction makes adaptive transmission feasible in diverse wireless communication systems.     

基于LDPC码和BPPM的无线光通信系统性能研究

为了克服光在弱湍流大气条件传输时,光强闪烁造成的突发错误,在已知信道衰落信息(CSI)和未知信道衰落信息(NCSI)条件下,研究了基于低密度奇偶校验(LDPC)码和二进制脉冲位置调制(BPPM)的无线光通信系统性能,论述了BPPM光通信信道对称性的特点,并与基于LDPC码和OOK调制的光通信系统性能进行了比较。结果表明,在NCSI和CSI情况下,基于LDPC码和BPPM的光通信系统相对于基于LDPC码和OOK调制的系统均有更优异的差错性能;在NCSI情况下,基于LDPC码和BPPM的光通信系统的性能相对于CSI情况下的系统性能损失并不大;且采用LDPC码BPPM相对于LDPC码OOK调制的光通信系统,随着湍流强度的增大系统性能损失较小。因此,基于LDPC码和BP-PM的光通信系统在不需要信道衰落信息估计的情况下,能获得较大的编码增益,便于工程实现,在无线光通信中将有一定的应用前景。     

Exploiting Multiuser Diversity With Imperfect One-Bit Channel State Feedback

It has been well recognized that significant throughput gains can be leveraged in multiuser wireless communication systems by exploiting multiuser diversity with a smart scheduler. This scheduler collects channel state information (CSI) from all users and allocates the resources to the user(s) experiencing favorable channel conditions. However, for a frequency-division-duplex system with a large number of users, how to efficiently collect the required CSI will be a challenging task, especially when the feedback links are of limited capacity. In this paper, we propose a scheduling algorithm to exploit multiuser diversity with possibly imperfect one-bit channel state feedback. The basic idea is to define a threshold lambda and let each user report one-bit information to the scheduler about the comparison between its measured channel fading level and lambda. Correspondingly, the scheduler uses these feedback bits to classify all users into two sets and assigns the channel to one user belonging to the set experiencing favorable channel conditions. Several implementation schemes are developed by attacking the optimization of lambda under different system configurations, covering both the case when the one-bit feedback is perfect and those when the one-bit feedback is imperfect. Computer simulations show that when the user number is large, say, more than ten users, the proposed scheduling supports significantly larger data rate over the round-robin scheduling, while in comparison with the optimum scheduling with complete CSI, the performance loss is limited if the one-bit feedback is of high reliability. In addition, our studies show that we can effectively enhance the robustness against feedback imperfectness by incorporating the feedback reliability into optimization of lambda     

An Efficient CSI Feedback Scheme for MIMO-OFDM Wireless Systems

For antenna-array-based multiple-input multiple- output orthogonal-frequency-division-multiplexing (MIMO-OFDM) wireless systems, gain in channel throughput reduced through sufficient feedback of the channel state information (CSI) is significant, particularly when the number of transmit antennas is larger than the number of receive antennas. In this letter, we demonstrate that, in such scenarios, (1) the CSI of each OFDM sub-carrier can be parameterized into a short bit stream by a proposed low-complexity QR decomposition on the corresponding MIMO channel matrix, (2) the overall CSI can be reliably represented by a proposed parameter interpolation on the above bit streams of only a fraction of sub-carriers, and (3) a MIMO-OFDM system with a low-rate CSI feedback parameterized above can provide a channel throughput comparable to the channel capacity.