一种MIMO时变信道下的低反馈量有限反馈预编码方法

本文提出了一种新颖的适用于MIMO时变信道环境的有限反馈预编码方法.该方法利用子空间跟踪算法自适应跟踪时变信道的发射预编码矩阵,同时结合Jacobi迭代算法有效降低了有限反馈预编码的反馈量.通过计算机仿真,该方法在ARI时变信道模型,以及更实际的Jake's时变信道模型下的跟踪性能得到很好的验证.在更低的反馈量下,该方法可以获得比传统的存在反馈延时的Grassmannian预编码方法更好的系统容量性能,而且该方法在相同的反馈量下,无论是收敛性能还是误码率性能都明显优于将Jacobi迭代算法直接应用于时变信道跟踪的预编码方法.     

协作多点传输中一种基于特征子信道的干扰对齐预编码矩阵优化方案

由于普遍采用频率复用因子为1的方式组网,多小区干扰已经成为限制无线蜂窝系统容量的主要因素.协作多点传输技术可以有效抑制蜂窝系统中多小区干扰问题,提高小区边缘吞吐率.近年来,作为协作多点传输的一种实现形式,干扰对齐技术受到广泛关注,其优势在于可以获得多小区系统最优的发送自由度.本文提出了一种在干扰对齐技术下基于特征子信道的预编码矩阵优化方案.本方案通过选取与最好的特征子信道对准的预编码矢量发送信号,使得信号经历最好的一组特征子信道,改善接收端的信号强度,从而达到提高系统吞吐量的目的.仿真结果表明,相比已有的预编码矩阵的设计方案,本文提出的方案在系统的吞吐量方面有明显的提升.     

MIMO多中继系统中基于不完全信道状态信息的预编码算法研究

针对配置多个中继节点的多输入多输出（Multiple-input Multiple-Output,MIMO）系统,提出一种不完全信道状态信息（Imperfect channel state information,CSI）下基于最小均方误差（Minimum Mean-Squared Error,MMSE）准则的预编码设计方案,该方案综合考虑各个节点天线之间的相关性及信道估计误差。由于各个中继端预编码矩阵之间可视为相互独立,故将约束优化问题转化为若干半正定规划问题,继而通过内点法或CVX工具箱求解预编码矩阵,然后采用求导法推导出接收端处理矩阵。最后利用迭代算法交替迭代来联合优化预编码矩阵和接收端处理矩阵,直到算法收敛。仿真结果表明,与未能综合考虑各个节点的预编码算法相比,本文提出的算法能明显改善系统BER性能。      

一种适用于协作多点联合传输的自适应PMI反馈方法

作为下一代无线通信系统的关键技术,协作多点传输能够有效降低相邻小区之间的干扰,提高小区边缘用户的频谱效率。在频分双工系统中,其性能依赖于各基站获取的基于有线反馈的信道状态信息的准确性。然而,所有协作基站都获得相同精确程度的信道状态信息会导致用户终端的反馈开销随着协作基站数线性增长。本文关注于如何在性能增益和反馈开销之间取得折中,提出一种适用于协作多点联合传输的自适应预编码矩阵索引反馈方法。用户终端根据不同协作基站下行链路的信道质量指示的相对大小,自适应的用不同的比特数表示不同强度链路的预编码矩阵索引,即用较多比特数表示具有较高信道质量指示的预编码矩阵索引,反之亦然。仿真结果表明,与传统反馈方案相比,该方法在几乎不牺牲频谱效率的前提下,大幅降低了反馈开销。      

一种空间相关MIMO信道下的空时预编码方案

在实际的多输入多输出（MIMO：Muhiple Input Multiple Output）移动通信系统中，信道的空间相关性通常会使得现有的空时编码方案难以获得最佳的误码性能。为了解决该问题，本文提出了一种适用于MIMO空间相关信道的空时预编码方案。该方案在完成对发射信号的空时编码后，针对发射天线的空间相关矩阵设计了合理的预编码矩阵，进而能够在保证分集增益的前提下克服相关信道的不利影响。仿真结果证实，该方案能够在一定程度上有效提高整个系统的误码性能。     

基于格规约辅助的MIMO系统GMD-TH预编码方案

现有的GMD-TH(Geometric Mean Decomposition-Tomlison Harashima)预编码方案在发射端未对获得MIMO(Multiple-Input-Multiple-Output)信道增益矩阵优化,因而其误码率和分集增益无法获得令人满意的效果.为此,在原有MIMO系统GMD-TH预编码的基础上,提出梁种基于格规约辅助的GMD-TH预编码方案.该方案采用基于格规约的算法对信道矩阵进行优化,经过优化的信道矩阵其列向量之间具有更好的正交性并且向量的长度更短,并且采用优化的信道矩阵提高了GMD-TH预编码MIMO系统的分集增益.仿真结果表明:相比于传统的线性预编码方案,该预编码方案有效地提高了MIMO分集增益,相同误码率下,信噪比降低3dB以上,具有实用价值.     

有限反馈MIMO系统中一种基于双码本的功率分配与预编码方案

研究表明，有限反馈预编码能有效利用信道信息提高MIMO系统频谱效率，在此基础上，在发射端进行功率控制将使频谱效率得到进一步提升．为了最大化有限反馈MIMO系统的频谱效率，本文提出了一种基于自适应调制的功率分配与预编码方案．为了减少反馈量，该方案通过双码本方式来反馈预编码矩阵和功率控制矢量，并给出了一种基于K-均值聚类的功率码本设计方法．仿真结果表明，该方案的反馈信息量少，且获得了较好频谱性能。     

多用户MIMO系统在有限反馈下结合预编码与调度算法的研究

捅要：研究了多用户多输入多输出（MIMO）系统在有限反馈下的一种结合单位预编码与用户调度算法的方案。在该系统中每一用户都具有多个接收天线,该方案具有较大的多用户分集增益和较小的计算复杂度,从而能够减少系统中的用户间干扰。每个用户独立地决定自己的天线合并矢量,并将最优波束矢量以及对应的估计信干噪比通过有限反馈信道反馈给基站,此时基站获取了相应的用户信道信息和用户间干扰信息。基于接收到的反馈信息,基站从预定义的码本中选取和速率最大的最优子集进行系统预编码,然后依照预编码矩阵调度欧氏距离最小的用户且用户个数不超过发送天线的数目。仿真结果显示该方案有效地改善了系统速率,特别是在用户数目较多或者信道环境较好的情况下。     

一种用于MlMO系统的双码本预编码方法

时分复用(TDD)模式的通信系统可以通过有效利用信道的互易性,即通过上行链路对下行信道信息进行估计,并利用估计得到的信息对MIMO通信系统进行预处理或者预编码,从而进一步提升MIMO系统的信道容量和性能。然而,对于频分复用(FDD)的通信系统,由于上下行频段的不对称使得信道互易性原则不再适用,此时欲对信道进行预处理,则必须通过终端对下行信道信息进行上行反馈。反馈方式可分为直接对下行信道信息进行量化反馈的显式反馈和将量化后的信道空间以码本形式进行反馈的隐式反馈。对信道空间量化的隐式反馈进行研究,并在此基础上提出了一种双码本预编码的方法,并进一步对所提方案进行了理论分析和仿真验证。     

一种基于子空间跟踪的认知MIMO预编码算法

针对多用户认知MIMO下行系统,提出一种适用于慢衰落信道环境的预编码算法。该算法基于子空间跟踪思想,利用信道时域相关性,通过递推方式自适应更新预编码矩阵来跟踪和适应慢衰落信道环境的变化。分别从算法收敛速度、收敛精度、跟踪性能以及计算复杂度四个方面进行了理论推导和仿真分析。结果表明,所提算法可以有效的跟踪信道环境的动态变化,与传统算法相比,在保证系统性能的同时显著降低了处理复杂度。     

Precoder Partitioning in Closed-loop MIMO Systems

We study unitary precoding for multistream MIMO systems with partial channel state information at the transmitter. We introduce a quantization scheme in which the full space of non-equivalent precoding matrices is partitioned into Grassmannian and orthogonalization parts. The Grassmannian part is used for maximizing the power after precoding and the orthogonalization part is used for removing cross talk between the data streams. We show that orthogonalization improves the attainable capacity when the receiver is linear. We give a parametrization for the non-equivalent orhogonalization matrices and a metric which measures the orthogonality of the transmission. Optimal orthogonalization codebooks for two-stream transmission are presented. When feedback is limited, the optimal partitioning of feedback bits between Grassmannian and orthogonalization parts becomes an issue. In correlated scenarios, the number of feedback bits may be significantly reduced by investing bits into the orthogonalization part.     

Limited feedback unitary precoding for orthogonal space-time block codes

Orthogonal space-time block codes (OSTBCs) are a class of easily decoded space-time codes that achieve full diversity order in Rayleigh fading channels. OSTBCs exist only for certain numbers of transmit antennas and do not provide array gain like diversity techniques that exploit transmit channel information. When channel state information is available at the transmitter, though, precoding the space-time codeword can be used to support different numbers of transmit antennas and to improve array gain. Unfortunately, transmitters in many wireless systems have no knowledge about current channel conditions. This motivates limited feedback precoding methods such as channel quantization or antenna subset selection. This paper investigates a limited feedback approach that uses a codebook of precoding matrices known a priori to both the transmitter and receiver. The receiver chooses a matrix from the codebook based on current channel conditions and conveys the optimal codebook matrix to the transmitter over an error-free, zero-delay feedback channel. A criterion for choosing the optimal precoding matrix in the codebook is proposed that relates directly to minimizing the probability of symbol error of the precoded system. Low average distortion codebooks are derived based on the optimal codeword selection criterion. The resulting design is found to relate to the famous applied mathematics problem of subspace packing in the Grassmann manifold. Codebooks designed by this method are proven to provide full diversity order in Rayleigh fading channels. Monte Carlo simulations show that limited feedback precoding performs better than antenna subset selection.     

Antenna selection and power control in MIMO systems with continuously varying channels

Existing limited feedback methods for Multiple- Input Multiple-Output (MIMO) systems focus largely on block fading conditions whereas, in practice, channels change continuously over time. In this paper, we investigate adaptive transmission over a continuously varying channel (CVC). A combination of Kalman channel tracking, truncated channel inversion (TCI) antenna selection, and power control, is used to achieve bit error rates that are lower than those obtained with existing schemes. Using Kalman filtering for pilot-based channel tracking as well as decision-directed channel tracking, our scheme achieves about 3.6dB gain over the eigen-mode antenna selection scheme and 1.5dB gain over the unitary precoding scheme, when operating at a BER of 10?5.     

Throughput-Optimal Precoding and Rate Allocation for MISO Systems With Noisy Feedback Channels

A throughput metric is considered for a multiple-input single-output (MISO) system with noisy feedback of channel state information (CSI). The goal is to optimize a precoding matrix with a medium-access control layer metric. The problem is a nonlinear multidimensional optimization. Results show that the optimal precoding turns into beamforming when the signal-to-noise ratio (SNR) of CSI feedback is sufficiently large. A necessary condition for the optimality of beamforming under the throughput metric is determined, and the necessary and sufficient condition is numerically found based on the Gauss-Chebyshev Quadrature method. Next, the rate allocation for beamforming and spatial diversity is analyzed. Then, a two-mode transmission scheme is proposed such that the transmitter is engaged in either the beamforming mode or the spatial diversity mode depending on the SNR of the CSI feedback. It is shown that at a fairly high SNR of CSI feedback, the rate allocation needs to be reduced, while at a low SNR of CSI feedback, the allocated rate should be increased. It is shown that when the SNR of CSI feedback is lower than a threshold, there always exists an SNR of the transmitted signal such that the CSI feedback can be viewed as the real CSI solely for the purpose of rate allocation. The result also shows that the throughput of two-mode transmission is almost the same as the throughput of the optimal precoding scheme, even with a low SNR and large feedback delay.     

Adaptive bit allocation scheme in multi-cell cooperative block diagonalization systems with delayed and limited feedback

In multi-cell cooperative multi-input multi-output(MIMO) systems,base station(BS) can exchange and utilize channel state information(CSI) of adjacent cell users to manage co-channel interference.Users quantize the CSIs of desired channel and interference channels using finite-rate feedback links,then BS can generate cooperative block diagonalization(BD) precoding matrices using the obtained quantized CSI at transmitter to supress co-channel interference.In this paper,a novel adaptive bit allocation scheme is proposed to minimize the rate loss due to imperfect CSI.We derive the closed-form expression of rate loss caused by both channel delay and limited feedback.Based on the derived rate loss expression,the proposed scheme can adaptively allocate more bits to quantize the better channels with smaller delays and fewer bits to worse channels with larger delays.Simulation results show that the proposed scheme yields higher performance than other allocation schemes.     

一种前向放大中继通信系统中的有限反馈中继预编码方案

 本文首先给出了前向放大中继通信系统中,在给定QoS要求下的最优中继预编码矩阵的设计. 进而,针对实际通信系统中反馈信道带宽的限制,给出了一种有限反馈中继预编码码本的设计方案. 计算机仿真表明,本文所提新码本有限反馈中继预编码方案的系统性能随着反馈比特数的增加接近完全反馈下的系统性能,且在相同反馈比特数下的系统容量和误码率均明显优于将传统MIMO通信系统中的Grassmannian码本或随机码本直接应用于中继预编码的有限反馈方案.     

基于码本的有限反馈非酉矩阵预编码多用户MIMO系统

该文提出了一种基于码本的有限反馈非酉矩阵预编码下行多用户MIMO系统。该方案根据用户反馈的信道信息SINR在发送端进行调度和预编码来提高系统容量。预编码的码本设计依据格拉斯曼空间装箱原理,并将码本中的向量按其相关性构成非酉矩阵来提高预编码增益和抑制多用户共道干扰。新方案反馈量少、复杂度低,在相同情况下比传统的单用户MIMO系统和基于码本的酉矩阵预编码多用户MIMO系统都具有更好的性能。     

Precoding for Orthogonal Space–Time Block-Coded OFDM Downlink: Mean or Covariance Feedback?

This paper presents linear and nonlinear precoding design for error-rate improvement in orthogonal space–time block-coded (OSTBC) multiple-input–multiple-output (MIMO) orthogonal frequency-division-multiplexed (OFDM) downlink, where both the conditional mean of the channel gain matrix and the channel gain covariance matrix may be available at the transmitter. The conditional means of the channel matrix are derived for a general transmit-antenna-correlated frequency-selective fading MIMO channel with estimation errors and feedback delay. Mean-feedback linear precoding and nonlinear Tomlinson–Harashima precoding (THP) are developed to maximize the signal-to-noise power ratio (SNR). The intuition that when the mean feedback becomes accurate the mean-feedback precoding outperforms covariance precoding is confirmed. Dual-mode precoding is also proposed, in which the novel mean-feedback precoding or covariance precoding is adaptively chosen at the receiver. The precoding-mode switching metric is the maximized SNR, which is an indicator of the error rate. The receiver calculates its metric, selects the mode that achieves a higher SNR, and decides whether mean feedback is necessary. Our proposed precoders (both mean feedback and adaptive) significantly reduce the system error rate. Nonlinear precoding is shown to outperform linear precoding. Adaptive precoding outperforms both mean-feedback precoding and covariance precoding if individually applied in OSTBC OFDM.      

基于协方差反馈和空时分组码预编码的自适应QAM调制

基于多天线相关信道协方差反馈,提出一种低复杂度自适应正交幅度调制(QAM)机制以提高系统发送速率。根据最小误比特率(BER)准则,建立了最优空时分组码(STBC)预编码方案。并在保证系统可靠性要求下,针对不同的信道衰落条件选取合适的QAM星座图级别。结果表明所建议机制具有低实现复杂度、低反馈带宽开销和高数据流通率等优点。     

Design of Multiuser Downlink Linear MIMO Precoding Systems With Quantized Feedback

We treat the problem of sum-rate maximization via scheduling and linear precoding in multiuser (MU) multiple-input–multiple-output (MIMO) systems with quantized feedback. We formulate the optimal quantized linear precoder design problem and provide the numerical procedure for finding the solution. We also provide a simple scheduling scheme that exploits the MU diversity gain. With the aim of sum-rate maximization, we further address the quantization codebook design based on the capacity measure by introducing a new distance metric. It is demonstrated that, with or without scheduling, the proposed optimal linear precoding scheme achieves significant gain over the conventional linear precoder designs with similar feedback overhead, such as the zero-forcing precoder. Moreover, although the proposed quantization codebook design improves the performance of other existing MU-MIMO precoding schemes, the performance gain offered by it is typically much higher when it is coupled with the proposed linear precoding design.