基于信道质量干扰比量化准则的有限反馈波束形成

波束形成技术是MIMO无线通信系统的关键技术之一,它能够使系统有效性与可靠性都得到显著地提高,但它需要系统所有在线用户反馈其全部瞬时变化的信道状态信息,这将使拥挤的无线频谱资源更加紧张.为了降低系统所需的反馈量,本文结合LTE系统MIMO下行链路环境,将采用DFT基码本对信道状态信息进行量化,仅需要用户给基站反馈其最优码本索引;同时本文也首次提出基于信道质量干扰比为量化准则的一种新波束形成方案,并给出了质量干扰比的详细计算公式.此量化准则能够兼顾到量化信道的质量信息和子信道之间的互干扰信息,比传统的量化准则更具优良的性能.仿真结果表明,本文提出方案不仅可以明显降低系统反馈量,而且其性能超过随机波束形成,尤其在低信噪比场景下甚至优于特征波束形成的系统性能.理论分析和仿真验证表明本文方案是一种比较好的波束形成方案.     

Channel Feedback Quantization for High Data Rate MIMO Systems

In this work, we study a multiple-input multiple-output (MIMO) wireless system where the channel state information is partially available at the transmitter through a feedback link. Based on singular value decomposition, the MIMO channel is split into independent sub-channels. Effective feedback of the required spatial channel information entails efficient quantization/encoding of a unitary matrix. We propose two schemes for quantizing unitary matrices via Givens rotations and examine the performance for a scenario where the rates allocated to the sub-channels are selected according to their corresponding gains. Numerical results show that the proposed schemes offer a significant performance improvement as compared to that of MIMO systems without feedback, with a negligible increase in the complexity     

Channel Magnifying Limited Feedback Technique for FDD Massive MIMO Systems

A limited feedback system, so-called, channel magnifying (CM) is proposed for a downlink (DL) frequency-division duplex (FDD) massive multiple-input multiple-output (MIMO) system. Although massive MIMO system has received significant research interest as a key technology for beyond 4G wireless communications systems, it has a number of issues that needs to be technically addressed. Among such issues is the difficulty of acquiring channel state information at transmitter for an FDD massive MIMO system which cannot exploit channel reciprocity as in a time-division duplex system. The proposed CM technique makes it possible to support a few user equipments in DL FDD massive MIMO system by finding a balance between spatial resources and channel quantization error (CQE). By choosing a subchannel with low CQE, CM can secure multiplexing gain at high SNR based on a fixed size codebook. Two types of subchannel indicator alignment (SIA) schemes are introduced for efficient interference nulling for the proposed CM technique. Specifically, we discuss how to maximize the sumrate of CM through genie added SIA and minimum CQE based SIA. Simulation results show that the sum rate of the proposed CM has a higher multiplexing gain than that of random vector quantization, especially when the number of transmit antennas is sufficiently large.     

有限反馈MISO-OFDM系统中最大化容量的波束成形

该文提出了一种在有限反馈条件下多输入单输出正交频分复用(MISO-OFDM)系统中基于最大化容量的波束成形方案。将OFDM符号的所有子载波分成若干簇,从码本中选择能够最大化该簇每个子载波平均容量的码字,作为该簇的波束成形向量。在低信噪比区域,求出了最优波束成形向量的闭合解;鉴于在高信噪比区域无法求出闭合解,基于簇内不同子载波信道频响之间的相关性提出了一种简化的次优算法以降低搜索最优解时的计算复杂度。仿真结果表明,在典型参数设置下,所提方案能够取得比其他已有方案更高的容量和更低的误码率。     

存在反馈周期和反馈时延的自适应调制MIMO系统

该文研究了反馈信道信息存在的反馈周期和时延对自适应调制MIMO系统的影响,分析了基于奇异值分解的等效并行衰落信道特性,并且给出了接收机的检测方法和一种简化的检测顺序。研究表明,等效并行衰落信道在一定的时间间隔内具有较大的相关性;接收机采用迫零和干扰消除方法可明显提高系统性能。     

基于有限反馈的MIMO系统自适应调制

该文利用信道的特征矢量,将MIMO高斯信道转化成混合信道,结合功率分配给出了自适应调制方案,使系统容量在发射功率和服务质量约束下达到最大。发射端所需的波束成形和功率分配等信息利用有限的比特反馈到发射端。仿真结果表明：在小信噪比的情况下,该自适应调制方案对系统容量有明显改善。     

Optimal Power Allocation with Limited Feedback of Channel State Information in Multi-User MIMO Systems

In this paper,an expression for the user’s achievable data rate in the multi-user multiple-input multiple-output(MU-MIMO)system with limited feedback(LF)of channel state information(CSI)is derived.The energy efficiency(EE)is optimized through power allocation under quality of service(QoS)constraints.Based on mathematical equivalence and Lagrange multiplier approach,an energy-efficient unequal power allocation(EEUPA)with LF of CSI scheme is proposed.The simulation results show that as the number of transmitting antennas increases,the EE also increases which is promising for the next generation wireless communication networks.Moreover,it can be seen that the QoS requirement has an effect on the EE of the system.Ultimately,the proposed EEUPA with LF of CSI algorithm performs better than the existing energy-efficient equal power allocation(EEEPA)with LF of CSI schemes.     

基于有限反馈的MIMO系统鲁棒性均匀信道分解

 在MIMO系统中,当假设发送端和接收端已知完美信道状态信息的情况下,均匀信道分解(UCD)被证明在误码率性能上是最优的且是容量无损的.然而在实际系统中,接收端或发送端获得的信道信息通常存在着一定的估计误差.本文提出一种基于有限反馈的鲁棒性均匀信道分解方案,在设计方案中充分考虑了信道估计误差的影响,并利用香农率失真定理和广义Lloyd矢量量化算法(GLA)得到了信道估计误差的近似值,使得本文提出的方案有很好的实用性.仿真结果表明与传统均匀信道分解方案相比,本文提出的鲁棒性均匀信道分解方案有效地降低了系统的误码平层,提高了系统容量.     

Efficient QR-Based Multi-Mode Precoding for Limited Feedback Multi-User MIMO Systems

This paper proposes QR-based criteria for efficient selection of multi-mode precoders in multi-user multiple-input multiple-output (MU-MIMO) precoded systems with a limited feedback. In particular, a MU-MIMO QR based precoding scheme is developed which takes advantage of the overall precoder design among users in conjunction with QR detection. A selection criterion is developed to identify the precoder that yields the maximum free distance. Efficient mode selection and tree search strategies are adopted to reduce the complexity associated with the identification of the optimum precoder. To further improve the detection performance in correlated channels, an efficient QR-based MMSE V-BLAST detector is proposed. Computer simulations confirm that the proposed QR-based precoding scheme attains the performance of existing schemes with a significantly lower complexity level.     

Adaptive MIMO Beamforming Algorithm Based on Gradient Search of the Channel Capacity in OFDM-SDMA Systems

This letter proposes an adaptive beamforming algorithm for uplink access in broadband wireless networks employing orthogonal frequency-division multiplexing with spacedivision multiple access (OFDM-SDMA) technologies. The proposed algorithm seeks, iteratively, the optimal transmit weight vectors that directly maximize the OFDM-SDMA channel capacity for each user in the system, using gradient search of the channel capacity. The analysis and simulation show that the capacity of OFDM-SDMA systems with the transmit weights obtained using the proposed adaptive beamforming algorithm is substantially higher than the one based on conventional approaches such as eigen-beamforming.     

Opportunistic Beamforming with Limited Feedback

This work investigates the following question: subject to strictly limited (finite-rate) feedback in a multi-user multi-antenna system, what channel state information (CSI) should we send back to the transmitter, and how should it be used? Considering the class of single-beam systems, we suggest a combination of beamforming (array gain) and multi-user diversity. It has been shown that in single antenna systems, one bit of feedback per user can capture almost all gains available due to multi-user diversity, therefore we propose and analyze a compound strategy that uses one bit for multi-user diversity and any further feedback bits for beamforming. We obtain the scaling laws of this compound strategy, showing that it scales as well as any single-beam system with full transmit-CSI.     

协作多基站多用户MIMO系统中有限反馈比特分配方案

For the cooperative multi-cell systems with multi-user MIMO, a new limited feedback bit allocation scheme is proposed to minimize the rate loss caused by quantization error. In the proposed scheme, the Channel State Information (CSI) feedback of cell-edge user for the local service cell and the adjacent interference cell are separately quantized. Based on the upper bound of the rate loss of cell-edge user due to the limited feedback, the number of feedback bits for quantized CSI of the local service cell and the adjacent cell are optimized with the fixed total bits of the limited feedback. The simulation shows that our proposed scheme of feedback bits allocation efficiently decreases the interference and increases the rate of systems compared with that of equal bits allocation and those of other allocations .     

Performance Evaluation of Adaptive MIMO-OFDM Systems with Limited Feedback Using Measurement Based Channel Models

Performance evaluation of enhanced link adaptation method for MIMO-OFDM systems with limited feedback using measurement based channels and a stochastic channel model is presented in this paper. In particular, impact of practical channel estimation and feedback errors on link performance is analyzed. An adaptive spatial and modulation scheme selection process is based on the effective signal-to-interference-plus-noise ratio (ESINR). Mutual information effective SINR mapping method is applied for calculating ESINR values due to its capability of accurate estimation of the error rate performance of large variety of MIMO channels. Numerical results show that simple adaptive systems that switch between diversity/multiplexing or beamforming/multiplexing schemes obtain relatively good performance in realistic 2 × 2 MIMO channels. It is also shown that the imperfect channel estimation and feedback errors can have significant impact on the link performance. Furthermore, it is noticed that using the stochastic channel model in performance simulations can give rather pessimistic results compared to true measurement data.     

Game-Theoretic Joint Power Allocation and Beamforming for Cognitive MIMO Systems with Finite Feedback

In the paper, we consider the imperfect channel state information (CSI) in practical cognitive MIMO systems. We first analyze the feedback of quantified CSI from the primary user (PU) and propose a joint power allocation and beamforming algorithm via game theory. Compared with the game under the condition of perfect CSI, new utility function and cost function are constructed under imperfect CSI. We analyze the error introduced from the uniformly quantified CSI and obtain new constraints. Besides, existence of the Nash equilibrium in case of both perfect CSI and imperfect CSI are proven. We propose a new iterative algorithm to reach the Nash equilibrium (NE). Simulation results show that the proposed algorithm can converge quickly.     

SLNR-Based Transmit Beamforming for MIMO Wiretap Channel

In this paper, a novel closed-form transmit beamforming scheme is proposed for multiple-input multiple-output (MIMO) wiretap channel. The proposed method follows the idea of maximizing signal-to-leakage-plus-noise ratio and does not need additional power allocation. An efficient approach is further developed to implement the generalized eigenvalue decomposition required by our method when n _r  < n _t (n _t , n _r and n _e denote the antenna numbers at the transmitter, the intended receiver and the eavesdropper, respectively), which has lower complexity than the existing approaches. Moreover, we identify some shortcomings of the generalized singular value decomposition (GSVD) based beamforming method, a common beamforming scheme for MIMO wiretap channel. Finally, simulation results show that the proposed method significantly outperforms GSVD-based method when n _e  < n _t and has almost the same performance as GSVD-based method when n _e  ≥ n _t .     

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.     

Transmission Scheme for 2D Antenna Array MIMO Systems with Limited Feedback

Three-dimensional (3D) multiple-input multiple-output (MIMO) systems exploit spatial richness and provide another degree of freedom to transmit signals and eliminate spatial interference. Currently, however, there is no 3D codebook for two-dimensional (2D) antenna array MIMO systems with limited feedback. In this paper, based on the existing 2D codebook, we present a limited feedback and transmission scheme for 2D antenna array MIMO systems. In this scheme, the mobile station (MS) has imperfect channel knowledge, and the base station (BS) only acquires partial information relating the channel instantiation. MS must feed back two channel state information (CSI) instances, i.e., the horizontal and vertical CSIs. After receiving the two CSI instances, the BS interpolates a new vertical precoding vector using the vertical CSI. Then, the BS re-constructs a 3D beamforming vector using horizontal and vertical precoding vectors and compensates the reported horizontal channel quality indicator. System level simulation is employed, and the simulation results show that the proposed method improves the system spectral efficiency and the cell-edge SE significantly.     

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。     

Index Assignment for Quantized Beamforming MIMO Systems

Index assignment (IA) technique is introduced to quantized beamforming systems. The feedback channel in these systems is modeled as a discrete memoryless channel (DMC), and the diversity and array gains with feedback errors are derived. Based on the analytical results, IA scheme is designed to provide a redundancy-free protection against feedback errors. Simulation results show that good IA scheme improves the array gain and symbol error rate (SER).     

存在信道估计误差的有限反馈多用户多输入多输出传输性能分析

该文针对存在信道估计误差的有限反馈多用户多输入多输出(MIMO)传输性能进行分析。基于量化微元逼近理论得出了多用户迫零波束赋型系统容量的下界;该下界表明：存在信道估计误差时,随着码本尺寸的增大,高信噪比条件下系统容量趋于一有限值;并且,信道估计误差越大,系统容量随码本尺寸收敛越早。进一步分析了采用多用户选择分集的系统容量性能,分析表明,当用户数量趋于无穷大时,和容量有界。该文结果和现有文献的结论不同,并经由仿真结果进行验证。