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 .     

基于动态反馈与功率分配的干扰对齐方法

在多用户MIMO系统中,信道状态的精确性严重影响干扰对齐技术的性能。该文针对信道状态信息(CSI)有限反馈导致的干扰泄露问题,提出一种基于动态反馈与功率分配的干扰对齐方法。首先从理论上分析了系统和速率与信道状态反馈比特分配和功率分配之间的关系,得到了在动态反馈和功率分配条件下系统和速率的解析表达式;在此基础上,以系统容量提升为目标,对反馈比特分配和功率分配优化问题进行建模;并根据信道的准静态特性利用库恩塔克条件(KKT)对该问题进行求解,得到功率和反馈比特分配方案。仿真结果表明,与单独考虑信道状态动态反馈条件下的干扰对齐技术相比,提出的方法能够有效减少干扰泄漏强度,提高系统和速率。     

MU-MISO Transmission with Limited Feedback

Downlink multi-user transmission techniques that allow several users to be served simultaneously in frequency and time have been recognized as a promising means to increase system capacity. However they require accurate channel state information (CSI) at the base station (BS) such that appropriate signal processing can be performed to separate multiple users in the space domain. In practice however, CSI cannot be perfectly known to the BS due to the capacity-limited feedback channel in FDD systems for instance. In this letter we investigate multiuser multiple-input single-output (MU-MISO) transmission with limited feedback. In particular we identify the useful information that the BS needs to deal with inter-user interference and a codebook design is derived accordingly. In addition, an adaptive threshold-based feedback approach is proposed, where only users with good feedback quality are allowed to do feedback. It is shown that the adaptive scheme can effectively improve the system performance given a fixed number of feedback bits.     

协同分布式天线系统中低复杂度天线单元选择与自适应比特分配算法

For multi-user cooperative Distributed MIMO (D-MIMO) systems, a low-complexity Remote Radio Unit (RRU) selection and adaptive bit partition algo-rithm is proposed to maximize the transmission Signal-to-Interference-Noise Ratio (SINR). Considering limited feedback, each user can adaptively select an RRU clus-ter to maintain the best communication quality. Under this condition, only one codebook is utilized for quanti-zing the Channel State Information (CSI) with variable dimensions, which effectively reduces the codebook storage amount. Furthermore, we propose an adaptive bit partition algorithm, which separately allocates bits to quantize the desired channels and interference channels. The optimal solution is achieved through an optimiza-tion theory to minimize the effect of inter-cell interfer-ence. Simulation results show that the proposed algo-rithm substantially improves the performance compared to other non-adaptive schemes.     

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。      

Performance analysis of adaptive limited feedback coordinated zero‐forcing multicell systems in time‐varying channels: A practical approach

In this paper, coordinated limited feedback zero‐forcing beamforming multicell system in time‐varying channel is studied. In time‐varying channels, the actual rate loss arising from error in channel quantization of both interuser and intercell inferences is quantified in this work. Using the actual degradation of rate loss, a limited feedback bit allocation is proposed to reduce interuser and intercell interference simultaneously with feedback update period as one of the parameters. Furthermore, the scaling law of bits required to maintain a constant rate loss is formulated in varying channel conditions for a given feedback update period. Simulation results demonstrated the practical feedback requirement in multicell systems in the presence of both intercell and interuser interference over conventional schemes to maintain a constant rate offset. The key finding from the proposed feedback allocation is that practically much higher allocation of feedback bits and feedback scaling are required in time‐varying channels to reduce throughput degradation.     

Interference alignment based on optimizing bit allocation with limited feedback in interfering MIMO-MAC networks

A novel limited feedback for interference alignment algorithm was proposed based on the disadvantages of traditional limited feedback interference alignment schemes for MIMO uplink.Unlike the traditional linear interference alignment,an iterative scheme was presented.The pre-coder was designed through maximizing the target signal to the interference leakage and noise ratio,and interference suppression matrix was designed by maximizing the signal to interference plus noise ratio for each data stream,which can come over rate loss in low SNR.In addition,the bit allocation of traditional limited feedback schemes were based on the mean of interference leakage,which were not perfect and make the total number of the effective bits less than the bits provided by system.Thus,a novel bit allocation scheme was provided that improving bit utilize and decreasing quantization error.Theoretical analysis and simulation experiments show that compare to the existed classical algorithms,the algorithm was a stepwise enhancement of signal quality for iterative search in perfect CSI which was better than linear algorithm and has a larger space with no need for strictly align interference in limited feedback CSI which reduces the influence of quantization error more.The effective bit distribution and bit utilization are improved by bit allocation,resulting in improved performance in limited feedback.     

Hybrid beamforming design based on unsupervised machine learning for millimeter wave systems

This article proposes a hybrid beamforming design with reduced channel state information (CSI) feedback. We use a beam sweeping procedure to provide channel measurements and feed a CSI report scheme. Thereby, the base station (BS) can perform an adequate estimation of the channel characteristics with reduced signaling overhead. Consequently, we need short pilot sequences and very few precoding matrix indicators (PMIs) to properly describe channel behavior. Moreover, we also evaluate different user selection strategies based on unsupervised machine learning framework that exploits the channel information provided by the proposed beam sweeping scheme. Our performance evaluation indicates that the user selection based on fuzzy c‐means is able to efficiently explore the reduced CSI. The proposed hybrid beamforming scheme reduces the multi‐user interference and achieves significant gains in total data rate as channel conditions and interference environment becomes more challenging.     

Channel quality-to-interference ratio quantization criterion for limited feedback beamforming in MIMO systems

The discrete Fourier transform (DFT)-based codebook is employed in this paper to quantize channel state information so that the amount of feedback can be reduced in the multiple input multiple output (MIMO) downlink of long term evolution (LTE) system. And a novel beamforming (BF) scheme based on the proposed channel quality-to-interference (QIR) quantizing criteria is developed, which uses only the index of the optimal codebook for the beamforming at the base station (BS), and dramatically reduces the amount of feedback. The proposed BF scheme jointly considers the influences of the quality of the quantized channels and the mutual interference among the sub-channels. The extensive simulation results verify that throughput of the proposed BF scheme is better than that of the random BF with a little feedback, and that of the eigen-beamforming even under low signal noise ratio (SNR) scenario.     

多用户MIMO-OFDM系统低速率CSI反馈方法及信道容量分析

该文针对闭环多用户MIMO-OFDM系统提出一种基于线性预测的低速率CSI (Channel State Information)反馈方法。根据相关带宽将OFDM子载波划分成多个子带,移动台对每个子带的CSI作线性预测,并对预测误差进行量化编码后反馈给基站;基站使用相同的线性预测滤波器将反馈来的预测误差恢复成CSI,然后在每个子带上通过迫零-波束赋形实现多用户空间复用。同时,该文还在采用注水定理分配发射功率的条件下,从理论上分析了下行链路信道容量。数值仿真结果显示,每个反馈数据的实部或虚部仅用1bit量化时,本方法仍能够以较高的精度恢复CSI。与目前3GPP LTE标准所采用的基于码书的反馈方案相比,该方法能够在反馈开销相同情况下,有效地抑制同信道干扰,大幅提高系统容量。     

Adaptive minimum bit feedback for sum-rate analysis in multi-user distributed antenna systems

The paper mainly studies the sum-rate performance of limited feedback (LFB) block-diagonalization (BD) in multi-user distributed antenna system (DAS). As the channel state information (CSI) fed back to base station (BS) is limited, multi-user interference (MUI) is caused inevitably because of the quantization error. Considering the influence of the MUI to the capacity of DAS, we propose a parameter of effective sum-rate ratio (ESRR) to denote the capacity offset under the condition of the BS can get perfect CSI and limited CSI first, then we confirm that the approximated ESRR is very close to actual ESRR got through simulations. After that, based on the approximated ESRR, an adaptive minimum bit feedback scheme which can effectively reduce the overhead of feedback channel and the complexity of the system is proposed. Simulation results show the effectiveness of the proposed scheme.     

A Strategy to Reduce the Signaling Requirements of CQI Feedback Schemes

Resource allocation in OFDMA systems involves the application of adaptive modulation/coding (AMC). AMC provides the flexibility to match the modulation-coding scheme (MCS) to each user’s channel conditions. In order to accurately determine the appropriate MCS level, full channel state information (CSI) is required by the base-station (BS). Full CSI feedback requires bandwidth to be reserved in the OFDM frame which is occupied by the users to transmit channel quality information (CQI) to the BS. Allocations and deallocations of channels reserved for CQI feedback require expensive signaling messages between the base stations and the mobile stations, and therefore should be minimized. In this paper, we show that the amount of bandwidth consumed for this type of signalling can be significantly reduced by selecting the users with the lowest average SNR to periodically feedback their instantaneous channel state information. These benefits become more evident at low values of the user velocity.     

Power control for delay constrained multi‐channel communications using outdated CSI

In this paper, we study the power allocation scheme for a single user, multi‐channel system, e.g., orthogonal frequency‐division multiplexing (OFDM) systems, under time‐variant wireless fading channels. We assume the receiver feeds back perfectly estimated channel state information (CSI) to the transmitter after a processing delay. The objective of the power allocation is to maximize throughput subject to quality‐of‐service (QoS) constraint. The QoS measure of our consideration is a triplet of data rate, delay, and delay bound violation probability. A two‐step sub‐optimal power allocation scheme is proposed to address the impact of outdated CSI. In the first step, the total transmission power that can be used within one block is determined according to the summation of the channel gains of all the channels. In the second step, the total transmission power is allocated among all the channels. The proposed power control scheme is less sensitive to the feedback delay. Compared to the optimal power allocation scheme designed for the perfect CSI scenario, it has lower computational complexity while achieving comparable capacity. Copyright © 2010 John Wiley & Sons, Ltd.     

Systematic Codebook Designs for Quantized Beamforming in Correlated MIMO Channels

The full diversity gain provided by a multi-antenna channel can be achieved by transmit beamforming and receive combining. This requires the knowledge of channel state information (CSI) at the transmitter which is difficult to obtain in practice. Quantized beamforming where fixed codebooks known at both the transmitter and the receiver are used to quantize the CSI has been proposed to solve this problem. Most recent works focus attention on limited feedback codebook design for the uncorrelated Rayleigh fading channel. Such designs are sub-optimal when used in correlated channels. In this paper, we propose systematic codebook design for correlated channels when channel statistical information is known at the transmitter. This design is motivated by studying the performance of pure statistical beamforming in correlated channels and is implemented by maps that can rotate and scale spherical caps on the Grassmannian manifold. Based on this study, we show that even statistical beamforming is near-optimal if the transmitter covariance matrix is ill-conditioned and receiver covariance matrix is well-conditioned. This leads to a partitioning of the transmit and receive covariance spaces based on their conditioning with variable feedback requirements to achieve an operational performance level in the different partitions. When channel statistics are difficult to obtain at the transmitter, we propose a universal codebook design (also implemented by the rotation-scaling maps) that is robust to channel statistics. Numerical studies show that even few bits of feedback, when applied with our designs, lead to near perfect CSI performance in a variety of correlated channel conditions.     

空间相关莱斯衰落信道下基于部分信道信息的多用户预编码与调度算法

在空间相关的莱斯衰落信道模型下,针对多用户MIMO(Multiple-Input Multiple-Output)系统潜在的多用户分集增益和空间分集增益,该文提出了一种基于部分信道信息的多用户预编码与调度算法。结合部分瞬时信道信息和统计信息,利用约束最大似然估计对各用户信道矢量进行估计,然后利用估计的各用户信道调度多个用户进行预编码。仿真结果表明,该方案以较少的反馈开销,获得了较大的性能增益。     

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.     

部分支撑集辅助的压缩感知CSI反馈方法

在频分双工大规模多输入多输出系统中,基于压缩感知的信道状态信息（CSI,channel state information）反馈方法因正确重构CSI小幅度元素的支撑集合造成巨大的反馈开销.为降低基于压缩感知的CSI反馈的开销,提出一种部分支撑集辅助的压缩感知CSI反馈方法.提出方法将CSI的一部分小幅度元素的支撑集与压缩CSI一同反馈回基站.基站无需重构反馈回基站的CSI小幅度元素的支撑集,压缩CSI所需的测量次数（反馈开销）得以极大降低.分析与仿真结果表明,相对于传统的基于CS的CSI反馈方法,提出方法在确保CSI重构精度与可达和速率情况下,能有效降低CSI反馈开销和CSI重构的计算复杂度.     

TDD-MIMO系统中由I/Q不平衡引起的信道非互易性补偿方法

分析了I/Q不平衡引起的信道互易性丧失对系统容量的影响,提出了一种基于信道状态测量的补偿算法。该算法中基站(BS)和移动台(MS)各自对接收信道进行测量而得到上下行链路的信道状态信息(CSI),然后将2条链路的CSI汇集到BS处,BS计算出分别用于BS和MS的校准矩阵。仿真结果表明所提算法能够有效地对I/Q不平衡进行校准,使信道互易性得到保持,从而使系统容量的损失得以弥补。     

相关瑞利信道下差分空时检测协同分集方案

本文提出了一种采用分布式差分空时分组编码和检测的协同分集方案,在不需要信道状态信息(CSI)的情况下可以实现满分集和全速率发射,并推导了相关瑞利信道下该方案误码率(BER)性能上限的解析表达式。传统的差分空时分组编码对整个码块进行差分,而协同分集下的任何一个协同用户进行差分编码时都不知道整个码块的信息。本文所提出的差分BPSK调制方案,通过将两协同用户的信息分别被调制到相互正交的实轴和虚轴上,从而将码块的联合差分转化为各用户独立差分。分析了在协同用户间不同的信道状态信息(CSI)和协同用户到接收用户不同的CSI情况下本文所提出发射方案的性能。仿真结果表明本文所提出的方案获得了明显的分集增益,同时也较好的吻合了理论分析的结果。     

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.