LTE基于码本选择预编码研究

本文研究了LTE系统中基于码本选择的预编码发送方案,对LTE基于码本选择的预编码方案进行了推导,提出了基于性于比(SINR)的码本选择算法,并对其进行了性能仿真.仿真结果表明,在4根发射天线和2根接收天线配置下,基于码本的预编码简单实用,且能够在在满足LTE性能需求的基础上大大提高系统性能.     

LTE基于码本选择预编码研究

本文研究了LTE系统中基于码本选择的预编码发送方案,对LTE基于码本选择的预编码方案进行了推导,提出了基于性干比（SINR）的码本选择算法,并对其进行了性能仿真。仿真结果表明,在4根发射天线和2根接收天线配置下,基于码本的预编码简单实用,且能够在在满足LTE性能需求的基础上大大提高系统性能。     

MIMO系统在慢衰落信道中的实用反馈方案

提出了一种可变码长编码方式,其应用于慢衰落信道中的基于有限反馈迫零预编码的多用户MIMO系统.该方案利用了慢衰落信道中每个配置单天线的用户连续两次从码本中选择的量化信道矢量的相关性来对码本进行有效编码,从而减少了需反馈的统计平均比特数,节省了信道带宽.在满足一定条件的慢衰落信道中,在需要反馈的比特数相同的情况下,新方案比应用基于量化的天线合并算法每用户多天线的多用户MIMO系统在信道总容量方面具有更好的性能,且该编码对应于二叉树形结构的叶子,故基站可自动识别其编码,可用于实际的通信系统之中.     

线性接收机方案下的分层码本有限反馈预编码

该文分析了线性接收机方案下传统Grassmann码本的性能缺失,并通过分析酉阵扰动对系统误码率及信道容量性能的影响,建立应用于多天线系统线性接收机方案下的由Grassmann主码本与酉阵扰动子码本共同构成的分层码本体系,与之对应的是两步码字选择准则。对于子码本的建立,该文提出了基于旋转矩阵和基于householder变换两种方案,并通过计算机仿真阐释了分层码本体系对于传统单码本预编码在系统误码率及信道容量等方面的性能优势。     

基于OSTBC的有限反馈预编码设计

D.J.Love等在文献[7]中提出了基于正交空时码的有限反馈多天线系统,并且设计了至今保持最优性能的码本。这一系统已经被IEEE国际标准所采用。针对这一系统,本文基于文献[16]中的预编码设计准则,提出了新的预编码设计方案。相对于Love等在[7]中提出的码本,本文提出的新的码本在性能上有了提高。并且本文新的码本还具有快速选码方案,在其性能损失较小的情况下,有效降低了选码复杂度。     

基于零向波束形成和旁瓣抑制的MIMO-OFDM系统码本设计

MIMO系统可提高通信系统的容量和频谱的利用率,支持高速数据传输,是下一代无线传输系统的关键技术。在MIMO系统中,当发射端已知信道状态信息时,利用有限反馈的预编码可提高系统的性能。已有的码本设计方法,有的实现较为复杂,有的受到信道的限制。根据零向准则波束形成器的原理,结合降低天线副瓣的方法,实现了一种MIMO—OFDM系统预编码码本的设计。将此码本用于MIMO—OFDM系统有限反馈的预编码中,经仿真结果验证了码本设计方法的有效性。该码本的设计复杂度低,具有较好的性能。     

LTE中下行MU-MIMO的物理层技术分析

LTE基站可以针对终端的空间特性采用单用户MIMO(SU-MIMO)和多用户MIMO(MU-MIMO)技术。LTE网络的用户数随着商用化的进程逐年递增,MU-MIMO技术也成为运营商关注的热点。论文分析了LTE中下行MU-MIMO的物理层技术,侧重设计了基于终端双码本反馈的方法,研究了独立码本预编码和联合迫零(ZF)预编码算法。通过计算机仿真比较了2种算法的吞吐率性能,结果表明低SNR时独立码本预编码优于联合ZF预编码,而高SNR时联合ZF预编码优于独立码本预编码。     

TD-LTE基站智能天线性能分析

多天线技术是移动通信技术发展的重要趋势,是实现移动通信系统高容量、高频谱效率的重要手段.LTE系统引入了多向多天线技术,并定义了多种多天线的工作模式,其中基于非码本预编码的智能天线是TD-LTE的核心技术,可以充分利用TDD的上下行信道互易性,显著提升系统的吞吐量和频谱效率.本文从理论上给出了2天线及8天线在TD-LTE系统性能上的差异,并且分析了8天线智能天线技术在实际应用中可能面临的挑战,从广播信道的无功率加权、下行链路质量估计补偿、SRS协调发送、多用户配对算法优化、接收机性能增强等角度对智能天线的产品实现进行优化,可提升性能10％～30％.     

一种提高部分反馈预编码MIMO系统安全性能方案

MIMO(Mutiple-In Mutiple-Out,多输入多输出)技术是无线移动通信领域智能天线技术的重大突破。无线通信的广播特性容易引起信息泄露,开展研究提高无线通信网络安全性能的课题十分必要。目前,MIMO系统中的预编码技术已成为无线通信领域的研究热点。本文将介绍一种加密技术控制下通过旋转码本提高预编码系统安全性能的方案。通过仿真,验证了该方案可以使无线通信可靠性接近1,并且该方案的设计思想可以推广到其他预编码系统中。     

一种基于码本的SLNR算法

在有限反馈系统中,针对用户端向基站反馈完整的信道状态信息对上行链路传输能力要求较高、设备实现较为复杂的问题,给出一种基于码本的最大化SLNR及SFR的PF调度算法。该算法通过码本量化预编码矩阵,基站仅需用户端反馈码本编号即可获得预编码矩阵,从而大大降低了信息反馈量。仿真结果表明,算法保证了系统和边缘用户性能,但是,作为降低反馈量的代价,与引入码本前相比,算法性能略微下降。     

Quantized Principal Component Selection Precoding for Spatial Multiplexing with Limited Feedback

In this paper, for spatial multiplexing with limited feedback, a quantized principal component selection (QPCS) precoding scheme is proposed that achieves comparable capacity to the closed-loop multiple-input multiple-output (MIMO) and furthermore adapts to various fading channel conditions without any additional feedback bits and transmit channel state information (CSI). We propose a systematic design method for a codebook consisting of a finite number of unitary matrices based on a maximizing minimum distance criterion in the one- dimensional angular domain and show that the method outperforms the Grassmannian subspace packing method in various fading channel conditions. The proposed QPCS precoding scheme allows for adjustment of the precoding matrix based on limited feedback information on the principal vectors approximating a MIMO channel in the angular domain according to various channel conditions. Furthermore, for practical implementation of the QPCS precoding scheme, we propose a structured precoder optimization procedure and show that the proposed procedure induces a negligible capacity loss compared with the exhaustive precoder optimization, even with considerably reduced complexity.     

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.     

Interpolation Based Unitary Precoding for Spatial Multiplexing MIMO-OFDM With Limited Feedback

Spatial multiplexing with linear precoding is a simple technique for achieving high spectral efficiency in multiple-input multiple-output orthogonal frequency division multiplexing (MIMO-OFDM) systems. Linear precoding requires channel state information for each OFDM subcarrier, which can be achieved using feedback. To reduce the amount of feedback, this paper proposes a limited feedback architecture that combines precoder quantization with a special matrix interpolator. In the proposed system, the receiver sends information about a fraction of the precoding matrices to the transmitter and the transmitter reconstructs the precoding matrices for all the subcarriers. A new interpolator is proposed inspired by spherical interpolation that respects the orthogonal columns of the precoding matrices and the performance invariance to right multiplication by a unitary matrix. The interpolator is parameterized by a set of unitary matrices; a construction of a suitable set is briefly described. Simulations illustrate the performance of limited feedback precoding with coding, estimation or prediction error, and time variation for bit error rate (BER), mutual information, and mean squared error (MSE)     

Limited feedback unitary precoding for spatial multiplexing systems

Multiple-input multiple-output (MIMO) wireless systems use antenna arrays at both the transmitter and receiver to provide communication links with substantial diversity and capacity. Spatial multiplexing is a common space-time modulation technique for MIMO communication systems where independent information streams are sent over different transmit antennas. Unfortunately, spatial multiplexing is sensitive to ill-conditioning of the channel matrix. Precoding can improve the resilience of spatial multiplexing at the expense of full channel knowledge at the transmitter-which is often not realistic. This correspondence proposes a quantized precoding system where the optimal precoder is chosen from a finite codebook known to both receiver and transmitter. The index of the optimal precoder is conveyed from the receiver to the transmitter over a low-delay feedback link. Criteria are presented for selecting the optimal precoding matrix based on the error rate and mutual information for different receiver designs. Codebook design criteria are proposed for each selection criterion by minimizing a bound on the average distortion assuming a Rayleigh-fading matrix channel. The design criteria are shown to be equivalent to packing subspaces in the Grassmann manifold using the projection two-norm and Fubini-Study distances. Simulation results show that the proposed system outperforms antenna subset selection and performs close to optimal unitary precoding with a minimal amount of feedback.     

基于遗传搜索和有限反馈的酉阵设计

在MIMO系统中,酉阵主要应用于差分空时调制（DUSTM）及有限反馈预编码两个方面。DUSTM主要应用于无法获得信道状态参数（CSI）情况下,而有限反馈预编码则应用于完全或部分可获得CSI情况下。文中提出了一种基于实型＂频率＂系数的遗传搜索方法,即使在系统维数很大的情况下,运用该方法获得酉阵设计的码字性能优于由整型＂频率＂系数穷举搜索而获得的码字。     

Multimode basis selection for spatial multiplexing with limited feedback

Proposed is multimode basis selection in which the active bases are selected at the receiver from the common unitary basis set known at both receiving and transmitting ends, conveyed to the transmitter using limited feedback, and assembled into a precoding matrix at the transmitter. It is shown that the proposed multimode basis selection scheme outperforms both multimode antenna selection and Grassmannian multimode precoding in capacity in correlated fading channels without additional complexity and feedback.     

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.      

Design of reduced complexity feedback precoding for MIMO‐OFDM

Beamforming technique is applied to significantly increase the performance of a MIMO system, if the channel information (CI) of the communication system is available at the transmitter. For the transmitter to obtain the entire CI, however, a considerable reverse‐link bandwidth is required. To save the bandwidth, a limited‐rate closed‐loop system, therefore, uses a predetermined codebook which is derived from the CI. The codebook consists of a finite number of precoders out of which the index of the best one is transmitted from the receiver to the transmitter using only a few bits saving substantial bandwidth. However, the amount of bits that need to be fed back can still be significantly large for MIMO‐OFDM systems when the precoding matrix index (PMI) for each subcarrier should be transmitted. Such per‐subcarrier precoding scheme has high feedback overhead and also incurs huge computational cost to determine the best PMI for each subcarrier. We, therefore, propose a per‐band precoding scheme that precodes a band (group) of subcarriers by only one precoder. More importantly, we develop, for the proposed per‐band scheme, reduced‐complexity precoding selection methods that lead to the design of efficient receivers. The effectiveness of the proposed scheme is investigated through computer simulations. Copyright © 2014 John Wiley & Sons, Ltd.     

Adaptive precoding and power allocation in distributed antenna systems with limited feedback

This paper introduces the limited feedback precoding into the distributed antenna system and proposes to adapt the predetermined orthogonal space time block codes to the available channel state information at the transmitter. The optimal representation of precoding information, namely the precoder, with least bits therefore becomes the key problem. Inspired by the characteristics of the distributed antenna system, we focus our work on the precoder construction, adaptable in response to the large and small scale fading, such that the symbol error probability is significantly reduced over that of a fixed, non‐adaptive, independent and identically distributed precoder codebook design. Furthermore, a suboptimal power‐loading strategy is presented by minimizing the derived tight upper bound on the average pairwise error probability of the precoded orthogonal space time block codes, which approaches the optimal performance asymptotically without additional channel knowledge other than the available feedback information. We prove that the proposed precoded orthogonal space time transmission scheme can achieve full diversity order. In particular, the robustness of our proposed transmission scheme to channel estimation error and feedback delay is respectively investigated in some detail, and numerical results show that it obviously improves the link reliability and obtains substantial gains even with few bits of feedback in comparison with conventional antenna selection scheme. Copyright © 2013 John Wiley & Sons, Ltd.