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。     

MIMO-OFDM系统中基于簇的有限反馈方法

本文针对有限反馈MIMO-OFDM波束形成系统,研究了基于簇的有限反馈方法.利用簇内相邻子载波信道的频率相干性,提出了一种低复杂度的信道均值分簇法,该方法以平均信道响应在给定码本中选择最佳簇波束形成向量.此外,利用簇波束形成向最之间的剩余相关性,还提出了递归反馈和基于格的反馈两种反馈速率降低方法,即将前一簇码字的邻域内码字作为当前簇的新码本,从而大幅度降低反馈比特数.仿真结果表明信道均值分簇法能以较低的计算复杂度获得较佳的BER性能,反馈降低方法相对传统分簇方法能进一步降低反馈速率,递归反馈方法有一定性能损失,而基于格的反馈方法性能损失可忽略不计.     

MIMO-OFDM系统中反馈降低算法

本文针对MIMO-OFDM波束赋形系统中基于子载波递归反馈编码算法反馈负载过大和基于簇递归反馈编码算法出现跟踪错误传播问题,提出一种新的周期性递归反馈编码算法。该方法利用簇波束向量之间的冗余相关性进一步降低反馈量。同时,通过周期性地搜索初始码书避免了跟踪错误的传播问题。另外,还提出了一种修订分簇反馈降低方法,接收端只当码字发生变化时才反馈最优码字及其子载波索引给发送端,否则不反馈任何信息。仿真结果表明这两种方法可以有效地降低反馈量,并且保证了系统性能。     

基于信道相关统计及有限反馈的多基站协作波束成型

为减少多基站协作波束成形系统中基站间的信息交换量和用户信道信息反馈量,提出一种基站联合利用有限反馈信道参数和信道空间相关矩阵估计信道矢量函数的方法;基于所估计的信道矢量函数,提出一种多基站协作扩展迫零波束成形算法。相比已有的多基站协作波束成形方案,所提算法能在反馈/回程开销和系统性能之间取得更合理的折中,计算仿真验证了所提算法的有效性。     

排列模式索引调制正交频分复用系统

多模索引调制正交频分复用系统(MM-OFDM-IM)在索引调制正交频分复用系统的基础上采用不同星座集对系统中的全部子载波进行索引调制,能有效地提高系统的子载波利用率和频谱效率.但全部子载波的利用影响了系统的子载波间抗干扰能力,导致误码率性能下降.针对这一问题,该文提出排列模式索引调制正交频分复用系统(PM-OFDM-IM).该系统在MM-OFDM-IM的基础上重新引入静默子载波,既能保证系统较高的频谱效率,又能提高系统的误码率性能.同时该文提出一种基于幅值相移键控的分类映射模式,即按半径大小排列的星座集分类模式(PCC-R),该模式能够良好结合系统传输的额外信息.最后仿真结果验证,该系统能够更优地均衡系统的频谱效率和误码率性能,且所提分类映射方案可以达到更优的系统性能.     

MIMO-OFDMA波束成形的反馈设计

MIMO-OFDMA的波束成形算法中,接收机需要反馈信道状态信息。当系统用户、子载波、天线数增加时,反馈量随之线性增长。为了限制反馈量在一个固定的限度之下,文中提出一种预设门限的反馈算法。接收机在确定每个子载波上的最优波束成形矢量及相应的信噪比之后,只有当信噪比大于门限时,才反馈最优波束成形矢量的索引及相应的信噪比值。该算法可以大大降低反馈量,同时保证频谱利用率几乎不变。     

基于部分信道信息的自适应资源分配

在保证一定吞吐量前提下,为了降低传输功率,文中提出了一种基于部分信道反馈信息的多用户OFDM系统子载波分配方案。该方案利用少量已反馈子载波的状态信息来线性推导未反馈子载波的信道增益。仿真结果表明,在平坦瑞利信道衰落中,相比传统方式该方案在不增加反馈开销的前提下该方案能够大幅度地改善系统的性能,提高资源利用率。     

一种基于子载波合并的多播资源调度算法

针对无线OFDM多播系统,该文提出一种基于子载波合并的多播资源调度算法。该算法通过提前将子载波分组,避免了不必要的子载波配对;自适应地选择子载波合并\非合并,在分集和复用两种模式中选取最优方案;同时根据子载波功率分配的特点,将其解耦为配对子载波集合内功率分配和集合间功率分配两个子问题,进一步优化了算法性能。仿真结果表明,与现有方案相比,所提方案能够在复杂度较低的情况下,较好地提升系统性能。     

排列模式索引调制正交频分复用系统

多模索引调制正交频分复用系统(MM-OFDM-IM)在索引调制正交频分复用系统的基础上采用不同星座集对系统中的全部子载波进行索引调制,能有效地提高系统的子载波利用率和频谱效率。但全部子载波的利用影响了系统的子载波间抗干扰能力,导致误码率性能下降。针对这一问题,该文提出排列模式索引调制正交频分复用系统(PM-OFDM-IM)。该系统在MM-OFDM-IM的基础上重新引入静默子载波,既能保证系统较高的频谱效率,又能提高系统的误码率性能。同时该文提出一种基于幅值相移键控的分类映射模式,即按半径大小排列的星座集分类模式(PCC-R),该模式能够良好结合系统传输的额外信息。最后仿真结果验证,该系统能够更优地均衡系统的频谱效率和误码率性能,且所提分类映射方案可以达到更优的系统性能。     

超宽带太赫兹通信中天线结构设计及其波束色散影响分析

为克服超宽带太赫兹通信中的波束色散,当前已设计了多种基于实时延(TTD)的天线结构,但其功耗大、复杂度高。为解决这一问题,该文提出一种低功耗、低复杂度的基于串行等间距延时器的稀疏射频链天线结构。通过联合优化延时器时延和移相器相位,可以改变子载波波束方向,实现多载波波束扩展与波束聚拢,从而为不同分布场景的用户服务。具体而言,在所提天线结构下,提出一种优化延时器时延和移相器相位的方案将多载波波束扩散到不同方向,实现波束扩展,以服务分布在不同方向上的用户。然后,通过调节时延和相位使所有载波波束对准同一方向,实现波束聚拢,以服务分布在同一方向上的用户。仿真结果表明了所设计天线结构和所提优化方案的有效性。     

Power allocation for multi-antenna systems with space-time block coding in the presence of estimation error and delayed feedback

Based on imperfect channel state information with channel estimation error at the receiver and delayed feedback at the transmitter, a suboptimal power allocation (PA) scheme to minimize bit error rate (BER) under a power constraint is developed for beamforming multi-antenna systems with space-time block coding. The proposed scheme is based on a so-called compressed signal-to-noise ratio criterion, where a single compressed factor is utilized, and it can be used to generalize some existing schemes by setting the compressed factor to different forms. A closed-form compressed factor is derived to minimize the BER, and the resultant close-form expression of power allocation is attained. This closed-form expression is computational efficient and can obtain the BER performance close to the existing optimal approach which requires numerical search. Simulation results show that the proposed scheme can provide BER lower than the equal power allocation scheme. However, due to the impact of both estimation error and delayed feedback, it has performance degradation when compared to the PA scheme with estimation error or delayed feedback only.     

Sparse Bayesian Learning Approach to Adaptive Beamforming Assisted Receivers

In this letter, a new adaptive beamforming assisted receiver based on sparse Bayesian learning is proposed. We consider a general probabilistic Bayesian learning framework for obtaining sparse solutions to adaptive beamforming assisted receivers to improve the performance of an adaptive beamforming assisted receiver based on the minimum mean squared error (MMSE) scheme. Simulation experiments show that the bit error rate (BER) performance of the sparse Bayesian beamforming receiver shows an outstanding BER performance compared to MMSE beamforming receivers     

Distributed beamforming in wireless relay networks with quantized feedback

This paper is on quantized beamforming in wireless amplify-and-forward (AF) relay networks. We use the generalized Lloyd algorithm (GLA) to design the quantizer of the feedback information and specifically to optimize the bit error rate (BER) performance of the system. Achievable bounds for different performance measures are derived. First, we analytically show that a simple feedback scheme based on relay selection can achieve full diversity. Unlike the previous diversity analysis on the relay selection scheme, our analysis is not aided by any approximations or modified forwarding schemes. Then, for highrate feedback, we find an upper bound on the average signalto- noise ratio (SNR) loss. Using this result, we demonstrate that both the average SNR loss and the capacity loss decay at least exponentially with the number of feedback bits. In addition, we provide approximate upper and lower bounds on the BER, which can be calculated numerically.We observe that our designs can achieve both full diversity as well as high array gain with only a moderate number of feedback bits. Simulations also show that our approximate BER is a reliable estimation on the actual BER. We also generalize our analytical results to asynchronous networks, where perfect carrier level synchronization is not available among the relays.     

Space-Time Adaptive MMSE Multiuser Decision Feedback Detectors With Multiple-Feedback Interference Cancellation for CDMA Systems

In this paper, we propose a novel space-time minimum mean square error (MMSE) decision feedback (DF) detection scheme for direct-sequence code-division multiple access (DS-CDMA) systems with multiple receive antennas, which employs multiple-parallel-feedback (MPF) branches for interference cancellation. The proposed space-time receiver is then further combined with cascaded DF stages to mitigate the deleterious effects of error propagation for uncoded schemes. To adjust the parameters of the receiver, we also present modified adaptive stochastic gradient (SG) and recursive least squares (RLS) algorithms that automatically switch to the best-available interference cancellation feedback branch and jointly estimate the feedforward and feedback filters. The performance of the system with beamforming and diversity configurations is also considered. Simulation results for an uplink scenario with uncoded systems show that the proposed space-time MPF-DF detector outperforms existing schemes such as linear, parallel DF (P-DF), and successive DF (S-DF) receivers in terms of bit error rate (BER) and achieves a substantial capacity increase in terms of the number of users, compared with the existing schemes. We also derive the expressions for MMSE achieved by the analyzed DF structures, including the novel scheme, with imperfect and perfect feedback and expressions of signal-to-interference-plus-noise ratio (SINR) for the beamforming and diversity configurations with linear receivers.     

Adaptive Beamforming with Dimension Reduction in Spatially Correlated MISO Channels

In this paper, we propose an adaptive beamforming scheme that generates the beam weight using dominant eigenvectors of the spatial covariance matrix. The number of eigenvectors used for the generation of beam weight is determined to maximize the signal-to-noise ratio (SNR) for given feedback constraints (e.g., the amount of feedback information and feedback delay). It is shown that the conventional limited feedback beamforming and eigen-beamforming are special cases of the proposed scheme. Simulation results show that the proposed beamforming scheme can effectively be applied to spatially correlated channels.     

时变信道中非理想反馈条件下波束成形的误码率分析

在基于信道信息有限反馈的无线多入单出系统中,发射机可采用简单的波束成形技术实现发射分集增益和阵列增益。已有的相关研究大多包含块衰落信道、准确信道估计或无反馈延迟等理想假设。该文建立了更为实际的Jakes时变信道中存在信道估计误差和反馈延迟的系统模型,分析了方形和矩形正交幅度调制星座图的平均误码率。研究表明：误码率的理论分析和仿真结果完全相符;增加反馈比特数可提高阵列增益,但不能增加分集增益;在慢变信道中,波束成形要显著优于正交空时分组码;误码率受信道估计误差和反馈延迟影响,且对后者尤为敏感。     

Enhanced Unitary Beamforming Scheme for Limited-Feedback Multiuser MIMO Systems

In this letter, we consider practical downlink multiuser multiple-input multiple-output systems where each user has multiple antennas and codebook-based limited channel feedback is available at base station. We propose a preprocessing scheme for downlink transmission where a unitary beamforming matrix is used. Firstly, we propose how to construct the unitary matrix that maximizes sum rate. Secondly, we propose a codebook-based channel feedback method for the proposed beamforming method. Numerical results show that the proposed scheme outperforms conventional zero-forcing beamforming scheme in terms of sum rate.     

Adaptive minimum bit-error rate beamforming

An adaptive beamforming technique is proposed based on directly minimizing the bit-error rate (BER). It is demonstrated that this minimum BER (MBER) approach utilizes the antenna array elements more intelligently than the standard minimum mean square error (MMSE) approach. Consequently, MBER beamforming is capable of providing significant performance gains in terms of a reduced BER over MMSE beamforming. A block-data adaptive implementation of the MBER beamforming solution is developed based on the Parzen window estimate of probability density function. Furthermore, a sample-by-sample adaptive implementation is considered, and a stochastic gradient algorithm, referred to as the least bit error rate, is derived. The proposed adaptive MBER beamforming technique provides an extension to the existing work for adaptive MBER equalization and multiuser detection.     

高速铁路无线通信中基于正交空时码的格型正交重构算法

分析了基于正交空时码的开环和闭环MIMO系统,并着重研究了高铁场景下速度对正交空时码的影响：高速移动导致的快时变信道将会破环正交空时码的正交结构,降低由此获得的分集增益,从而引起了误码率性能的降低。提出了格型正交重构算法,通过givens变换对正交空时码进行码内正交重构;算法在恢复码内正交性的同时,也改变了发射端波束成形方向。因此,在高速移动场景下,所提算法使发射端获得了波束成形的阵列增益以及与用户静止时相同的分集增益。从系统性能仿真中看出,所提算法提升了高铁场景下基于正交空时码MIMO系统的误码性能。