基于相控阵的信道探测技术研究

本文基于相控阵天线快速扫描能实现波束跟踪的原理,利用控制相控阵波束跟踪应用程序,研发了利用相控阵进行移动场景下信道测试的应用设备。利用此设备在城区校园内和郊区高速公路上开展实地的信道测量工作,建立了路径损耗和阴影衰落的统计模型对大尺度衰落特性进行了分析和研究,为高速移动场景下的无线信道测量提高参考。     

无线通信中基于线性预测和参数反馈的下行波束形成方法

本文基于最大信号噪声比准则(MSNR，Maximum Signal Noise Ratio)提出一种新的下行波束形成方法，即根据天线收发的互易性原理，通过上行波束形成得到的上行天线阵列矢量，变换而得到下行天线阵列矢量，再通过移动端Rake接收机的合作，反馈回每一径的衰落系数，考虑到存在传输和处理时延并且在此时间内信道有明显变化时，反馈的衰落系数存在误差。因此，本文采用了信道预测的方法，估计即时衰落系数，这样求得期望信号的相关矩阵，计算相应的最大广义特征值所对应的特征矢量，进行下行波束形成。     

频率选择性衰落信道下长扩频序列CDMA 系统的盲波束成形

本文提出一种适用于频率选择性衰落信道下长扩频序列CDMA系统，基于DMMSE准则的盲波束成形方案。该方案反频率选择性衰落信道分解成若干个非频率选择性衰落子信道，不需要训练符号开销，回避跟踪衰落信道参数，以所需用户的扩频序列作为永久性训练序列。仿真结果验证了其抵抗远近效应、抑制多址干扰的能力。     

Lutz信道模型下卫星CDMA系统的容量分析

针对CDMA卫星系统,详细分析了卫星对地多波束模型中用户受到的多址干扰,仿真得到了理想功控下卫星接收端信号的信干比与单波束容量的关系。在非理想功控条件下,针对阴影衰落和多径衰落对信号功率的影响进行了详细的理论分析和数学推导,独立于编码调制方式,从功率角度着重分析了Lutz信道模型下的卫星容量计算方法,增加了计算模型的普适性。最后仿真得到Lutz信道模型下卫星接收端的最低解调门限与单波束容量的关系。通过对比,得出了复杂信道下卫星实际容量与理论容量的偏差,为卫星波束设计、功率分配等问题提供了参考。     

波动双径衰落信道下的圆阵方向调制方法

为了实现对三维空间中任意期望方向的物理层安全通信,提出了一种基于均匀圆阵的方向调制发射机系统模型,使用迫零波束形成发射有用信号的同时用零空间映射法发射随机的人工噪声干扰非期望方向.针对当前对毫米波信号小尺度衰落刻画较好的波动双径（fluctuating two-ray,FTR）衰落信道,推导了本文模型的平均安全容量（average secrecy capacity,ASC）下界、安全中断概率（secrecy outage probability,SOP）上界的闭合表达式,对比了系统安全性能关于信噪比的变化曲线.仿真结果表明,阵元数为64,信噪比为10 dB,圆阵方向调制在期望方向可以无失真地恢复信号,非期望方向误差向量幅度（error vector magnitude,EVM）高达100%,误比特率高于40%.本文方法能在三维空间实现无需密钥的定向安全传输,提高信噪比可改善其在FTR衰落信道下的安全性能.     

波束形成和解码的联合迭代接收技术

SDMA技术是提高频谱利用率的有效技术，但在判决反馈最小均方波束形成中，在深衰落之后期望用户波束会指向干扰用户,严重影响了波束形成的性能。文中提出在波束形成和信道解码之后，经过编码、交织，重新生成波束形成的参考信号，再进行波束形成，有效克服了深衰落之后的波束指向错误。计算机仿真结果证明了该方法的有效性。     

一种利用窄波束成形提高短波信道相干带宽的方法

短波通信信道具有频率选择衰落特性,相干带宽是描述单载波系统平坦衰落信道带宽的重要参数。提高相干带宽有利于系统使用更大的带宽来实现高数据速率传输并且有助于减少接收机的复杂度。该文提出一种利用发射端窄波束成形在空间上将发射能量集中于主路径且抑制其它路径信号来提高短波信道相干带宽的方法,并利用自适应技术来建立及保持点对点之间的窄波束通信。理论研究表明,该方法能有效地提高信道相干带宽。该文以中距离短波通信为例,计算了使用不同发射天线数时的相干带宽值,计算结果证实了该方法的有效性。     

在相关多入多出(MIMO)信道中的可变速率多用户分集技术

在本篇文章中,我们研究基于MIMO虚拟信道表述的可变速率多用户分集技术.为实现慢衰落相关信道中的多用户分集,本文在发射端采用随机波束成形技术,并提出一种新的利用MIMO虚拟信道来产生随机波束成形矩阵的方法.同时,我们对期望用户的子信道采用自适应编码调制技术,以进一步提高系统性能.     

TD-LTE网络8阵元和2阵元天线应用研究

由于TD-LTE具有上下行对称的信道互易性,非常适用于采用8阵元波束赋形天线提高小区边缘用户吞吐量。但是,在网络实际建设中,由于8阵元天线受限条件较多,部分站点具有一定的建设难度。本文首先对TD-LTE 8阵元和2阵元天线性能及建设难度进行了对比分析,然后通过仿真分析了2阵元天线和8阵元天线混合组网的可行性,并在此基础上,对未来TD-LTE的网络建设提出了相关建议。     

6G超大规模天线信道波束域传播特性分析

为了探明6G超大规模天线信道在波束域的信道传播特性,重点开展了无线信道测量实验与测量数据分析.首先,在典型城市场景下,开展了sub-6 GHz频段配置超大规模均匀线阵的视距(light-of-sight, LoS)和非视距(non-LoS, NLoS)信道测量实验;其次,划分若干子阵列,并分别使用高分辨率参数提取算法估计多径幅度、时延和角度;然后,根据不同的子阵列角度分辨率构建虚拟波束,将对应不同虚拟波束角度的多径进行分组及叠加.通过比较不同子阵列在相同角度对应的虚拟波束系数,发现传统天线域的空间非平稳特性在波束域表现不明显.通过分析不同虚拟波束的信道系数,可观察到波束域稀疏特性,并且LoS场景的稀疏特性更加明显.针对不同场景下各虚拟波束的多径数目、功率和均方根时延扩展进行拟合分析,可知分别服从高斯分布、高斯分布、均匀分布.此外,分析发现LoS场景下虚拟波束间相关性较低,而NLoS场景下虚拟波束间相关性随波束分辨率提高而增大.     

Novel Transmit Beamforming Schemes for Time-Selective Fading Multiantenna Systems

Transmit beamforming has been widely adopted for wireless systems with multiple transmit antennas. For a block fading channel, the Grassmannian beamformer has been shown to provide very good performance for finite rate feedback. However, the original Grassmannian beamformer does not take the time domain correlation of the channel fading into consideration. In this paper, based on a first-order autoregressive (AR1) dynamic fading model, we develop two new classes of beamforming algorithms that exploit the interframe correlations in the channel fading. We first introduce an algorithm based on a standard predictive vector quantization (PVQ) approach, and the resulting PVQ beamformer accomplishes superior power delivery at the receiver. However, the error performance of the PVQ beamformer is not satisfactory at high signal-to-noise ratios, and it also has a high implementation complexity. To resolve these issues, we then develop a novel successive beamforming (SBF) algorithm. The new SBF scheme uses the knowledge of the previous fading blocks to aid the beamforming codebook design of the current fading block. The beamforming codebook is constructed based on the successive partition of the surface of a spherical cap. The new SBF scheme accomplishes nearly the same performance as that of the PVQ beamformer, and it has a much simpler implementation. Through numerical simulations, we demonstrate that the proposed beamformers outperform the other previously proposed beamformers at various fading scenarios     

Minimum-span constant modulus array for a smart antenna testbed

The realisation of an efficient algorithm for a real-time smart antenna testbed based on DECT technology is considered. The testbed is based on a multistage constant modulus array, a blind adaptive beamformer and an adaptive signal canceller that removes the captured signal from the input signals. Based on the Wiener model of convergence, a reduced form of the CM array has been developed that takes advantage of the reduced rank of the input correlation matrix for additional stages. Computer simulations confirm the functionality of this algorithm, and convergence and sensitivity to fading are also discussed.     

Adaptive joint beamforming and B‐MMSE detection for CDMA signal reception under multipath interference

The combination of antenna array beamforming with multiuser detection can effectively improve the detection efficiency of a wireless system under multipath interference, especially in a fast‐fading channel. This paper studies the performance of an adaptive beamformer incorporated with a block‐wise minimum mean square error(B‐MMSE) detector, which works on a unique signal frame characterized by training sequence preamble and data blocks segmented by zero‐bits. Both beam‐former weights updating and B‐MMSE detection are carried out by either least mean square (LMS) or recursive least square (RLS) algorithm. The comparison of the two adaptive algorithms applied to both beamformer and B‐MMSE detector will be made in terms of convergence behaviour and estimation mean square error. Various multipath patterns are considered to test the receiver's responding rapidity to changing multipath interference. The performance of the adaptive B‐MMSE detector is also compared with that of non‐adaptive version (i.e. through direct matrix inversion). The final performance in error probability simulation reveals that the RLS/B‐MMSE scheme outperforms non‐adaptive B‐MMSE by 1–5 dB, depending on the multipath channel delay profiles of concern. The obtained results also suggest that adaptive beamformer should use RLS algorithm for its fast and robust convergence property; while the B‐MMSE filter can choose either LMS or RLS algorithm depending on antenna array size, multipath severity and implementation complexity. Copyright © 2004 John Wiley & Sons, Ltd.     

The constant modulus array for cochannel signal copy and directionfinding

The constant modulus (CM) array is a blind adaptive beamformer capable of recovering a narrowband signal among several cochannel sources without using a pilot or training signal. It is a conventional weight-and-sum adaptive beamformer whose weights are updated by the constant modulus algorithm. An adaptive signal canceller follows the beamformer to remove the captured signal from the array input and to provide an estimate of its direction vector. Based on a Wiener model, we investigate the steady-state properties of the CM array and the signal canceller. For mutually uncorrelated sources and noise, it is shown that the signal canceller exactly removes the source captured by the array. Thus, identical stages of the CM array and signal canceller may be used in a multistage system to recover several cochannel sources. Computer simulations are presented to verify the analytical results and to illustrate the transient behavior of the system     

Misadjustment and tracking analysis of the constant modulus array

The constant modulus (CM) array is a blind adaptive beamformer that can separate cochannel signals. A follow-on adaptive signal canceler may be used to perform direction finding of the source captured by the array. In this paper, we analyze the convergence and tracking properties of the CM array using a least-mean-square approximation. Expressions are derived for the misadjustment of the adaptive algorithms, and a tracking model is developed that accurately predicts the behavior of the system during fades. It is demonstrated that the adaptive canceler contributes more to the overall misadjustment than does the adaptive CM beamformer. Computer simulations are presented to illustrate the transient properties of the system and to verify the analytical results     

一种基于恒模算法的多用户盲波束形成新方法

提出了一种适用于多用户的盲波束形成新方法.该方法基于最小二乘恒模算法,当最小二乘恒模算法收敛于某一信号后,我们导出了算法的权向量与其它用户波达方向的关系,进而在不同用户的波达方向上形成初始增益,从而实现对所有用户的波束形成.文中给出了算法的并行和串行实现方案.     

A Low-Complexity Multiuser Detector for Asynchronous CDMA QPSK Adaptive Antenna Array Systems

This paper proposes a low complexity joint space-time multiuser detection algorithm for asynchronous DS/CDMA antenna array systems. The proposed multiuser detector is composed of an adaptive antenna array, used as a linear beamformer, and a sliding window decorrelator. A QPSK modulation scheme is used in order to increase bandwidth efficiency. Numerical results are given in terms of Bit Error Rate (BER) under the assumption of a frequency-selective Rayleigh slow fading channel. In particular, the proposed receiver is shown to be near-far resistant, even in worst fading cases, and to exploit completely array introduction while maintaining acceptable computational complexity. The proposed architecture avoids linear filter realization of the decorrelator, which is impractical in the case of a large number of users, and operates with relatively short data frames instead of the complete information sequence. Finally, this receiver is very flexible to changes in timing configuration.     

Performance of a wireless MC-CDMA system with an antenna array in afading channel: reverse link

We propose a multicarrier code-division multiple-access (MC-CDMA) system employing an antenna array at the base station, and analyze the performance of the MC-CDMA system in a fading channel. An optimal beamformer not requiring explicit direction of arrival estimation or training signals is derived for the MC-CDMA system in the reverse link, and is shown to reduce interferences from other users significantly, thus increasing the system's user capacity. Also, it is shown that the MC-CDMA approach can achieve better performance than the direct-sequence CDMA approach when both approaches employ the code filtering technique for the antenna array     

The performance of a -steering beamformer in correlated Rayleigh fading

We consider the performance of a -steering adaptive beamformer in a distributed Rayleigh fading environment. To determine the beamformer coefficients, the signal and interference array response vectors must be estimated. An estimator that makes use of the knowledge of the spatial distribution of the signal is proposed and compared with other estimators that do not use this information. As the angular spread of the signal increases, the value of this information is reduced, until, for the case of isotropic scattering, the estimate that uses the spatial distribution is equivalent to the estimate that does not. Performance results are presented that compare the performance of the different estimators. We derive closed-form expressions for the signal-to-interference-plus-noise ratio (SINRO) and probability of error for the case where the training data are orthogonal.     

Optimal antenna diversity signaling for wide-band systems utilizingchannel side information

Optimal multi-antenna wide-band signaling schemes are derived for multipath channels assuming perfect channel state information at the transmitter. The scheme that minimizes the bit-error probability in the single-user case is a rank-one space-time beamformer which focuses the signal transmission in the direction of the most dominant channel mode. Several suboptimal variations are discussed for multiuser applications. The optimal signaling scheme given channel statistics at the transmitter is also derived. The optimal scheme in this case is a full-rank space-time beamformer that transmits on all channel modes. Analysis and simulation results are used to compare the schemes proposed in this paper. Finally, we discuss the optimal signaling scheme when a delayed version of the channel state is available at the transmitter. It is shown that in this case the optimal scheme is a rank-1 beamformer when the channel variations are sufficiently slow and is a full rank beamformer in a sufficiently fast fading channel