MIMO系统的波束成形技术与开关算法结合的性能分析

提出了一种在多用户情况下多输入多输出系统用开关算法结合波束成形技术的方法.对3种波束成形技术的方案研究表明,波束成形的处理过程时开关算法的判决准则有重要影响.从仿真结果可以看到,空时分组码的自适应波束成形算法与开关算法结合时在误码率为10-2时,信噪比比迫零算法结合和奇异值分解算法结合时分别提高7dB和8 dB左右.     

网络MIMO中的基站联合传输技术

协作多点（CoMP）是未来无线通信中的关键技术之一。多个小区基站通过对信道状态信息（CSI）和用户数据信息的不同程度共享,实现多基站协作传输,将传统意义上的干扰转化为对用户有用的信号,从而提高频谱效率,极大改善小区边缘用户的性能。块对角（BD）算法通过在干扰信道的零空间上设计合适的预编码向量,完全抵消干扰。本文将BD算法用于多小区联合传输,从空间维的角度剖析了其干扰抵消和宏分集增益的工作原理,并将其与迫零（ZF）算法,最小均方误差（MMSE）算法,时分多址（TDMA）以及单用户特征值波束成形算法比较,分析了各种算法的优劣。同时将协作BD算法与非协作BD算法比较。仿真结果表明BD算法优于上述预编码算法,协作传输优于非协作传输。
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基于多目标遗传算法的星载天线干扰抑制算法

提出了一种基于多目标遗传算法的星载天线干扰抑制算法，该算法在射频端通过调节权系数进行输出功率判决从而实现波束形成。文中引入多目标优化问题Pareto最优解的概念，采用了无支配性排序遗传算法（NSGA-Ⅱ）来搜索干扰调零权值的Pareto最优解集，充分发挥这种先进多目标遗传算法的高内在并行性、强鲁棒性以及能够不断优化最优解集的优势，较好地兼顾了星载天线干扰抑制时干扰抑制深度与主波束保形这一对矛盾问题。最后提出了归一化双目标函数加权选择最优调零权的方法从Pareto最优解集中选择一组符合决策者偏好的最优调零权。计算机仿真实验证明，文中所提出的算法具有较好的干扰抑制能力和主波束保形效果。     

分布式发射天线V-BLAST信号的排序干扰抵消检测

针对V-BLAST信号通过分布的发射天线进入信道,该文提出了一种由于发射天线地域上的分布性引起的各发射天线发射信号不同时到达接收天线的V-BLAST排序干扰抵消(Order Interference Cancellation, OIC)检测算法。计算机仿真显示,该算法适用于任意个数的接收天线,在较高信噪比(20dB)的条件下,性能优于直接迫零算法3dB以上。在相同信噪比条件下,分布发射天线V-BLAST的排序干扰抵消检测算法比集中发射天线V-BLAST的排序干扰抵消检测算法有着更好的性能。     

基于联合用户分组及天线选择的大规模MIMO波束成形

大规模MIMO系统中,当小区用户数与基站天线数较大时,各用户的信道条件不尽相同,提出一种适用于大规模MIMO下行链路的基于联合用户分组及天线选择的迫零波束成形算法。将用户分成两组,选择信道条件较优的一组用户来接收信号,并为每一个发送数据流选择最优的基站天线组合进行通信,以较小的性能损失,换取大规模MIMO 射频电路的成本与功耗的大幅度降低。仿真结果证明,该算法能够较好地实现系统性能与硬件复杂度的折中。     

基于分布式天线的全双工中继系统最大化和速率波束成形设计

基于分布式天线的全双工中继系统结合了全双工中继两跳同时同频传输的能力和分布式天线高效覆盖的特性,为提升小区边缘和严重阴影衰落区域的频谱效率提供了一种有效途径。在自干扰抵消非理想的多用户场景下,利用分布式多天线波束成形可实现对系统中自干扰和多用户干扰的联合抑制。为此,该文首先建立了在各分布式天线节点独立发射功率约束下最大化多用户端到端和速率的最优化系统模型,进而提出一种双层迭代算法,解决原问题的非凸性求解难题。仿真结果验证了算法的有效性,表明在多用户分布式天线全双工中继系统中,所提波束成形设计能够有效抑制自干扰和多用户干扰,显著提高系统频谱效率。     

一种改进的多用户下行MIM0系统用户选择算法

为了克服多用户下行MIMO系统采用迫零波束成形算法时基站天线的数目要大于等于所有同时工作的用户接收天线的数目之和的缺点,丈中提出了一种改进的多用户下行MIMO系统用户选择算法。该算法具有较低的算法复杂度,性能优于常规的基于贪婪算法的用户选择方法。     

同时同频全双工场景中的射频域自适应干扰抵消

考虑同时同频全双工无线收发信机的射频域自干扰抵消技术,现有研究多集中于利用手动方式调整自干扰估计信号的参数。针对这一问题,该文提出一种射频域的自适应干扰抵消算法。以正交、同相参考支路构成的自干扰估计结构为基础,利用梯度下降法搜索支路的最优权矢量,估计出自干扰信号,实现了射频域的自适应干扰抵消,并且给出了该算法的收敛性分析。分析与仿真表明,当迭代步长越大或统计时间越短时,算法的收敛速度越小。在100倍符号周期的统计时间,0.3的归一化步长,80 dB干信比以及0 dB信噪比的仿真条件下,该文提出的射频域自适应干扰抵消算法可以实现约100 dB的自干扰抑制。     

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。     

LTE-Advanced异构网中载波选择与波束成形联合算法

为解决LTE-Advanced异构网内宏小区及微小区间的相互干扰问题,提出了载波选择与波束成形联合算法.在高密度、多低功率微基站节点的异构网中,通过关闭部分基站的部分载波,并在开启载波的基站天线上进行波束成形,使接收端受到的干扰大幅度降低.另外,基于效用函数的约束,对载波、波束分别计算,使原本二维联合优化问题变成两个一维优化问题,降低了算法复杂度.仿真结果表明该方案增加了异构网中边缘用户及整个系统的吞吐量.     

On the Optimality of Beamforming with Quantized Feedback

The ergodic capacity of a fading vector channel with multiple transmit antennas and a single receive antenna is explored. Perfect channel information is assumed to be available at the receiver while the transmitter has only partial knowledge of the direction of the user's channel vector based on quantized feedback. We present necessary and sufficient conditions for the optimality of beamforming in such systems. The conditions are applicable to all quantized feedback scenarios regardless of the channel distribution, number of transmit antennas, number of quantization vectors or transmit power. The optimality conditions are closely related to the iteration conditions of the Lloyd algorithm, revealing an interesting link between the optimality of beamforming and the optimality of the vector quantizers. Using the conditions, we prove the capacity optimality of beamforming for several quantized feedback scenarios such as the antenna-selection scheme. We also point out examples of quantized feedback scenarios where beamforming is not optimal. We find that for the independent identically distributed Rayleigh fading channel with more than a single bit of quantized feedback, there is no capacity benefit from increasing the number of antennas beyond the number of quantization vectors. Extensions of the necessary and sufficient optimality condition to the multiple-input multiple-output case are also provided.     

全双工MIMO中继系统高性能的波束成型算法

为了更好地抑制全双工多入多出技术(MIMO)中继系统的自干扰,提高信息传输速率,提出了一种新型的波束成型组合算法。该组合在源节点和目的节点采用奇异值分解的波束成型向量,而在中继站采用基于最大化信干噪比接收波束成型来抑制中继端的自干扰,以及最大化信泄噪比发射波束成型矩阵来减少中继发送端泄漏到中继接收机的信号组合波束成型算法。为了降低复杂度,随后引入了交替迭代结构来进一步优化中继接收和发送波束成型矩阵,并比较了不同组合波束成型方案的和速率性能。仿真结果表明,与现存的波束成型组合相比,当干噪比较低时,提出的组合算法能够提供更优的和速率性能。     

Tradeoff between diversity gain and interference suppression in a MIMO MC-CDMA system

In this paper, the uplink of an asynchronous multi-carrier direct-sequence code-division multiple-access (MC-DS-CDMA) system with multiple antennas at both the transmitter and the receiver is considered. We analyze the system performance over a spatially correlated Rayleigh fading channel with multiple-access interference (MAI), and evaluate the antenna array performance with joint fading reduction and MAI suppression. Assuming perfect channel knowledge available at the transmitter, maximal ratio transmission is employed to weight the transmitted signal optimally in terms of combating signal fading. At the receiver, adaptive beamforming reception is adopted to both suppress MAI and combat the fading. Note that while correlations among the fades of the antennas in the receive array reduce the diversity gain against fading, the array still has the capability for interference suppression. We examine the effect of varying the number of transmit and receive antennas on both the diversity gain and the interference suppression.     

Transmitter optimization and performance gain for multiple-input single-output systems with finite-rate direction feedback

Consider finite-rate channel-direction feedback in a system with multiple transmit but single receive antennas. We investigate how the transmitter should be optimized for symbol error rate with finite-rate feedback, and how the symbol error rate and outage probability improve as a function of the number of feedback bits. It is found that when the number of feedback directions is equal to or larger than the number of transmit antennas, transmit beamforming is optimal. Otherwise, the antennas should be divided into two groups, where antenna selection is used in the first group to choose the strongest channel, and equal power allocation is used in the second group. At high signal to noise ratio (SNR), the optimal power allocation between these two antenna groups is proportional to the number of antennas in each group. Based on high SNR analysis, we quantify the power gain of each feedback bit. It is shown that the incremental gain increases initially and diminishes when the number of feedback bits surpasses the logarithm (base 2) of the number of transmit antennas.     

Transmitter diversity effect in TDMA/TDD mobile radio transmission

Transmitter diversity, which employs a single transmit/receive antenna at the portable stations and two transmit/receive antennas at the base station, is experimentally investigated for a TDMA/TDD (time division duplex) mobile radio system. Experimental results show that transmitter diversity can significantly improve the BER (bit error rate) performance of the portable station, due to AWGN (additive white gaussian noise), delay spread, and CCI (cochannel interference) in Rayleigh fading environments.<>     

Transmitter optimization and optimality of beamforming for multiple antenna systems

We solve the transmitter optimization problem and determine a necessary and sufficient condition under which beamforming achieves Shannon capacity in a linear narrowband point-to-point communication system employing multiple transmit and receive antennas with additive Gaussian noise. We assume that the receiver has perfect channel knowledge while the transmitter has only knowledge of either the mean or the covariance of the channel coefficients. The channel is modeled at the transmitter as a matrix of complex jointly Gaussian random variables with either a zero mean and a known covariance matrix (covariance information), or a nonzero mean and a white covariance matrix (mean information). For both cases, we develop a necessary and sufficient condition for when the Shannon capacity is achieved through beamforming; i.e., the channel can be treated like a scalar channel and one-dimensional codes can be used to achieve capacity. We also provide a waterpouring interpretation of our results and find that less channel uncertainty not only increases the system capacity but may also allow this higher capacity to be achieved with scalar codes which involves significantly less complexity in practice than vector coding.     

Weighted adaptively grouped multilevel space time trellis codes

In existing grouped multilevel space-time trellis codes (GMLSTTCs), the groups of transmit antennas are predefined, and the transmit power is equally distributed across all transmit antennas. When the channel parameters are perfectly known at the transmitter, adaptive antenna grouping and beamforming scheme can achieve the better performance by optimum grouping of transmit antennas and properly weighting transmitted signals based on the available channel information. In this paper, we present a new code designed by combining GMLSTTCs, adaptive antenna grouping and beamforming using the channel state information at transmitter (CSIT), henceforth referred to as weighted adaptively grouped multilevel space time trellis codes (WAGMLSTTCs). The CSIT is used to adaptively group the transmitting antennas and provide a beamforming scheme by allocating the different powers to the transmit antennas. Simulation results show that WAGMLSTTCs provide improvement in error performance of 2.6 dB over GMLSTTCs.     

大规模MIMO相关信道下的联合天线分组和天线选择

在大规模多输入多输出系统中,针对密集部署的大型天线阵列之间的强相关性会抑制天线选择增益效果的问题。在系统下行链路场景下建立空间相关信道模型,提出了基于天线分组的天线选择算法。根据瞬时信道相关矩阵将天线阵列划分为若干组,保证各组内天线之间相关性较强。在完成天线分组的基础上,基于信道矩阵列范数准则在各组发射天线与接收天线之间构成的子信道矩阵中选择天线,进而构造有效发射天线与接收天线之间的信道矩阵。仿真分析了所提天线选择算法对系统遍历和速率的影响,结果表明,在基站天线数为32、接收天线数为2、选择天线数为2、天线相关因子为0.9的假设下,当信噪比为10 dB时,与基于相邻天线分组的天线选择算法相比,所提算法使系统和速率约提高了27.5%,且所提算法若要与最优天线选择算法达到相同的和速率,仅需将其信噪比提升1～2 dB即可。     

Mitigation of the inter-node interference in multi-antenna full-duplex networks

When multiple single-antenna half-duplex (HD) nodes communicate with a full-duplex (FD) base station equipped with multiple transmit and receive antennas, the uplink nodes would generate inter-node interference (INI) on the downlink nodes. We propose a base station assisted INI suppression scheme. The scheme is to design a proper amplify-and-forward matrix at the base station so that the sum achievable rate of the uplink and downlink is maximized. We derive the close-form expression of the amplify-and-forward matrix, which is the product of a parameter optimization and a zero-forcing beamforming (ZFBF) matrices. Finally, we investigate the performance of the proposed INI suppression scheme in a single cell.