OFDMA系统中多波束机会波束成形研究

机会波束成形（Opportunistic Beamforming,OBF）技术通过加大、加快信道的波动来提高多用户分集增益,将之使用在正交频分复用多址接入（OFDMA）系统中,可以使得系统的吞吐量最大化。在OFDMA系统中,通过对所有子载波进行分簇,可以实现在牺牲系统性能很少的情况下,大大降低系统反馈量。研究了对分簇OFDMA系统使用OBF技术来提高系统吞吐量,并进一步研究了在训练阶段使用多个随机加权向量进行多波束成形,然后从中选择最好的一个来实现数据阶段的波束成形。仿真证明,采用多波束机会波束成形技术能提高OFDMA系统吞吐量。     

MIMO系统中随机波束成形算法的研究

文章研究了MIMO(多输入多输出)系统中的随机波束成形(RBF)算法,仿真结果表明:随着用户数的增加,随机波束成形技术可以用最小的反馈获得相干波束成形情况下的吞吐量,并可获得MIMO系统慢衰落信道中下行链路的多用户分集增益和复用增益.     

基于多波束的机会波束成形在OFDMA系统中的使用

机会波束成形技术可以使信道的波动变大、变快，从而增大多用户分集增益。它同样也适用于OFDMA系统。对于OFDMA系统而言，我们还可以通过对所有子载波进行分簇，从而在牺牲系统性能很少的情况下，大大地减少系统反馈量。为了进一步提高系统吞吐量，还可以在每个时隙使用多个随机加权向量，然后基站从中选择一个最好的向量进行机会波束成形。     

毫米波大规模MIMO系统混合波束成形技术综述

毫米波通信和大规模多输入多输出（Multiple-Input Multiple-Output,MIMO）技术是5G的关键候选技术,在提高5G系统各项性能指标上潜力巨大。混合波束成形作为毫米波大规模MIMO系统中的关键点,能在系统性能和实现复杂度上取得较好平衡,受到业界和学术界广泛关注。首先给出了混合波束成形经典系统模型和常用信道模型,根据信道状态信息获取方式的不同,从基于理想信道条件和基于波束配对两个方面分析和归纳了现有的混合波束成形方案,最后指出了混合波束成形未来发展趋势以及尚未解决的难点。     

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.     

一种基于虚拟子载波和导频子载波的多天线OFDM系统波束成形算法

在无线通信系统中,正交频分复用(OFDM)和多天线技术结合可以获得频率分集和空间分集,波束成形是其中的关键技术之一.针对多天线系统的波束成形这一主题,一些学者已经提出了很多波束成形算法,用以提高系统的接收性能.本文提出了一种新的波束成形算法,该算法有效地利用了OFDM系统中的虚拟子载波资源.新算法在加性高斯白噪声单径和多径信道中的仿真结果均表明该算法使系统性能得到了显著提高.     

中继辅助MIMO系统中基于有限反馈的分布式波束成形

对协作式中继辅助的多输入多输出通信系统中基于有限反馈的联合波束成形问题进行了研究.首先推导了接收信噪比的下界,并基于该下界建立联合波束成形的数学模型.其次推导了分布式波束成形的最优方案.最后针对该最优方案,设计了一个迭代算法,优化得到分布式波束成形码本,从而分布式中继站的波束成形可基于码本序号的有限反馈来进行.实验结果仿真表明,所设计的联合波束成形码本在反馈开销很小的情况下获得明显的性能增益.     

协作式中继增强MIMO通信系统的联合波束成形

本文对协作式中继辅助的多输入多输出(Multiple-input Muhiple-output,MIMO)通信中的联合波束成形问题进行了研究.首先推导了接收信噪比的下界,并基于该下界建立了联合波束成形的数学模型;然后推导了分布式波束成形的最优方案;最后设计了联合优化信源的多天线波束成形和协作式中继的分布式波束成形的算法,从而获得联合波束成形方案.计算机实验仿真表明所提联合波束成形方案获得明显的性能增益.     

星载DBF发射多波束阵列的互调波束影响

研究了DBF发射多波束阵列中固态功率放大器(SSPA)非线性引起的互调波束的影响。简化了Shimbo模型,在仿真中验证了其精确性。基于简化的Shimbo模型描述了SSPA的非线性特性,分析了DBF发射多波束阵列在多波束多载波条件下的互调波束特性,主要研究了SSPA三阶互调的影响;并对输入回退(IBO)与互调波束以及放大器效率的关系进行了理论分析与仿真,为DBF系统中SSPA线性度与效率的折衷选择提供了依据。     

波束形状损耗对多波束搜索雷达的影响

多波束搜索雷达采用多个同时接收波束对每一个发射波束的照射空域进行覆盖,其波束形状损耗计算和影响与单波束搜索模式存在差异。文中研究了多波束搜索雷达中波束形状损耗对系统检测性能的影响,推导了通用的计算方法,并利用仿真方法研究了不同系统参数配置下的检测性能。由仿真结果可知,波束形状损耗一般随同时接收多波束数目的增加而减小。     

Opportunistic Beamforming and Scheduling for OFDMA Systems

Orthogonal frequency-division multiple access (OFDMA) is an attractive technique for exploiting multiuser diversity in the downlink of a cellular system. This paper addresses three problems in multiuser diversity for OFDMA systems. First, we propose a way to significantly reduce the amount of channel state information (CSI) feedback without sacrificing performance too much, by selective and adaptive feedback. Second, we propose a way to increase the cell throughput and fairness by applying an opportunistic beamforming scheme to orthogonal frequency-division multiplexing. This beamforming scheme increases the frequency fading rate, which increases the multiuser diversity effect. Thirdly, we deal with the issue of fairness and quality-of-service (QoS) in opportunistic systems by proposing a modified proportional fair (PF) scheduler for OFDMA. Key features in the scheduler are that it incorporates QoS classes into the PF scheduler and that it has a tunable fairness level. Extensive simulation results are presented to evaluate the performance of the proposed schemes. The opportunistic beamforming scheme performed well in comparison with several other schemes. The modified PF scheduler was able to give users different QoS, based on their requirements, while still exploiting multiuser diversity     

Opportunistic eigenbeamforming: Exploiting multiuser diversity and channel correlations

Multiuser diversity is an inherent form of diversity present in any time-varying system with several users. An opportunistic scheduler has to be used in order to exploit this type of diversity. A scheme that increases the effective dynamic range of the channel by deploying multiple antennas at the transmitter is called opportunistic beamforming. Opportunistic beamforming increases the degree of multiuser diversity in several scenarios, including correlated channels. Nevertheless, multiuser diversity can also be combined with other transmit schemes that have proven to be effective in correlated channels, such as eigenbeamforming. Eigenbeamforming is a point-to-point link transmit technique that could easily be combined with an opportunistic scheduler to extract multiuser diversity. We refer to the joint use of eigenbeamforming with an opportunistic scheduler as opportunistic eigenbeamforming. In this work, we show that the available multiuser diversity with opportunistic eigenbeamforming is larger than the one achieved when opportunistic beamforming is employed using the proportional fair scheduler under different degrees of correlation in the channel. In the present work we have considered a single-cell scenario.     

Performance of OFDMA Multicell Systems with Opportunistic Beamforming

The opportunistic beamforming (OB) technique in multicellular OFDMA networks is investigated in this paper. Three cross-layer radio resource management (RRM) algorithms for OFDMA operational scenarios are considered. These algorithms build upon typical network planning practices for OFDMA systems. The first two implement an OFDMA network with opportunistic rate adaptation while the third one aims at a network which guarantees QoS provision through power control on carrier basis. The RRM algorithms are based on typical OFDMA resource allocation targets such as minimization of transmit power, maximization of throughput and interference averaging. Then, the OB concept is combined with the considered RRM algorithms and a comparative performance analysis between the two types of networks (omni and opportunistic beamforming) is performed in terms of throughput, blocking probability and fairness. The paper aims at providing a useful insight into the way the OB technique affects the performance of different OFDMA networks based on large scale simulations. The simulation results suggest that OB is preferred for OFDMA systems with opportunistic rate adaptation rather than power controlled systems which offer QoS provision. According to the presented results, OB provides to OFDMA systems with opportunistic rate adaptation a ??13% throughput gain and ??75% gain in terms of blocking probability. In addition, it is shown that the combination of OB with interference averaging RRM algorithms has a minor beneficial impact only on the system fairness.     

Adaptive opportunistic fair scheduling over multiuser spatial channels

We consider the problem of opportunistic fair scheduling (OFS) of multiple users in downlink time-division multiple-access (TDMA) systems employing multiple transmit antennas and beamforming. OFS is an important technique in wireless networks to achieve fair bandwidth usage among users, which is performed on a per-frame basis at the media access control layer. Multiple-transmit-antenna beamforming provides TDMA systems with the capability of supporting multiple concurrent transmissions, i.e., multiple spatial channels at the physical layer. Given a particular subset of users and their channel conditions, the optimal beamforming scheme can be calculated. The multiuser opportunistic scheduling problem then refers to the selection of the optimal subset of users for transmission at each time instant to maximize the total throughput of the system subject to a certain fairness constraint on each individual user's throughput. We propose discrete stochastic approximation algorithms to adaptively select a better subset of users. We also consider scenarios of time-varying channels for which the scheduling algorithm can track the time-varying optimal user subset. We present simulation results to demonstrate the performance of the proposed scheduling algorithms in terms of both throughput and fairness, their fast convergence, and the excellent tracking capability in time-varying environments.     

Opportunistic beamforming based on multiple weighting vectors

In order to improve the throughput of the opportunistic beamforming, the authors generalize the opportunistic beamforming by using multiple random weighting vectors at each time slot. The base station chooses the best weighting vector and performs the opportunistic beamforming with this optimum vector. For the case of equally strong independent fast Rayleigh fading channels, the throughput of the proposed scheme is analytically approximated and the optimum number of random weighting vectors per time slot is obtained. Numerical results demonstrate that the proposed scheme considerably improves the throughput compared to the conventional opportunistic beamforming for a low/realistic number of users.     

机会波束成形系统中一种新的调度方案

主要研究机会波束成形系统中的调度算法,将宽带无线IP网络调度算法应用于机会波束成形系统。提出了一个保证用户QoS要求的新方案。数值分析表明,所提的方案较之传统的方案在保证系统吞吐量和用户间公平性的同时,也保证了用户的QoS。     

基于机会通信的多天线蜂窝系统小区间干扰协调

通过研究有限反馈波束赋形蜂窝系统中小区间同信道干扰变化的特性,提出了一种基于机会通信的有限权值机会波束方法。该方法通过在预先设计的有限数量的赋形权值码本中随机选择赋形权值,使目标小区的干扰变化与邻小区用户调度无关,从而使干扰测量和速率预测更为准确,降低了反馈时延和干扰变化造成的中断率。理论分析和仿真均表明:采用正比公平调度的有限权值机会波束与有限反馈波束赋形具有相近的发射速率,从而系统吞吐量得到提高。     

A study of opportunism for multiple-antenna systems

Recently proposed opportunistic beamforming exploits the multiuser diversity to reduce the feedback by not requiring the precoding information used for closed-loop schemes to be known at the transmitter. Opportunism could also be beneficially employed for other multiple-antenna transmission techniques like cophasing and antenna selection. For opportunistic beamforming and antenna selection, we give closed-form expressions for throughput that closely approximate the performance of these schemes with a Proportionally Fair scheduler (PFS) at low signal-to-noise ratios (SNRs). For large number of transmit antennas, opportunistic cophasing has similar performance as opportunistic beamforming. Asymptotic dependence of the required number of users to achieve the gains of opportunism on the number of transmit antennas is exponential for opportunistic beamforming (and cophasing for large numbers of transmit antennas), and at best linear for opportunistic antenna selection. For multiple-antenna receivers, we additionally examine an opportunistic multiple-input multiple-output (MIMO) scheme that transmits multiple data streams simultaneously to the same user.     

Transmit Diversity Versus Opportunistic Beamforming in Data Packet Mobile Downlink Transmission

We compare space-time coding (transmit diversity) and random "opportunistic" beamforming in a space-division multiple access/time-division multiple access single-cell downlink system with random packet arrivals, correlated block-fading channels, and non-perfect channel state information at the transmitter due to a feedback delay. Our comparison is based on system stability. The ability of accurately predicting the channel signal-to-noise ratio dominates the performance of opportunistic beamforming, even under the optimistic assumption that the sequence of beamforming matrices is perfectly known a priori by the receivers. Our results show that the relative merit of opportunistic beamforming versus space-time coding strongly depends on the channel Doppler bandwidth. Therefore, previous naive conclusions on the fact that transmit diversity always hurts the system performance under multiuser-diversity scheduling should be taken with great care