采用线性迫零接收机的多天线多用户分集性能分析

采用线性迫零接收机的多天线系统中,同时传输的多个数据流,其信噪比并不相互独立,这使得分析多用户分集的性能非常困难。该文通过分布假设检验和最大似然参数估计方法,采用伽马分布逼近所有数据流信噪比最大、最小值以及均值的概率分布,并基于此进一步分析了多天线多用户分集的系统性能。仿真结果表明,前述逼近的准确度较高,该文的分析方法能够较好地衡量多用户分集增益的大小。     

正交空时分组码在瑞利衰落信道中的性能估计

根据Alamouti最大似然译码方法，给出了在正交空时分组码传输的衰落信道条件下接收机输出瞬时信噪比的一般表达式，分析了瑞利衰落信道条件下引入正交空时分组码的多天线系统的符号差错性能，研究表明采用正交空时码传输信号，增加发送天线数量和接收天线数量都可以得到更大的分集增益；在接受天线数量一定的情况下，增加发送天线的数量可以带来更大的分集增益，但当发送天线数量增加到一定程度后，再增加发送天线数量就不能带来明显的分集改善了。     

发射相关瑞利衰落信道V-BLAST错误性能分析

研究了发射相关瑞利衰落信道对垂直分层空时码最大似然接收机比特差错性能的影响。研究发现,最大似然接收机平方欧氏距服从自由度为2×NR的中心卡方分布。在此基础上给出成对差错概率和比特差错概率联合界新的精确表示。用其近似公式定量分析发送相关性对最大似然接收机分集增益及信噪比损失的影响。分析表明,发射相关性不改变最大似然接收机所获得的分集增益,信噪比损失会随着相关系数不同而变化。仿真结果验证了理论分析的正确性。     

相关MIMO信道下空间分集系统中多用户分集性能

多用户MIMO(Multi-input multi-output)系统中,研究空间相关对联合空间分集和多用户分集的性能影响.推导了接收天线之间存在相关的多用户MIMO STBC(Space-time block coding)和MIMO MRT/MRC(Maximum-ratio-transmis-sion/maximal-ratio-combining)系统的中断容量、平均容量表达式,并由此进行性能分析.计算机仿真验证了理论分析的有效性.结果表明,相对于闭环的多用户MI-MO MRT/MRC系统,空间相关下开环的多用户MIMO STBC系统具有较低的平均容量和较高的调度增益.     

基于最大比合并的MIMO分集系统递增天线选择算法

MIMO分集系统天线选择技术可以在不增加系统射频链路的情况下,达到与全天线几乎相同的分集增益.针对发送端采用最大比发送,接收端采用最大比合并的MIMO分集系统,提出了一种递增天线选择方法,每次增加一根天线,并使得它与已选出的天线结合起来具有最大的信噪比增益.相对于对所有可用天线集进行遍历的最优算法,它减小了需要搜索的范围和每次搜索的计算量,降低了复杂度;相对于功控天线选择算法,它考虑了新增天线与已选出天线集之间的相关性,改善了性能.仿真结果表明,在误比特率、信道容量和信噪比增益方面,此算法和最优算法性能相近,且不随可用天线数和选出天线数的改变而改变.     

多用户MIMO系统在有限反馈下结合预编码与调度算法的研究

捅要：研究了多用户多输入多输出（MIMO）系统在有限反馈下的一种结合单位预编码与用户调度算法的方案。在该系统中每一用户都具有多个接收天线,该方案具有较大的多用户分集增益和较小的计算复杂度,从而能够减少系统中的用户间干扰。每个用户独立地决定自己的天线合并矢量,并将最优波束矢量以及对应的估计信干噪比通过有限反馈信道反馈给基站,此时基站获取了相应的用户信道信息和用户间干扰信息。基于接收到的反馈信息,基站从预定义的码本中选取和速率最大的最优子集进行系统预编码,然后依照预编码矩阵调度欧氏距离最小的用户且用户个数不超过发送天线的数目。仿真结果显示该方案有效地改善了系统速率,特别是在用户数目较多或者信道环境较好的情况下。     

多用户MIMO—CDMA系统下行链路设计及MATLAB仿真

MIMO(Multi-Input Multi-Output)技术是最可能应用到未来无线通信系统中的关键技术之一,多用户MIMO-CDMA是MIMO技术领域研究的最新发展。文章概述了多用户MIMO-CDMA下行链路的系统结构,提出了一种基于帧结构的可供DSP实现的多用户MIMO-CDMA全系统的仿真模型,接收端采用MRC最大似然接收提高了系统的接收分集增益,最后对不同用户数和不同PN长度条件下的多用户MIMO-CDMA系统进行了性能分析和评价。     

用于移动通信系统的发送分集技术

在分析接收分集技术最大比率接收合并(MRRC)方案的基础上，介绍了一种两分支发送分集方案。该方案采用2个发射天线、1个接收天线，可提供与1个发送天线、2个接收天线情况下的MRRC方案相同的分集增益。由于发送符号采用了正交性设计，该发送分集方案在接收端可以将不同的发送符号分离开来，分别进行最大似然检测。仿真结果表明，运用分集技术可大大改善无线通信系统的性能，且该方案和一发两收的MRRC方案性能相似、计算复杂性相同。该发送分集方案能更好地应用于移动通信系统。     

空间相关莱斯衰落信道下基于部分信道信息的多用户预编码与调度算法

在空间相关的莱斯衰落信道模型下,针对多用户MIMO(Multiple-Input Multiple-Output)系统潜在的多用户分集增益和空间分集增益,该文提出了一种基于部分信道信息的多用户预编码与调度算法。结合部分瞬时信道信息和统计信息,利用约束最大似然估计对各用户信道矢量进行估计,然后利用估计的各用户信道调度多个用户进行预编码。仿真结果表明,该方案以较少的反馈开销,获得了较大的性能增益。     

基于中继放大转发的多天线基站下行传输性能分析

研究基站具有多根天线、中继与用户具有单天线的中继放大转发多用户系统,利用正交空时分组编码和机会主义调度策略同时获得空间分集和多用户分集.针对自适应增益和固定增益转发策略,首先导出独立不同分布瑞利衰落下系统中断概率和误码率的闭合表达式;随后根据高信噪比下系统的渐近性能确定系统的分集阶数,分析空间分集和多用户分集对系统性能的联合影响,并与单跳多天线基站下行传输系统进行性能比较;最后利用计算机仿真验证理论分析结果的正确性.     

多天线的发送分集系统中多用户分集的研究

研究了如何通过选择发送天线和天线组合来提高整个多用户多天线系统的性能。本文分别讨论了STOBC、选择式发送分集、相位加权等各种发送信号结构下系统的调度性能。和其它发送信号结构相比,STOBC对于调度性能有着很大危害。信道反馈信息越多,调度性能就越好。不同的发送天线选择和组合策略得到不同的性能。本文讨论了三种选择策略:max-max, max-sum和max-min。max-max的调度性能优于其它两种策略。因此,发送信号的结构和选择策略都严重影响着多用户分集。     

The impact of antenna diversity in packet data systems with scheduling

It has been observed through simulations of some specific scheduling algorithms that multiuser diversity gains in packet data systems with channel-aware scheduling can be reduced in the presence of any form of link diversity, such as transmit antenna diversity or wideband multipath diversity. We establish that asymptotically, in the limit of large number of transmit antennas and users, the maximum throughput achieved by any optimal scheduling algorithm in the presence of transmit diversity under signal-to-noise-ratio-only feedback can be infinitely worse than that of a system with no diversity. Our results are general and are independent of any particular scheduling algorithm.     

The diversity gain of transmit diversity in wireless systems withRayleigh fading

In this paper, we study the ability of transmit diversity to provide diversity benefit to a receiver in a Rayleigh fading environment. With transmit diversity, multiple antennas transmit delayed versions of a signal to create frequency-selective fading at a single antenna at the receiver, which uses equalization to obtain diversity gain against fading. We use Monte Carlo simulation to study transmit diversity for the case of independent Rayleigh fading from each transmit antenna to the receive antenna and maximum likelihood sequence estimation for equalization at the receiver. Our results show that transmit diversity with M transmit antennas provides a diversity gain within 0.1 dB of that with M receive antennas for any number of antennas. Thus, we can obtain the same diversity benefit at the remotes and base stations using multiple base-station antennas only     

Opportunistic Feedback for Multiuser MIMO Systems With Linear Receivers

A novel multiuser scheduling and feedback strategy for the multiple-input multiple-output (MIMO) downlink is proposed in this paper. It achieves multiuser diversity gain without substantial feedback requirements. The proposed strategy uses per-antenna scheduling at the base station, which maps each transmit antenna at the base station (equivalently, a spatial channel) to a user. Each user has a number of receive antennas that is greater than or equal to the number of transmit antennas at the base station. Zero-forcing receivers are deployed by each user to decode the transmitted data streams. In this system, the base station requires users' channel quality on each spatial channel for scheduling. An opportunistic feedback protocol is proposed to reduce the feedback requirements. The proposed protocol uses a contention channel that consists of a fixed number of feedback minislots to convey channel state information. Feedback control parameters including the channel quality threshold and the random access feedback probability are jointly adjusted to maximize the average throughput performance of this system. Multiple receive antennas at the base station are used on the feedback channel to allow decoding multiple feedback messages sent simultaneously by different users. This further reduces the bandwidth of the feedback channel. Iterative search algorithms are proposed to solve the optimization for selection of these parameters under both scenarios that the cumulative distribution functions of users are known or unknown to the base station     

Antenna diversity in multiuser data networks

We consider the use of multiple antennas at the transmitter and/or the receiver to provide open-loop spatial diversity in a multiuser wireless data network. With channel quality information (CQI) available to the transmitter, and by always scheduling the transmission to the active user having the best channel conditions at the time of scheduling, another form of diversity, termed multiuser diversity, is obtained in a data system. This paper provides an analysis of the interaction between these two forms of diversity. From a network point of view, we prove that the asymptotic sum rate, in the limit of a large number of active homogeneous users and subject to the same average total transmit power, is inversely related to the number of transmit antennas for independent and identically distributed (i.i.d.) flat Rayleigh fading channels. In the case of i.i.d. flat Rician fading, the asymptotic sum rate also depends inversely on the number of transmit antennas, but directly on the number of receive antennas. Numerically, we show that the total diversity gain is also constrained by finite CQI quantization and channel fading statistics.     

On Scheduling for Multiple-Antenna Wireless Networks Using Contention-Based Feedback

Multiuser diversity gain is an effective technique for improving the performance of wireless networks. This gain can be exploited by scheduling the users with the best current channel conditions. However, this kind of scheduling requires that the base station (or access point) knows some kind of channel quality indicator (CQI) information for every user in the system. When the wireless link lacks channel reciprocity, each user must feed back this CQI information to the base station. The required feedback load makes exploiting multiuser diversity extremely difficult when the number of users becomes large. To alleviate this problem, this paper considers a contention-based CQI feedback where only users whose channel gains are larger than a threshold are allowed to transmit their CQI information through a spread-spectrum based contention channel. Considering the capture effect in this contention channel, it is shown that i) the multiuser diversity gain can be exploited regardless of the number of transmit antennas at the base station and ii) the total system throughput exponentially approaches that of the full feedback scheme as the spreading code length of the contention channel linearly increases. In addition, it is also shown that multiuser diversity can be maintained with the feedback delay of time-variant channels. We also consider the issue of differentiated rate scheduling, in which the base station gives different rates to different subsets of mobiles. In this scenario, mobiles feed back their CQI with some access probability, and we show this technique causes only a negligible throughput loss compared to the case without supporting differentiated rate.     

Performance Analysis of Multiuser Diversity in MIMO Systems with Antenna Selection

In this paper, a framework is presented to analyze the performance of multiuser diversity (MUD) in multiuser point-to-multipoint (PMP) MIMO systems with antenna selection. Based on this framework, the tight closed-form expressions of outage capacity and average symbol error rate are derived for the multiuser transmit antenna selection with maximal-ratio combining (TAS/MRC) system, by which we show how and with what characteristics antenna selection gains, MIMO antenna configurations and fading gains impact on the system performance, with an emphasis on the study of multiuser diversity influence. From both theoretical and simulation results, our study shows that in multiuser PMP TAS/MRC systems an diversity order equals to the product of the number of transmit antennas, number of receive antennas and number of users can be achieved; what's more, users plays a key role in the system performance and can be viewed as equivalent 'virtual" transmit antennas, which is the source of the multiuser diversity inherent exists in the multiuser system. This kind of diversity can be efficiently extracted in the design of multiantenna systems.     

A random beamforming technique in MIMO systems exploiting multiuser diversity

A random beamforming technique for multiple-input multiple-output (MIMO) systems that simultaneously obtains downlink multiuser diversity gain, spatial multiplexing gain and array gain by feeding back only effective signal-to-noise ratios (SNRs) is described. In addition, power control using waterfilling is employed to improve the throughput of our method in correlated channels. In a slow fading channel, we prove that the throughput of the proposed method converges to that of eigen beamforming when many users are in a cell. The number of users required to achieve capacity bound increases with the number of antennas and SNR was determined. However, the capacity bound is achieved even with a small number of users, e.g., 16 users in a cell, when the SNR is low, e.g., 0 dB, and the number of transmit and receive antenna is small, e.g., two. We also find that the effect of waterfilling is more noticeable in correlated channels.     

A simple transmit diversity technique for wireless communications

This paper presents a simple two-branch transmit diversity scheme. Using two transmit antennas and one receive antenna the scheme provides the same diversity order as maximal-ratio receiver combining (MRRC) with one transmit antenna, and two receive antennas. It is also shown that the scheme may easily be generalized to two transmit antennas and M receive antennas to provide a diversity order of 2M. The new scheme does not require any bandwidth expansion or any feedback from the receiver to the transmitter and its computation complexity is similar to MRRC     

分布式天线系统的多用户分集

分布式天线系统(DAS)由于其具有更大的覆盖范围,较好的功率效率,以及更高的系统容量在近年来备受关注,但很多文献都只是基于以上几个方面进行分析,而没有考虑多用户分集。该文提出了一种考虑用户公平性的最小容量损失轮询调度算法,并以该算法为基础,从多用户分集的角度研究了基于迫零波束成形算法的下行分布式天线系统的信道容量。结果表明,在天线数目和总功率相同的情况下,采用分布式天线系统可以得到比集中放置天线系统(CAS)更大的多用户分集增益。最小容量损失轮询调度算法明显优于普通轮询调度算法,该算法对DAS和CAS都适用。