多天线无线数据通信系统中多用户分集的研究

研究当接收天线不少于发送天线时多输入多输出(MIMO)系统的多用户分集能力。首先从理论上分析了发送天线个数等于1和2时最大似然接收和迫零接收系统的平均吞吐量和调度增益,以及仿真分析了发送天线个数大于2时系统性能。理论分析和仿真表明：在多用户的MIMO系统中,接收的平均信噪比、用户个数、收发天线个数、接收机的结构等对于多用户分集有很大的影响。当发送天线个数为1时,接收天线较少(1,2,3)和平均信噪比为．10dB时调度增益很大,但调度增益随着天线个数和发送功率增大急剧下降。和最大似然接收相比,迫零接收具有更大的多用户分集增益,因此迫零接收机的吞吐量可以很容易超过最大似然接收机。     

多天线系统中的多用户分集方案的性能分析及比较

在多天线多用户无线通信系统中,可以利用用户之间彼此独立的随机衰落信道的特点,结合随机波束成型技术,或者空间分集技术,或者天线选择技术,获得多用户分集增益。然而不同多用户分集方案的系统性能不尽相同。该文分析了在多输入单输出(MISO)信道中目前几种典型的多用户分集方案的性能,给出系统吞吐率的定量表达式,并综合比较了不同用户数和不同天线数对各种多用户分集方案性能的影响,为实际多天线多用户系统设计提供了有益的参考。     

分布式移动通信系统中多用户分集分析

分析了分布式移动通信系统中,Rayleigh-lognormal复合信道下,发送分集对下行链路多用户分集的影响,发现基于最大比发送(MRT)和基于选择发送分集(STD)调度算法获得了几乎相同的多用户分集增益。该结论表明,在分布式移动通信系统中,采用基于发送分集的调度方案并不能有效地提高系统的吞吐量。     

空间相关对发射分集系统的多用户分集性能的影响

研究空间相关对发射分集结合调度策略的多用户系统下行链路性能的影响.推导出发射天线之间存在空间相关的条件下,TxAA(Transmit Adaptive Array)、STBC(Space-Time Block Coding)结合调度策略的多用户系统平均容量的表达式,在此基础上研究其平均容量.通过仿真验证理论分析的有效性.结果表明,发射天线之间的空间相关性会提高多用户系统的平均容量,另外TxAA在各种相关条件下的系统平均容量都优于STBC.     

天线分集技术

在无线通信中,受多径干扰的影响,传输信号有较大的失真。采用空间分集技术,通过使用多发送天线和多接收天线系统(MIMO系统),可以从一定程度上保证传输信号的质量。空时编码技术从编码的角度,为空间分集技术提供了发送信号的策略。     

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.     

无线移动通信中的协作分集技术

介绍了一种可用于蜂窝网等多种无线网络的新型的协作分集技术,在多用户的环境中,它允许具有单天线的移动终端共享彼此的天线,形成虚拟的多发射天线,从而得到分集增益.从系统模型、系统性能、协作方式等方面介绍了这项技术,并分析了存在的问题.     

基于不同调度算法的多用户分集MIMO中继信道容量分析

采用平均容量性能来定量分析多用户分集多输入多输出(MIMO)中继信道,中继节点使用放大转发(Amplify-and-forward,AF)协议将接收到的源节点的发送信号重发给目的节点.研究了多用户MIMO中继信道的不同调度算法,即容量公平、最大特征根、最小特征根调度策略和空间独立性调度算法,结果表明空间独立性调度算法有...     

第三代移动通信系统中的发送分集技术

发送分集技术简单来说就是发送端利用多天线结构对发送信号引入空间分集。利用反馈的信道状态信息，可以使系统在获得分集增益的同时，又获得额外天线增益。本文介绍了第三代移动通信中的发送分集技术。     

无线通信中的协作分集技术

协作分集技术是在多用户环境下,具有单天线的多个终端可以共享彼此的天线,形成一个虚拟的多天线系统,来实现上行链路的发送分集。协作分集技术能够提高系统的吞吐量,减少发送功率,并能降低系统的误码率,因此受到了广泛的关注。     

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.     

Diversity order of decode-and-forward MIMO relaying with transmit antenna selection

This article analyzes the diversity order of several proposed schemes, where the transmit antenna selection (TAS) strategies are combined with low-complexity decode-and-forward (DF) protocols in the multiple-input multiple-output (MIMO) relaying scenario. Although antenna selection is a suboptimal form of beamforming, it enjoys the advantages of tractable optimization and low feedback overhead. Specifically, this article proposes schemes that combine TAS strategies with fixed decode-and-forward (FDF) and selection decode-and-forward (SDF) protocols. Following that, the asymptotic expressions of outage probabilities are derived and the diversity order of the proposed schemes analyzed. These kinds of combination of transmit antenna selection strategies and low-complexity decode-and-forward protocols can achieve partial diversity order in the MIMO relaying scenario. The numerical simulations verify the analysis.     

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.     

Low complexity per-antenna rate and power control approach for closed-loop V-BLAST

Previous studies have shown that per-antenna rate and power control can greatly increase the data throughput of vertical Bell Labs layered space-time (V-BLAST), while an extra transmit antenna selection can provide additional diversity advantage. We combine the transmit antenna selection with power and rate control for each antenna. We derive a simple criterion for minimum bit-error rate (BER) or minimum total transmit power when the data throughput is constant over time. Zero-forcing and zero-forcing successive interference cancellation detections are considered. For practical implementation, we also present a fast algorithm that gives near-optimal performance with very low complexity. Simulation results show that the proposed closed-loop BLAST outperforms the open-loop V-BLAST significantly in terms of BER performance, especially when the antennas exhibit strong fading correlations.     

MIMO Broadcast Scheduling with Limited Feedback

We consider multiuser scheduling with limited feedback of partial channel state information in MIMO broadcast channels. By using spatial multiplexing at the base station (BS) and antenna selection for each user, we propose a multiuser scheduling method that allocates independent information streams from all M transmit antennas to the M most favorable users with the highest signal-to-interference-plus-noise ratio (SINR). A close approximation of the achievable sum-rate throughput for the proposed method is obtained and shown to match the simulation results very well. Moreover, two reduced feedback scheduling approaches are proposed. In the first approach, which we shall refer to as selected feedback scheduling, the users are selected based on their SINR compared to a predesigned threshold. Only those selected users are allowed to feed back limited information to the BS. The resultant feedback load and achievable throughput are derived. It will then be demonstrated that with a proper choice of the threshold, the feedback load can be greatly reduced with a negligible performance loss. The second reduced feedback scheduling approach employs quantization for each user, in which only few bits of quantized SINR are fed back to the BS. Performance analysis will show that even with only 1-bit quantization, the proposed quantized feedback scheduling approach can exploit the multiuser diversity at the expense of slight decrease of throughput.     

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.     

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.     

Rate scheduling in multiple antenna downlink wireless systems

We consider scheduling strategies for multiantenna and multibeam cellular wireless systems for high-speed packet data services on the downlink. We establish a fundamental connection between the stability region of the queuing system and the set of feasible transmission rates, which provides the basis for the scheduling algorithm proposed in this paper. Transmission using adaptive steerable beams and fixed sector beams are considered and average delay versus throughput results are obtained through simulations for the proposed scheduling scheme in each case. While in single antenna systems multiuser diversity gains are achieved by the scheduling algorithms that transmit to a single user in each scheduling interval, our results show that with multiple antennas, transmitting to a carefully chosen subset of users has superior performance. The multiantenna scheduling problem is closely related to the problem of coordinated scheduling for transmission through multiple base stations, where a user can receive signals from several base stations simultaneously. We consider the special case when three single-antenna base stations are allowed to cooperate and transmit to the users in the triangular region between the base stations and propose scheduling strategies that demonstrate significant gains.