MIMO系统在慢衰落信道中的实用反馈方案

提出了一种可变码长编码方式,其应用于慢衰落信道中的基于有限反馈迫零预编码的多用户MIMO系统.该方案利用了慢衰落信道中每个配置单天线的用户连续两次从码本中选择的量化信道矢量的相关性来对码本进行有效编码,从而减少了需反馈的统计平均比特数,节省了信道带宽.在满足一定条件的慢衰落信道中,在需要反馈的比特数相同的情况下,新方案比应用基于量化的天线合并算法每用户多天线的多用户MIMO系统在信道总容量方面具有更好的性能,且该编码对应于二叉树形结构的叶子,故基站可自动识别其编码,可用于实际的通信系统之中.     

单中继多用户MIMO系统中继预编码新方案

针对单中继多用户多输入多输出(MIMO)通信系统中用户间干扰问题,提出了一种可以有效抑制用户间干扰的中继接收和预编码方案。该方案在中继端对信号进行迫零(ZF)接收,利用最大化信漏噪比(SLNR)准则设计中继端预编码向量,并针对难以获得最优解的情况,利用迭代解出次优解。仿真结果表明,与中继端ZF预编码相比,SLNR预编码更好地抑制了用户间干扰,相同和速率性能下,基站发送端功率比ZF预编码减少4 dB,具有实用价值。     

协同MIMO系统中的自适应用户数选择算法研究

提出了一种在协同多入多出(Co-MIMO,collaborative MIMO)系统中的自适应用户数选择算法.基站向调度器报告各个移动台从本基站接收到的信号、干扰及噪声功率,调度器据此计算各个基站支持用户数的最佳方案,反馈给相应的基站.自适应方案根据信道的变换灵活改变基站和移动台之间的从属关系,达到有效降低小区边缘用户的小区间干扰、优化小区边缘系统容量的目的.仿真结果证明了在发射端多用户预编码下自适应支持用户数选择算法相对于支持固定用户数的优势.     

基于码本的有限反馈非酉矩阵预编码多用户MIMO系统

该文提出了一种基于码本的有限反馈非酉矩阵预编码下行多用户MIMO系统。该方案根据用户反馈的信道信息SINR在发送端进行调度和预编码来提高系统容量。预编码的码本设计依据格拉斯曼空间装箱原理,并将码本中的向量按其相关性构成非酉矩阵来提高预编码增益和抑制多用户共道干扰。新方案反馈量少、复杂度低,在相同情况下比传统的单用户MIMO系统和基于码本的酉矩阵预编码多用户MIMO系统都具有更好的性能。     

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。     

多用户MIMO系统下行链路的随机多波束复用技术

该文提出了一种随机多波束多用户复用技术,充分利用多用户分集以及基站多天线的空间自由度来提高系统吞吐量。不同于传统的随机波束形成技术,该技术首先在给定预编码码本内随机选取一个码字,然后调度多个空分复用用户以及其余预编码矩阵。该文采用了一种逐次调度的方式,第一次训练调度一个主发送用户并确定一个次发送预编码矩阵,通过第二次训练选择次发送用户,这种方式能以很小的反馈开销有效控制复用用户之间的相互干扰。同时,该文所提技术能进一步推广到用户具有不同天线配置的异构情形。仿真结果表明,该文技术在具有不同相关性的信道环境下都能获得较优的系统吞吐量。     

利用有向图实现多用户下行多天线系统的用户选择

在多用户下行多天线系统中,基站可利用信道状态信息将独立的数据流同时发送到多个用户以得到更高的信道容量.由于在实际系统中基站很难实时得到各用户的完全信道状态信息,可以基于有限的信道反馈量进行用户选择来提高系统的和速率.提出了一种部分信道状态信息下利用有向图实现多用户下行多天线系统用户选择的算法.该算法将用户选择的问题变成了在有向图中寻找闭合环的问题,可以大幅度降低用户选择的复杂度;克服了常规的随机波束成形算法只有在用户数非常多的时候才能有效工作的弱点,在用户数少或中等的时候能得到比其更好的性能.因此该算法更适合在实际系统中应用.     

一种多用户多中继MIMO通信系统迭代预编码方案

该文针对多中继MIMO通信系统多用户传输时的用户间干扰问题,提出了一种可以有效抑制多用户干扰的发射端与中继端迭代预编码的方案。该方案首先在中继端进行多中继联合的迫零预编码,然后将基站与多用户之间等效为一个点到多点的MIMO信道,采用最大化信漏噪比的预编码方法,在发射端进一步消除用户间的干扰,同时避免了噪声放大的影响,进而通过中继端预编码与发射端预编码的多次迭代,更好地保持了多中继并行传输引入空间分集所带来的性能优势,有效地抑制了用户间干扰,获得了更高的系统和容量。     

带内全双工多用户MIMO系统中的用户选择

考虑一种多用户MIMO的传输设计,配置发射天线阵列和接收天线阵列的蜂窝基站可以工作在带内全双工传输模式。在该全双工通信方案中,基站的下行发射信号对基站的上行接收产生显著的干扰,即自干扰。这里,下行预编码处理和上行发射协方差矩阵处理将被依次进行,以简化全双工的设计。其次,为了进一步改善上、下行信道的和速率性能,我们提出一种尝试性的下行用户选择方案,其基本思想是:当某一个下行用户的信道矩阵的范数较小时,关闭该下行用户的数据流。计算机仿真结果表明,在基站下行总发射功率受限时,在低的和中等的下行信噪比区域,用户选择有助于提高下行和速率;在高的上行信噪比区域,简化的用户选择使得上行和速率明显提高。      

基于块对角化的格基规约GMD矢量预编码

研究了对多用户多输入多输出下行链路进行块对角化后,使用格基规约算法的几何均值分解矢量预编码的实现方法。根据块对角化思想将多用户多天线信道分解为等价并行子信道,基于等价子信道给出了单个用户的几何均值分解矢量预编码的传输方案,通过使用格基规约算法分别结合连续干扰消除和垂直分层空时编码两种方法,求解矢量预编码中的扰动矢量。仿真表明,提出的方法误码率性能优于块对角化矢量预编码算法2 dB以上,而且能在不降低系统性能的前提下降低计算复杂度。     

MU-MISO Transmission with Limited Feedback

Downlink multi-user transmission techniques that allow several users to be served simultaneously in frequency and time have been recognized as a promising means to increase system capacity. However they require accurate channel state information (CSI) at the base station (BS) such that appropriate signal processing can be performed to separate multiple users in the space domain. In practice however, CSI cannot be perfectly known to the BS due to the capacity-limited feedback channel in FDD systems for instance. In this letter we investigate multiuser multiple-input single-output (MU-MISO) transmission with limited feedback. In particular we identify the useful information that the BS needs to deal with inter-user interference and a codebook design is derived accordingly. In addition, an adaptive threshold-based feedback approach is proposed, where only users with good feedback quality are allowed to do feedback. It is shown that the adaptive scheme can effectively improve the system performance given a fixed number of feedback bits.     

Low complexity MIMO scheduling with channel decomposition using capacity upperbound

In multiuser MIMO systems, the base station schedules transmissions to a group of users simultaneously. Since the data transmitted to each user are different, in order to avoid the inter-user interference, a transmit preprocessing technique which decomposes the multiuser MIMO downlink channel into multiple parallel independent single-user MIMO channels can be used. When the number of users is larger than the maximum that the system can support simultaneously, the base station selects a subset of users who have the best instantaneous channel quality to maximize the system throughput. Since the exhaustive search for the optimal user set is computationally prohibitive, a low complexity scheduling algorithm which aims to maximize the capacity upper bound is proposed. Simulation results show that the proposed scheduling algorithm achieves comparable total throughput as the optimal algorithm with much lower complexity.     

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     

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.     

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.     

Improved transmit steering for MIMO-OFDM downlinks with distributed base station antenna arrays

Space-division multiple-access (SDMA) is a communication technique that enables a base station to communicate with several mobile users simultaneously. The ability of the base station to spatially separate several users depends on the pairwise cross correlations between the channel matrices of the users (the inter-user correlation). In this paper, we propose an improved steering downlink multiple-input-multiple-output-orthogonal frequency-division multiplexing (OFDM) system that reduces both the inter-user correlation and the near-far problem resulting in a significant enhancement in system performance. In this system, several base station multiantenna arrays are distributed in a given area. Each array communicates with the base station via optical fiber links, and all transmitter signal processing is performed at the base station. Multiantenna users are spatially separated such that only a subset of the users is served by each tone of the OFDM symbol. The served users are selected based on an algorithm that reduces the inter-user correlations. Distributing the arrays around the users also balances the channel matrix leading to significant reduction in the effect of the near-far problem. The channel matrix of each user is assumed correlated and Ricean distributed. Several data symbols can be spatially multiplexed to each user over each OFDM tone with high reliability and with good total system capacity.     

Exploiting Multiuser Diversity With Imperfect One-Bit Channel State Feedback

It has been well recognized that significant throughput gains can be leveraged in multiuser wireless communication systems by exploiting multiuser diversity with a smart scheduler. This scheduler collects channel state information (CSI) from all users and allocates the resources to the user(s) experiencing favorable channel conditions. However, for a frequency-division-duplex system with a large number of users, how to efficiently collect the required CSI will be a challenging task, especially when the feedback links are of limited capacity. In this paper, we propose a scheduling algorithm to exploit multiuser diversity with possibly imperfect one-bit channel state feedback. The basic idea is to define a threshold lambda and let each user report one-bit information to the scheduler about the comparison between its measured channel fading level and lambda. Correspondingly, the scheduler uses these feedback bits to classify all users into two sets and assigns the channel to one user belonging to the set experiencing favorable channel conditions. Several implementation schemes are developed by attacking the optimization of lambda under different system configurations, covering both the case when the one-bit feedback is perfect and those when the one-bit feedback is imperfect. Computer simulations show that when the user number is large, say, more than ten users, the proposed scheduling supports significantly larger data rate over the round-robin scheduling, while in comparison with the optimum scheduling with complete CSI, the performance loss is limited if the one-bit feedback is of high reliability. In addition, our studies show that we can effectively enhance the robustness against feedback imperfectness by incorporating the feedback reliability into optimization of lambda     

Opportunistic Space-Division Multiple Access With Beam Selection

In this paper, a novel transmission technique for the multiple-input multiple-output (MIMO) broadcast channel is proposed that allows simultaneous transmission to multiple users with limited feedback from each user. During a training phase, the base station modulates a training sequence on multiple sets of randomly chosen orthogonal beamforming vectors. Each user sends the index of the best beamforming vector and the corresponding signal-to-interference-plus-noise ratio for that set of orthogonal vectors back to the base station. The base station opportunistically determines the users and corresponding orthogonal vectors that maximize the sum capacity. Based on the capacity expressions, the optimal amount of training to maximize the sum capacity is derived as a function of the system parameters. The main advantage of the proposed system is that it provides throughput gains for the MIMO broadcast channel with a small feedback overhead, and provides these gains even with a small number of active users. Numerical simulations show that a 20% gain in sum capacity is achieved (for a small number of users) over conventional opportunistic space division multiple access, and a 100% gain (for a large number of users) over conventional opportunistic beamforming when the number of transmit antennas is four.     

Opportunistic Scheduling in Multiuser OFDM Systems with Clustered Feedback

This paper proposes a random access based feedback protocol for multiuser orthogonal frequency division multiple access (OFDMA) systems with clustered feedback, where users are assigned to clusters of subcarriers based on the feedback of channel state information to the base station. To reduce feedback requirements, the proposed protocol employs a contention channel with a fixed number of feedback minislots to carry feedback for each cluster. Users send a feedback message with some probability in the appropriate feedback slot if their average channel quality on a cluster is above a threshold. Both arithmetic and geometric averages are used and compared as channel quality measures. The threshold and feedback probability are jointly adjusted to maximize the estimated average sum rate of all users based on two different limited feedback information. Numerical simulations illustrate that it is possible to achieve multiuser diversity gain despite collisions in the feedback channel and with only a few feedback opportunities.