Design of Multiuser Downlink Linear MIMO Precoding Systems With Quantized Feedback

We treat the problem of sum-rate maximization via scheduling and linear precoding in multiuser (MU) multiple-input–multiple-output (MIMO) systems with quantized feedback. We formulate the optimal quantized linear precoder design problem and provide the numerical procedure for finding the solution. We also provide a simple scheduling scheme that exploits the MU diversity gain. With the aim of sum-rate maximization, we further address the quantization codebook design based on the capacity measure by introducing a new distance metric. It is demonstrated that, with or without scheduling, the proposed optimal linear precoding scheme achieves significant gain over the conventional linear precoder designs with similar feedback overhead, such as the zero-forcing precoder. Moreover, although the proposed quantization codebook design improves the performance of other existing MU-MIMO precoding schemes, the performance gain offered by it is typically much higher when it is coupled with the proposed linear precoding design.      

一种基于信干比门限反馈的MIMO下行系统自适应传输策略

以降低系统反馈为目标,提出一种在多天线下行系统基于信干比门限反馈的自适应传输策略.基于正交随机波束成形,提出仅当用户最大信干比超过门限时才将最大信干比和对应的波束序号反馈给基站,并给出了系统平均总速率与该信干比门限的关系表达式.仿真结果表明,通过选取有效的信干比门限,可在几乎不降低总速率的情况下,系统反馈极大降低.     

Improved Multiuser MIMO Unitary Precoding Using Partial Channel State Information and Insights from the Riemannian Manifold

Multiple-input multiple-output (MIMO) systems can be leveraged to increase capacity in fading channels. Especially in multiuser downlink communication systems, it has been shown that knowledge of channel state information at the transmitter (CSIT) is critical to leverage the capacity gain available from multiple antennas. When duplexing is performed using time division, CSIT can often be successfully obtained when channel reciprocity is available. CSIT acquisition, however, is much more difficult in frequency division duplexing. Sending feedback on the uplink has been shown to be a powerful technique to improve downlink performance in single user MIMO systems. The basic idea is to restrict the CSIT to a B bit codebook so that the mobiles can easily transmit these bits on the uplink. In this paper, we consider the multiuser downlink model with unitary precoding when there is a codebook consisting of 2^B unitary matrices that the precoder is restricted to lie in. This codebook is designed offline and known to both the basestation and all users. Each user sends back signal-to-interference plus noise ratio (SINR) information along with binary feedback about the unitary precoder. Based on the CSIT received on the uplink, the basestation selects one of the unitary matrices in the codebook to maximize the sum-rate. For this set-up, we first analyze the sum-rate performance of the unitary precoding scheme. We then show that the codebook of unitary precoders represents a collection of points in a special kind of manifold and show how the achievable sum-rate performance relates to the minimum distance of the codebook points in this space. Finally, we present a framework for constructing the codebook to maximize this minimum distance. Monte Carlo simulation results are presented to show the sum-rate performance of the proposed codebook design.     

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.     

一种多用户MIMO/OFDM下行链路保障QoS的自适应传输机制

在用户QoS参数限制下,该文提出一种在多用户MIMO/OFDM下行链路使系统总速率最大的资源分配机制。基站应用空分多址接入,使得每个子载波可支持多个用户,应用线性预编码方法抵消用户间的干扰,提出保障QoS的自适应功率、比特分配方案。该文并提出两种可应用到实际系统的低复杂度的比特加载和比特去除算法,仿真结果表明,该两种方法性能可非常接近最优遍历算法。     

System‐Level Performance of Limited Feedback Schemes for Massive MIMO

To implement high‐order multiuser multiple input and multiple output (MU‐MIMO) for massive MIMO systems, there must be a feedback scheme that can warrant its performance with a limited signaling overhead. The interference‐to‐noise ratio can be a basis for a novel form of Codebook (CB)‐based MU‐MIMO feedback scheme. The objective of this paper is to verify such a scheme's performance under a practical system configuration with a 3D channel model in various radio environments. We evaluate the performance of various CB‐based feedback schemes with different types of overhead reduction approaches, providing an experimental ground with which to optimize a CB‐based MU‐MIMO feedback scheme while identifying the design constraints for a massive MIMO system.     

Sum-rate analysis of multiuser MIMO system with zero-forcing transmit beamforming

In this letter, we present the exact sum-rate analysis of the multiuser multiple-input multiple-output (MIMO) systems with zero-forcing transmit beamforming (ZFBF). We develop the analytical expressions of the ergodic sum-rate for two low-complexity user selection strategies for the dual-transmit-antenna scenario. Based on the analytical results, we examine the parameter optimization problem to properly trade off between channel power gain and directional gain in term of maximizing the ergodic sum rate.     

Performance analysis of discrete feedback schemes for downlink multiuser diversity in OFDMA systems

This paper analyzes discrete feedback of various feedback schemes for multiuser diversity in a multiple subchannel environment. In these schemes of full feedback, fixed feedback, adaptive feedback, and threshold-based feedback, users report a quantized discrete value to indicate their modulation and coding scheme instead of feeding back the full values of the channel state. A closed-form expression of the sum-rate capacity is developed to evaluate the feedback schemes. By comparing the feedback schemes in terms of the sum-rate capacity and the feedback load, this paper provides guidance for the selection of a feedback scheme. Additionally, a mixed-feedback scheme is proposed to reduce the amount of allocated feedback resources in the threshold-based feedback scheme, where the base station adjusts the capacity-feedback tradeoff by controlling two parameters, namely the number of subchannels and a threshold.     

Performance of vector perturbation multiuser MIMO systems with limited feedback

This paper considers the multiuser multiple-input multiple-output (MIMO) Rayleigh fading broadcast channel. We consider the case where the multiple transmit antennas are used to deliver independent data streams to multiple users via a multiuser technique known as vector perturbation. We propose lattice-theoretic and rate-distortion based approaches to analyze the performance of these systems, taking into account the practical restrictions imposed by limited feedback and training. We show that performance is primarily determined by the ratio between the number of users and the number of transmit antennas. We then propose a new practical low-complexity low-rate feedback scheme, and show that the performance approaches the ideal rate-distortion based scheme.     

Multiuser MIMO OFDM Based TDD/TDMA for Next Generation Wireless Communication Systems

Multiple-input multiple-output (MIMO) wireless technology in combination with orthogonal frequency division multiplexing (MIMO OFDM) is an attractive air-interface solution for next-generation wireless local area networks (WLANs), wireless metropolitan area networks (WMANs), and fourth-generation mobile cellular wireless systems. In this paper, one multiuser MIMO OFDM systems with TDD/TDMA was proposed for next-generation wireless mobile communications, i.e., TDD/TDMA 4G, which can avoid or alleviate the specific limitations of existing techniques designed for multiuser MIMO OFDM systems in broadband wireless mobile channel scenarios, i.e., bad performance and extreme complexity of multiuser detectors for rank-deficient multiuser MIMO OFDM systems with CDMA as access modes, extreme challenges of spatial MIMO channel estimators in rank-deficient MIMO OFDM systems, and exponential growth complexity of optimal sub-carrier allocations for OFDMA-based MIMO OFDM systems. Furthermore, inspired from the Steiner channel estimation method in multi-user CDMA uplink wireless channels, we proposed a new design scheme of training sequence in time domain to conduct channel estimation. Training sequences of different transmit antennas can be simply obtained by truncating the circular extension of one basic training sequence, and the pilot matrix assembled by these training sequences is one circular matrix with good reversibility. A novel eigenmode transmission was also given in this paper, and data symbols encoded by space–time codes can be steered to these eigenmodes similar to MIMO wireless communication systems with single-carrier transmission. At the same time,, an improved water-filling scheme was also described for determining the optimal transmit powers for orthogonal eigenmodes. The classical water-filling strategy is firstly adopted to determine the optimal power allocation and correspondent bit numbers for every eigenmode, followed by a residual power reallocation to further determine the additional bit numbers carried by every eigenmode. Compared with classical water-filling schemes, it can also obtain larger throughputs via residual power allocation. At last, three typical implementation schemes of multiuser MIMO OFDM with TDMA, CDMA and OFDMA, i.e., TDD/TDMA 4G, VSF-OFCDM and FuTURE B3G TDD, were tested by numerical simulations. Results indicated that the proposed multiuser MIMO OFDM system schemes with TDD/TDMA, i.e., TDD/TDMA 4G, can achieve comparable system performance and throughputs with low complexity and radio resource overhead to that of DoCoMo MIMO VSF-OFCDM and FuTURE B3G TDD.     

Sum-Rate Analysis of Downlink Channels with 1-Bit Feedback

The sum-rate capacity of a single-input single-output (SISO) downlink with Rayleigh flat fading channels and K users, grows as log log K when optimal scheduling is employed. However, the optimal scheduling requires that the full channel state information (CSI) for all users be available to the transmitter. In this work it is shown that the same rate growth holds even if the feedback rate from the users to the transmitter is reduced to 1-bit per fading block. A simple analysis for this setup is presented, resulting in a closed form expression for the achievable ergodic sum-rate. The mechanism of setting a sub-optimal threshold is elucidated by simple lower and upper bounds to the sum-rate. Among the insights afforded by the sum-rate expression and the bounds, is that application of the sub-optimal threshold demonstrates the same scaling law as the optimal full CSI scheme, asymptotically with the number of users K     

Performance analysis of joint switched diversity and adaptive modulation

In this paper, a simple and practical system based on a switched diversity scheme with adaptive modulation is presented. This system provides a reduced number of channel estimation while offering the optimum spectral efficiency given by a selection diversity system. In addition, the switching threshold is easily manipulated so as to make an efficient use of the tradeoff between spectral efficiency and channel estimation overhead. An extension of switched diversity into a multiuser scheduling is later also considered. This switch-based multiuser access scheme results in a lower average feedback load than that for the optimal selection-based multiuser scheme. Numerical results show that we can obtain a trade-off between spectral efficiency and the feedback load by choosing the switching threshold appropriately.     

Performance Analysis of Scheduling in Multiuser MIMO Systems with Zero-Forcing Receivers

Despite its low-complexity, the zero-forcing receiver is known to suffer from noise enhancement to restore the spatially multiplexed data in a single-user MI MO system. Nevertheless, in the multiuser system, the poor-channel avoidance property of the scheduling technique provides a natural way to overcome the drawback of noise enhancement (Heath et al., 2001). In this paper, we present an analytical framework to evaluate the performance of the zero-forcing receiver operating in the multiuser MIMO system with user scheduling. Using the order statistics technique, we derive closed-form expressions for the sum-rate capacity of the multiuser MIMO system that employs the simple spatial multiplexing at the transmitter and zero-forcing processing at the receiver with a number of scheduling algorithms. These closed-form expressions hold for an arbitrary finite number of users and facilitate efficient numerical evaluations for cases of practical interest. In addition, the tractable analysis provides insight into how the scheduling technique affects the performance of the multiuser MIMO system under scalar feedback and vector feedback. The results are also extended to the case of heterogonous users with unequal average SNR.     

Zero‐forcing and codebook based beamforming scheme for practical usage of multiuser MIMO‐OFDM with uplink channel sounding

Since the concept of the multiuser multiple input multiple output (MU‐MIMO) system has been introduced for enhancement of capacity and flexibility, it has been accepted in various wireless standards. To enjoy the benefits of the MU‐MIMO system, full or partial channel information is necessary at the transmitter, but how to use the full or partial feedback information in the practical system perspective has not been investigated well. In this paper, we analyze the interference of full usage concurrent transmission codebook based on the MU‐MIMO systems and also investigate the usage of channel information for a codebook based scheme and a zero‐forcing beamforming (ZFBF) scheme. Based on the analytic results, we propose two adaptive schemes for the practical usage perspective in MU‐MIMO‐OFDM systems. Firstly, we propose an adjustable uplink channel sounding scheme, which depends on the number of users in a given cell/sector in frequency division duplexing system, with ZFBF MU‐MIMO‐OFDM systems. Secondly, we propose an adaptive switching scheme, which depends on signal‐to‐noise ratio, between the codebook based scheme and the ZFBF scheme. The performance of the proposed scheme is evaluated with computer simulations, and the simulation results show that the proposed scheme provides the enhanced throughput over entire signal‐to‐noise‐ratio regions. Copyright © 2014 John Wiley & Sons, Ltd.     

Low Complexity Iterative-QR (IQR) Precoder Design Based on Structure Blocks for Massive MIMO System

Massive multiple-input multiple-output (MIMO) technology is a promising technique having a high spectral efficiency for future wireless systems. Counterintuitively, the practical issues of transmitted signals are being attenuated by fading, propagation limitations, and implement non-linear precoding are solved by enlarging system dimensions. However, the computational complexity of precoding grows with the system dimensions. The existence block diagonalization (BD) precoding, which completely pre-cancels the multiuser interference is very complicated to implement with the use of a large number of BS antennas, since it considers full multiplexing order. Motivated by the high performance of the BD and generalized for the case when the users have multiple antennas, we propose a structure blocks based on iterative QR decompositions (IQRDs) to compute the precoding scheme. The proposed BLIQR-based precoder designed partitioned the channel matrix into capable square-wise blocks matrix and the IQRDs are applied to the blocks channel matrix. The channel matrix is partitioned such that it can fulfill the multiplexing order for the use in Massive MIMO. The computational complexity of the proposed design is effectively reduced and the sum-rate performance is improved, especially in large number of BS antennas. The performance of the proposed scheme achieves a good trade-off between throughput and computational complexity.     

Threshold-based opportunistic scheduling for ergodic rate guarantees in wireless networks

In this paper, we propose an opportunistic downlink scheduling scheme that exploits multiuser diversity in a wireless network with threshold-based limited feedback. We assume that each user has its own ergodic rate requirement. The design objective of our scheme is to determine the values of thresholds with which heterogeneous ergodic rate requirements of all users are satisfied. In our analysis, we present a formula to check the feasibility of given ergodic rate requirements, and then obtain the feasible thresholds that realize them. We also obtain the optimal thresholds that maximize the ergodic sum-rate of the network while guaranteeing the ergodic rate requirements. Through numerical studies and simulations, we show the usefulness of our scheme and analysis.     

User Selection Criteria for Uplink Spatial Multiplexing MIMO Systems

We study in this paper multiuser uplink scheduling algorithms for Multiple-Input Multiple-Output (MIMO) systems, where the multiusers compete for the MIMO Channel and the scheduler selects one user at a time based on a certain criterion. Then the selected user spatially multiplexes his data over the transmit antennas. This spatial multiplexing (SM) scheme provides high data rates while the multiuser diversity obtained from scheduling improves the performance of the uplink system. At the receiver, the Vertical-Bell-Labs LAyered Space Time architecture (V-BLAST) is used to detect the information layers. The main contribution of this paper is proposing and comparing the performance of several scheduling criteria for MIMO uplink scheduling. In addition, novel V-BLAST capacity bounds based on random matrix theory is presented. Furthermore, we investigate suboptimal schedulers and compare their performance. The main results of this study show that the scheduler that maximizes the optimal MIMO capacity doesn’t work well for a V-BLAST system. Instead, the optimal scheduler that maximizes the V-BLAST capacity is derived and analyzed. In addition, we look into scheduling for SM with sphere decoding and we find that in this case, using MIMO capacity as the scheduling criterion performs the best.     

Optimization of Training and Scheduling in the Non-Coherent SIMO Multiple Access Channel

Channel state information (CSI) is important for achieving large rates in MIMO channels. However, in time-varying MIMO channels, there is a tradeoff between the time/energy spent acquiring channel state information (CSI) and the time/energy remaining for data transmission. This tradeoff is accentuated in the MIMO multiple access channel (MAC), since the number of channel vectors to be estimated increases with the number of users. Furthermore, the problem of acquiring CSI is tightly coupled with the problem of exploiting CSI through multiuser scheduling. This paper considers a block-fading MAC with coherence time T, n uncoordinated users-each with one transmit antenna and the same average power constraint, and a base station with M receive antennas and no a priori CSI. For this scenario, a training-based communication scheme is proposed and the training and multiuser-scheduling aspects of the scheme are jointly optimized. In the high-SNR regime, the sum capacity of the non-coherent SIMO MAC is characterized and used to establish the SNR-scaling-law optimality of the proposed scheme. In the low-SNR regime, the sum-rate of the proposed scheme is found to decay linearly with vanishing SNR when flash signaling is incorporated. Furthermore, this linear decay is shown to be order-optimal through comparison to the low-SNR sum capacity of the non-coherent SIMO MAC. A by product of these SNR-asymptotic analyses is the observation that non-trivial scheduling (i.e., scheduling a strict subset of trained users) is advantageous at low SNR, but not at high SNR. The sum-rate and per-user throughput are also explored in the large-n and large-M regimes. Non-coherent capacity, training, multiple access channel, multiuser scheduling, opportunistic scheduling.     

Feedback threshold with guaranteed QoS in multiuser OFDM systems

A threshold setting scheme is proposed based on the resource management and limited feedback theory in multiuser orthogonal frequency division multiplexing (OFDM) systems. In adaptive resource allocation, the factors denoting the quality of service (QoS) and fairness are both considered as the user weight. From the aspect of feedback outage probability, the proposed algorithm sets the threshold for each user related to its weight, which brings enough feedback to the user with greater weight. Analysis and simulation results show that, compared with the threshold ignoring weights, the proposed scheme has much lower feedback load while with better QoS.     

Duality, achievable rates, and sum-rate capacity of Gaussian MIMO broadcast channels

We consider a multiuser multiple-input multiple- output (MIMO) Gaussian broadcast channel (BC), where the transmitter and receivers have multiple antennas. Since the MIMO BC is in general a nondegraded BC, its capacity region remains an unsolved problem. We establish a duality between what is termed the "dirty paper" achievable region (the Caire-Shamai (see Proc. IEEE Int. Symp. Information Theory, Washington, DC, June 2001, p.322) achievable region) for the MIMO BC and the capacity region of the MIMO multiple-access channel (MAC), which is easy to compute. Using this duality, we greatly reduce the computational complexity required for obtaining the dirty paper achievable region for the MIMO BC. We also show that the dirty paper achievable region achieves the sum-rate capacity of the MIMO BC by establishing that the maximum sum rate of this region equals an upper bound on the sum rate of the MIMO BC.