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。      

Some results on the sum-rate capacity of MIMO fading broadcast channels

We study the ergodic sum-rate capacity of the fading MIMO broadcast channel which is used to model the downlink of a cellular system with N/sub t/ transmit antennas at the,base and K mobile users each having N/sub r/ receive antennas. Assuming perfect channel state information (CSI) for all users is available at the transmitter and the receivers, we evaluate the sum-rate capacity numerically using the duality between uplink and downlink. Assuming Nt K, we also derive both upper and lower bounds on the sum-rate capacity to study its increase rate due to multi-user diversity. Finally, we compare three transmission schemes which use the single-user-MIMO scheme (SU-MIMO), ranked known interference (RKI) and zero-forcing beamforming (ZFB), respectively, to transmit to a selected set of users in order to approach the sum-rate capacity. We show that both ZFB and RKI outperform SU-MIMO in a cellular downlink scenario. when many mobile users are present.     

一种天线分集的多载波CDMA上行链路方案

多载波系统能克服多径信道频率选择性的影响，而应用多天线发射和接收能获得分集增益。本文提出了一种两天线发、多天线收的多载波CDMA上行链路系统方案。在发射端，空时块码被用来进行发射分集。对于多载波CDMA上行链路系统，不同的用户的信号经历了不同的多径衰落，信号间的正交性不再能够保持。本文提出了一种接收机方案，并讨论了几种联合检测方法来恢复所有用户在两个编码时刻发送的信息符号，并给出计算机仿真结果。     

A novel prefiltering technique for downlink transmissions in TDD MC-CDMA systems

We discuss a prefiltering technique for interference mitigation in the downlink of a time division duplex (TDD) multicarrier code-division multiple access (MC-CDMA) system. The base station (BS) is equipped with multiple transmit antennas, and channel state information (CSI) is obtained at the transmitter side by exploiting the channel reciprocity between uplink and downlink transmissions. The prefiltering coefficients are designed so as to minimize a proper cost function that depends on the signal-to-interference-plus-noise ratios (SINRs) at the mobile terminals (MTs). The resulting scheme allows using a simple despreading receiver, thereby eliminating the need for channel estimation and equalization. Numerical results show the advantages of the proposed scheme over some existing solutions.     

Adapting a downlink array from uplink measurements

In a frequency division duplex system, a transmitter adaptive antenna array can potentially improve the performance of a wireless downlink, but because the uplink and downlink channels have different wavelengths, and, therefore, different responses, direct downlink adaptation based on channel estimates of the uplink is generally not feasible. Instead, there has been some interest in adaptations that require only the second-order statistics of the uplink and downlink to be similar. These algorithms derive adaptive weights from the covariance of the received signal to apply to a downlink transmitter array. We make two contributions to this area. First, we introduce an array configuration employing M+1 elements with log-periodic spacing that comprises two overlapping subarrays, each with M elements, that are scaled versions of each other, with the scale factor equal to the ratio of the uplink wavelength to the downlink wavelength. This array has identical beampatterns at the two wavelengths, thus helping to fulfill the requirement that the uplink and downlink second-order statistics be the same. Second, we demonstrate that obtaining a good estimate of the uplink covariance matrix is not essential for the successful operation of the adaptive scheme. Even when the mobile is at rest and the uplink information comprises only a single snapshot from the receiver array, an adaptive scheme can improve the SNR     

分布式目标下行波束形成

研究了智能天线在下行链路的应用。利用上行链路接收数据,建立分布式目标信道模型。估计移动用户的发射角、扩展角度和发射功率以及下行信道相关矩阵,确定下行链路的波束形成方法。即对频分复用（FDD）系统进行上行相关矩阵变换,构建下行信道协方差矩阵,利用最大合并比准则获得下行链路权值。计算机仿真证实了这种方法的有效性。     

Tradeoff between diversity gain and interference suppression in a MIMO MC-CDMA system

In this paper, the uplink of an asynchronous multi-carrier direct-sequence code-division multiple-access (MC-DS-CDMA) system with multiple antennas at both the transmitter and the receiver is considered. We analyze the system performance over a spatially correlated Rayleigh fading channel with multiple-access interference (MAI), and evaluate the antenna array performance with joint fading reduction and MAI suppression. Assuming perfect channel knowledge available at the transmitter, maximal ratio transmission is employed to weight the transmitted signal optimally in terms of combating signal fading. At the receiver, adaptive beamforming reception is adopted to both suppress MAI and combat the fading. Note that while correlations among the fades of the antennas in the receive array reduce the diversity gain against fading, the array still has the capability for interference suppression. We examine the effect of varying the number of transmit and receive antennas on both the diversity gain and the interference suppression.     

New transmit schemes and simplified receivers for MIMO wireless communication systems

In this paper, optimized transmit schemes for multiple-input multiple-output (MIMO) systems with simplified receivers are proposed for the downlink of high-speed wireless communication systems. In these systems, MIMO signal preprocessing is performed at the transmitter or base station with the receiver at the mobile station having a simplified structure that requires only limited signal processing. An important potential application for our transmit MIMO techniques is in the downlink of high-speed wireless communication systems with Vertical Bell Laboratories Layered Space-Time (V-BLAST) or a similar technique utilized in the uplink, creating a high-speed duplex system with a simplified mobile station transceiver structure. Two approaches are introduced and these depend on whether or not receive diversity is employed at the receiver. Both methods require that channel state information be available at the transmitter. In addition, some important associated issues such as peak-to-average power ratio requirements at the transmitter and robustness to downlink channel errors are also investigated and various solutions are proposed. Simulation results are provided and these show that performance improvement can be achieved when compared with other MIMO transmit schemes.     

AoA-Based Channel Estimation for Massive MIMO OFDM Communication Systems on High Speed Rails

Channel estimation is a well-known challenge for wireless orthogonal frequency division multiplexing(OFDM)communication systems with massive antennas on high speed rails(HSRs).This paper investigates this problem and design two practicable uplink and downlink channel estimators for orthogonal frequency division multiplexing(OFDM)communication systems with massive antenna arrays at base station on HSRs.Specifically,we first use pilots to estimate the initial angle of arrival(AoA)and channel gain information of each uplink path through discrete Fourier transform(DFT),and then refine the estimates via the angle rotation technique and suggested pilot design.Based on the uplink angel estimation,we design a new downlink channel estimator for frequency division duplexing(FDD)systems.Additionally,we derive the Cramér-Rao lower bounds(CRLBs)of the AoA and channel gain estimates.Finally,numerical results are provided to corroborate our proposed studies.     

A transmitter diversity scheme for wideband CDMA systems based onspace-time spreading

We present a transmit diversity technique for the downlink of (wideband) direct-sequence (DS) code division multiple access (CDMA) systems. The technique, called space-time spreading (STS), improves the downlink performance by using a small number of antenna elements at the base and one or more antennas at the handset, in conjunction with a novel spreading scheme that is inspired by space-time codes. It spreads each signal in a balanced way over the transmitter antenna elements to provide maximal path diversity at the receiver. In doing so, no extra spreading codes, transmit power or channel information are required at the transmitter and only minimal extra hardware complexity at both sides of the link. Both our analysis and simulation results show significant performance gains over conventional single-antenna systems and other open-loop transmit diversity techniques. Our approach is a practical way to increase the bit rate and/or improve the quality and range in the downlink of either mobile or fixed CDMA systems. A STS-based proposal for the case of two transmitter and single-receiver antennas has been accepted and will be included as an optional diversity mode in release A of the IS-2000 wideband CDMA standard     

Single-user and multiuser adaptive maximal ratio transmission forRayleigh channels

In maximal ratio transmission, the base station adjusts the complex weights of its transmit antennas to compensate for downlink channel gains in order to produce signal reinforcement-diversity-at a desired mobile that may have only a single antenna. To make the method adaptive, the complex weights are obtained from the instantaneous complex gains in the uplink; however, delay and frequency offset between uplink measurements and downlink transmission reduce the correlation between the gains in the two directions. This paper provides an analysis of the effects of gain decorrelation and of the number of antennas, and an assessment of the effect of fading when the method supports multiple users. It demonstrates that large reductions in downlink transmit power are achievable, and it examines simple transmission models to see how well they support the technique     

Spatial spectrum approach for pilot spoofing attack detection in MIMO systems

In this study, a spatial spectrum method is proposed to cope with the pilot spoofing attack (PSA) problem by exploiting the of uplink–downlink channel reciprocity in time-division-duplex multiple-input multiple-output systems. First, the spoofing attack in the uplink stage is detected by a threshold derived from the predefined false alarm based on the estimated spatial spectrum. When the PSA occurs, the transmitter (That is Alice) can detect either one or two spatial spectrum peaks. Then, the legitimate user (That is Bob) and Eve are recognized in the downlink stage via the channel reciprocity property based on the difference between the spatial spectra if PSA occurs. This way, the presence of Eve and the direction of arrival of Eve and Bob can be identified at the transmitter end. Because noise is suppressed by a spatial spectrum, the detection performance is reliable even for low signal-noise ratios and a short training length. Consequently, Bob can use beamforming to transmit secure information during the data transmission stage. Theoretical analysis and numerical simulations are performed to evaluate the performance of the proposed scheme compared with conventional methods.     

Zero-forcing methods for downlink spatial multiplexing in multiuser MIMO channels

The use of space-division multiple access (SDMA) in the downlink of a multiuser multiple-input, multiple-output (MIMO) wireless communications network can provide a substantial gain in system throughput. The challenge in such multiuser systems is designing transmit vectors while considering the co-channel interference of other users. Typical optimization problems of interest include the capacity problem - maximizing the sum information rate subject to a power constraint-or the power control problem-minimizing transmitted power such that a certain quality-of-service metric for each user is met. Neither of these problems possess closed-form solutions for the general multiuser MIMO channel, but the imposition of certain constraints can lead to closed-form solutions. This paper presents two such constrained solutions. The first, referred to as "block-diagonalization," is a generalization of channel inversion when there are multiple antennas at each receiver. It is easily adapted to optimize for either maximum transmission rate or minimum power and approaches the optimal solution at high SNR. The second, known as "successive optimization," is an alternative method for solving the power minimization problem one user at a time, and it yields superior results in some (e.g., low SNR) situations. Both of these algorithms are limited to cases where the transmitter has more antennas than all receive antennas combined. In order to accommodate more general scenarios, we also propose a framework for coordinated transmitter-receiver processing that generalizes the two algorithms to cases involving more receive than transmit antennas. While the proposed algorithms are suboptimal, they lead to simpler transmitter and receiver structures and allow for a reasonable tradeoff between performance and complexity.     

Multisite throughput of a mobile data radio link

In a single-site system, the uplink channel (from the mobile units to the site) has a throughput of 0.184 if pure ALOHA transmission protocol is used. The downlink channel throughput (from the site to the mobile units) for a single-site system is one because the transmissions are scheduled and no conflict occurs. To widen the service area and to increase system throughput for the single-site system without using additional frequency channels, multiple transmitter/receiver sites can be used. Throughput in both directions (uplink and downlink) and optimal site assignment in the downlink channel are calculated using an iterative method. Formulas are derived for two- and three-site systems. For systems with more than three sites, the three-site formulas can be used in an approximate method. Results produced by this method are within 1 or 2% of those generated by software simulations. For simplicity, FM capture is assumed to depend on the received signal level difference between the two strongest signals. Calculations are given assuming the message error rates are known     

Capacity Improvement for TDD-MIMO Systems via AR Modeling Based Linear Prediction

The quality of channel state information (CSI) affects the performance of multiple input multiple output (MIMO) systems which employ multi-elements antenna arrays at both the transmitter and the receiver. In a time division duplex (TDD) systems, the CSI for downlink can be obtained from uplink channel using reciprocity principal. However, the performance of a MIMO system can be degraded due to channel impairments especially in fast fading scenarios when the CSI obtained from uplink is used for downlink transmission. In this paper, we study performance of autoregressive (AR) modeling based MIMO channel prediction under varying channel propagation conditions (mobile speed, multipath number and angle spread) and prediction filter order. Our simulation results show that using the predicted CSI for downlink provides capacity improvement compared to conventional method.     

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.     

Analysis of smart antenna outage in UTRA FDD networks

The outage occurring from beamformer-based smart antennas is analysed for an antenna array deployed in an urban macro-cell environment. Spatio-temporal channel data obtained from a small, urban cell and from within the UTRA frequency allocation has enabled beamforming performance to be determined from the measured uplink and downlink frequency bands. The analysis shows a sub-optimum uplink beamformer performance is observed for 8% of occasions and a 25% downlink beamforming outage for an eight-element uniformly spaced linear array     

Space-time communication for OFDM with implicit channel feedback

We consider wideband communication (e.g., using orthogonal frequency-division multiplexed (OFDM) systems) over a typical cellular "downlink," in which both the base station and the mobile may have multiple antennas, but the number of antennas at the mobile is assumed to be small. Implicit channel feedback can play a powerful role in such systems, especially for outdoor channels, which typically exhibit narrow spatial spreads. A summary of our findings is as follows: a) Implicit channel feedback regarding the covariance matrix for the downlink space-time channel can be obtained, without any power or bandwidth overhead, by suitably averaging uplink channel measurements across frequency. Since this approach relies on statistical reciprocity, it applies to both time-division duplex (TDD) and frequency-division duplex (FDD) systems. Using such covariance feedback yields significantly better performance at lower complexity than conventional space-time or space-frequency codes, which do not employ feedback; b) We provide guidelines for optimizing antenna spacing in systems with covariance feedback. Theoretical investigation of a hypothetical system with completely controllable channel eigenvalues shows that the optimal number of channel eigenmodes is roughly matched to the (small) number of receive antenna elements. Thus, while antenna elements in conventional systems without feedback should be spaced far apart in order to ensure uncorrelated responses, the optimal antenna spacing with covariance feedback is much smaller, thereby concentrating the channel energy into a small number of eigenmodes.     

一种新的有效支持智能天线应用的多址接入协议

本文研究将智能天线应用于分组无线网络中,提出了支持智能天线应用的自适应时隙分配多址接入协议(ASAMA).该协议采用时分双工(TDD)方式,每一帧开始时用户节点依次发送训练序列,基站的智能天线据此计算出各用户节点的空间特征(Spatial Signature).基于波束形成的信干噪比最大化准则,设计了逐点优化与全局优化两种不同复杂度的时隙分配算法.由基站对上下行业务的时隙进行动态分配,在保证通信质量的前提下,使每个时隙容纳多个数据分组,以充分实现信道的空分复用(SDMA).对该协议的信道利用率进行了近似分析,并利用仿真方法考察其性能.结果表明,ASAMA协议能有效支持智能天线应用并具有很高的信道利用率与良好的时延性能.