频率选择性衰落环境中MIMO系统信道容量研究

提出了一种频率选择性环境中多输入多输出系统信道容量的分析方法.该方法基于接收均匀圆阵构建了蕴含天线间距、散射角大小和多径数等模型物理参数的衰落空间相关模型。分析了MIMO OFDM系统信道衰落的统计特性。在此基础上利用Wishart分布的性质详细推导了任意天线数的MIMO OFDM系统的信道容量及其上下限。该方法回避了已有方法需要求取信道衰落相关特征值概率密度函数的问题，降低了运算量；并且可以有效地分析在频率选择性环境中模型物理参数对信道容量的影响。仿真结果表明，随着天线间距的增大，系统的平均信道容量也逐渐增大；但当天线间距增大到一定程度后,信道容量变化不明显，散射角越大，信道容量的增长速率越快，当接收信噪比较高时,平均信道容量的上下限接近于其实际值。     

On the approximation of MIMO capacity

Information theoretical optimization problems related to the resource allocation of wireless cellular and ad hoc networks are not solvable in closed form due to the nonlinear logarithmic representation of the Shannon capacity. This paper introduces functional approximations to the MIMO capacity over flat Rayleigh fading channels, which allow for analytical solutions to network resource optimization problems. The precision of the suggested approximations is assessed and is shown to provide a very close match to the exact capacity expression.     

Downlink capacity of interference-limited MIMO systems with joint detection

The capacity of downlink cellular multiple-input multiple-output (MIMO) systems, where co-channel interference is the dominant channel impairment, is investigated in this paper, mainly from a signal-processing perspective. Turbo space-time multiuser detection (ST MUD) is employed for intracell communications and is shown to closely approach the ultimate capacity limits in Gaussian ambient noise for an isolated cell. Then, it is combined with various multiuser detection methods for combating intercell interference. Among various multiuser detection techniques examined, linear minimum-mean-square-error (MMSE) MUD and successive interference cancellation are shown to be feasible and effective. Based on these two multiuser detection schemes, one of which may outperform the other for different settings, an adaptive detection scheme is developed, which together with a Turbo ST MUD structure offers substantial performance gain over the well-known V-BLAST techniques with coding in this interference-limited cellular environment. The obtained multiuser capacity is excellent in the high to medium signal-to-interference ratio scenario. Nonetheless, numerical results also indicate that a further increase in system complexity, using base-station cooperation, could lead to further significant increases of the system capacity. The asymptotic multicell MIMO capacity with linear MMSE MUD preprocessing is also derived, and this analysis agrees well with the simulation results.     

On the capacity of multiuser MIMO networks with interference

Maximizing the total mutual information of multiuser multiple-input multiple-output (MIMO) systems with interference is a challenging problem. In this paper, we consider the power control problem of finding the maximum sum of mutual information for a multiuser network with mutually interfered MIMO links. We propose a new and powerful global optimization method using a branch-and-bound (BB) framework, coupled with a novel reformulation-linearization technique (RLT). The proposed BB/RLT guarantees finding a global optimum for multiuser MIMO networks with interference. To reduce the complexity of BB/RLT, we propose a modified BB variable selection strategy to accelerate the convergence process. Numerical examples are also given to demonstrate the efficacy of the proposed solution.     

闭环MIMO的容量损失研究

首先建立了闭环MIMO(multiple-input-multiple-output)的系统模型,推导出了闭环MIMO容量损失与反馈信道速率之间的数学关系。通过仿真这个数学关系,得出了在不同信噪比、天线数、中断概率下,容量损失相对于反馈信道速率变化规律的相关结论。这个数学关系和这些结论,为实际的闭环MIMO系统设计提供了一定的基础依据,对综合考虑信噪比、天线数、中断概率、闭环MIMO容量损失和反馈信道速率的系统设计有参考意义。     

On the capacity of MIMO relay channels

We study the capacity of multiple-input multiple- output (MIMO) relay channels. We first consider the Gaussian MIMO relay channel with fixed channel conditions, and derive upper bounds and lower bounds that can be obtained numerically by convex programming. We present algorithms to compute the bounds. Next, we generalize the study to the Rayleigh fading case. We find an upper bound and a lower bound on the ergodic capacity. It is somewhat surprising that the upper bound can meet the lower bound under certain regularity conditions (not necessarily degradedness), and therefore the capacity can be characterized exactly; previously this has been proven only for the degraded Gaussian relay channel. We investigate sufficient conditions for achieving the ergodic capacity; and in particular, for the case where all nodes have the same number of antennas, the capacity can be achieved under certain signal-to-noise ratio (SNR) conditions. Numerical results are also provided to illustrate the bounds on the ergodic capacity of the MIMO relay channel over Rayleigh fading. Finally, we present a potential application of the MIMO relay channel for cooperative communications in ad hoc networks.     

Maximising capacity of MIMO systems with receive antenna subarray formation

Antenna subarray formation is a novel RF pre-processing technique that reduces hardware complexity of multiple-input multiple-output systems while alleviating the performance degradation of conventional antenna selection techniques. With this method, each RF chain is allocated to a linear combination of the responses of a subset of the available antenna elements, which is performed in the radio frequency domain. A novel, analytical, suboptimal algorithm is introduced for receive antenna subarray formation based on instantaneous channel information that maximises the effective channel capacity.     

MIMO MC-CDMA communications for future cellular systems

We propose an efficient multiple-input multiple-output concept based on space-time turbo coded modulation and layered spatial multiplexing architectures for cellular multicarrier code-division multiple access systems. We design appropriate receiver algorithms, and compare their performance to competing schemes in a single-cell system. We then evaluate the performance of the scheme in a seven-cell system with universal frequency reuse. The proposed MIMO scheme improves throughput significantly compared to the corresponding single-antenna communications even in the presence of spatial correlation     

An approximate capacity distribution for MIMO systems

In this letter, we derive the exact variance of the capacity of a multiple-input multiple-output (MIMO) system. This enables an investigation of the accuracy of a Gaussian approximation to the capacity foreshadowed by various central limit theorems. We confirm recent results which state that the capacity variance appears to converge to a limit independent of absolute antenna numbers, but dependent on the ratio of the numbers of receive to transmit antennas. The Gaussian approximation itself is surprisingly good, even in the worst cases giving satisfactory results.     

High capacity with limited spectrum in cellular systems

The rapid growth of wireless telecommunication establishes the need for capacity evolution methods for the cellular systems of today. The authors discuss some important aspects of creating high-capacity cellular networks that operate with a limited amount of frequency spectrum. Macrocells are initially used to build a network with cost-effective wide-area coverage. By decreasing site-to-site distance and tightening frequency reuse, the capacity of the macro network can be increased substantially. Subsequently, micro- and picocells are added, creating a multilayered cellular network. Methods such as power control, efficient frequency allocation, and traffic control between the layers are employed to exploit the full potential of the network     

System capacity of F-TDMA cellular systems

We study the system capacity of cellular systems with time-division multiple access, slow time-frequency hopping (F-TDMA), and conventional single-user processing at the receivers. System capacity is formally defined as the maximum of the product of the number of users per cell times the user spectral efficiency for a given maximum outage probability. We adopt an information-theoretic definition of outage as the event that the mutual information of the block-interference channel resulting from a finite number of signal bursts spanned by the transmission of a user code word falls below the actual code rate, because of fading, shadowing, and interference. Starting from this definition, we develop a general framework which naturally takes into account many different aspects of F-TDMA cellular systems like channel reuse, channel utilization, waveform design, time-frequency hopping, voice activity exploitation, handoff, and power control strategies. Most importantly, our analysis does not rely on the choice of a particular coding scheme and can be applied to a very large class of systems in order to find guidelines for capacity-maximizing system design. A numerical example based on a typical urban mobile environment shows that there is a considerable capacity gap between actual F-TDMA systems and the limits predicted by our analysis. However, this gap can be filled by carefully designed (practical) systems, which make use of conventional single-user processing and simple coded modulation schemes     

On the capacity of MIMO broadcast channels with partial side information

In multiple-antenna broadcast channels, unlike point-to-point multiple-antenna channels, the multiuser capacity depends heavily on whether the transmitter knows the channel coefficients to each user. For instance, in a Gaussian broadcast channel with M transmit antennas and n single-antenna users, the sum rate capacity scales like Mloglogn for large n if perfect channel state information (CSI) is available at the transmitter, yet only logarithmically with M if it is not. In systems with large n, obtaining full CSI from all users may not be feasible. Since lack of CSI does not lead to multiuser gains, it is therefore of interest to investigate transmission schemes that employ only partial CSI. We propose a scheme that constructs M random beams and that transmits information to the users with the highest signal-to-noise-plus-interference ratios (SINRs), which can be made available to the transmitter with very little feedback. For fixed M and n increasing, the throughput of our scheme scales as MloglognN, where N is the number of receive antennas of each user. This is precisely the same scaling obtained with perfect CSI using dirty paper coding. We furthermore show that a linear increase in throughput with M can be obtained provided that M does not not grow faster than logn. We also study the fairness of our scheduling in a heterogeneous network and show that, when M is large enough, the system becomes interference dominated and the probability of transmitting to any user converges to 1/n, irrespective of its path loss. In fact, using M=/spl alpha/logn transmit antennas emerges as a desirable operating point, both in terms of providing linear scaling of the throughput with M as well as in guaranteeing fairness.     

On the design of MIMO block-fading channels with feedback-link capacity constraint

In this paper, we propose a combined adaptive power control and beamforming framework for optimizing multiple-input/multiple-output (MIMO) link capacity in the presence of feedback-link capacity constraint. The feedback channel is used to carry channel state information only. It is assumed to be noiseless and causal with a feedback capacity constraint in terms of maximum number of feedback bits per fading block. We show that the hybrid design could achieve the optimal MIMO link capacity, and we derive a computationally efficient algorithm to search for the optimal design under a specific average power constraint. Finally, we shall illustrate that a minimum mean-square error spatial processor with a successive interference canceller at the receiver could be used to realize the optimal capacity. We found that feedback effectively enhances the forward channel capacity for all signal-to-noise ratio (SNR) values when the number of transmit antennas (n/sub T/) is larger than the number of receive antennas (n/sub R/). The SNR gain with feedback is contributed by focusing transmission power on active eigenchannel and temporal power waterfilling . The former factor contributed, at most, 10log/sub 10/(n/sub T//n/sub R/) dB SNR gain when n/sub T/>n/sub R/, while the latter factor's SNR gain is significant only for low SNR values.     

Exact capacity distributions for MIMO systems with small numbers of antennas

It is well known that multiple input multiple output (MIMO) systems offer the promise of achieving very high spectrum efficiencies (many tens of bit/s/Hz) in a mobile environment. The gains in MIMO capacity are sensitive to the presence of spatial correlation introduced by the radio environment. In this letter we consider the capacity outage performance of MIMO systems in correlated environments. For systems with large numbers of antennas Gaussian approximations are very accurate. Hence, we concentrate on systems with small numbers of antennas and derive exact densities and distribution functions for the capacity, which are simple and rapid to compute.     

MIMO capacity with interference

System capacity is considered for a group of interfering users employing single-user detection and multiple transmit and receive antennas for flat Rayleigh-fading channels with independent fading coefficients for each path. The focus is on the case where there is no channel state information at the transmitter, but channel state information is assumed at the receiver. It is shown that the optimum signaling is sometimes different from cases where the users do not interfere with each other. In particular, the optimum signaling will sometimes put all power into a single transmitting antenna, rather than divide power equally between independent streams from the different antennas. If the interference is either sufficiently weak or sufficiently strong, we show that either the optimum interference-free approach, which puts equal power into each antenna, or the approach that puts all power into a single antenna is optimum and we show how to find the regions where each approach is best.     

Simulation results of the capacity of cellular systems

We study the capacity of cellular systems with interference-adaptation dynamic channel allocation (DCA) through a set of heuristics that evaluate the required number of channels for some mobile traffic pattern. In particular, we evaluate the improvement in the reuse factor given the knowledge of the mobiles' locations. Assuming that the mobiles' locations are sampled from the uniform random distribution or are fixed on a uniform grid, we show the effect of a number of parameters, such as the number of mobiles per cell, the minimum allowable signal-to-interference ratio, and the limited knowledge of mobiles' locations. We also investigate the effect of shadow fading and signal-based power control. Although previous papers have proposed various heuristics with varying performance, we present heuristics that are shown to give the maximum packing results based on our assumptions. In particular, the single-interferer assumption, used throughout our work, is justified and the optimality of the square-root signal-based power control is proven     

Inter-intra cellular pilot contamination mitigation for heterogeneous massive MIMO cellular systems

Telecommunication Systems - Massive multiple-input-multiple-output (MIMO) technology has been advocated as one of the most advanced and promising technologies for catering to the high data...     

Antenna-assisted round robin scheduling for MIMO cellular systems

Packet scheduling is investigated for the downlink of a multiple-input multiple-output (MIMO) cellular system. We propose an antenna-assisted round robin scheduling (AA-RRS) scheme to improve the conventional RRS scheme. The AA-RRS scheme provides fair channel access chance to users as the RRS scheme, whereas it increases the system capacity through the effective use of multiple antennas in achieving a diversity effect from multiple users. Simulation results show that the AA-RRS scheme achieves a significant capacity gain over the RRS scheme. We also investigate the effects of long-term power control on the system capacities for the proposed scheduling scheme.     

使用MIMO技术的固定中继蜂窝网容量分析

为固定中继蜂窝网络提供了一种新的性能分析方案,该方案以引入未来将广泛使用的MIMO天线系统为出发点,通过分析在不存在固定中继和存在固定中继两种情况下系统信道容量的变化,考察固定中继蜂窝网络架构的优越性。通过SIMO MISO和MIMO各种情况下的信道容量计算,运用蒙特卡罗方法,得出最后的数值结果。     

On the MIMO channel capacity of multidimensional signal sets

In this contribution, the capacity of multiple-input multiple-output (MIMO) systems using multidimensional phase-shift keying/quadratic-amplitude modulation signal sets is evaluated. It was shown that transmit diversity is capable of narrowing the gap between the capacity of the Rayleigh-fading channel and that of the additive white Gaussian noise channel. However, because this gap becomes narrower when the receiver diversity order is increased, for higher order receiver diversity, the performance advantage of transmit diversity diminishes. A MIMO system having full multiplexing gain has a higher achievable throughput than the corresponding MIMO system designed for full diversity gain, although this is attained at the cost of a higher complexity and a higher signal-to-noise ratio. The tradeoffs between diversity gain, multiplexing gain, complexity, and bandwidth are studied.