基于频谱效率的低复杂度自适应MIMO系统

在一定的误比特率(BER)下,使用自适应MQAM调制可以最大化MIMO系统的频谱效率。通过不同的算法可以得到自适应调制MIMO系统的离散速率频谱效率(DRSE)。为了进一步提高频谱效率,提出了一种基于DRSE,在不同的算法之间进行切换的低复杂度自适应机制。研究了在两种2×2信道下即:独立同分布(i.i.d)的平坦瑞利衰落信道和空间相关的瑞利衰落信道,自适应MIMO系统在正交空时块编码(OSTBC)和空间复用(SM)之间切换的自适应机制。仿真结果表明,通过选择一种最优的调制模式及发射模式的组合方式,此切换算法可以使系统的频谱效率得到有效提高,同时只增加有限的反馈信息,降低了系统复杂度。     

有限反馈MIMO系统中一种基于双码本的功率分配与预编码方案

研究表明，有限反馈预编码能有效利用信道信息提高MIMO系统频谱效率，在此基础上，在发射端进行功率控制将使频谱效率得到进一步提升．为了最大化有限反馈MIMO系统的频谱效率，本文提出了一种基于自适应调制的功率分配与预编码方案．为了减少反馈量，该方案通过双码本方式来反馈预编码矩阵和功率控制矢量，并给出了一种基于K-均值聚类的功率码本设计方法．仿真结果表明，该方案的反馈信息量少，且获得了较好频谱性能。     

基于正交变换的广义多载波系统自适应传输方案

针对基于正交变换的广义多载波系统(OT-GMC),提出一种正交变换模式和调制编码方式联合自适应的单载波混合传输方案。该方案以最大化频谱效率为优化准则,依据接收信号的信干噪比(SINR)自适应切换正交变换模式和调制编码方式。当正交变换采用恒等变换时,对子带数据进行独立的链路自适应,提高了频谱效率。当正交变换采用离散傅里叶变换时,发射信号具有较低的峰均比,提高了发射端的功放效率。该传输方案实质上获得了单载波混合传输系统内频谱效率和峰均比的最优化折中性能。仿真表明,多径信道下该混合传输方案的吞吐量性能优于单一传输方案。     

不完美反馈自适应多天线跨层系统的性能分析

为提高网络无线通信的频谱利用率,提出一种自适应闭环多输入多输出(MIMO)跨层设计方案。该方案是融合物理层的自适应调制技术(AM)和数据链路层的自动重传技术(ARQ);发射端利用反馈信息,自适应选择调制模式和发射权矢量,并自动重传数据,形成三重反馈的闭环MIMO跨层系统。推导了不完美反馈下的系统频谱效率和中断概率闭合表达式,分析了时延对跨层系统性能的影响。仿真结果表明:对比单发/单收(SISO)跨层系统以及开环MIMO跨层系统,自适应闭环MIMO跨层系统的性能有明显提高。     

Kerdock预编码与自适应调制联合优化性能研究

自适应技术在多输入多数出(MIMO)系统中能提高无线通信系统的可靠性和吞吐量,在无线通信系统中有着广阔的应用前景.设计的Kerdock码书与调制方式的自适应MIMO系统通过有限量化反馈信道采用传输信道容量最大化准则实时选择码字、按比特分配算法选择每个发送符号的调制方式,经仿真实验得出该有限量化反馈预编码与调制技术相结合的自适应系统对误码性能大的改善,并在低信噪比时呈现出好的分集增益特性.     

MIMO—OFDM系统下基于频谱利用率的自适应发射方案选择

本文提出并分析了一种MIMO-OFDM系统下,考虑相关空间信道的自适应发射方案选择算法.采用比特交织卷积编码,在以下三种发射模式间进行选择:空间复用、空间分集和复用/分集的混合模式.我们通过对MIMO-OFDM系统中采用迫零(ZF)接收机在高信噪比条件下的错误概率的简化分析,得出空间相关的多天线系统下的自适应选择算法,在给定的误码率条件下可获得使频谱利用率最大化的编码速率、调制模式和发射模式,并反馈到发射端,有效改善了系统的性能.     

基于Ka频段的自适应MIMO调制算法

这里研究了一种应用于Ka频段卫星通信信道的自适应多输入多输出(MIMO,multiple-Input multiple-Out-put)调制算法,不同于一般Ka频段卫星通信的信道衰减补偿算法,该算法主要是能根据不同天气条件,利用MIMO系统的接收信噪比自适应的转换调制方式,以维持一定的目标比特率为目标,从而获取较高的系统吞吐量。仿真结果表明,该算法能有效应对Ka频段固定卫星通信信道下的严重信道衰落,兼顾系统性能和频谱效率,得到较高的吞吐量。     

一种MIMO系统中的新型资源分配算法

本文提出了一种MIMO(Multi-Input and Multi-Output)系统中的新型自适应调制和功率分配算法,在奇异值分解的基础上通过注水算法进行功率分配,此后根据所分配的功率与信道状态信息来确定自适应调制的门限及相应的调制阶数,并在自适应调制后通过功率修正因子来弥补常见算法中功率不能得到有效利用的问题,仿真结果表明该算法可以得到较高的频谱效率.     

LTE中结合自适应调制编码和HARQ的跨层设计北大核心

通常链路自适应技术均是基于分层的思想来设计的,往往使得局部性能最优化,但系统的整体性能却未达到最优。文章在LTE(长期演进)网络架构基础上,针对物理层的AMC(自适应调制编码)和MAC(介质访问控制)层的HARQ(混合自动请求重传)两种自适应技术提出一种跨层设计方案。分析了跨层耦合参数与系统性能之间的关系,并给出了详细的推导过程和具体的表达式。然后给出了平均时延和平均误包率约束条件下的跨层优化问题。仿真结果表明,该跨层设计能进一步提高系统的平均频谱效率;在给定业务QoS(服务质量)需求下,可以根据信道质量来选择最佳的最大重传次数和AMC方式,使得系统的平均频谱效率最大化。     

帧长和信道预测对自适应调制系统性能的影响

该文主要分析了在自适应调制系统中,帧长自适应和信道预测对系统性能的影响。推导了信道预测和没有信道预测的自适应调制系统的频谱效率公式,分析结果表明对于自适应调制系统,信道预测的系统频谱效率是最优的,非信道预测系统根据信道情况选择最优帧长可以获得该系统的最优系统频谱效率,但它是低于信道预测的频谱效率的,而且是随多普勒频率的增大而逐渐减小的。     

一种基于LDPC链路自适应技术的研究

为了设计信道纠错能力强、系统频带利用率高的无线链路自适应传输技术,文中研究了基于LDPC信道纠错码的自适应编码调制OFDM系统模型以及相应的传输方案,并设计了四区间自适应OFDM通信机制;提出了利用代价函数计算传输模式切换阈值的方法.仿真显示所设计的自适应OFDM系统具有良好的链路适应能力,可以得到优化的系统频带利用率和平均误码率性能.     

On spectral efficiency of low-complexity adaptive MIMO systems in Rayleigh fading channel

Adaptive MQAM modulation is used to maximize spectral efficiency of Multiple-Input Multiple-Output (MIMO) systems while keeping bit error rate (BER) under a target level. Closed-form expressions of the average spectral efficiency, coined as discrete-rate spectral efficiency (DRSE), are derived for adaptive modulation MIMO systems using different algorithms. To further enhance the spectral efficiency, a low complexity adaptation scheme is suggested to switch across different algorithms based on the DRSE. In the current letter, we investigate the adaptation scheme that switches between Orthogonal Space-Time Block Codes (OSTBC) and spatial multiplexing with zero-forcing (ZF) detection for MIMO systems with two transmit antennas. Two types of operating environment are considered: flat Rayleigh fading channel without spatial correlation and spatially correlated Rayleigh fading channel with transmit correlation.     

Novel Link Adaptation Schemes for OFDM System

1　IntroductionRecently,intenseinteresthasbeenfocusedonthetransmissiontechniqueswhichcansupporthighdataratetransmissionoverwirelesschannels.OneofthemainrequirementsonthetransmissiontechniqueistheabilitytocombattheInter SymbolInterfer ence (ISI) ,amajorprobleminthehighdataratetransmissionovermultipathfadingchannels.OFDMisthemostpromisingsolutiontothisproblem[1～ 3] .ThecombinationoflinkadaptationandOFDMisaneffectivewaytoimprovethespectralefficiency[4～ 7] .TheoptimalwaytoachievetheShannon…     

On a MIMO-based open wireless architecture: space-time complementary coding [Accepted from Open Call]

This article proposes a MIMO-based open wireless architecture for next-generation wireless systems. The use of space-time complementary coding in the proposed OWA scheme contributes to a great improvement in system performance due to the joint spatial diversity and multiplexing advantages achieved by the STCC-MIMO scheme. The proposed STCC-MIMO-based OWA scheme offers an unified platform that integrates various attractive features, such as link adaptation, spatial diversity and multiplexing trade-off, and accurate channel estimation. The application of innovative pair-wise complementary codes and offset stacking spread spectrum in the STCC-MIMO scheme helps to boost the overall spectral efficiency due to its near interference-free operation. MIMO systems play an extremely important role in 4G wireless, and therefore it is desirable that the OWA support different MIMO configurations. This article shows that the proposed STCC-MIMO scheme offers great flexibility to implement a MIMO system suitable for different diversity and multiplexing requirements     

An Enhanced Link Adaptation Scheme Based on Cooperative Interference Prediction for MIMO-OFDM Systems

Dynamic inter-cell interference (ICI) causes channel quality indicator mismatch such that system throughput is reduced. In this paper, an enhanced link adaptation scheme based on cooperative interference prediction is proposed to reduce the mismatch. The scheme predicts the dynamic ICI within a cluster of base stations (BSs) for improving the link adaptation accuracy. With the dynamic ICI prediction, more accurate modulation and coding scheme can be selected without increasing the overhead over the air. System level simulations show that the proposed scheme significantly improves the system spectral efficiency (SE), especially the cell-edge SE.     

A link adaptation scheme for the downlink of mobile hotspot

A link adaptation (LA) scheme for the downlink of mobile hotspot, which is supported by an IEEE 802.16e or mobile WiMAX network, is proposed. The mobile WiMAX uses orthogonal frequency division multiple access (OFDMA) in its physical layer. The main function of the LA scheme is to select an appropriate burst profile, which includes a multiple input multiple output (MIMO) transmission mode, modulation technique, and coding scheme. We formulate a discrete optimization problem for the LA scheme by maximizing the throughput. An algorithm to implement the LA scheme is also proposed. Numerical results show that the proposed LA scheme exhibits good performance. Copyright © 2011 John Wiley & Sons, Ltd.     

Cross‐layer design for MIMO systems over spatially correlated and keyhole Nakagami‐m fading channels

Cross‐layer design is a generic designation for a set of efficient adaptive transmission schemes, across multiple layers of the protocol stack, that are aimed at enhancing the spectral efficiency and increasing the transmission reliability of wireless communication systems. In this paper, one such cross‐layer design scheme that combines physical layer adaptive modulation and coding (AMC) with link layer truncated automatic repeat request (T‐ARQ) is proposed for multiple‐input multiple‐output (MIMO) systems employing orthogonal space‐‐time block coding (OSTBC). The performance of the proposed cross‐layer design is evaluated in terms of achievable average spectral efficiency (ASE), average packet loss rate (PLR) and outage probability, for which analytical expressions are derived, considering transmission over two types of MIMO fading channels, namely, spatially correlated Nakagami‐m fading channels and keyhole Nakagami‐m fading channels. Furthermore, the effects of the maximum number of ARQ retransmissions, numbers of transmit and receive antennas, Nakagami fading parameter and spatial correlation parameters, are studied and discussed based on numerical results and comparisons. Copyright © 2009 John Wiley & Sons, Ltd.     

On the Performance of Link Adaptation Techniques in MIMO Systems

Recent research results on multiple antennas system namely the multiple input multiple output (MIMO) system show that huge potential be it the spectral efficiency or the enhancement in transmission reliability can be achieved. In this paper, with the assumption that the channel is narrowband and the channel coefficients are known at both link ends, we propose and evaluate several link adaptation (LA) algorithms applied to the spatial multiplexing MIMO systems. Depending on the requirements, the LA technique is designed to optimize either the system spectral efficiency or the bit error rate (BER). We also consider the potential use of the maximum eigen beamforming in supporting concurrent users. The results show that efficient use of the channel information could lead to a significant improvement in the system performance.     

A cross layer approach based on queuing and adaptive modulation for MIMO systems

A cross layer approach based on queuing and adaptive modulation for MIMO systems has been presented. To realize the system, an adaptive modulation scheme using MIMO systems combined with finite-length queuing at the data link layer has been designed. A finite state Markov chain for MIMO channels has been constructed to persue the queuing analysis. The packet loss rate, the average throughput, and the average spectral efficiency have been computed. Furthermore, the packet error rate for adaptive modulation to minimize the packet loss rate and maximize the average throughput is extracted. The obtained results present the advantages of using the proposed cross layer architecture.