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。     

AQAM系统最佳信噪比门限值的研究 及其在IEEE802.16e中的应用

 自适应调制根据瞬时信道信噪比选择适当的调制方式,能够改善系统的BER性能以及平均吞吐量性能.在常用于高速移动环境的六径瑞利衰落信道模型下,研究自适应调制系统的最佳信噪比门限值,使系统的平均吞吐量最大,同时能够保证系统达到目标BER性能.得到的最佳信噪比门限值可以用于ITU-R M.1225 Vehicular Test A 模型的自适应调制系统.给出了最佳信噪比门限值用于IEEE802.16e OFDM自适应调制系统的性能仿真,并对仿真结果进行了分析.     

自适应Turbo-QAM编码调制及其在 平坦Nakagami衰落信道的吞吐量性能

本文提出了一种通用的自适应编码调制系统吞吐量性能分析方法.基于各固定编码调制方式的误码率(BER)性能,使每种编码调制方案对应于一个信道平均信噪比(SNR)范围,再应用拉格朗日函数法得到使自适应编码调制系统吞吐量性能最大的信噪比转换门限,可得到系统的平均吞吐量性能.数值分析结果表明,相对于自适应M进制正交幅度调制(MQAM),自适应Turbo编码MQAM系统吞吐量性能有显著提高,并且该性能受瞬时误码率要求的影响较小.Nakagami信道中,在相同平均信道信噪比条件下,随着m的增大,系统吞吐量性能提高缓慢.     

基于马尔可夫过程的卫星移动信道模型及长期预测方法

卫星移动信道可被描述为基于有限状态马尔可夫过程的衰落模型,该文分析了卫星信道的可预测性,然后基于加权预测思想提出了一种卫星移动信道长期预测方法,该方法在当前信道采样的基础上进行二次采样,采样频率大于马尔可夫状态转移速率的2倍,利用信道状态的相关性和信道状态转移概率信息来加权预测未来长期内的信道状态,并依据自回归预测模型给出信道预测输出值,仿真结果表明,采用此方法对卫星信道未来的信道状态进行预测,在信噪比较高时均方误差能够达到10-2量级,在自适应传输过程中可以降低系统平均误比特率,且能够提高系统吞吐量性能,这对卫星移动通信系统的自适应传输和自适应资源分配都具有一定的指导意义。     

New Approach for Optimized Resources Allocation in DVB- S2 Networks

自适应编码调制技术根据信道环境自适应地调整传输参数来提高频谱效率,但是卫星网络中信道的时变特征对该技术的使用造成了一些限制.提出一种基于预测的信噪比估计算法,来应对回路延时带来的不利影响.在预测值基础上,提出一种新的编码调制方案选择算法来选择更强健的编码调制方案.最后,提出一种滑动窗口技术来应对编码调制方案的频繁转换及其引起的系统性能振荡.仿真结果表明,新方案能够使网络资源得到更有效的利用.     

LTE-Advanced系统中的自适应调制技术

自适应调制技术是LTE-Advanced系统自适应调制编码(AMC)技术的一项重要研究内容.根据均方误差函数的下凸性,在不同信道模型下,通过迭代算法校正得到新型MCS等级(包含256QAM高阶调制)的通用β值,并与传统MCS等级进行性能对比.仿真结果显示,新型MCS等级在高信噪比时提供的系统吞吐量更高,且优化后的β值使CQI估计更准确.     

一种H.264加权预测MAD的码率控制算法

本文提出一种H.264宏块层加权预测MAD值的码率控制算法来提高视频的质量并减少计算的复杂度.在详细分析了时间—空间相关性和物体的运动特征后,当前宏块的平均绝对差值(MAD)通过前一帧时间相关性模型预测的MAD值和由当前帧的空间相关性模型预测的MAD值加权得到.所得到的MAD值更加准确,能适应不同类型帧序列.实验结果表明,与JVT-G012相比,提出的方法在相同条件下码率有所下降,更加接近目标码率.峰值信噪比(PSNR)平均可以提高0.22dB.     

基于BICM的高效编码协作方案及性能分析

 提出一种基于比特交织编码调制(BICM)的高效编码协作(CC)方案,该方案中协作用户编码比特分为两帧,每一帧都通过BICM调制发送,通过结合比特交织器满足高阶调制星座符号的不同比特经历的衰落独立。一方面,论文分析了该协作方案在不同信道下的成对错误概率,并进一步推导了错误比特概率上界。理论分析以及仿真结果都表明：准静态衰落信道下,基于BICM的高谱效率编码协作方案仍然可以获得完全分集;而快衰落信道下,编码协作没有分集。另一方面,论文针对提出CC方案中映射方式以及交织器的设计进行研究,得出GRAY映射仍然是协作用户采用的最佳映射,且两帧的交织器不同更利于提高性能。     

面向DVB-S2系统的最优相关间隔帧同步方法

针对卫星通信第二代卫星数字电视广播（Digital Video Broadcasting-Satellite 2,DVB-S2）系统中可变的编码调制方式和物理层帧长,以及接收机需要工作在低至-2 dB的信噪比条件的要求,提出了一种基于DVB-S2帧头结构的最优相关间隔相关检测和双阈值峰值检测帧同步方法,并采用状态机控制PLS（Physical Layer Signaling）码解码,通过解出帧头信息快速跟踪下一帧头位置。仿真结果表明,该方法性能明显优于传统的SOF+PLSC（Start of Frame and Physical Layer Signaling Code）算法,并且能够适用于低信噪比条件和可变的编码调制方式。     

基于无线信道预测的自适应调制

该文研究了在平坦Rayleigh衰落信道下,借助于无线信道预测对一个基于非编码的MQAM自适应调制进行优化设计的问题。主要是通过采用无偏二阶估计的方法去研究一个已知的预测误差方差对最优化传输特性的影响。推导出基于预测的信噪比和预测误差方差的数据速率的最优解,以此对数据速率进行调整,使得频谱效率在误比特率的约束条件下达到最大化。通过仿真给出的数值解表明它能更好地将链路自适应地快速调整到信道的真实条件,提高整个系统的性能。     

Adaptive modulation systems for predicted wireless channels

When adaptive modulation is used to counter short-term fading in mobile radio channels, signaling delays create problems with outdated channel state information. The use of channel power prediction will improve the performance of the link adaptation. It is then of interest to take the quality of these predictions into account explicitly when designing an adaptive modulation scheme. We study the optimum design of an adaptive modulation scheme based on uncoded M-quadrature amplitude modulation, assisted by channel prediction for the flat Rayleigh fading channel. The data rate, and in some variants the transmit power, are adapted to maximize the spectral efficiency, subject to average power and bit-error rate constraints. The key issues studied here are how a known prediction error variance will affect the optimized transmission properties, such as the signal-to-noise ratio (SNR) boundaries that determine when to apply different modulation rates, and to what extent it affects the spectral efficiency. This investigation is performed by analytical optimization of the link adaptation, using the statistical properties of a particular, but efficient, channel power predictor. Optimum solutions for the rate and transmit power are derived, based on the predicted SNR and the prediction error variance.     

An Introduction to Adaptive QAM Modulation Schemes for Known and Predicted Channels

A major disadvantage with fixed modulation (nonadaptive) on channels with varying signal-to-noise ratio (SNR) is that the bit-error-rate (BER) probability performance is changing with the channel quality. Most applications require a certain maximum BER and there is normally no reason for providing a smaller BER than required. An adaptive modulation scheme, on the contrary, can be designed to have a BER which is constant for all channel SNRs. The spectral efficiency of the fixed modulation is constant, while it, in general, will increase with increasing channel SNRs for the adaptive scheme. This in effect means that the average spectral efficiency of the adaptive scheme is improved, while at the same time the BER is better suited to the requirement of the application. Thus, the adaptive link becomes much more efficient for data transmission. The major disadvantage is that the transmitter needs to know the channel SNR such that the best suitable modulation is chosen and the receiver must be informed on the used modulation in order to decode the information. This leads to an increased overhead in the system as compared with a fixed modulation system. In this paper, we introduce adaptive modulation systems by presenting some of the simpler adaptive quadrature amplitude modulation schemes and their performance for both perfectly known and predicted channels.     

IEEE 802.11n: Joint modulation‐coding and guard interval adaptation scheme for throughput enhancement

IEEE 802.11n is a high‐speed wireless broadband local area networking standard. IEEE 802.11n‐based devices are using some kind of adaptive modulation‐coding (AMC) scheme to adjust its transmission rate according to the radio channel condition. In these devices, however, the concept of guard interval adaptation is not been considered. Normally, orthogonal frequency division multiplexing (OFDM) technology‐based systems are using the guard interval much greater than the length of the channel impulse response. However, many previous works have shown that the choice of the larger guard interval is inefficient in terms of achievable throughput. IEEE802.11n supports using two guard intervals (short = 400 ns or long = 800 ns). Indeed, the shorter guard interval evidently results in intersymbol interference (ISI) and intercarrier interference (ICI), but the gain offered by shortened guard interval may exceed the loss caused by interference. In this paper, we propose a novel but simple solution for the guard interval adaptation joint with an adaptive modulation‐coding scheme to optimize the throughput performance of a wireless local area network (WLAN) system. This paper aims to analyze the effect of joint adaptive modulation‐coding and the guard interval (JAMCGI) algorithm on the WLAN system under bit‐error‐rate (BER) constraints. Simulation results and their analysis show a significant increase in the throughput performance of the WLAN system with our proposed algorithm.     

A quantitative comparison of ad hoc routing protocols with and without channel adaptation

To efficiently support tetherless applications in ad hoc wireless mobile computing networks, a judicious ad hoc routing protocol is needed. Much research has been done on designing ad hoc routing protocols and some well-known protocols are also being implemented in practical situations. However; one major imperfection in existing protocols is that the time-varying nature of the wireless channels among the mobile-terminals is ignored; let alone exploited. This could be a severe design drawback because the varying channel quality can lead to very poor overall route quality in turn, resulting in low data throughput. Indeed, better performance could be achieved if a routing protocol dynamically changes the routes according to the channel conditions. In this paper, we first propose two channel adaptive routing protocols which work by using an adaptive channel coding and modulation scheme that allows a mobile terminal to dynamically adjust the data throughput via changing the amount of error protection incorporated. We then present a qualitative and quantitative comparison of the two classes of ad hoc routing protocols. Extensive simulation results indicate that channel adaptive ad hoc routing protocols are more efficient in that shorter delays and higher rates are achieved, at the expense of a higher overhead in route set-up and maintenance.     

MC-CDMA系统中自适应调制算法的性能分析

自适应调制能提高系统的频谱利用率和改善系统的误码率性能。该文提出了一种将自适应调制算法与MC-CDMA技术相结合的方案。采用这种方案的自适应MC-CDMA系统在获得频率分集增益和有效频谱利用率的同时,还能获得很好的误码率性能。文章分析了提出的自适应MC-CDMA系统的性能,并通过仿真结果说明了合并方式和信道环境都将影响自适应调制算法在MC-CDMA中应用的有效性。     

基于非编码信息匹配的自适应Turbo TCM编码调制方案

本文提出了一种通过调整非编码信息量,并与信号映射相结合来改变编码、调制模式的自适应Turbo TCM编码调制方案,称之为基于非编码信息匹配的自适应Turbo TCM编码调制方案.由于将Turbo码和TCM编码调制技术相结合得到的Turbo TCM编码调制方案的带宽效率高,所以,在慢时变无线衰落信道中,自适应Turbo TCM编码调制的平均频谱效率也将很高.给出了该方案的工作原理、设计方法,并通过蒙特卡洛(Monte Carlo)仿真研究了该方案的性能.与现有编码调制方案相比,该方案具有频谱效率高、易于设计和实现的优点.     

Channel adaptive CQI reporting schemes for HSDPA systems

HSDPA (high-speed downlink packet access) is an evolved UMTS packet scheme that delivers increased user peak data rates and quality of service. A key technique supporting HSDPA is adaptive modulation and coding (AMC), in which the modulation scheme and the coding rate are changed adaptively according to the downlink channel quality reported by the user equipment (UE). Therefore, the channel quality indication (CQI) reporting scheme is directly related to the accuracy of AMC and the performance of HSDPA. This letter proposes channel adaptive CQI reporting schemes in which UEs report the CQI value intelligently by using information about channel quality. With the proposed schemes, the battery capacity of UE can be conserved and the uplink interference can be lowered by filtering off redundant CQI reports or the transmission error rate can be lowered by fast CQI reports.     

一种基于BOC信号的导航发射信道预失真技术

由于卫星导航系统中导航卫星发射信道的记忆非线性效应,使带宽受限的二进制偏移载波（BOC）调制信号失真加剧,导航系统性能将受到严重影响。针对这个问题,建立并分析了导航发射信道模型,提出将等效简化信道模型的主体部分（HPA及其前后置滤波器）等效为Wiener—Hammerstein模型的思想,同时设计了一种针对此模型的基于直接学习结构的自适应LMS预失真方案。仿真结果表明,该方案能有效抑制BOC信号的带外频谱扩展,优化星座图,可以很好地消除导航信道对BOC信号的失真影响,在卫星导航系统中有着重要的意义和良好的应用前景。