STC-OFDM系统中的信道估计算法

针对稀疏信道特性的STC-OFDM系统,提出一种基于参数模型的导频信道估计方法。利用最小二乘算法估计出导频所在位置的频率响应,用时间平均相关代替统计平均相关,得到信道相关矩阵的估计值。通过基于特征值分解的低秩建模,估计出信道的多径数目,利用旋转不变子空间方法估计出各个延迟的时间参数,在频率和时间域上插值平滑得到信道的频率响应。该方法不需要已知信道的先验信息,仿真结果证明了该方法的有效性。     

基于导频的R-LS和M-MMSE联合估计算法研究

为提高长期演进(LTE)上行链路SC-FDMA系统的信道估计精度,在研究导频在SC-FDMA系统信道估计中应用的基础上,提出了基于导频的信道估计算法.设置一定的误比特率,在正则化最小二乘(R-LS)和失配最小均方误差(M-MMSE)所做初始信道估计之后,提取合适的数据与原来的导频相结合并增加导频数量,再次进行迭代信道估计,在SCME信道模型下仿真获得该算法在LTE上行链路系统的误比特率和均方误差结果.由理论分析和仿真结果可知,该算法有效且可行,可以提高上行链路系统的信道估计精度而且降低系统误比特率.     

基于正则总体最小二乘准则的OFDM系统信道估计算法

基于导频的最小二乘(LS)估计方法的优点是结构简单,容易实现,但对噪声和干扰敏感。通过对OFDM系统中由于信道的时变性产生的载波间干扰进行了分析,在此基础上提出正则总体最小二乘(TLS)信道估计算法。根据Tikhonov正则化原理,导出了总体最小二乘正则化解法的计算公式。与一般的LS信道估计相比,TLS方法同时考虑了信道噪声和信道时变特性,所以该算法不仅在一般的信道条件下具有良好的性能而且也适合快变信道条件下的OFDM系统。仿真结果表明,该算法具有很好的均方误差和误比特率性能。     

MIMO-OFDM 系统时域参数化信道估计算法研究

针对MIMO-OFDM 系统信道估计问题,提出了一种在时域内进行最小二乘信道估计的方案．它的基本 思想是利用发送端与接收端导频序列所形成的时域信号来计算信道的时域响应信息．为了获得LS 信道估计的最小 均方误差,本文给出了一种最优导频序列设计方案,它要求每根发射天线中的导频序列为等间隔排列,不同发射天 线之间的导频序列位置相互交错．本文还提出了一种基于LS 准则的参数化信道估计方法,对比传统的LS 信道估 计算法,本文算法能够大幅度地提高信道估计的精度;对比传统的LSPCE 算法,本文算法可以有效地降低其计算复 杂度．仿真实验和性能分析验证了所提方法的有效性．     

基于压缩感知的稀疏信道估计方法

针对OFDM系统中传统最小二乘(LS)信道估计方法需要大量导频估计精度却不高的问题,提出基于压缩采样匹配追踪(CoSaMP)的压缩感知信道估计新方法.利用发送信号,接收信号和信道的频城关系建立基于压缩感知的数学模型,再采用CoSaMP算法对信道进行重构.仿真结果表明,与LS算法相比,基于CoSaMP的压缩信道估计方法能利用少量的导频信号达到与之相比拟的信道估计性能,提高了频谱利用率;与现有压缩感知信道估计算法(基追踪(BP)与正交匹配追踪(OMP)相比,在使用相同导频数目条件下,具有更好的信道估计性能和更低的计算复杂度.     

MIMO-OFDM系统中基于小波包的信道估计方法

对多输入多输出-正交频分复用(MIMO-OFDM)系统中基于导频辅助的最小二乘(LS)信道估计算法进行研究,针对LS算法对噪声影响比较敏感的缺点,提出了一种基于小波包去噪的信道估计方法,对导频符号的信道响应进行去噪处理后,再做内插估计.根据该方法的思想,基于长期演进(LTE)协议进行计算机仿真与分析,结果表明该方法比传统的LS估计算法具有更好的性能,能够有效减小信道噪声的影响,提高信道估计精度.     

OFDM系统中利用整体最小二乘法的信道估计

基于导频的最小二乘(LS)估计方法的优点是结构简单、容易实现,但对噪声和干扰敏感.为了抑制噪声和干扰,引入了整体最小二乘法(TLS)进行信道估计,并给出了整体最小二乘法的公式.该方法同时考虑了信道噪声和信道时变特性.理论分析和仿真结果表明,该算法能有效消除噪声和干扰,较好地恢复传输信号.     

基于最小二乘平滑算法的时变信道盲辨识

针对基展开模型的时变信道盲辨识问题,提出了联合阶数估计的最小二乘平滑算法.该算法充分利用其投影误差矩阵包含的时变信道阶数和系数信息,在信道辨识前先进行阶数精确估计,克服了该子空间类盲辨识方法对信道阶数敏感的缺点,改善了辨识效果;与线性预测类方法相比,该方法不需要知道输入信号的统计特性.理论推导和仿真结果验证了该算法的有效性.     

一种基于二维导频的信道估计改进算法

针对充满噪声干扰和多径效应的无线信道环境,提出一种基于二维导频的信道估计改进算法.该算法采用二维导频设置图案,利用导频点处充分记录的信道信息,提取信道参数的最优初始估计.在此基础上,利用优化的LMMSE算法,修正初始信道估计中的估计误差.理论分析及仿真实验表明,在多径时变衰落信道下,改进后的信道估计方法有效提高了系统的估计性能.     

基于平滑l0范数的OFDM系统稀疏信道估计

信道估计是无线通信系统的关键技术.为了提高信道估计的精确性,针对现有压缩传感的信道估计算法需预先知道信道稀疏度的问题,提出了一种新的基于压缩传感理论的正交频分复用系统信道估计平滑l0范数(Smoothed l0 Norm,SLO)算法,并引入双曲正切函数对SL0算法进行改进.仿真结果表明,在相同条件下,与传统最小二乘(LS)以及匹配追踪(MP)和压缩采样匹配追踪(CoSaMP)算法比较,改进算法收敛速度快,估计值均方误差小,降低了系统复杂度.     

各向异性扩散平滑滤波的改进算法

图像的噪声过滤和增强是数字图像处理中非常重要的组成部分.在图像处理过程中,为了既有效地去除噪声,又能够较好地保持图像的边缘和重要的细节信息,在Perona-Malik各向异性扩散模型（PM模型）的基础上,通过对变分方法的扩散方程中扩散系数的改进,提出了一个对噪声图像更有效更具有适应性的去噪扩散模型.该模型针对不同的梯度大小采用了不同的扩散系数.在实际处理过程中该模型不仅能够有效地保持图像的边缘,而且还能够克服PM模型对小尺度噪声敏感和部分边缘和细节失真的问题.实验结果表明,改进的扩散模型的性能优于PM模型,是一种较为理想的保边缘平滑模型.     

Doubly selective fading channel estimation in MIMO OFDM systems

Recent researches in the information theory have demonstrated that MIMO commu nication systems have the potential to provide the linear increase in the channel capacity and frequency spectral efficiencies compared with single-input single-output (SISO communication systems. MIMO is one of the major development aspects in the new generation mobile communication technology, in which plenty spatio-temporal signa processing problems need to be resolved. Especially when MIMO channels are time…     

A cross-layer adaptive transmission scheme combined with SR-ARQ over correlated fading channels

Current adaptive transmission schemes all assume independent block fading. However, in slow fading channels, it is highly possible for consecutive block transmissions to be correlated. In this paper, we propose a cross-layer adaptive transmission scheme, which is an optimum combination of modulation format and packet size, combined with selective repeat automatic repeat request (SR-ARQ) to improve the throughput in correlated slow fading channels. We apply a multi-state Markov system model to analyze the system performance and to optimize the selection of modulation levels at the physical layer and packet sizes at the data link layer in a correlated slow fading channel, which is also described by a finite-state Markov chain. A general closed-form expression of the average throughput for this cross-layer adaptive transmission scheme is presented. Simulation results show that our adaptive transmission scheme combined with SR-ARQ can obtain a good performance in correlated slow fading channels.     

正交频分复用系统中基于人眼视觉系统的自适应图像传输

针对传统的图像传输方法,提出了一种基于人眼视觉系统的自适应图像传输方案。在OFDM系统中,通过将自适应技术与信源分割及信道特性相结合的方法,实现在频率选择性慢衰落信道中高质量的图像传输。为验证该方法的良好性能,三种不同图像源被用于仿真程序中。仿真结果及理论分析可以证明,自适应图像传输与传统的图像传输相比,可大幅地提高接收图像的峰值信噪比。     

一种MPEG-4 FGS流媒体自适应传输系统

针对MPEG-4FGS流媒体提出一种质最自适应传输系统,采用3种质量平滑机制,即GOP质量平滑、帧质量平滑和FEC差错控制,保证在丢包情况下的连续GOP以及每个GOP内连续帧的质量稳定。模拟结果表明,自适应传输系统能和Internet丢包环境下平滑连续GOP的质量和GOP内连续帧的质量。     

Utility-based scheduling in wireless multi-hop networks over non-deterministic fading channels

In this paper, an innovative scheduling scheme is proposed for interference-limited wireless multi-hop networks with non-deterministic fading channels. The scheduling problem is considered as a network utility maximization (NUM) problem subject to link rate constraints. By jointly taking into account of the link scheduling and the statistical variations of signal and interference power, the convex sets for the NUM are derived. Two types of non-deterministic fading channels (i.e., Rayleigh fading channel and Ricean fading channel) are characterized into our NUM models as examples. To solve the convex optimization problem, the subgradient projection method based on dual decomposition is employed. Then, a heuristic algorithm is designed for the TDM mode wireless multi-hop networks by minimizing the discrepancy between the expected network cost and the optimal one in each timeslot. At last, the source–destination session rate and network utility are evaluated in a dedicated wireless multi-hop network scenario. The numerical results demonstrate that the session rates convergence and the network utility is improved by our proposed scheme.     

Robust progressive image transmission over OFDM systems using space-time block code

A joint source-channel coding (JSCC) scheme for robust progressive image transmission over broadband wireless channels using orthogonal frequency division multiplexing (OFDM) systems with spatial diversity is proposed for the application environments where no feedback channel is available such as broadcasting services. Most of current research about JSCC focuses on either binary symmetric channels (BSC) or additive white Gaussian noise (AWGN) channels. To deal with fading channels in most previous methods, the fading channel is modeled as two state Gilbert-Elliott channel model and the JSCC is normally aimed at the BER of bad channel status, which is not optimal when the channel is at good status. By using diversity techniques and OFDM, the frequency selective fading effects in broadband wireless channels can be significantly decreased and we show that subchannels in OFDM systems approach Gaussian noisy channels when the diversity gain gets large; as a result, the system performance can be improved in terms of throughput and channel coding efficiency. After analyzing the channel property of OFDM systems with spatial diversity, a practical JSCC scheme for OFDM systems is proposed. Simulation results are presented for transmit diversity with different numbers of antennas and different multipath delay and Doppler spread. It is observed from simulations that the performance can be improved more than 4 dB in terms of peak signal-to-noise ratio (PSNR) of the received image Lena and the performance is not very sensitive to different multipath spread and Doppler frequency.     

Clipping distortion mitigation in OFDM systems over fading channels by using DFT-based method

Saeedi et al. have recently proposed a DFT-based method (DBM) for Orthogonal Frequency Division Multiplexing (OFDM) systems to compensate for the performance degradation caused by clipping distortion. They have analyzed the performance of the DBM only in an additive white Gaussian noise channel. But, it is known that the main advantage of an OFDM system is its robustness in fading channels where this system turns a frequency selective channel into a set of parallel flat non-selective fading sub-channels. In this paper, we analyze the performance of the DBM over a frequency selective fading channel with flat fading Rician sub-channels. Our results show that similar to additive white Gaussian noise channel case, the DBM can properly mitigate the clipping distortion in OFDM signals over fading channels. Furthermore, the performance of a Hybrid method that consists of a channel coding method and the DBM is also examined over fading channels.     

Unifying eigen-mode MIMO transmission

Multiple-input multiple-output (MIMO) technique plays a key role in improving the spectrum and power efficiency in future mobile communication systems. Exploiting a unified MIMO transmission scheme that can adapt with various channel conditions is well motivated both in theory and practical applications. An eigen-mode based closed-loop MIMO transmission over frequency selective fading MIMO channels, which considers receive correlation, transmit correlation and line of sight (LOS) components, is investigated by maximizing the upper bound of channel capacity under the assumption that the channel is partially known at the transmitter and perfectly known at the receiver. Based on the eigen-mode transmission, several key techniques including linear precoding, stream selection, virtual spatial hopping and online capacity estimation are proposed, and a unified MIMO transmission scheme is established. Both computer simulation and field test results show that the proposed scheme can significantly improve the spectral efficiency and link reliability under various channel conditions.     

Construction of an objective function for optimization-based smoothing

An objective function for optimization-based smoothing is proposed for both linear and quadratic triangular and quadrilateral elements. Unlike currently published objective functions that are used to perform smoothing or untangling separately, this objective function can be used to untangle and smooth a mesh in a single process. The objective function is designed in such a way that it is easy and straightforward to be extended to higher order elements. The objective function has higher order continuous derivatives that make it suitable for optimization techniques. It has been shown empirically that the proposed function only has one minimum. With the integration of the proposed new objective function into our optimization-based smoothing algorithm, our combined Laplacian/optimization smoothing scheme provides us with satisfactory high quality meshes.