大气激光通信下的OFDM导频信道估计方法

针对大气激光通信下的OFDM(正交频分复用)系统,对几种基于导频的信道估计方法进行了比较分析,并给出了仿真结果。仿真结果表明,SVD-LMMSE(基于奇异值分解的线性最小均方误差)算法在提高信道估计精度的同时,可以降低系统计算的复杂度;增大CP(循环前缀)可以非常有效地提高系统性能。     

A comparative study of iterative channel estimators for mobile OFDM systems

We investigate two iterative channel estimators for mobile orthogonal-frequency division multiplexing. The first estimator is based on iterative filtering and decoding whereas the second one uses an a posteriori probability (APP) algorithm. The first method consists of two cascaded one-dimensional Wiener filters, which interpolate the unknown time-varying two-dimensional frequency response in between the known pilot symbols. As shown, the performance can be increased by feeding back the likelihood values at the output of the APP-decoder to iteratively compute an improved estimate of the channel frequency response. The second method applies two APP estimators, one for the frequency and the other one for the time direction. The two estimators are embedded in an iterative loop similar to the turbo decoding principle. As shown in detail, this iterative estimator is superior and its performance is independent of whether the chosen time-frequency pilot grid satisfies the two-dimensional sampling theorem or not. The bit-error rate as a function of the signal-to-noise ratio is used as a performance measure. In addition, the convergence of the iterative decoding loop is studied with the extrinsic information transfer chart.     

一种适用于终端移动的OFDM无线局域网的信道估计方法

本文提出了一种适用于具有移动终端的OFDM无线局域网的信道估计方法.该方法采用了卡尔曼滤波算法进行了信道估计,并利用导频进行信道跟踪.将基于训练序列的信道估计结果作为Kalman滤波器的初始值和观测值,用基于导频的信道估计结果来计算Kalman滤波器参数.并利用导频进一步跟踪信道在时间上的变化.Simulink仿真结果表明,该算法比基于导频的信道估计方法和基于训练序列的信道估计方法效果都要好.     

OFDM系统中信道估计方法的研究

LS算法是OFDM系统信道估计方法中计算复杂度最低性能最差的方法.根据有限长度h的能量分布,提出了一种改进算法.通过仿真,与LS算法和性能较好的LMMSE比较,验证了改进后的LS算法性能优于最初的LS算法,并可在实际中得到较好的应用.     

Totally blind APP channel estimation for mobile OFDM systems

A new two-dimensional blind channel estimation scheme for coherent detection of orthogonal frequency-division multiplexing (OFDM) signals in a mobile environment is presented. The channel estimation is based on the a posteriori probability (APP) calculation algorithm. The time-variant channel transfer function is completely recovered without phase ambiguity with no need for any pilot or reference symbols, thus maximizing the spectral efficiency of the underlying OFDM system. The phase ambiguity problem is solved by using a 4-QAM (quadrature amplitude modulation) scheme with asymmetrical arrangement. The results clearly indicate that totally blind channel estimation is possible for virtually any realistic time-variant mobile channel.     

Two EM-type channel estimation algorithms for OFDM with transmitter diversity

We study channel estimation for orthogonal frequency-division multiplexing (OFDM) systems utilizing transmitter diversity and operating over multipath fading channels. Two expectation-maximization (EM)-type algorithms are introduced and compared with each other in terms of convergence rate. At each iteration and for every OFDM link, the EM-type algorithms partition the problem of estimating a multi-input channel into independent channel estimations for each transmit-receive antenna pair, therefore avoiding the matrix inversion encountered in the joint least-square estimation. The EM-type algorithms can also be used to efficiently implement a recently proposed algorithm, termed the significant-tap-catching estimator, so that the system performance is more robust to different multipath channel delay profiles.     

A reduced complexity channel estimation for OFDM systems withtransmit diversity in mobile wireless channels

A reduced complexity channel estimation for OFDM systems with transmit diversity is proposed by exploiting the correlation of the adjacent subchannel responses. The sizes of the matrix inverse and the FFTs required in the channel estimation at every OFDM data symbol are reduced by half of the existing method for OFDM systems with nonconstant modulus subcarrier symbols or constant modulus subcarrier symbols with some guard tones. The complexity reduction of half FFTs size and some matrix multiplications is still achieved for constant modulus subcarrier symbols with no guard tones. The price for the complexity reduction is a slight BER degradation and for the channels with small relative delay spreads, the BER performance of the reduced complexity method becomes quite comparable to the existing method. An alternative approach for the number of significant taps required in the channel estimation is described which achieves a comparable performance to the case with the known suitable number of significant taps. A simple modification which reduces the lost leakage of the nonsample-spaced channel paths is also proposed. This modification achieves a substantial performance improvement over the existing method without any added complexity     

基于直接判决和导频跟踪的OFDM系统快时变信道估计

提出了一种基于直接判决的OFDM系统的快时变信道估计方法。采用了直接判决算法进行信道估计，并从中选择有效的估计结果，联合导频信号进行信道跟踪。将基于训练序列的信道估计结果作为直接判决算法的初始值，利用传输信号直接判决的统计特性进行了信道估计，并利用改进的导频算法进一步地跟踪信道在时间上的变化。Simulink仿真结果表明，该估计算法适用于时变信道，比基于导频的信道估计方法和基于训练序列的信道估计方法效果都要好。     

基于PSD估计的OFDM系统多径信道估计方法

本文通过功率谱估计(PSD)估计来估计无线多径信道,然后在系统中进行信道均衡.仿真结果证明,PSD估计信道的均方误差优于最小二乘估计和最小均方误差估计的均方误差;通过信道均衡,系统性能基本接近于信道已知条件下的性能.     

一种时变信道条件下OFDM系统的信道估计方法

针对正交频分复用（OFDM）系统，本文提出了一种时变条件下的信道估计方法。文中假设数据以包为单位进行传输，导频序列以叠加的方式和数据序列相结合，在得到对应于数据包内每帧OFDM信号的信道冲激响应之后，再在一个包内对这些信道冲激响应进行加权平均，从而提高信道估计的精度。     

短波信道质量评估技术综述

短波信道质量评估设备按技术体制分为两大类,分别是独立信道探测系统和嵌入式探测系统。介绍了两类系统的基本原理、目前研究和应用现状以及存在的问题,指出应将两类技术进行结合,根据通信业务进行针对性的信道分析以提高评估效率。最后对其发展方向进行展望,以期为短波实时信道估值方面的研究提供参考。     

A class of mobile motion prediction algorithms for wireless mobile computing and communications

This paper describes a class of novel mobile motion prediction algorithms for supporting global mobile data accessing. Traditionally, mobility and routing management includes functions to passively keep track of the location of the users/terminals and to maintain connections to the terminals belonging to the system. To maintain uninterrupted high-quality service for distributed applications, it is important that a mobile system be more intelligent and can anticipate the change of the location of its user. We propose an aggressive mobility and routing management scheme, called predictive mobility management. A class of mobile motion prediction algorithms predicts the future location of a mobile user according to the user's movement history, i.e., previous movement patterns. By combining this scheme with mobility agent functions, the service and user routing data are actually pre-connected and pre-assigned at the locations to which the user is moving. Thus, the user can immediately receive service or data with virtually the same efficiency as at the previous location, i.e., without encountering a large data structure handover delay before service or data is available.     

一种新的基于导频信号和离散小波变换的OFDM移动通信系统信道估计方法

针对在时变无线信道条件下工作的正交频分复用通信系统,本文提出了一种通过小波滤波器插值来进行信道传递函数估计的方法.此方法利用离散小波变换消噪以显著地消除接收到的导频信号中的加性白高斯噪声项.本文所提出的方法适用于所有线性调制OFDM系统.     

一种有效降低OFDM系统信道估计复杂度的方法

在应用于OFDM系统的信道估计方法中,维纳滤波是一种估计性能较好的算法,但是该方法计算复杂度大,不利于实现.本文基于块状导频方式,提出了选取相关子载波的相关系数法来简化维纳滤波的信道估计方法,该方法在估计性能损失不大的情况下,有效地降低了计算复杂度.文中在慢时变信道环境下对该方法进行了性能仿真,并对仿真结果进行了讨论.     

Kalman filtering for power estimation in mobile communications

In wireless cellular communications, accurate local mean (shadow) power estimation performed at a mobile station is important for use in power control, handoff, and adaptive transmission. Window-based weighted sample average shadow power estimators are commonly used due to their simplicity. In practice, the performance of these estimators degrades severely when the window size deviates beyond a certain range. The optimal window size for window-based estimators is hard to determine and track in practice due to the continuously changing fading environment. Based on a first-order autoregressive model of the shadow process, we propose a scalar Kalman-filter-based approach for improved local mean power estimation, with only slightly increased computational complexity. Our analysis and experiments show promising results.     

Downlink adaptive array algorithms for cellular mobile communications

The capacity of wireless downlink communication to mobile receivers in a dense urban environment is limited primarily by co-channel interference. Downlink adaptive arrays can be used to mitigate this limitation by maximizing the power transmitted to desired in-cell mobiles in the reference cell while minimizing power to co-channel mobiles in neighboring cells. This is accomplished by using uplink measurements to estimate downlink covariance matrices and then solving a generalized eigenvalue problem. Several algorithms are proposed to adaptively estimate the optimal solution and are evaluated using a simplified signal model that allows efficient deterministic performance calculations.     

基于本地人工信道的新型OFDM信道估计方法

介绍了一种新颖的基于本地人工信道的信道估计方法,并对该方法进行进一步的奇异值分解和简化,使其具有较少的存储量和计算量,具有很大的实际应用价值。对相关方法进行了仿真和比较,在误比特率性能上与传统线性最小均方误差估计相差小于1 dB。     

一种OFDM系统中的盲信道估计算法

该文利用OFDM系统传输的信息符号为有限字符集和各子载波的相互独立特性,提出了用伪导频符号(PPS)进行信道的盲估计算法。与用于做信道估计的导频符号不同,PPS传输的是有用的数据,因此提高了系统的带宽利用率,PPS的平均功率增加3dB或6dB可有效抑制信道的加性高斯噪声。对信道盲估计的均方误差(MSE)和由此算法获得的信道的状态信息对无编码的OFDM系统进行解调的误比特率进行了仿真,结果表明提出的算法是有效的并具有很好的灵活性。     

基于遗传算法的短波OFDM信道估计导频优化方案

短波信道具有时域稀疏性,压缩感知理论应用于短波OFDM信道估计可以改善估计性能以及减少导频开销。然而信道估计性能和信道的可恢复性都与导频的放置有着密切关系,为了进一步提高信道重构精度,本文以最小化测量矩阵的互相关为导频优化目标,提出一种基于遗传算法的导频优化方案,并设计了相应的交叉算子和变异算子,以产生新个体,保证种群的多样性。仿真结果表明,相比随机搜索,该方案可以得到更小的互相关值,更高的重构精度以及更高的频带利用率。     

Iterative receiver for joint detection and channel estimation in OFDM systems under mobile radio channels

In this paper, an iterative receiver for a joint data-detection and channel-estimation scheme is presented for orthogonal frequency-division multiplexing systems, which incorporates iterative decoding in the receiver. In the proposed scheme, a maximum a posteriori-based decoder and a channel estimator provide more reliable information on the coded bits for each other in an iterative manner. We first consider a practical implementation issue for the optimal minimum mean squared error two-dimensional (2-D) channel estimator as an essential element in the iterative receiver. To reduce the complexity of the 2-D estimator as suited to the iterative receiver, we focus on rigorously investigating how a separable estimator must be designed so that its structure may become asymptotically equivalent to that of the optimal 2-D estimator. Furthermore, we derive an analytical expression of the iterative process to evaluate a convergence performance as a function of the number of iterations and discuss its convergence property. Our simulation results demonstrate that the proposed iterative receiver achieves a near-ideal performance with only a few iterations under time-variant multipath fading channels.