Performance Analysis of UWB Channels for Wireless Personal Area Networks

Ultra-wide band (UWB) communication is one of the most promising technology for high data rate networks over short-range communication. The ultra-wide bandwidth offers pulses with very short duration that provides frequency diversity and multipath resolution. Ultra-wide band (UWB) channels raise new effects in the receiver, the amplitude fading statistics being different compared to the conventional narrow band wireless channels. This review paper focuses on modeling of ultra-wide band channels, especially for simulation of personal area networks and also discusses the benefits, application potential and technical challenges in wideband communication. The concept of Orthogonal Frequency Division Multiplexing (OFDM) has recently been applied in wireless communication systems due to its high data rate transmission capability with high bandwidth efficiency and its robustness to multi-path delay. UWB OFDM communication was proposed for physical layer in the IEEE 802.15.3a standard which covers wideband communication in wireless personal area networks. Since the channel model for multicarrier UWB communication is different from that of plain ultra-wide band channel, a novel modification method in UWB channel model is proposed with specific center frequency and multipath resolution. Moreover, dynamic channel estimation is necessary before demodulation of UWB OFDM signals since the radio channel is time varying and frequency selective for wideband systems. The performance of the proposed method is statistically analyzed using LS and MMSE based channel estimation methods.     

基于STBC的MB-OFDM系统性能研究

在对多带（MB）OFDM UWB通信系统进行研究分析的基础上,提出一种基于空时分组码（STBC）的多输入多输出（MIMO）MB-OFDM超宽带通信系统,并详细介绍了系统模型和空时码的编译码原理,对系统性能进行了仿真。仿真结果表明,在改进的S-V多径信道环境下,和传统空时分组码相结合的MB-OFDM系统不仅具有更高的信道容量和分集增益,而且误码性能有所提高。     

A Technique for Frequency Synchronization in Multi-Band OFDM in Realistic UWB Channel

This paper presents a highly accurate frequency offset estimation algorithm for multi-band orthogonal frequency division multiplexing (MB-OFDM) systems effective for realistic ultra-wideband (UWB) environment. The proposed algorithm derives its estimates based on phase differences in the received subcarrier signals of several successive OFDM symbols in the preamble. We consider different carrier frequency offsets and different channel responses in different bands to keep the analysis and simulation compatible for practical multi-band UWB scenario. Performance of the proposed algorithm is studied by means of bit error rate (BER) performance of MB-OFDM system. In order to compare the variance of the synchronizer to that of the theoretical optimum, we derive the Cramer–Rao lower bound (CRLB) of the estimation error variance and compare it with the simulated error variance both in additive white Gaussian noise and UWB channel model (CM) environments, CM1–CM4. Next, we modify the estimation algorithm by proposing a multi-band averaging frequency offset synchronization (MBAFS) scheme. We establish superior BER performance with MBAFS compared to our first scheme. We calculate modified CRLB for MBAFS and compare it with simulation results for CM1–CM4. Both analysis and simulation show that MBAFS algorithm can estimate the carrier frequency offset effectively and precisely in UWB fading channels for MB-OFDM applications. We also analyze the computational complexity of both the proposed algorithms in order to verify their feasibility of implementation in practical UWB receiver design.     

信息管理中UWB系统信道估计与均衡算法及实现

通过介绍UWB技术的发展现状,着重阐述了MB-OFDM UWB通信系统的物理层帧结构和信道模型。在分析最小二乘(LS)信道估计算法和基于快速傅里叶变换(FFT)信道估计算法的基础上,针对MB-OFDM UWB信号帧结构特征提出了一种基于FFT变换和Hannan-Quinn(HQ)准则的改进算法,即FFT-HQ信道估计算法。该文也提出了一种基于信道估计的自适应均衡算法,利用仿真可以看出此均衡算法在性能上优于传统的基于LS算法调整均衡器系数的方法。     

Over-sampling basis expansion model aided channel estimation for OFDM systems with ICI

The rapid variation of channel can induce the intercarrier interference in orthogonal frequency-division multiplexing （OFDM） systems. Intercarrier interference will significantly increase the difficulty of OFDM channel estimation because too many channel coefficients need be estimated. In this article, a novel channel estimator is proposed to resolve the above problem. This estimator consists of two parts： the channel parameter estimation unit （CPEU）, which is used to estimate the number of channel taps and the multipath time delays, and the channel coefficient estimation unit （CCEU）, which is used to estimate the channel coefficients by using the estimated channel parameters provided by CPEU. In CCEU, the over-sampling basis expansion model is resorted to solve the problem that a large number of channel coefficients need to be estimated. Finally, simulation results are given to scale the performance of the proposed scheme.     

修正Kalman滤波多带UWB信道估计改进方法

针对多带超宽带(UWB)系统中修正Kalman滤波算法复杂度高的缺陷,提出一种低复杂度的修正Kalman滤波改进方法。该方法中UWB信道采用自回归模型(AR)建模,利用导频跟踪时变信道衰减因子,通过Kalman滤波和频域分段最小均方误差(MMSE)算法同时跟踪信道的时域相关性和频域相关性,提高了系统性能,降低了计算复杂度。仿真结果表明,和修正的Kalman滤波方法相比,在估计精度损失很小的情况下,所提方法极大降低了计算复杂度,提高了系统整体的估计性能。     

一种改进的快变信道展开模型

 在高速移动正交频分复用系统（OFDM）中,信道的快速变化引起载波间干扰,降低系统性能,且使信道的准确估计变得更为困难. 由于在接收端待估计的信道参数多于接收信号样点数,信道估计方程无确定解. 为了解决这一问题,通常将快变信道展开为基函数叠加的近似表达形式,信道估计问题变成对展开系数的估计,待估计的参数大大减少. 本文分析并比较了常用的基展开模型的特点,指出了它们的不足,并在此基础上提出一种改进的快变信道展开模型,该模型的核心思想是在过采样基础上通过基线补偿减小吉布斯效应的影响,从而减小展开误差. 该模型性能与信道的统计性质无关,且在展开基的变化赶不上实际信道变化速度时,展开误差不会明显增大,优于CE-BEM, GCE-BEM, KL-MEM等模型,可用于高速移动OFDM系统的快变信道估计.     

Efficient time domain threshold for sparse channel estimation in OFDM system

A novel efficient time domain threshold based sparse channel estimation technique is proposed for orthogonal frequency division multiplexing (OFDM) systems. The proposed method aims to realize effective channel estimation without prior knowledge of channel statistics and noise standard deviation within a comparatively wide range of sparsity. Firstly, classical least squares (LS) method is used to get an initial channel impulse response (CIR) estimate. Then, an effective threshold, estimated from the noise coefficients of the initial estimated CIR, is proposed. Finally, the obtained threshold is used to select the most significant taps. Theoretical analysis and simulation results show that the proposed method achieves better performance in both BER (bit error rate) and NMSE (normalized mean square error) than the compared methods has good spectral efficiency and moderate computational complexity.     

Wavelet-Based Semiblind Channel Estimation for Ultrawideband OFDM Systems

Ultrawideband (UWB) communications involve very sparse channels, because the bandwidth increase results in a better time resolution. This property is used in this paper to propose an efficient algorithm that jointly estimates the channel and the transmitted symbols. More precisely, this paper introduces an expectation-maximization (EM) algorithm within a wavelet-domain Bayesian framework for semiblind channel estimation of multiband orthogonal frequency division multiplexing based UWB communications. A prior distribution is chosen for the wavelet coefficients of the unknown channel impulse response to model a sparseness property of the wavelet representation. This prior yields, in maximum a posteriori estimation, a thresholding rule within the EM algorithm. We particularly focus on reducing the number of estimated parameters by iteratively discarding “insignificant” wavelet coefficients from the estimation process. Simulation results using UWB channels that were issued from both models and measurements show that, under sparseness conditions, the proposed algorithm outperforms pilot-based channel estimation in terms of the mean square error (MSE) and bit error rate (BER). Moreover, the estimation accuracy is improved, whereas the computational complexity is reduced compared with traditional semiblind methods.      

办公室环境下的超宽带信道测量与建模

依据广泛的频域信道测量数据,提出了符合中国超宽带(UWB)技术频率使用规定的办公室室内信道模型。信道总体模型采用修正Saleh-Valenzuela (S-V)模型。在信道测量信号的后处理中,使用过渡带为高斯滚降特性的类高斯窗来提取符合中国超宽带频谱规范的测量信号。利用CLEAN算法从时域测量数据中提取高分辨率的离散信道响应,并为信道时域测量信号提出了一种基于小波分析的自动分簇算法,统计提取出了大尺度和小尺度信道模型参数。结果表明:提出的办公室超宽带信道模型和实测数据具有相近的时延扩展特性和平均多径数量,可以比IEEE 802.15.4a信道模型更好地反映中国办公室环境下的UWB信道特性。     

MIMO-OFDM快衰落信道的稀疏自适应感知估计

在多输入多输出（MIMO）-正交频分复用（OFDM） 系统中,怎样在较高频谱利用率的情况下对快时变信道进行较为准确的估计是一个具有挑战性的课题。该文在利用压缩感知理论可提高系统频谱利用率的基础上,提出了一种适合于快时变环境下MIMO-OFDM 系统的稀疏自适应信道估计方法。该方法不再受到奈奎斯特采样频率条件约束,避免了传统导频辅助信道估计方法频谱利用率低的缺点。该文方法通过构建多天线群时频结构特征稀疏基,利用多天线间和群时变OFDM符号内信道冲激响应具有更强稀疏性的特点,对MIMO-OFDM快衰落信道进行稀疏变换。由于实际MIMO-OFDM快衰落信道往往处于频率选择性、时变性和多种干扰并存的复杂环境,受到干扰的信道参数对系统而言是未知,采用该方法克服了现有基于压缩感知理论的信道估计方法需要预先知道信道冲激响应稀疏度才能重构信道参数的不足,在信道稀疏度未知道的情况下,运用稀疏自适应的方法来对不同时频结构特征的信道参数进行估计。仿真结果表明所提估计方法具有对快时变信道参数估计的鲁棒性和较高频谱利用率,且均方误差小。      

一种UWB-OFDM系统信道模型改进方案

简要介绍了OFDM技术的UW B系统,分析了Intel提供的UW B系统中常用的S-V模型,指出了其局限性,提出了一种较为合理的改进方案,使该模型能应用于UW B-OFDM系统,最后给出了该模型的统计特征和信道样本。     

Performance Analysis of Tikhonov Regularized LS Channel Estimation for MIMO OFDM Systems with Virtual Carriers

In this paper the effect of virtual carriers (VCs) on channel estimation performance is considered for multiple input multiple output (MIMO) orthogonal frequency division multiplexing (OFDM) systems. The link between the number of VCs and the inverse problem in a conventional LS channel estimation is identified. It is observed that a linear increase in VCs contributes to an exponential increase of the system inverse problem as the condition number of the inverse matrix is exponentially increased. A solution is proposed using a low complexity modified LS channel estimation. The performance analysis of the modified LS channel estimation, regarding the effect of VCs, show gains in both the mean square error (MSE) performance of channel estimation as well as bit error rate (BER) system performance compared to conventional LS. The performance is improved irrespective of the increasing number of VCs as the estimator is shown to be insensitive to the increase of VCs in the OFDM system. The complexity reduction is also considered and an approach is proposed showing a similar complexity as LS.     

Frequency-Dependent UWB Channel Characteristics in Office Environments

This paper reports frequency-dependent ultrawideband (UWB) channel characteristics. Measurements were performed in 103 receiver locations of six different office environments. From the measured data, the effect of frequency on the pathloss properties of a UWB signal is analyzed. After analyzing the pathloss behavior to propagation environments, the pathloss exponent variation models are developed in various environments as a function of frequency. Based on these models, pathloss prediction is performed, and the accuracy of the prediction is compared with those of existing pathloss models. In addition, the frequency-dependent UWB channel correlation characteristics are investigated. For the frequency correlation statistics of the UWB channel, double-slope models representing correlation coefficients are established with and without line-of-sight (LOS) paths. Using these correlation models, a channel gain estimation algorithm is proposed. The performance of the proposed estimation algorithm is evaluated with estimation parameters, and it is confirmed that the proposed estimation algorithm has better performance than the conventional algorithm using a linear interpolation algorithm.      

一种正交频分复用系统的非线性补偿技术

针对正交频分复用(OFDM)系统在功率放大器(PA)非线性较强时的性能问题,基于一种无线设备非线性与无线信道的联合估计技术,提出了一种基于训练序列的OFDM非线性信道估计与补偿技术.首先基于最小二乘(LS)算法进行发射机非线性与无线信道单位脉冲响应的联合估计,然后依次进行无线信道与发射机非线性的补偿.仿真结果显示,提出方法可逼近不考虑PA非线性时OFDM无线通信系统的完美均衡理论解析值.     

Fundamental Limit of OFDM Range Estimation in a Separable Multipath Environment

In this paper, orthogonal frequency division multiplexing (OFDM) for time-based range estimation (TBRE) in a separable multipath channel is investigated and analyzed with respect to its accuracy. First, the Cramer–Rao lower bound (CRLB) in a separable multipath channel is theoretically derived, and indicates a similar expression to that for a single path channel. The CRLB for non-data-bearing (NDB) OFDM transmission is compared to that for pseudo-noise (PN) transmission, demonstrating a large performance gap in favor of the NDB OFDM. Furthermore, the maximum likelihood estimator (MLE) for TBRE in a separable multipath channel is theoretically derived, also demonstrating a similar expression to that in a single path channel, except that several peaks instead of one peak are expected in a separable multipath channel corresponding to all arrival paths. The MLE for TBRE is then compared to the commonly used MLE for channel estimation, showing an equal performance in terms of mean square error when using an NDB OFDM transmission. Simulation results demonstrate a good agreement with our proposed theory.     

基于BP神经网络的OFDM系统信道估计

针对正交频分复用(Orthogonal Frequency Division Multiplexing,OFDM)系统中传统信道估计算法复杂度高或估计精度低的问题,给出一种基于反向传播(Back Propagation,BP)神经网络的信道估计方法.采用Simulink仿真工具构建OFDM信号采集平台,建立了基于BP神...     

IMM-based Kalman filter for channel estimation in UWB OFDM systems

Ultra-wide band (UWB) communication is one of the most promising technologies for high-data rate wireless networks for short-range applications. This paper proposes a blind channel estimation method namely Interactive Multiple Model (IMM)-based Kalman algorithm for UWB OFDM systems. IMM-based Kalman filter is proposed to estimate frequency selective time-varying channel. In the proposed method, two Kalman filters are concurrently estimating channel parameters. The first Kalman filter, namely the Static Model Filter (SMF) gives an accurate result when the user is static while the second Kalman filter namely the Dynamic Model Filter (DMF) gives an accurate result when the receiver is in moving state. The static transition matrix in SMF is assumed as an Identity matrix where as in DMF, it is computed using Yule–Walker equations. The resultant filter estimate is computed as a weighted sum of individual filter estimates. The proposed method is compared with other existing channel estimation methods.     

一种基于直接判决的OFDM快衰落信道估计新方法

为了解决正交频分复用（Orthogonal Frequency-Division Multiplexing，OFDM）无线局域网信道估计和跟踪问题，采用直接判决算法进行信道估计，并从中选择可靠的估计结果，结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信道估计，并利用改进的导频算法进一步跟踪信道在时间上的变化。Simulink仿真结果表明，该算法优于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。