频率选择性慢衰落信道下的快速频率估计

给出了在频率选择性慢衰落信道下适用于突发传输的两种快速频偏估计算法。针对慢衰落选择性信道下的频率估计问题，两种算法均给出了一个简单的闭式解，且算法实现无需任何信道幅度信息。第一种算法要求用于频率估计的训练序列的自相关矩阵具有对角特性，第二种算法使用周期的训练序列，两种算法所需的运算量大大低于最大似然估计算法。仿真结果显示，在较高信噪比下两种算法均具有较好的估计性能。     

频率选择性MIMO信道下的网格退化辅助检测

将应用于平坦衰落相关信道下的网格退化(LR, lattice reduction)算法推广到频率选择性衰落信道中.给出了在频率选择性衰落的相关信道中,采用网格退化算法辅助检测前后最小均方误差(MMSE)检测器的性能比较.实验表明:在具有频率选择性衰落的相关信道中,采用网格退化辅助检测算法能显著地提高系统的检测性能.     

OFDM系统频率和时间估计的循环平稳实现

提出了一个用于频率选择性衰落信道中传输的正交频分多路(OFDM)的定时和频率联合估计的算法。这种估计器利用了接收信号的二阶循环特性,而且不需要信道冲激响应的信息。计算机仿真表明,采用所提出的方案能显著地提高估计器的性能。     

一种改进的MIMO-OFDM的信道估计算法

针对频率选择性衰落信道下MIMO—OFDM的系统,基于瑞利衰落信道的模型,利用m序列的自相关性,提出了一种基于m序列做梳状导频的多输入多输出正交频分复用最小二乘算法,该算法可以避免对大矩阵求伪逆,以减少计算复杂度,从而提高了信道估计的计算性能。通过对该算法的误码率性能分析和计算复杂度分析,结果表明,相比传统经典最小二乘算法,所提出信道估计算法在中低信噪比下,有效提高了信道估计性能,适合于在实际应用中实现。     

基于多项式内插的MIMO时间-频率双选择性信道的信道估计

数据的高速率传输以及终端的高速移动,导致无线通信信道具有时间选择性与频率选择性两个特征。该文主要研究了数据分组传输方式下,基于导频符号辅助调制(PSAM)的多输入多输出(MIMO)时间-频率双选择性信道的信道估计问题。首先,将时间-频率双选择性MIMO信道,建模为一个随时间变化的多项式内插信道模型;然后,根据信道Doppler衰落速率、多项式模型中的误差项,确定出模型的阶数以及整个数据块的长度;最后,基于该多项式内插信道模型,提出了采用PSAM的MIMO双选择性信道估计方法。实验结果表明该算法在时间-频率双选择性衰落信道下具有较好的性能。     

无需信道估计的一种简单的差分分集OFDM传输方式

空时码在近几年得到了普遍关注,但空时码的应用通常都假定信道具有非频率选择性衰落特性,同时还假定信道状态也是已知的,这些都极大地限制了空时码的应用。该文给出了一种未知频率选择性衰落信道下的差分分集调制方法。应用正交频分复用技术的基本原理,把一个频率选择性衰落信道转换成了多个并行平坦衰落信道。在此基础上,给出了一种无需信道估计的简单的时空频分集结构.仿真结果表明,这种结构在频率选择性衰落信道中无需信道估计就能得到很好的结果.     

Turbo编码MIMO-OFDM系统中的EM迭代信道估计

该文对频率选择性衰落信道条件下的Turbo编码多入多出正交频分复用(MIMO-OFDM)系统的信道估计进行了研究,提出了一种将Turbo迭代译码与EM迭代信道估计相结合的方法。采用该方法的EM迭代信道估计性能得到了明显提高,且适合于非常恶劣的信道环境。另外,通过仿真可知,利用EM迭代信道估计算法的系统性能优于利用简化的最小二乘(LS)信道估计算法所获得的系统性能。     

基于PSAM的MIMO时间-频率双选择性信道的信道估计

数据的高速率传输以及终端的高速移动,导致无线通信信道具有时间选择性与频率选择性两个特征.本文主要研究了数据分组传输方式下,基于导频符号辅助调制(PSAM)的多输入多输出(MIMO)时间-频率双选择性信道的信道估计问题.首先,将时间-频率双选择性MIMO信道,建模为一个随时间变化的多项式内插信道模型;然后,根据信道Doppler衰落速率、多项式模型中的误差项,确定出模型的阶数以及整个数据块的长度;最后,基于该多项式内插信道模型,提出了采用PSAM的MIMO双选择性信道估计方法.实验结果表明本文的算法在时间-频率双选择性衰落信道下具有较好的性能.     

多天线OFDM系统信道估计算法的改进

提出一种针对多天线OFDM系统中基于LS准则的信道估计的改进方法。该方法使用梳状训练序列估计出一半的信道频率响应,然后利用抛物线插值对剩余一半进行估计。分析和仿真表明该信道估计算法可以抵抗信道的严重频率选择性衰落。     

一种用于IEEE802.16d的联合信道估计算法

针对IEEE802.16d帧结构提出一种结合前导训练序列和导频的信道估计算法。算法通过前导完成初始精确的信道估计,再使用判决反馈方式提高导频估计准确度,并根据前后符号导频估计结果得到反映信道变化的时变因子,利用它来调整前导估计和导频估计比例,联合完成信道估计。仿真结果表明,与传统的信道估计方法相比,此算法能够更有效的对抗频率选择性衰落和更大的多普勒频移,改善系统的性能。     

频选快衰落信道的Turbo均衡算法

针对频选快衰落信道,本文提出卡尔曼滤波信道跟踪、软输出判决反馈均衡及软输入软输出信道解码迭代处理的Turbo均衡算法以充分利用已获得的信息,实现信道估计、信道均衡与信道解码的迭代更新,并克服传统判决反馈均衡器误差传播的缺陷.仿真表明,本算法能有效地跟踪快衰落信道,经两次迭代就可获得较为满意的码间干扰消除效果.     

Maximum-likelihood synchronization, equalization, and sequenceestimation for unknown time-varying frequency-selective Rician channels

This paper develops a receiver structure to perform jointly maximum-likelihood (ML) synchronization, equalization, and detection of a linearly modulated signal transmitted over a time-varying frequency-selective Rician-faded channel, corrupted by additive white Gaussian noise (AWGN). The receiver is particularly suited to a fast-fading channel, where other receivers that rely on estimating the channel cannot track it quickly enough. The signal mean and autocovariance are needed, and a scheme is proposed for estimating these quantities adaptively. The receiver processes the specular and diffuse components (corresponding to the signal mean and autocovariance) separately. Processing the known specular component is the classical detection problem. The unknown diffuse component is processed by predictors. We show that the predictors can achieve synchronization in a novel manner, if synchronization is required. A union bound on the receiver's bit-error rate (BER) is derived, and it tightly bounds simulated BERs in fast-fading at high signal-to-noise ratios (SNRs)     

Extended MLSE receiver for the frequency-flat, fast-fading channel

This paper develops a maximum likelihood sequence estimation (MLSE) receiver for the frequency-flat, fast-fading channel corrupted by additive Gaussian noise when linear modulations (M-ASK, M-PSK, and M-QAM) are employed. This paper extends Ungerboeck's derivation of the extended MLSE receiver for the purely frequency-selective channel to the time-selective channel. Although the new receiver's structure and metric assume ideal channel state information (CSI) at the receiver, the receiver structure can be used wherever high-quality CSI is available. The receiver is maximum likelihood for a variety of channels, including Ricean, Rayleigh, lognormal, and additive white Gaussian noise (AWGN) channels. Bounds on the receiver's bit error rate (BER) are deduced for ideal and pilot tone CSI for fast Rayleigh fading. A crude lower bound is developed on the BER of predictor-based receivers for the same channel. This paper offers insight into matched filtering and receiver processing for the fast-fading channel and shows how pilot symbols and tones should be exploited     

Joint estimation of carrier frequency offset and statistical parameters of the multipath fading channel

In this paper, we focus on the joint estimation of the carrier frequency offset and statistical parameters of the multipath mobile channel. By modeling the multipath fading channel with a complex bandpass autoregressive (AR) model, we show how the estimates for the frequency offset, Doppler spread, and power profile of the multipath channel can be extracted from the estimated complex AR coefficients obtained via the expectation-maximization algorithm. A main advantage of the proposed joint estimator is that while it has a capability of performing equally well in all scattering environments, it can provide accurate estimates even in high-mobility channel conditions. We also demonstrate how the complexity of the estimator can be significantly reduced, while only slightly trading off performance, by applying the mean field approximation technique. Moreover, we derive a fully adaptive joint synchronization and channel-estimation scheme, as well as a novel Kalman-smoother-based frequency-error detector that can be used in feedback frequency-recovery schemes and is particularly well suited for fast-fading channel conditions. Finally, we revisit the Cramer-Rao lower bound analysis, and show how the Fisher information matrix can be conveniently computed in the presence of a frequency-selective Rayleigh fading channel.     

DPSK versus pilot-aided PSK MAP equalization for fast-fadingchannels

This letter compares pilot-aided phase-shift keyed (PSK) and differential PSK (DPSK) modulation when using new maximum a posteriori joint channel estimation and equalization receivers with frequency-selective fast-fading channels. We conclude that pilot-aided PSK has superior bit-error rate performance in this case. However, at low signal-to-noise ratio, performance is similar, and DPSK is competitive due to reduced receiver complexity     

PSP array processing for multipath fading channels

The performance of per-survivor processing (PSP) algorithms with stochastic gradient estimators is investigated for frequency-selective channels and array measurements. The focus is on the degree to which the underlying channel structure can be reliably exploited by such algorithms in tracking the time-varying channel and detecting the data     

Frequency-domain correlative coding for MIMO-OFDM systems over fast fading channels

Multiple-input multiple-output (MEMO) antennas combined with orthogonal frequency division multiplexing (OFDM) are very attractive for high-data-rate communications. However, MEMO-OFDM systems are very vulnerable to time-selective fading as channel time-variation destroys the orthogonality among subchannels, causing inter-carrier interference (ICI). In this letter, we apply frequency-domain correlative coding in MEMO-OFDM systems over frequency-selective, fast-fading channels to mitigate ICI. We derive the analytical expression of the carrier-to-interference ratio (CIR) to quantify the impact of time-selective fading and demonstrate the effectiveness of correlative coding in mitigating ICI in MEMO-OFDM systems.     

无线OFDM系统频率选择性衰落信道下的SOVA译码算法

本文对正交频分复用(OFDM)系统频率选择性衰落信道中,信道状态信息(channel state information 简称CSI)确知,以及CSI不能或不必精确估计时的SOVA译码算法进行了研究,给出了OFDM系统频率选择性信道BPSK调制时SOVA译码算法的数学描述.在此基础上,我们提出了两种SOVA译码算法:一种是基于盲信道估计的SOVA算法;另一种是不必训练和信道估计的差分SOVA算法,该算法可以在CSI完全未知的情况下进行可靠的SOVA译码.仿真结果说明该算法具有良好的性能.     

Channel estimation for the uplink of a DS-CDMA system

This paper deals with channel estimation in the uplink of a direct-sequence code-division multiple-access system operating in a multipath environment. The parameters of interest are the delays and the complex attenuations incurred by the signal echoes along the propagation paths. We propose an iterative approach for estimating the channel parameters of a new user entering the system. The method is based on the space-alternating generalized expectation-maximization algorithm and exploits a training sequence. In comparison to other estimation algorithms, it reduces a complicated multidimensional optimization problem to a sequence of one-dimensional problems. In addition, it can be effectively used in applications over fast-fading channels. Computer simulations are employed to assess the performance of the proposed scheme. It is found that it is resistant to multiuser interference and has accuracy close to the Cramer-Rao lower bound even with very short training sequences.     

MIMO-OFDM系统的信道估计算法

基于训练序列的信道估计方法在慢衰落信道中呈现出较好的性能，但是这种方法不适用于快衰落环境，为了能够及时跟踪信道的变化，通常采用基于导频符号的信道估计方法。将针对采用连续传输方式的MIMO—OFDM系统，讨论基于导频符号的信道估计方法并进行了计算机仿真。