一种基于基扩展模型的OFDM频域快时变信道估计方法

现有的OFDM系统快时变信道估计方法均为时域估计方法,假定信道为整数倍采样信道,利用理想的等间隔均匀导频恢复信道时域响应函数,进而采用基扩展模型拟合并估计其时变特性。而实际的信道通常为非整数倍采样信道,频带内可用的导频也并非假定的理想模式。此时,恢复的信道时域响应出现能量泄漏,导致算法性能大大受限。本文提出一种频域快时变信道估计方法,利用信道频域时变传输函数辅助估计,从而估计得到信道频域响应矩阵。该方法在频域实现,性能不受信道时域响应能量泄露影响。仿真实验从误码率、均方误差两方面分别验证了该方法的有效性。     

Estimation of time-varying channels for pilot-assisted OFDM systems

Orthogonal frequency division multiplexing (OFDM) systems encounter performance degradations because of the time-varying (TV) channels common in wireless environments. The channel variations within one OFDM symbol introduce intercarrier interference. In this case, the frequency domain channel matrix is no longer diagonal, hence the corresponding channel estimation is challenging. In this article, two novel TV channel estimation approaches are proposed for the pilot-assisted OFDM systems, where the channel is approximated by the high-order linear model or the piece-wise linear model in time domain. The least square estimation is derived for the two kinds of channel approximations. The simulation is performed based on realistic TV channels with a fairly high Doppler spread. The results show the significant decreasing of the estimation mean square error using the proposed approaches.     

Intercarrier Interference Suppression for OFDMA Uplink in Time- and Frequency-Selective Fading Channels

This paper investigates intercarrier interference (ICI) suppression and channel estimation for the uplink of an orthogonal frequency-division multiple-access (OFDMA) system in a time- and frequency-selective fading channel. In such a doubly selective channel, channel variations within each OFDMA block disrupt the orthogonality among subcarriers and leads to ICI. We develop an appropriate signal model for the OFDMA uplink in a doubly selective fading channel and propose a minimum mean square error (MMSE) scheme and an MMSE successive detection (MMSE-SD) scheme to suppress ICI. It is shown that the MMSE scheme is the optimal linear scheme in terms of maximizing achievable data rate and that the MMSE-SD scheme is able to further remove ICI and exploit the Doppler diversity embedded in time-varying channels. As an essential component in ICI suppression, channel estimation is also considered. A basis expansion model (BEM) is formulated for the OFDMA uplink channel, and a pilot-aided channel-estimation algorithm is developed to track users' channels in the time domain. Simulation results are presented to illustrate the overall performance improvements that can be obtained from using the proposed ICI suppression and channel-estimation schemes.      

OFDM系统基于导频的联合信道估计与干扰抵消算法

对OFDM(orthogonal frequency-division multiplexing)系统中由于时频双选信道产生的载波间干扰进行了分析,并在此基础上提出了一种新的OFDM导频符号结构及信道估计方法,从而实现了在一个OFDM符号内时域信道估计和干扰抵消。仿真结果表明:提出的信道估计与干扰抵消相结合的联合算法不仅可以给出精度较高的信道信息,而且可以明显提高系统的性能。     

Deterministic multiuser carrier-frequency offset estimation for interleaved OFDMA uplink

In orthogonal frequency-division multiple access (OFDMA), closely spaced multiple subcarriers are assigned to different users for parallel signal transmission. An interleaved subcarrier-assignment scheme is preferred because it provides maximum frequency diversity and increases the capacity in frequency-selective fading channels. The subcarriers are overlapping, but orthogonal to each other such that there is no intercarrier interference (ICI). Carrier-frequency offsets (CFOs) between the transmitter and the receiver destroy the orthogonality and introduces ICI, resulting in multiple-access interference. This paper exploits the inner structure of the signals for CFO estimation in the uplink of interleaved OFDMA systems. A new uplink signal model is presented, and an estimation algorithm based on the signal structure is proposed for estimating the CFOs of all users using only one OFDMA block. Diversity schemes are also presented to improve the estimation performance. Simulation results illustrate the high accuracy and efficiency of the proposed algorithm.     

双选择信道下OFDM系统中一种基于新Kalman滤波估计的Turbo均衡

在OFDM系统中,信道的快速时变性破坏了子载波间的正交性,从而导致子载波间干扰(ICI),降低了系统性能。该文针对双选择信道的时变特性,提出了一种新的Kalman滤波信道估计算法,将其应用于过采样的复指数基扩展模型(OCE-BEM),从而将一个OFDM符号周期内信道参数时变的问题转化为参数时不变问题,同时,将这种新的Kalman滤波器与基于ICI抑制的低复杂度LMMSE Turbo均衡器相结合,并辅以循环冗余码校验(CRC)控制算法迭代次数,从而不需要更多的导频符号,在保证算法性能的基础上,减小算法的计算时延和复杂度。理论分析和仿真结果表明,该文给出的方法在双选择信道下能够有效地跟踪信道变化并抑制ICI影响。     

Iterative channel estimation and pilot design rules for high‐mobility comb‐pilot OFDM system

With wireless communications in high‐mobility environment becoming popular, this poses a big challenge for communication systems based on the comb‐pilot OFDM, such as IEEE 802.11p, since it has not the enough pilots to estimate the time‐ and frequency‐selective channel accurately. In this paper, several comb‐pilot schemes and three comb‐pilot design rules are proposed to meet the Nyquist criterion for sampling the vehicle‐to‐vehicle (V2V) channel and the requirements of second‐order statistic of V2V channel. Based on the proposed pilot schemes, an iterative channel estimation method from the CE‐BEM model is proposed, together with three ICI cancellation methods. After thorough simulation, the effectiveness of the comb‐pilot design rules, the proposed channel estimation method, and intercarrier interference (ICI) cancellation methods is verified. Compared with other channel estimation methods, the proposed method performs better. The simulation results also reveal that the channel order L+1 has a great impact on the performance of the comb‐pilot OFDM system.     

A time-domain estimation method of rapidly time-varying channels for OFDM-based LTE-R systems

This paper addresses the performance of fast doubly selective fading channel estimation combined with Inter-Carrier Interference (ICI) cancellation for Long Term Evolution (LTE) communication platform in the High Speed Railway (HSR) environment. We consider the Channel Impulse Response (CIR) coefficients with a critical Doppler frequency shift and multi-path fading that were taken from the WINNER II channel model and the D2a propagation scenario, where the conditions of HSR are analyzed. As multi-path fading increases and the channel varies in the order of the symbol period, we first propose a novel approach for designing a pilot symbol structure in the time domain. Then, we describe the deployment of the proposed pilot symbol structure to estimate the channel in the time domain. Channel information corresponding to the data positions is obtained by linear interpolation. In each OFDM symbol, the slope and the initial value for establishing an interpolation function are estimated to adapt to the time variation of the channel. An accurate estimate of channel state information is used for the purpose of ICI cancellation. The simulation results show that the channel estimated by our proposed method can follow the real channel well, even in a very high Doppler frequency. The estimation method in terms of Mean Squared Error (MSE) significantly outperforms the state-of-the-art methods. The combination of our channel estimator with several interference cancelers provides a considerably better system performance than that achieved when frequency channel estimation is used.     

Joint channel estimation and data detection for high rate orthogonal time frequency space systems

This paper proposes a new pilot pattern in the delay-Doppler (DD) domain for the orthogonal time frequency space (OTFS) system. In contrast to the embedded-pilot schemes, guard intervals are not used so as increase the spectral efficiency. Also, compared to the superimposed design where data symbols and pilots are arranged on the entire DD grid, in the proposed rearrangement, the number of pilots used is only spread over a subgrid of the DD grid. Hence, the interference of pilots with data symbols is reduced. Afterwards, an algorithm for channel estimation (CE) and symbol detection in the DD domain benefiting from the sparsity of the DD channel is designed. The sparse CE step is formulated as a specific marginalization of the maximum a posteriori (MAP) criterion by providing a Bayesian approach via the mean-field approximation that involves the variational Bayesian expectation maximization (VB-EM) algorithm. Detection of data symbols is done using a low complexity MP algorithm. We also propose an interference cancellation (IC) scheme to mitigate contamination of data by pilots that is run after each CE step. To achieve a high CE accuracy, based on the mean mutual incoherence property (MIP), a pilot optimization problem for OTFS is formulated and develop a simulated annealing-based algorithm to solve it. Finally, simulation results show that the proposed scheme achieves a good compromise between spectral efficiency, complexity, and performance in terms of bit error rate (BER) and normalized mean square error (NMSE) when compared to literature benchmarks.     

数字调幅广播系统中基于滑动窗口的二维信道估计

针对DRM数字广播系统,提出一种基于滑动窗口的二维信道估计方法.该方法首先对接收到的导频信号所在信道进行最小二乘估计,然后进行时间方向插值估计导频子载波上所有位置处的信道频率响应,最后通过一个滑动窗口在多径扩展域和多谱勒扩展域分别进行处理.对于快速时变信道,通过调整窗口的滑动步数,可以有效处理窗口内信道路径数和路径延迟可能变化的情况,从而显著降低了子载波间干扰和高斯白噪声的影响.理论分析和仿真表明,与现有信道估计相比,该方法具有估计精确、易于实现的优点.     

Doppler Spread Estimation for Broadband Wireless OFDM Systems Over Rician Fading Channels

In this paper, we present a new Doppler spread estimation algorithm for broadband wireless orthogonal frequency division multiplexing (OFDM) systems with fast time-varying and frequency-selective Rayleigh or Rician fading channels. The new algorithm is developed by analyzing the statistical properties of the power of the received OFDM signal in the time domain, thus it is not affected by the influence of frequency-domain inter-carrier interference (ICI) introduced by channel variation within one OFDM symbol. The operation of the algorithm doesn’t require the knowledge of fading channel coefficients, transmitted data, or signal-to-noise ratio (SNR) at the receiver. It is robust against additive noise, and can provide accurate Doppler spread estimation with SNR as low as 0 dB. Moreover, unlike existing algorithms, the proposed algorithm takes into account the inter-tap correlation of the discrete-time channel representation, as is the case in practical systems. Simulation results demonstrate that this new algorithm can accurately estimate a wide range of Doppler spread with low estimation latency and high computational efficiency.     

Channel estimation for multi‐antenna mobile worldwide interoperability for microwaves access downlink partial usage of subchannels systems

In order to improve channel estimation performance, a new time domain least square method is proposed for the multi‐antenna worldwide interoperability for microwaves access system at the downlink partial usage of subchannels mode. We show that the pilots in the proposed method double those of the regular time domain least square method by establishing a hypothetical channel estimation model instead of increasing the additional pilots, in which the frequency outputs of virtual pilots in the current symbol are reconstructed at the positions where the actual pilots of the adjacent symbol are located. We also show that the border effect incurred by the uneven limited pilots, which degrades the performance of channel estimation, can be reduced by using SVD as effectively as regularization. Simulations demonstrate that the proposed method using SVD or regularization can perform better than the conventional approaches, and especially over the pedestrian‐B channel for four transmits, it only incurs the performance loss of about 2.5 dB at a 0.001 of BER compared to the ideal case, whereas the regular methods cease to be effective due to too rare pilots allocated for each transmit and cannot approach this BER value over the considered Eb ∕ N0 range. Copyright © 2013 John Wiley & Sons, Ltd.     

适用于高速移动场景的信道估计方法

目前存在的各种信道估计方法,通常假设信道参数在一个正交频分复用（OFDM）符号内是恒定不变的。但是在高速场景中,信道在一个OFDM符号内呈现出较为明显的变化,这时子载波间干扰（ICI）就会影响传统的信道估计方法,使估计性能明显下降。本文提出通过幂级数基的形式来建模无线信道变化的方法,将时变的信道估计转变成有限参数变量在幂级数基上的投影,这大大减少了估计信道参数的个数。理论分析及仿真结果表明,该方法能有效地跟踪和估计时变的信道参数,具有良好性能。     

Firefly Algorithm for Joint Estimation of Frequency Offsets and Channel in OFDMA Uplink

This paper addresses the problem of joint maximum-likelihood estimation of carrier frequency offsets, and channel responses in an orthogonal frequency division multiple access (OFDMA) uplink, which is a computationally-intensive, multiparameter, multidimensional problem. We propose to apply a recently-developed meta-heuristic algorithm called the firefly algorithm (FA) for the estimation of required parameters. We also introduce a new initialization procedure for the FA algorithm based on the separability of cost functions, which drastically reduces its computational complexity. A separate pilot OFDMA symbol is transmitted prior to the data symbols to estimate the parameters. The proposed technique is shown to achieve excellent performance at reduced computational complexity, especially at low signal-to-noise ratio. Also, it can be applied to OFDMA systems with any type of carrier assignment schemes. The significant features of the proposed estimation method are substantiated through extensive computer simulation studies and are compared with an important classical estimation technique available for the problem, to highlight its superiority.     

基于部分数据的叠加序列慢时变信道估计

针对OFDMA通信系统,提出了一种基于部分数据的叠加序列慢时变信道估计算法,并在接收端给出了数据恢复的方法。时变信道采用复指数基扩展模型来描述,对OFDMA系统的导频序列进行了精心设计。提出在频域减去一个基于部分数据的序列,从而使发送数据经过反傅里叶变换后,特定位上的数据能量变低,进而大大减少了数据在信道估计时产生的误差。由于在发射端信号已经产生了畸变,在接收端采用特定的方法对数据进行补偿,消除了这种影响。实验仿真证明该方法在一定程度上提高信道估计的性能。     

Comb type pilot aided channel estimation in OFDM systems with transmit diversity

Transmit diversity can be applied to OFDM systems by adopting space time code. Since the received signal is the overlapped signals transmitted from different transmit antennas, channel estimation is a rather challenging task for space time coded OFDM (ST-OFDM) systems. Pilot structure can help the receiver to effectively separate the overlapped signals and perform accurate channel estimation. In this paper, we propose three different channel estimation algorithms based on specially designed comb type pilots inserted in frequency domain. One of our proposed algorithms is performed in frequency domain and the other two are performed in time domain. Such comb type pilot based algorithms can provide higher bandwidth efficiency than common significant-tap-catching algorithm using training block pilots. Numerical analyzes and computational simulation show that our proposed estimation schemes have the same good performance while the time domain methods have relatively simple structure.     

Polynomial Estimation of Time-Varying Multipath Gains With Intercarrier Interference Mitigation in OFDM Systems

In this paper, we consider the case of a high-speed mobile receiver operating in an orthogonal frequency-division multiplexing (OFDM) communication system. We present an iterative algorithm for estimating multipath complex gains with intersubcarrier interference (ICI) mitigation (using comb-type pilots). Each complex gain variation is approximated by a polynomial representation within several OFDM symbols. Assuming knowledge of delay-related information, polynomial coefficients are obtained from time-averaged gain values, which are estimated using the least-square (LS) criterion. The channel matrix is easily computed, and the ICI is reduced by using successive interference suppression (SIS) during data symbol detection. The algorithm's performance is further enhanced by an iterative procedure, performing channel estimation and ICI mitigation at each iteration. Theoretical analysis and simulation results for a Rayleigh fading channel show that the proposed algorithm has low computational complexity and good performance in the presence of high normalized Doppler spread.     

一种基于二维处理的OFDM信道估计算法

该文提出了一种新的信道估计算法,用于无线移动信道下的正交频分复用(OFDM)系统。该算法对接收的导频信号(Pilot)分别在多径展宽域和多普勒展宽域进行处理,显著地降低了子载波间干扰和高斯白噪声的影响。此外,多普勒展宽域处理的滤波器是动态设计的,具有良好的自适应性。仿真结果表明,在不同的多普勒频偏下,该算法都有良好的性能。     

一种适用于快衰落信道的ICI抑制方案

针对高速移动环境下多普勒频偏造成信道的快衰落和正交频分复用(OFDM)系统中子载波间干扰(ICI)的问题,提出了一种适合快衰落环境的OFDM系统子载波间干扰抑制算法。此算法用线性变化模型来近似一个OFDM符号周期内的信道冲激响应,并以此为基础采用迭代MMSE均衡方法抑制载波间干扰。分析和仿真结果表明,此方法能有效地保证载波间的正交性,从而改善了OFDM系统的误码率(BER)性能。