Analysis and design of joint CFO/channel estimate techniques for a cooperative STBC‐OFDM system

This paper studies the joint estimation technique of carrier frequency offset (CFO) and channel information for a distributed decode‐and‐forward (DF) cooperative space‐time block‐coded (STBC) orthogonal frequency division multiplexing (OFDM) system. For the considered relay system, we provide theoretical analysis of the effects upon the output signal‐to‐noise ratio (SNR), which is caused by the CFO/channel estimation error. Based on the provided analytical results, a joint CFO/channel estimation scheme is then developed, where the CFO estimate is achieved by a multiple‐dimensional linear search algorithm. Furthermore, we propose an alternative estimation solution with iteration approach being designed for the CFO estimation prior to the channel estimation. In contrast to the former estimator, the iterative method enjoys the advantage of the substantially reduced implementation complexity without sacrificing the estimate performance. The conducted computer simulation results verify the effectiveness of the proposed schemes.     

Performance Investigation of Space‐Time Block Coded Multicarrier DS‐CDMA in Time‐Varying Channels

In this letter, we evaluate the system performance of a space‐time block coded (STBC) multicarrier (MC) DS‐CDMA system over a time selective fading channel, with imperfect channel knowledge. The average bit error rate impairment due to imperfect channel information is investigated by taking into account the effect of the STBC position. We consider two schemes: STBC after spreading and STBC before spreading in the MC DS‐CDMA system. In the scheme with STBC after spreading, STBC is performed at the chip level; in the scheme with STBC before spreading, STBC is performed at the symbol level. We found that these two schemes have various channel estimation errors, and that the system with STBC before spreading is more sensitive to channel estimation than the system with STBC after spreading. Furthermore, derived results prove that a high spreading factor (SF) in the MC DS‐CDMA system with STBC before spreading leads to high channel estimation error, whereas for a system with STBC after spreading this statement is not true.     

A new subspace method for blind estimation of selective MIMO‐STBC channels

In this paper, a new technique for the blind estimation of frequency and/or time‐selective multiple‐input multiple‐output (MIMO) channels under space‐time block coding (STBC) transmissions is presented. The proposed method relies on a basis expansion model (BEM) of the MIMO channel, which reduces the number of parameters to be estimated, and includes many practical STBC‐based transmission scenarios, such as STBC‐orthogonal frequency division multiplexing (OFDM), space‐frequency block coding (SFBC), time‐reversal STBC, and time‐varying STBC encoded systems. Inspired by the unconstrained blind maximum likelihood (UML) decoder, the proposed criterion is a subspace method that efficiently exploits all the information provided by the STBC structure, as well as by the reduced‐rank representation of the MIMO channel. The method, which is independent of the specific signal constellation, is able to blindly recover the MIMO channel within a small number of available blocks at the receiver side. In fact, for some particular cases of interest such as orthogonal STBC‐OFDM schemes, the proposed technique blindly identifies the channel using just one data block. The complexity of the proposed approach reduces to the solution of a generalized eigenvalue (GEV) problem and its computational cost is linear in the number of sub‐channels. An identifiability analysis and some numerical examples illustrating the performance of the proposed algorithm are also provided. Copyright © 2009 John Wiley & Sons, Ltd.     

Joint low‐complexity equalization and CFO estimation and compensation for UWA‐OFDM communication systems

Frequency synchronization has a great importance in preserving the performance of the underwater acoustic (UWA) orthogonal frequency division multiplexing (OFDM) systems. The carrier frequency offset (CFO) estimation can be blind or data‐aided. In this paper, the Zadoff‐Chu (ZC) sequences are used for OFDM synchronization in UWA communications, and they are compared with different data‐aided algorithms. We propose a low‐complexity algorithm for CFO estimation based on ZC sequences. Also, a joint equalization and CFO compensation scheme for UWA‐OFDM communication systems is presented. Simulation results demonstrate that the proposed CFO estimation algorithm allows estimation of the CFO accurately with a simple implementation in comparison with the traditional schemes. Also, the performance of the UWA‐OFDM system can be preserved in the presence of frequency offsets.     

Packet‐Reduced Ranging Method with Superresolution TOA Estimation Algorithm for Chirp‐Based RTLS

In this paper, a packet‐reduced ranging method using a superresolution time of arrival estimation algorithm for a chirp‐based real‐time locating system is presented. A variety of ranging methods, such as symmetric double‐sided two‐way ranging (SDS‐TWR), have been proposed to remove the time drift due to the frequency offset using extra ranging packets. Our proposed method can perform robust ranging against the frequency offset using only two ranging packets while maintaining almost the same ranging accuracy as them. To verify the effectiveness of our proposed algorithm, the error performance of our proposed ranging method is analyzed and compared with others. The total ranging performance of TWR, SDS‐TWR, and our proposed TWR are analyzed and verified through simulations in additive white Gaussian noise and multipath channels in the presence of the frequency offset.     

On the blind channel identifiability of multiple‐input multiple‐output space–time block code systems using Joint Approximate Diagonalization of Eigenmatrices

Channel identifiability for multiple‐input multiple‐output space–time block code (MIMO‐STBC) systems using Joint Approximate Diagonalization of Eigenmatrices (JADE) is studied in this paper. Compared with the previous blind MIMO‐STBC channel estimation methods in literature, the method proposed in this paper is more suitable for non‐cooperative scenario because it needs less prior information and can be applied to a general class of STBCs. The main contribution of the paper consists in the theoretical proof that, although the sources transmitted by different antennas of MIMO‐STBC systems are not independent, they can be retrieved from the received data by directly using JADE in most cases. The conclusion is also demonstrated by a simulation. This shows that the classical JADE algorithm can be applied to a wider range of situations rather than strictly independent sources. Copyright © 2013 John Wiley & Sons, Ltd.     

Compressed sensing channel estimation for STBC‐SM based hybrid MIMO‐OFDM system for visible light communication

This paper presents the idea of sparse channel estimation using compressed sensing (CS) method for space–time block coding (STBC), and spatially multiplexing (SM) derived hybrid multiple‐input multiple‐output (MIMO) Asymmetrically clipped optical‐orthogonal frequency division multiplexing (ACO‐OFDM) optical wireless communication system. This hybrid system accounts multiplexing gain of SM and diversity gain of STBC technique. We present a new variant of sparsity adaptive matching pursuit (SaMP) algorithm called dynamic step‐size SaMP (DSS‐SaMP) algorithm. It makes use of the inherent and implicit structure of SaMP, along with dynamic adaptivity of step‐size feature which is compatible with the energy of the input signal, thus the name dynamic step size. Existing CS‐based recovery algorithms like orthogonal matching pursuit, SaMP, adaptive step‐size SaMP, and proposed DSS‐SaMP were compared for hybrid MIMO‐ACO‐OFDM visible light communication system. The performance analysis is demonstrated through simulation results with respect to bit error rate, symbol error rate, mean square error, computational complexity, and peak‐to‐average power ratio. Simulation results show that the proposed technique gives improved performance and lesser computational complexity in comparison with conventional estimation algorithms.     

一种改进的Fitz频偏估计算法

针对通信接收中的载波同步问题,提出一种基于Fitz算法的改进载波频偏估计算法.分析了Fitz算法频偏估计方法中估计范围受限的原因,针对性地提出了改进方法,利用离散傅里叶变换(DFT)算法得到基准频偏,以此修正大间隔自相关函数估计频偏,保持了Fitz算法的估计精度优势,极大地扩展频偏估计范围,同时具有低的信噪比门限.最后给出了改进算法的实现步骤,并通过MATLAB仿真实验证明了算法的有效性.     

OFDM‐Based STBC with Low End‐to‐End Delay for Full‐Duplex Asynchronous Cooperative Systems

We propose a new space‐time block coding (STBC) for asynchronous cooperative systems in full‐duplex mode. The orthogonal frequency division multiplexing (OFDM) transmission technique is used to combat the timing errors from the relay nodes. At the relay nodes, only one OFDM time slot is required to delay for a pair‐wise symbol swap operation. The decoding complexity is lower for this new STBC than for the traditional quasi‐orthogonal STBC. Simulation results show that the proposed scheme achieves excellent performances.     

Performance of robust symbol‐timing and carrier‐frequency estimation for OFDM systems

In recent years, many maximum likelihood (ML) blind estimators have been proposed to estimate timing and frequency offsets for orthogonal frequency division multiplexing (OFDM) systems. However, the previously proposed ML blind estimators utilizing cyclic prefix do not fully characterize the random observation vector over the entire range of the timing offset and will significantly degrade the estimation performance. In this paper, we present a global ML blind estimator to compensate the estimation error. Moreover, we extend the global ML blind estimator by accumulating the ML function of the estimation parameters to achieve a better accuracy without increasing the hardware or computational complexity. The simulation results show that the proposed algorithm can significantly improve the estimation performance in both additional white Gaussian noise and ITU‐R M.1225 multipath channels. Copyright © 2008 John Wiley & Sons, Ltd.     

Block‐wise Alamouti schemes for OQAM‐OFDM systems with complex orthogonality

Offset quadrature amplitude modulation‐based orthogonal frequency division multiplexing (OFDM) systems cannot be directly combined with the Alamouti code because of the intrinsic imaginary interference. In this paper, we propose a block‐wise space‐frequency block coding (SFBC) scheme and a block‐wise space‐time block coding (STBC) scheme for offset quadrature amplitude modulation‐based OFDM systems, which achieve bit error rate performances that are close to OFDM systems. The proposed schemes satisfy the orthogonality condition of the Alamouti code in the complex field with guard band/intervals. To improve the spectral efficiency of the block‐wise SFBC scheme, we also consider the case without the guard band. It is observed that only the two innermost subcarriers do not satisfy the complex orthogonality condition when the guard band is removed. Then, a simple equalization scheme is proposed to independently equalize the two innermost subcarriers. Simulation results show that the block‐wise SFBC scheme works well under channels with mild‐to‐moderate frequency selectivity, and the block‐wise (STBC ) scheme suffers less than 1 dB loss under severe frequency selective channels at the bit error rate of 10 ^− 3, when only a simple one tap zero‐forcing equalizer is employed. Copyright © 2016 John Wiley & Sons, Ltd.     

无线局域网环境中OFDM系统频偏综合估计

本文提出了一种针对IEEE802．11a OFDM系统频偏综合估计算法，该算法分两个过程对OFDM频偏进行估计；捕获过程和跟踪过程，计算机仿真表明，该算法在不降低估计误差均方差的情况下，相对于其它使用前导字进行频偏跟踪的算法能够提供更大的频偏估计范围，具有更高的频率和更低的计算复杂度。     

基于CAZAC序列的水声通信技术研究

针对CI/OFDM系统对同步的敏感要求,提出了一种基于CAZAC序列的时频同步算法。该算法不仅可以实现定时同步,还可利用定时同步的相关结果和循环相关的原理实现小数倍频偏和整数倍频偏的估计与补偿。仿真和实验结果表明,在水声信道下,该算法具有良好的时频同步性能。     

一种高阶QAM信号载波频偏估计算法

给出了一种高阶QAM信号载波频偏估计算法。该算法先采用非数据辅助频偏估计器估算出一个粗略的频偏,并进行补偿,然后对补偿信号用非数据辅助频偏估计器进行频偏估计,并把两者的频偏估计进行相加。最后为了提高估计精度,对估计出来的频偏进行Kalman滤波,并得出精确的频偏估计。该算法具有频偏估计范围大、估计准确度高的特点。计算机仿真表明频偏估计范围可达0．5符号率,估计精度接近克拉美-罗限。     

一种基于自相关函数的载波频偏估计方法

为解决前向载波频偏估计中捕获范围和估计精度矛盾的问题,在最大似然准则的基础上,利用相位折叠现象使得自相关函数的频偏具有周期化规律,提出了一种利用最大似然估计中的自相关函数进行频偏估计的算法,该算法估计范围广,精度高且复杂度低,适于突发通信的频偏估计。最后进行了MATLAB仿真实验,仿真结果表明了该算法估计精度高、捕获范围广和计算复杂度低的性能。     

OFDM同步跟踪阶段的频偏估计算法

ＯＦＤＭ对栽频偏差是很敏感的，因此不仅在同步的捕获阶段，而且在同步的跟踪阶段，精确的频偏估计是很重要的。文献［３］提出了一种简化的相干判决反馈的迭代算法来估计同步跟踪阶段的载频偏差，对于１２８个子载波、ＱＰＳＫ调制的ＯＦＤＭ系统，在多径衰落信道下，当频偏小于子载波间隔的１５％时，频偏估计的标准方差小于１％，但是这个结果是在假设信道频率响应、定时偏差、载波相位等参数已知的条件下取得的，而当这些参数的估计存在误差时，由仿真结果可看出，频偏估计性能恶化了，因此，提出了采用差分判决反馈的频偏估计算法，虽然只能估计小于子载波间隔 ８％的频偏，但差分判决反馈算法不需要知道信道的参数，由于在同步跟踪阶段，频偏一般都较小，因此采用差分判决反馈算法可以很好地估计出同步跟踪阶段的频率偏差。     

低信噪比场景下Link-16系统的联合频偏估计算法

对于Link-16数据链终端平台在低信噪比(SNR)高动态场景下的多普勒频率偏移问题,该文设计一种新的数据结构,推导了该结构下的克拉默-拉奥下界(CRLB),并在此基础上提出一种联合频域变换与时域自相关运算的分步式频偏估计算法。其基本思想是,首先对接收信号做自相关处理,然后通过频域变换进行最大值索引,结合修正因子得到多普勒频移的粗估计值,再利用时域的改进L&R算法对接收信号进行细估计,根据两步估计算法得到最终的频偏估计值。算法应用蒙特卡罗实验仿真,仿真结果表明,与传统频偏估计算法相比,该算法的归一化均方误差更接近CRLB,在多普勒频偏为[–20 kHz, 20 kHz]时,估计精度可达10^–5。在低信噪比环境下,所提算法能达到较为理想的估计效果,适用于Link-16数据链通信。     

星载AIS信号的频偏估计

星载自动识别系统(AIS)中信号存在严重的多普勒频偏,影响信号的正确解调,而传统的基于快速傅里叶变换(FFT)的频偏估计算法其估计范围无法满足需要.为此,提出了一种改进的自相关-FFT频偏估计算法,通过构造辅助函数以确定信号频偏的正负号,在此基础上对基于FFT的频偏估计算法进行改进,从而扩大频偏估计范围,实现对大的多普勒频移的估计.仿真和实验结果均表明,改进算法的频偏估计范围扩大为原来的2倍,具有很高的估计精度,十分接近修正后的克拉美罗界,且算法简单、易于实现.     

一种改进的高动态QPSK信号解调算法

正交相移键控(QPSK)调制解调方式广泛应用于深空通信、数字卫星通信等高速系统中,高动态QPSK信号具有较大的多普勒载波频偏,针对现有的解调算法对QPSK信号频偏估计范围小、精度较低等问题,提出了一种改进的高动态QPSK信号解调算法。该算法利用基于Zoom-FFT的Quinn频率估计算法对载波频偏进行估计,将频偏估计分为粗估计和精估计两个过程,对估计所得频偏进行补偿后利用Costas环完成载波同步。在MATLAB中对算法进行了仿真,仿真结果表明,该算法能够实现对高动态大频偏QPSK信号的有效捕获,与传统的解调方式相比,该算法对载波频偏的估计范围更大,且具有更高的频偏估计精度和更好的误码性能。     

一种1090ES相位编码信号的同步算法

载波同步是对1090MHz扩展电文(1090MHz Extended Squitter,1090ES)的相位编码信号进行相干解调的前提和基础,研究针对这种调制方式的基于数据辅助的载波频偏估计算法。构建了1090ES相位编码信号的频偏估计模型,提出了一种基于遗传算法的频偏估计算法,详述了该算法的特点及其实现步骤,并通过MATLAB仿真,分析比较了该算法与Kay算法、Fitz算法及M&M算法的频偏估计性能。该算法的频偏估计范围可以达到采样频率的50%,在信噪比高于3 dB时,频偏估计的均方误差接近于修正的克拉美罗界,并且可以同时兼顾频偏估计精度和频偏估计范围。