自适应波束形成实现交织OFDMA上行链路的载波频偏补偿

在OFDMA系统中,采用交织子载波分配方法可以提高频率分集和系统容量,但是发射机和接收机之间的载波频率偏移会破坏子载波间的正交性,从而导致本用户的载波间干扰以及用户间干扰。该文分析了交织OFDMA上行链路的信号结构,提出了一种采用自适应波束形成器的载波频偏补偿方法,并给出了两种自适应波束形成器：匹配滤波器和MMSE方法。仿真结果验证了MMSE自适应波束形成器的误比特率性能优于匹配滤波器以及传统的循环卷积方法。     

一种基于子载波侦听的宽带无线网络接入控制策略

在基于正交频分多址技术的宽带网络中,通过把竞争分散到多个子信道能有效改善随机接入的性能.然而多信道特性也会使传统基于Request-To-Send(RTS)控制帧的随机接入协议面临子信道空闲、RTS传输时间延长、上行复杂的问题.针对这些问题,在本文所提出的接入方法中,用户在没有被分配到资源时通过在随机子载波上发送信号来请求传输机会,与此同时,接入点对这些信号侦听,并根据子载波忙闲状态确定哪些用户可以接入.理论分析及仿真结果表明,相比传统依靠RTS的接入方法,采用子载波侦听,系统将获得更高的吞吐量.     

基于OFDMA的WiMAX周期测距中的多用户频偏估计

针对WiMAX OFDMA上行链路的周期测距,提出了2种适用于多用户的载波频偏估计方法,分别采用PN码的时域及频域相关特性提取期望测距用户,并给出了多用户情况下CFO估计的理论结果.仿真实验证明,基于频域相关的方法更适合于在一个测距时隙内存在多个测距用户的情况,并且不易受定时偏差估计误差的影响.     

WiMAX OFDMA初始测距中的多用户频偏估计

测距过程是移动WiMAX标准中的重要过程,用于解决远近效应及同步问题。该文针对WiMAX OFDMA上行链路的初始测距,提出了两种适用于多用户的载波频偏估计方法。方法1采用参考测距信号与接收信号的时域相关提取期望测距信号,并利用相关结果的相位估计测距用户的频偏;方法2采用参考测距信号与接收到的两个相邻OFDM符号的频域相关提取期望测距信号,并利用两次结果的相位差估计测距用户的频偏。仿真实验证明基于频域相关的方法更适合于在一个测距时隙内存在多个测距用户的情况,并且不易受定时偏差估计误差的影响。     

基于ESPRIT算法的子载波交织型OFDMA上行链路频偏估计

频率同步问题对于OFDMA系统来说至关重要。本文针对子载波交织分配的OFDMA系统上行链路，利用其接收信号周期性的结构特征，提出一种基于ESPRIT算法的多用户频偏估计算法，通过子空间旋转不变的特点估计各用户的载波频偏。仿真表明，利用该算法进行频偏估计具有良好的准确性。     

MIMO-OFDM系统中基于虚子载波的低复杂度盲频偏估计

频偏估计是MIMO-OFDM系统设计的一个关键问题.本文在前人研究成果的基础上,提出了一种基于OFDM系统固有虚子载波结构的低复杂度载波频偏估计算法,不需要任何额外的导频(零或非零)资源,在小频偏范围内可以精确估计出频偏,在频偏范围较大时,可通过增加泰勒级数近似项和应用迭代机制继续获得很高的估计精度.     

基于虚拟子载波的盲信道估计改进算法

提出了一种改进算法，该算法基于虚拟子载波的子空间算法，利用循环前缀（CP）引入的信息冗余，由于OFDM传输方程不能直接推导子空间算法，故首先给出一种新的方法对OFDM传输方程作矩阵变换，从而得到一个结构简单的新方程，基于这个新方程推导子空间算法。系统分析及仿真结果表明，在信噪比为20dB的条件下，与已有算法相比可以有7dB的性能提高且在收敛速度上和已有算法相比．快500～800个符号。     

基于幅度比值和子空间的频率估计算法

为降低单频正弦复信号的频率估计误差,针对传统Rife算法在较低信噪比条件下幅值最大和次大谱线判断错误问题,利用RootMUSIC算法结果代替快速傅里叶变换(FFT)结果,确定幅值最大和次大谱线序号;同时针对Rife算法在部分频率区间估计误差较其他区间偏差大的问题,采用频移重估思路,进一步降低了整个频率估计区间的估计误差。仿真表明,在较低信噪比条件下,所提RM-Rife算法较Rife算法、Root MUSIC算法和M-Rife算法,具有更优的估计性能,可用于相关工程实践中。     

一种适用于OFDM-TDMA系统的载波频率偏移估计算法

该文提出了一种适用于OFDM-TDMA系统的载波频率偏移盲估计算法,它利用OFDM载波信道间内在的正交性,寻找与最小估计均方误差相对应的频偏来校正系统的频率偏移．通过计算机仿真表明,这种算法在频率选择性衰落信道下具有高的估计精度,且适用于时变环境下的频偏跟踪．     

基于DFT扩频的广义多载波频分多址上行链路传输方案——DFT-S-GMC

针对宽带移动通信上行链路传输,提出一种基于离散傅立叶变换(DFT)的广义多载波(GMC)频分多址方案--DFT-S-GMC.该传输方案采用DFT进行频域扩频,采用逆滤波器组变换(IFBT)实现频分复用和频分多址.理论分析和仿真表明,与现有主流上行多址方案(如OFDMA和DFT-S-OFDMA)相比,该方案具有较低的峰均比,并且对由上行用户间的定时和频率同步误差导致的多址干扰有较强的顽健性.     

Multiuser Frequency Synchronization for Interleaved-OFDMA Uplink Systems

This paper investigates the frequency synchronization in the uplink of the Orthogonal frequency- division multiple access （OFDMA） system with interleaved subcarrier assignment. In such a system, one of the key problems is the multiuser frequency synchronization, which focuses on the multiple-parameter estimation of the Carrier frequency offsets （CFOs）. In this paper, we propose a two-stage frequency offset estimation algorithm. The main advantage of the proposed method is that it can obtain the CFOs of all users simultaneously using only one OFDMA block. In addition, a novel CFO-compensation method is presented. Based on the inner signal structure of interleaved-OFDMA uplink, the new scheme uses adaptive beamformer to compensate the CFO of each user and isolate the signals of all users at the same time. Compared to the previously known methods, the proposed algorithm can provide accurate frequency synchronization and data detection without feeding CFO estimates back to active users for frequency adjustment.     

Joint Carrier-Frequency Offset and Channel Estimation for MIMO-OFDM Transmission

In this paper, we consider a multiple-input multiple-output (MIMO) orthogonal frequency division multiplexing (OFDM) system operating over frequency-selective fading channels. We propose a novel scheme for joint carrier-frequency offset (CFO) and channel estimation based on the expectation–maximization (EM) algorithm. Furthermore, the Cramer-Rao bounds (CRBs) for both CFO and channel estimators are exploited to evaluate the performance of the proposed scheme. Computer simulations show that the proposed algorithm achieves almost ideal performance compared with the CRBs for both channel and frequency offset estimations.

基于阵列天线的低复杂度OFDMA上行链路载频偏估计方法

 提出一种基于阵列天线的正交频分多址（OFDMA）上行链路载频偏闭式盲估计方法,基站端配备阵列天线,该方法基于各天线阵元的空域快拍之间存在的旋转不变性,结合子空间平滑的思想,可在低运算复杂度的条件下,利用波达方向矩阵法得到信号空间的原始基底,由此可进一步同时获得各子载波对应用户的波达方向（DOA）与载频偏的闭式估计.该方法支持任意子载波分配方案以及满负载系统,并且可根据当前系统负载动态调整估计复杂度.理论分析和仿真结果验证了本文方法的有效性.     

A hybrid joint ML technique for CFO,timing offset,and channel estimations in an OFDMA uplink

In this paper, we propose a novel hybrid joint maximum‐likelihood estimator for carrier frequency offset (CFO), timing offset, and channel response of all users in the uplink of an orthogonal frequency division multiple access (OFDMA) system. The proposed estimation method significantly reduces complexity of this multiparameter, multidimensional optimization problem, using the concept of separation of the different user signals by means of newly defined projection operators. This projection technique is combined with the alternating projection method available in the literature to arrive at a new hybrid algorithm that offers significant performance advantages in terms of computational complexity and estimator performance. The joint estimation of the CFOs, timing offsets, and channel coefficients for all active users together at the base station of the OFDMA uplink is a rarely addressed task. The proposed method also offers the flexibility of application to any subcarrier assignment scheme used in OFDMA systems. Extensive simulation studies corroborate the advantages of the new hybrid method for all three estimation requirements in the multiuser OFDMA uplink. Copyright © 2012 John Wiley & Sons, Ltd.     

任意子载波分配OFDMA上行链路CFO估计及补偿方法

正交频分多址(OFDMA)技术是正交频分复用技术(OFDM)的扩展。与OFDM一样,它对载波频偏(CFO)相当敏感。该文针对多用户任意子载波分配的OFDMA上行链路系统提出一种基于导频辅助的CFO估计方法,并在此基础上给出一种低复杂度CFO补偿方法。仿真结果表明,该方法计算复杂度低,性能良好。     

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.     

SOQPSK-TG信号的EM半盲载波频偏估计

针对成形偏移四相相移键控-TG(Shaped Offset Quadrature Phase Shift Keying-Telemetry Group version, SOQPSK-TG)信号在训练序列长度受限时频偏估计精度较低的问题,利用双二进制分解(Doubinary Decomposition, DBD)原理提出基于期望最大化(Expectation Maximization, EM)的SOQPSK半盲载波频偏(Carrier Frequency Offset, CFO)估计算法。为了确保EM算法收敛到预期性能范围,使用基于非线性四次方码元定时估计算法的非数据辅助频偏估计方法优化了EM算法初始点选择。仿真实验结果表明,该算法相比于使用训练序列进行数据辅助估计的方法,在不增加辅助数据数量的前提下能够进一步提升CFO估计的精度,并在较高信噪比下拥有接近序列总长度所对应的克拉美罗界(Cramér-Rao Bound, CRB)的优秀性能。     

Maximum-Likelihood Estimator for Coarse Carrier Frequency Offset Estimation in OFDM Systems

Orthogonal frequency division multiplexing (OFDM) systems are more sensitive to carrier frequency offset (CFO) compared to the conventional single carrier systems. CFO destroys the orthogonality among subcarriers, resulting in inter-carrier interference (ICI) and degrading system performance. To mitigate the effect of the CFO, it has to be estimated and compensated before the demodulation. The CFO can be divided into an integer part and a fractional part. In this paper, we investigate a maximum-likelihood estimator (MLE) for estimating the integer part of the CFO in OFDM systems, which requires only one OFDM block as the pilot symbols. To reduce the computational complexity of the MLE and improve the bandwidth efficiency, a suboptimum estimator (Sub MLE) is studied. Based on the hypothesis testing method, a threshold Sub MLE (T-Sub MLE) is proposed to further reduce the computational complexity. The performance analysis of the proposed T-Sub MLE is obtained and the analytical results match the simulation results well. Numerical results show that the proposed estimators are effective and reliable in both additive white Gaussian noise (AWGN) and frequency-selective fading channels in OFDM systems.

Carrier Frequency Offset estimation for interleaved OFDMA uplink based on subspace processing

This paper investigates Carrier Frequency Offset （CFO）, estimation in the uplink of the Orthogonal Frequency-Division Multiple Access （OFDMA） systems with the interleaved subcarrier assignment. CFOs between the transmitters and the uplink receiver will destroy orthogonality among different subcarriers, hence resulting in inter-carrier interference and multiuser interference. A two-stage frequency offset estimation algorithm based on subspace processing is proposed. The main advantage of the proposed method is that it can obtain the CFOs of all users simultaneously using only one OFDMA block. Compared with the previously known methods, it not only has a relatively low implementation complexity but is also suitable for random subchannel assignment.