ESTIMATION OF CARRIER FREQUENCY OFFSETS FOR MIMO SYSTEMS WITH DISTRIBUTED TRANSMIT ANTENNAS

The problem of estimating the carrier frequency offsets in Multiple-Input Multiple-Output (MIMO) systems with distributed transmit antennas is addressed. It is supposed that the transmit antennas are distributed while the receive antennas are still centralized, and the general case where both the time delays and the frequency offsets are possibly different for each transmit antenna is considered. The channel is supposed to be frequency flat, and the macroscopic fading is also taken into consideration. A carrier frequency offset estimator based on Maximum Likelihood (ML) is proposed, which can separately estimate the frequency offset for each transmit antenna and exploit the spatial diversity. The Cramer-Rao Bound (CRB) for synchronous MIMO (i.e., the time delays for each transmit antenna are all equal) is also derived. Simulation results are given to illustrate the per- formance of the estimator and compare it with the CRB. It is shown that the estimator can provide satisfactory frequency offset estimates and its performance is close to the CRB for the Signal-to-Noise Ratio (SNR) below 20dB.

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.

JOINT ESTIMATION OF CARRIER FREQUENCY OFFSET AND POWER DELAY PROFILE FOR MIMO-OFDM SYSTEMS

This paper develops a Cyclic Prefix(CP)based joint Maximum-Likelihood(ML)estima-tion algorithm of Carrier Frequency Offset(CFO)and Power Delay Profile(PDP)for Multi-InputMulti-Output Orthogonal Frequency Division Multiplexing(MIMO-OFDM)systems.However,theexact solution of the joint ML estimation is very complex since it needs a search over amulti-dimensional domain.Thus a simplified method is proposed to estimate the CFO and the PDPiteratively via the alternating-projection method which could induce the multidimensional searchproblem to a sequence of simple one-dimensional searches.Simulations show that the proposed algo-rithm is more accurate and robust than the existing algorithms.

PROPAGATOR BASED CLOSED-FORM BLIND CARRIER FREQUENCY OFFSET ESTIMATION FOR OFDM SYSTEMS

Most of the blind Orthogonal Frequency Division Multiplexing (OFDM) Carrier Frequency Offset (CFO) estimators necessitate large number of samples to ensure the estimation accuracy. However, the number of samples can not be selected too large because of the carrier offset drift. In this letter, a new closed form algorithm for blind OFDM CFO estimation in frequency-selective channel is proposed. This method utilizes the propagator obtained from data matrix and the diagonal loading technique, thus it has better performance even only using one or two OFDM blocks. Furthermore, the range of the CFO estimation which can be handled is overall transmission spectral. Simulation results confirm its effectiveness.

Efficient integer frequency offset estimation for OFDM systems

An efficient scheme of integer frequency offset estimate for orthogonal frequency division multiplexing （OFDM） systems is proposed based on a training symbol with several identicalparts. In this scheme, the received training symbol is first reshaped into several sub-symbols.It shows that the reshaping process in-troduees time diversity multiplexing.After a special fast Fourier transform （FFT） algorithm is applied to the sub-symbol,the integer frequency is estimated by finding the maximum magnitude of the resulting fre-quency domain signal.To improve the estimate performance,diversity combining methods are presented to makefull use of the multiple frequency domain sub-symbols.Compared to the traditional scheme, theproposed one has an improved estimate performance demonstrated by the computation simulation, while maintaining a very low complexity.

THE PERFORMANCE OF MT-CDMA SYSTEM IN THE PRESENCE OF CARRIER FREQUENCY OFFSET

In this letter,the sensitivity of an uplink Multi-Tone Code-Division Multiple Access (MT-CDMA) system to the Carrier Frequency Offset (CFO) is investigated. The analytical expression for the Bit Error Rate (BER) of uplink MT-CDMA in the presence of CFO is derived in a multipath Rayleigh fading channel which is verified through simulations. Both Maximal Ratio Combining (MRC) and Equal Gain Combining (EGC) are considered in combining multipath signals in the analysis. It is found that the BER performance can be improved with the number of multipath increasing in the presence of CFO.

Blind CFO estimation algorithm for OFDM systems by using generalized precoding and trilinear model

This paper discusses the blind carrier frequency offset （CFO） estimation for orthogonal frequency division multiplexing （OFDM） systems by utilizing trilinear decomposition and genera- lized preceding. Firstly, the generalized precoding is employed to obtain multiple covariance matrices which are requisite for the trilinear model, and then a novel CFO estimation algorithm is proposed for the OFDM system. Compared with both the joint diagonalizer and estimation of signal parameters via rotational invariant technique （ESPRIT）, the proposed algorithm enjoys a better CFO estimation performance. Furthermore, the proposed algorithm can work well without virtual carriers. Simulation results illustrate the performance of this algorithm,

A time domain multiple-CFOs and CIRs estimation algorithm over wireless multimedia sensor networks

Channel parameters estimation in an orthogonal for the receiver station is a multi-dimensional （MD） frequency division multiple access （OFDMA） system optimization problem, because every user node has a separate local oscillator and every transmitter to receiver link has individual carrier frequency offset （CFO） and channel impulse response （CIR） parameters. In order to reduce the computational complexity for MD optimization, a time domain CFOs and CIRs estimation algorithm over the OFDMA based wireless multimedia sensor networks （WMSN） is proposed in this paper. In this algorithm, the receiver station can decouple the signal from every node by correlation based on specially designed training sequences, so that the MD optimization problem is simplified to an 1-D optimal problem. It is proved that the multiple CFOs can be identified from the correlation result using the phase shift of the consecutive training se- quences. Based on the CFOs estimation result, the CIRs can then he estimated according to the minimum mean square error （MMSE） criterion. The theoretic analysis and simulation results show that the proposed algorithm can effectively decouple the signal from different user nodes and the bit error rate （BER） per- formance curves are close to the ideal estimation when the user number is not large.

一种带有CFO补偿的OFDMA上行链路Turbo接收机

提出了一种带有多用户载波频率偏移(CFO)联合补偿功能的迭代(Turbo)接收机,用于频率非同步环境下的正交频分多址(OFDMA)上行信号接收处理。基于频域Turbo均衡原理,该接收机实现了带有软干扰消除(SIC)的CFO补偿、最小均方误差(MMSE)频域均衡和最大后验(MAP)信道译码的联合迭代处理。理论分析和仿真结果均表明:与传统的非迭代接收机相比,文中所提Turbo接收机利用MAP译码器的软判决反馈信息实现了干扰重构和消除,在迭代中获得了更优的频率同步性能和误比特率性能;与现有几种典型的迭代接收机相比,文中所提Turbo接收机具有更优的误比特率性能和更低的实现复杂度。

空频码异步协作通信中的多CFO补偿

在协作通信中为了对抗同步误差,可基于OFDM技术在中继节点采用空频码实现协作分集和多径分集。OFDM可以抗定时误差,但对载波频率偏移非常敏感,对于协作通信,多个载波频率偏移的存在使得频率补偿很难实现。利用空频码结构和载波间干扰能量分布特点,提出在发射端调整空频码相对位置,在接收端进行多次频率搬移和相应的FFT变换,能有效对抗多个载波频率偏移。在此基础上采用Q抽头滤波算法,以较低的算法复杂度实现了接近于最小均方误差的性能。

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。

一种改进的OFDM系统CFO的盲估计方法

OFDM系统的载波频偏导致相邻OFDM符号相同子载波上的信号功率产生了差异。基于这个思想,利用OFDM信号的4阶矩与频偏满足余弦函数关系的特点,提出一种载波频偏估计的改进方法,即子载波功率差分估计方法。该方法具有简单的闭式解,避免了迭代搜索过程,降低了计算量,同时也适用于慢时变的频率选择性衰落信道。仿真实验表明,与现有方法相比,改进后的方法在降低计算复杂度的同时也具有良好的估计性能。

Pseudo-noise preamble based joint frame and frequency synchronization algorithm in OFDM communication systems

Frame and frequency synchronization are essential for orthogonal frequency division multiplexing （OFDM） systems. The frame offset owing to incorrect start point position of the fast Fourier transform （FFT） window, and the carrier frequency offset （CFO） due to Doppler frequency shift or the frequency mismatch between the transmitter and receiver oscil ators, can bring severe inter-symbol interference （ISI） and inter-carrier interference （ICI） for the OFDM system. Relying on the relatively good correlation charac-teristic of the pseudo-noise （PN） sequence, a joint frame offset and normalized CFO estimation algorithm based on PN preamble in time domain is developed to realize the frame and frequency synchronization in the OFDM system. By comparison, the perfor-mances of the traditional algorithm and the improved algorithm are simulated under different conditions. The results indicate that the PN preamble based algorithm both in frame offset estimation and CFO estimation is more accurate, resource-saving and robust even under poor channel condition, such as low signal-to-noise ratio （SNR） and large normalized CFO.

Carrier frequency offset and impulse noise estimation for underwater acoustic orthogonal frequency division multiplexing

The carrier frequency offset（CFO）and impulse noise always affect the performance of underwater acoustic communication_systems.The CFO and impulse noise could be estimated by using the null subcarriers to cancel the effects of the two types of interference.The null subcarriers estimation methods include optimal separate estimation and joint estimation.The separate estimation firstly estimates the CFO value and then estimates the impulse noise value.However,the CFO and impulse noise always affect each other when either of them is estimated separately.The performance could be improved by using the joint estimation.The results of simulations and experiments have showed that these two optimization methods have good performance and the joint estimation has better performance than the separate estimation method.There is 3 dB performance gain at the BER value of 10^（-2）when using the joint estimation method.Thus these methods could improve the system robustness by using the CFO compensation and impulse noise suppression.

Blind carrier frequency offset estimation for constant modulus signaling based OFDM systems: algorithm, identifiability, and performance analysis

Cartier frequency offset （CFO） estimation is critical for orthogonal frequency-division multiplexing （OFDM） based transmissions. In this paper, we present a low-complexity, blind CFO estimator for OFDM systems with constant modulus （CM） signaling. Both single-input single-output （SISO） and multiple-input multiple-output （MIMO） systems are considered. Based on the assumption that the channel keeps constant during estimation, we prove that the CFO can be estimated uniquely and exactly through minimizing the power difference of received data on the same subcarriers between two consecutive OFDM symbols; thus, the identifiability problem is assured. Inspired by the sinusoid-like cost function, curve fitting is utilized to simplify our algorithm. Performance analysis reveals that the proposed estimator is asymptotically unbiased and the mean square error （MSE） exhibits no error floor. We show that this blind scheme can also be applied to a MIMO system. Numerical simulations show that the proposed estimator provides excellent performance compared with existing blind methods.

Joint channel and carrier frequency offset estimation for multi-user MIMO-OFDM uplink transmissions

This article proposes a new algorithm of joint channel and carrier frequency-offset OCCFO） estimation for multi-user multi-input and multi-output orthogonal frequency division multiplexing （MIMO-OFDM） systems. A least square （LS） channel estimation and a carrier frequency offset （CFO） correlation estimation are combined in this contribution. CFOs are generally estimated using training sequences in a special synchronization timeslot. In this contribution, CFO estimation is further improved by taking advantages of channel estimation based on pilot symbols in traffic timeslots. The CFOs can be first obtained from the primary channel estimation. And then, with the knowledge of the CFOs estimated, channel estimation can be enhanced greatly. Computer simulation results indicate that the proposed JCCFO scheme is of good performance. Besides, the computational complexity is low.

Blind Carrier Frequency Offset Estimation via Power Spectrum Analysis in MIMO OFDM Systems

As a generalization of orthogonal frequency-division multiplexing （OFDM） systems, multi-input multi-output （MIMO） OFDM systems are very sensitive to carrier frequency offset （CFO）. This paper proposes a blind CFO estimation method based on power spectrum analysis, which has high bandwidth efficiency and is much less complex. This method can be used to estimate the residual CFO, which is less than half of the subcarrier spacing. The method uses a cosine cost function to get a closed-form CFO estimate. Simulation results illustrate that the method is effective for MIMO OFDM systems.

Frequency Synchronization in OFDM System

An accurate frequency synchronization method using the zadoff-chu (ZC) constant envelop preamble is analyzed, and a new preamble weighted by pseudo-noise sequence is used for orthogonal frequency division multiplexing (OFDM) systems. Using this method, frequency offset estimator range is greatly enlarged with no loss in accuracy. The range of the frequency estimation is ±30 of subcarrier spacing using ZC sequence as preamble. Simulations in the MATLAB for an AWGN channel show that the proposed method achieves superior performance to existing techniques in terms of frequency accuracy and range.

基于立方Farrow结构的插值细搜频偏估计算法

在高动态通信环境中,由相对运动引起的多普勒频移会在接收机下变频所得的基带信号中引入残留频偏,致使基带信号的相位发生偏离。对频偏进行精确估计是提升高动态环境通信质量的重要途径。传统方法通常通过对快速傅里叶变换(Fast Fourier Transform,FFT)后所得频谱进行一维搜索来估计载波频偏值,只能估计整数倍频点。为了进一步提高频偏估计的精度,本文提出了一种基于立方Farrow结构的插值细搜(Interpolation Detail Search, IDS)算法。仿真结果表明,此算法仅在16倍插值条件下就得到了逼近克拉美罗界(Cramer-Rao Bounds,CRB)的频偏估计性能,信噪比精度损失仅为1.2 dB,具有良好的估计性能。

Performance evaluation for PCC-OFDM systems impaired by carrier frequency offset over AWGN channels

Orthogonal frequency division multiplexing （OFDM） is sensitive to carrier frequency offset （CFO）, which destroys the orthogonality and causes inter-carrier interference （ICI）. ICI self-cancellation schemes based on polynomial cancellation coding （PCC-OFDM） can evidently reduce the sensitivity to CFO. In this paper, we analyze the performance of PCC-OFDM systems impaired by CFO over additive white gaussian noise （AWGN） channels. Two criteria are used to evaluate the effect of CFO on performance degradations. Firstly, the closed-form expressions of the average carrier-to-interference power ratio （CIR） and the statistical average ICI power, both of which reflect the desired power loss, are presented. Simulation and analytical results show that the theoretical expressions depend crucially on the normalized frequency offset and are hardly relevant to the number of subcarriers. Secondly, by exploiting the properties of the Beaulieu series, the effect of CFO on symbol error rate （SER） and bit error rate （BER） performance for PCC-OFDM systems are exactly expressed as the sum of an infinite series in terms of the charac- teristic function （CHF） of ICI. We consider the systems modulated with binary phase shift keying （BPSK）, quadrature PSK （QPSK）, 8-ary PSK （8-PSK）, and 16-ary quadrature amplitude modulation （16-QAM）, and all above modulation schemes are mapped with Gray codes for the evaluations of BER.