Low-Power and High-Accurate Synchronization for IEEE 802.16d Systems

Orthogonal frequency division multiplexing (OFDM) is a viable technology for high-speed data transmission by virtue of its spectral efficiency and robustness to multi-path fading. These advantages can be achieved only with good synchronization both in time and frequency. This paper proposes new efficient synchronization methods for an OFDM-based system, IEEE 802.16d. For the coarse time synchronization and the fractional carrier frequency offset (CFO) estimation, a disjoint architecture is proposed that performs auto-correlations separately to achieve more reliable frequency synchronization and to reduce overall hardware complexity and power consumption. In addition, for the fine symbol timing offset (STO) and the integer CFO, a new joint estimation method employing parallel cross-correlations between the received samples and the pre-rotated training sequences is proposed. Experimental results show significantly superior performance to the previous synchronization methods. A prototype synchronizer based on the proposed methods is designed with a 0.25- $mu$m CMOS process, which reduces power consumption by more than 60% compared to a conventional synchronizer.      

Improved time and frequency synchronization for dual-polarization OFDM systems

This article presents techniques for improved estimation of symbol timing offset (STO) and carrier frequency offset (CFO) for dual-polarization (DP) orthogonal frequency division multiplex (DP-OFDM) systems. Recently, quaternion multiple-input multiple-output OFDM has been proposed for high spectral efficiency communication systems, which can flexibly explore different types of diversities such as space, time, frequency, and polarization. This article focuses on synchronization techniques for DP-OFDM systems using a cyclic prefix, where the application of quaternion algebra leads to new improved estimators. Simulations performed for DP system methods show faster reduction of STO estimator variance with a double-slope line in the log-variance line versus signal-to-noise ratio (SNR) plot compared with single-polarization (SP) counterparts, and simulations for CFO estimates show a 3-dB gain of DP over SP estimates for same SNR values defined, respectively, for quaternion-valued or complex-valued signals. Cramer–Rao bounds for STO and CFO are derived for the synchronization methods, correlating with the observed gains of DP over SP OFDM systems.     

一种改进的OFDM盲同步算法

研究了OFDM系统中的最大似然估计(ML)算法,并使用数据循环移位以及多符号联合估计等技术对ML算法进行了改进,随后提出了一个综合性的新方案。仿真结果显示,在AWGN和瑞利多径衰落信道中,该方案可以有效提高符号定时偏差(STO)和载波频率偏移(CFO)的估计性能。     

OFDM盲载波间干扰抑制与同步算法研究

针对载波频偏引起的子载波间干扰问题,提出一种基于独立分量分析的OFDM载波频率同步算法。本算法直接实现载波频率同步,可以避免基于导频机制的频偏估计和由频偏估计误差带来的频谱效率降低与性能损失。首先建立含频偏OFDM独立分量分析模型,然后从最大似然原则得到分离的代价函数,结合自然梯度优化得到OFDM源信号实现载波频率同步。理论分析表明,提出的算法不仅具有基于最大似然的频偏补偿性能而且提高了系统的输出信噪比。最后,仿真分析证明了算法的有效性。      

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.     

OFDM-IDMA系统的迭代频偏估计及补偿

针对OFDM IDMA系统中载波频偏(CFO)带来的子载波之间的干扰问题,提出了在各用户具有相同频偏下的联合逐码片(构的信号作为虚拟的训练序列进行频域频偏估计,同时进行相应的时域频偏补偿.理论分析及实验仿真结果表明:基于逐码片迭代检测的近无频偏时的性能CBC)迭代检测的载波同步方法.该方法利用迭代检测中的外信息重OFDM IDMA系统的频偏估计和补偿方法能够使系统性能接     

Low-complexity block double-differential design for OFDM with carrier frequency offset

Carrier frequency offset (CFO) estimation for orthogonal frequency-division multiplexing (OFDM) has caught attention as OFDM systems have become widely adopted in recent years. In this paper, we design a novel double-differential (DD) codec with low computational complexity. Our design bypasses CFO and channel estimation, and is easy to implement at both transmitter and receiver. It also guarantees full multipath diversity, and reduces the peak-to-average power ratio from the number of subcarriers to the channel order. In addition, it is robust to CFO drifting. The closed form of the performance for our design is derived for OFDM transmissions over frequency-selective channels with CFO. Thorough simulation results corroborate our claims.     

Cyclostationarity-Based Frequency Synchronization for OFDM Systems Over Doubly-Selective Fading Channels

Orthogonal frequency division multiplexing (OFDM) systems are highly sensitive to carrier frequency offset (CFO), especially in doubly-selective fading environment. Cyclostationarity-based blind synchronization methods are appealing in high-data-rate applications and low signal-to-noise regions. However, the cyclostationarity has not been exploited for frequency synchronization of OFDM systems under doubly-selective fading channels. In this paper, we derive the close-form second order cyclic statistics of the received OFDM signal in presence of CFO, by modeling the doubly-selective fading channel with basis expansion model. Both transmitter-induced cyclostationarity and doubly-selective channel information are contained in the derived cyclic moments, and they are efficiently utilized for CFO estimation. Simulation results demonstrate that the proposed estimator provides significant improvements on frequency synchronization performance.     

基于判决反馈的OFDM载波频率同步环路

对OFDM系统中一种基于接收时域信号重构的载波频率盲估计算法进行了进一步的研究,分析了在这种方法中影响估计精度的4个方面的因素。为了减少载波频率估计的剩余误差,通过在频率补偿时增加反馈因子,提出了一种基于判决反馈的载波频率同步环路结构。通过调整同步环路的环路增益,可以获得更好的频率跟踪性能。如果信噪比达到9dB,即使存在较大的频率漂移,剩余频偏也被限制在子载波间隔的1%以内。     

Blind Frequency Synchronization in OFDM via Diagonality Criterion

In this paper, we address the problem of blind carrier frequency offset (CFO) estimation in orthogonal frequency-division multiplexing (OFDM) systems in the case of frequency-selective channels. CFO destroys the orthogonality between the carriers leading to nondiagonal signal covariance matrices in frequency domain. The proposed blind method enforces a diagonal structure by minimizing the power of nondiagonal elements. Hence, the orthogonality property inherent to OFDM transmission with cyclic prefix is restored. The method is blind since it does not require a priori knowledge of the transmitted data or the channel, and does not need any virtual subcarriers. A closed-form solution is derived, which leads to accurate and computationally efficient CFO estimation in multipath fading environments. Consistency of the estimator is proved and the convergence rate as a function of the sample size is analyzed as well. To assess the large sample performance, we derive the CramÉr–Rao bound (CRB) for the blind CFO estimation problem. The CRB is derived assuming a general Gaussian model for the OFDM signal, which may be applied to both circular and noncircular modulations. Finally, simulation results on CFO estimation are reported using a realistic channel model.     

Decision-directed fine synchronization in OFDM systems

A new decision-directed (DD) synchronization scheme is proposed for joint estimation of carrier frequency offset (CFO) and sampling clock frequency offset (SFO) in orthogonal frequency-division multiplexing (OFDM) systems. By exploiting the hard decisions, we report accurate estimators of residual CFO and small SFO. The performance analysis and simulation results indicate that the proposed novel DD scheme achieves much better performance than the conventional pilot-based schemes in both additive white Gaussian noise and frequency-selective channels.     

基于导频信号的MIMO-OFDM同步技术

MIMO-OFDM技术将成为第4代移动通信系统的关键技术,因MIMO-OFDM对时间和频率偏移非常敏感,因此MIMO-OFDM同步显得尤为重要。提出了一种新的MIMO-OFDM定时同步和频偏同步技术。以GCL序列为基础设计了一个新的符合MIMO-OFDM同步技术的导频序列,通过对该导频序列进行2次相关得到频率估计,并将所得频率运用到定时同步中,得到更为准确的时间估计。仿真结果表明,在相同的信噪比情况下,该方法可以使得系统的误码率和帧传送误码率相对传统方法得到进一步减小。     

Multiple Invariance MUSIC-Based Blind Carrier Frequency Offset Estimation for OFDM System with Multi-Antenna Receiver

In this paper, we address the problem of carrier frequency offset (CFO) estimation for orthogonal frequency division multiplexing (OFDM) systems with multi-antenna receiver. The received signal can be reconstructed to form data model with multi-invariance property, and then a multi-invariance MUSIC algorithm for CFO estimation is proposed. This algorithm has better performance of CFO estimation than ESPRIT method, multi-invariance ESPRIT method and trilinear decomposition algorithm, and also qualifies the estimation for both integer CFO and fractional CFO. Simulation results illustrate validity of this algorithm.     

A Blind CFO Estimator Based on Smoothing Power Spectrum for OFDM Systems

Carrier frequency offset (CFO) estimation is a critical problem in orthogonal frequency-division multiplexing (OFDM) systems. This letter proposes a blind CFO estimator based on smoothing the signal power spectrum. A closed-form CFO estimate is also presented, which greatly reduces the computational complexity of the proposed method. Analysis and simulation results show that the proposed estimator is very effective for OFDM systems.     

OFDM Carrier Synchronization Based on Time-Domain Channel Estimates

Carrier frequency synchronization is critical to the quality of signal reception in OFDM systems. This paper presents an approximate maximum-likelihood (ML) carrier frequency offset (CFO) estimation scheme based on time-domain channel estimates which retain the CFO information in the form of phase rotation. The proposed ML CFO estimate is investigated under static as well as time-varying fading channels. Statistical properties of the estimator are examined and Cramer-Rao lower bound (CRLB) is derived. Theoretical analysis and numerical simulations show that the proposed CFO estimator renders excellent performance with lower computational complexity. The proposed CFO estimate also has an advantage of allowing for more flexible pilot patterns     

CFO estimation schemes for differential OFDM systems

This paper proposes two blind carrier frequency offset (CFO) estimation schemes for differentially modulated orthogonal frequency division multiplexing (OFDM) systems. The proposed schemes estimate the fractional part of the CFO with only two consecutive OFDM blocks, and they exploit two implicit properties associated with differentially modulated OFDM (DOFDM) systems, i.e., the channel keeps constant over two consecutive OFDM blocks, and the DOFDM systems employ an M-ary phase-shift keying constellation. One of the schemes is based on the finite alphabet (FA) constraint and the other one is based on the constant modulus (CM) constraint. They provide a trade-off between the performance and computational complexity. The constrained Cramer-Rao lower bound is also derived. Several numerical examples are presented to validate the efficacy of the proposed schemes.     

Blind frequency-offset estimator for OFDM systems transmitting constant-modulus symbols

We address the problem of carrier frequency offset (CFO) synchronization in OFDM communications systems in the context of frequency-selective fading channels. We consider the case where the transmitted symbols have constant modulus, i.e., PSK constellations. A novel blind CFO estimation algorithm is developed. The new algorithm is shown to greatly outperform a previously published blind technique that exploits the fact that practical OFDM systems are not fully loaded. Further, the proposed algorithm is consistent even when the system is fully loaded. Finally, the proposed CFO estimator is obtained via a one-dimensional search, the same as with the existing virtual subcarrier-based estimator, but achieves a substantial gain in performance (10-dB SNR or one order of magnitude in CFO MSE).     

Research on carrier frequency offset estimation algorithm based on PN sequence preamble in OFDM system

Carrier frequency offset (CFO) due to Doppler frequency shift or frequency mismatch between the transmitter’s and receiver’s oscillators can introduce severe inter-symbol and inter-carrier interference into OFDM systems. A simplified OFDM system model is considered to analyze effects of CFO in theory and simulation. The article briefly reviews some traditional CFO estimation algorithms. Relying on relatively good correlation characteristic of pseudo-noise (PN) sequence, the PN preamble based algorithm of CFO estimation is developed. Performance characteristics of traditional and the new improved algorithms are simulated under different conditions. Results indicate that the PN preamble based algorithm of CFO estimation is more accurate, resource-saving and robust even under poor communications channel condition, such as low SNR and big normalized CFO.     

Joint Carrier Frequency Offset and Channel Estimation for Amplify-and-Forward Cooperative OFDM System

In this paper, we develop a new joint carrier frequency offset (CFO) and channel estimation algorithm for Amplify-and-Forward (AF) cooperative (CO)-OFDM systems. An expanded Basis Expansion Model (E-BEM) is proposed for time variant multipath channels. Based on the E-BEM, a joint OFDM channel estimation and synchronization method is derived. We also apply the space-frequency-block-code (SFBC) to AF CO-OFDM systems. Simulation results reveal that the proposed joint algorithm works well, leading to considerable gains of overall performance.     

FPGA Implementation of SDR Based CFO Estimation and Compensation Circuit for OFDM System

Based on software defined radio (SDR) architecture, this paper develops a reconfigurable CORDIC vectoring module (CVM) and CORDIC rotation module (CRM) in FPGA to implement the carrier frequency offset (CFO) estimation and compensation circuits of an orthogonal frequency division multiplexing (OFDM) system. The experimental results show that the proposed SDR-pipelined architecture can save power and hardware resource compared with conventional pipelined architecture, because the designed CVM and CRM modules can be reused in the processing modules of CFO estimation and compensation circuit. The performance trade-off for CVM and CRM implemented with different quantized float number in FPGA is presented. Furthermore, the hardware reconfiguration function of CVM and CRM is also validated.