一种无需导频的适用于差分OFDM系统的符号与采样钟联合同步方法

该文提出了一种适用于OFDM系统的联合符号和采样钟同步校正方法,其中同步校正是在数字域通过改变对接收过采样信号的插值(interpolation)和抽取(decimation)实现的。这种方法在发送端相邻载波间采用差分QPSK调制,在接收端利用QPSK的差分解调信号获得同步误差信号,从而获得关于OFDM符号同步和采样钟同步调整的算法,其特点是无需专门的同步导频信号。所提出算法的同步性能在高斯白噪声信道和多径衰落信道均得到验证。     

On the synchronization techniques for wireless OFDM systems

The latest research works on the synchronization scheme for either continuous transmission mode or burst packet transmission mode for the wireless OFDM communications are overviewed in this paper. The typical algorithms dealing with the symbol timing synchronization, the carrier frequency synchronization as well as the sampling clock synchronization are briefly introduced and analyzed. Three improved methods for the fine symbol timing synchronization in frequency domain are also proposed, with several key issues on the synchronization for the OFDM systems discussed.     

Joint estimation of symbol timing and carrier frequency offset of OFDM signals over fast time-varying multipath channels

In this paper, we present a novel joint algorithm to estimate the symbol timing and carrier frequency offsets of wireless orthogonal frequency division multiplexing (OFDM) signals. To jointly estimate synchronization parameters using the maximum likelihood (ML) criterion, researchers have derived conventional models only from additive white Gaussian noise (AWGN) or single-path fading channels. We develop a general ML estimation algorithm that can accurately calculate symbol timing and carrier frequency offsets over a fast time-varying multipath channel. To reduce overall estimation complexity, the proposed scheme consists of two estimation stages: coarse and fine synchronizations. A low complexity coarse synchronization based on the least-squares (LS) method can rapidly estimate the rough symbol timing and carrier frequency offsets over a fast time-varying multipath channel. The subsequent ML fine synchronization can then obtain accurate final results based on the previous coarse synchronization. Simulations demonstrate that the coarse-to-fine method provides a good tradeoff between estimation accuracy and computational complexity.     

Effect of imperfect noise estimation in timing synchronization

This paper calculates the performance of an orthogonal frequency division multiplexing (OFDM) system confronted with Rayleigh fading channels in terms of sync detection probability. We adopted a timing synchronization scheme employing simple repeated constant amplitude zero auto-correlation (CAZAC) training symbol with relevant correlation techniques, and analyzed the effect of imperfect noise estimation. Some probability expressions of OFDM timing synchronization for system measurement are evaluated.     

Low‐Complexity Symbol Timing Offset Estimation Schemes for OFDM Systems

In this paper, we propose three symbol synchronization schemes for Orthogonal Frequency Division Multiplex (OFDM) systems. The cyclic extension preceding OFDM symbols is of decisive importance for these schemes. The first scheme uses the phase‐differential coding of the received OFDM signal. The second and the third schemes use the length of the received OFDM signal. All three schemes make symbol synchronization possible, even though there is a frequency offset in the system. Simulation results show that these schemes can be used to synchronize an OFDM system over AWGN and multi‐path fading channels.     

A robust timing synchronization design in OFDM systems?part II: high-mobility cases

In this paper we consider the design of a robust timing synchronization algorithm for pilot-aided OFDM systems in high-mobility fading environments. We first analyze the impact of both mobility and timing errors on the performance of a pilot-aided OFDM system for frequency selective fading channels, by deriving an expression for channel estimation error variance. The analysis will show that, even for high levels of mobility, a pilot-aided channel estimator is considerably sensitive to timing errors, due to the impact of rotations in different bases. We then show how this sensitivity can be utilized to design a robust timing synchronization algorithm for mobile OFDM systems, without relying on synchronization training information. Theoretical results are then confirmed by simulating the performance of an OFDM system in high delay and Doppler spread fading environments. Finally, we show how the proposed mathematical framework and algorithm can be used to address timing synchronization in the presence of a frequency offset as well. The analysis of this paper is the extension of the derivations of Part I [8], the accompanying paper on the design of a robust timing synchronizer for low-mobility OFDM systems.     

Robust and Efficient OFDM Synchronization for FPGA-Based Radios

Orthogonal frequency division multiplexing (OFDM) is a popular modulation technique that can combat impulsive noise, is robust to multipath fading, is spectrally efficient, and can allow flexible allocation of spectrum. It has become a key standard in cognitive radio systems as well as an enabling technology for mobile data access systems. An OFDM receiver’s performance is heavily impacted by the accuracy of its symbol timing offset (STO) and carrier frequency offset (CFO) estimation. This paper proposes a novel OFDM synchronization method that combines robust performance with computational efficiency. FPGA prototyping is used to explore the trade-off between the number of computations to be performed and computation word length with respect to both synchronization performance and power consumption. Through simulation, the proposed method is shown to provide accurate fractional CFO estimation as well as STO estimation in a range of channels. In particular, it can yield excellent synchronization performance in the face of a CFO that is larger than many state-of-the-art synchronization implementations can handle. The system implementation demonstrates efficient resource usage and reduced power consumption compared with existing methods, and this is explored as a fine-grained trade-off between performance and power consumption. The result is a robust method suitable for use in low-power radios, enabling less precise analog front ends to be used.     

Pilot-free dynamic phase and amplitude estimations for wireless ICI self-cancellation coded OFDM systems

OFDM has the advantage over the conventional single-carrier modulation schemes in the presence of frequency-selective fadings. Nevertheless, intercarrier-interference (ICI) due to Doppler frequency drift, phase offset, local oscillator frequency drift, and sampling clock offset will be a severe problem in the wireless OFDM systems. Previous ICI self-cancellation coding schemes can greatly reduce the ICI, but they are very sensitive to the phase ambiguity, which is due to the composite effect of the phase offset, the multipath fading and the local oscillator frequency drift. In this paper, a novel receiver which combines the current ICI self-cancellation coding techniques with a new pilot-free joint phase/amplitude estimation and symbol detection scheme is proposed. Based on the energy modulation or the irregular symbol constellation, our new technique does not have any requirement of pilot symbols and it can operate on all kinds of phase error ranges. The proposed scheme is promising in comparison with other existing methods at different noise levels through OFDM simulations.     

Super-Imposed Training Scheme for Timing and Frequency Synchronization in OFDM Systems

In this paper, a super-imposed scheme for timing and frequency synchronization is proposed for orthogonal frequency division multiplexing (OFDM) systems. The proposed architecture includes a pre-defined pseudo-noise (PN) sequence, which is added to both the OFDM symbol and the cyclic prefix (CP) in time domain. In contrast to existing synchronization schemes, the proposed architecture has the advantage of better bandwidth utilization since it does not require extra pilot symbols. The peak-to-average power ratio (PAPR) is also significantly decreased. Both the timing and frequency offset estimators are derived using the maximum likelihood (ML) criterion. Efficiencies of the estimators are analyzed mathematically in AWGN channels. Simulation experiments are also conducted in frequency selective fading channels. Moreover, the optimal power allocation factor of the PN sequence is determined by minimizing the bit error rate using simulation experiments     

Joint Frequency and Symbol Synchronization Schemes for an OFDM System

This paper proposes two multi-stage joint symbol timing and carrier frequency synchronization schemes for an orthogonal frequency division multiplex (OFDM) system. Simultaneous estimation of symbol timing and frequency offset is derived from the maximum likelihood (ML) principle, assuming a cyclic prefix (guard interval) is inserted in each OFDM symbol. One of the proposed algorithms derives an initial frequency estimate in the first stage that reduces the frequency uncertainty to only two or three sub-carrier spacings. The timing information and a finer frequency estimate that has a resolution of a sub-carrier spacing are obtained in the second stage. The third stage provides an estimation for the residual fractional frequency error. The other algorithm bypasses the first stage for one can use the second stage alone to search for the timing and frequency offsets. However, the computing complexity of the second stage is higher than that of the first stage, thus the three-stage algorithm is a preferred choice unless the frequency uncertainty is small. Simulation results show that both algorithms yield excellent performance not only in white Gaussian channels but also in multipath fading channels.     

OFDM系统中的帧同步与频率跟踪算法

正交频分复用(OFDM)因其在抗频率选择性衰落和窄带干扰方面具有良好的性能而被广泛应用,但是OFDM系统比单载波系统对定时和频偏更加敏感。提出了一种简单有效的帧同步与频率跟踪算法,与传统的最大似然估计算法相比,实现更加容易,而且能保证精确的帧和频率同步。     

复杂多径信道下的OFDM抗干扰同步算法

时间、频率同步是OFDM系统的关键技术之一。针对存在干扰的复杂多径信道场景,本文提出了在一个OFDM符号内利用两个重复的CAZAC序列进行时间频率同步的新方法。该方法通过在频域生成训练序列并进行时域加窗,减小干扰的影响;通过设置相对门限值和多个相关峰联合判决,提高定时同步性能;利用相关值进行频偏估计,减小干扰对频率同步的影响。仿真表明,该方法能够有效地提高系统的同步精度与抗干扰能力,适用于存在干扰的复杂多径信道环境。      

Parameter estimation and error reduction for OFDM-based WLANs

We consider parameter estimation and error reduction for orthogonal frequency-division multiplexing (OFDM) based high-speed wireless local area networks (WLANs). We devise or select algorithms that can provide benefit to the overall system performance and can be efficiently implemented in real-time. In particular, first, we give a channel model which is especially useful for assessing the channel parameter estimation methods devised for OFDM-based WLANs. Second, we provide a sequential method for the estimation of carrier frequency offset (CFO), symbol timing, and channel response by exploiting the structure of the packet preamble specified by the IEEE 802.11a standard. Finally, to correct the residue CFO induced phase error using the pilot tones, we consider maximum-likelihood phase tracking and least-squares phase fitting approaches; to improve the channel estimation accuracy using the decoded data, we present a semiblind channel estimation method; to mitigate the sampling clock induced time delay error, we provide a sampling clock synchronization approach that obviates the need of an automatic frequency control clock recovery circuit. The overall system performance of using our algorithms is demonstrated via several numerical examples.     

Sampling Frequency Offset Estimation for Pilot-Aided OFDM Systems in Mobile Environment

In orthogonal frequency division multiplexing (OFDM) systems, most of the conventional sampling frequency offset (SFO) estimation methods work under the assumption of time-invariant or slow time-variant channels. In mobile environment, the time-variant channel significantly degrades the accuracy of SFO estimation. To solve the problem, we first analyze the properties of time-variant channels. If terminal moves within some tens of the wavelength of radio frequency (RF) signal, channel path delay almost remains unchanged. For most practical OFDM systems, our analysis indicates that channel path delay can be regarded as unchanged during the interval of some tens of OFDM symbols in time-variant channels. Based on the analysis, we propose a novel SFO estimation method for pilot-aided OFDM systems. Different from the conventional methods, the proposed method estimates SFO by detecting the variation of the symbol timing error caused by SFO. The detection is finished by implementing correlation between the channel impulse responses (CIRs) estimated by different OFDM symbols. Performance of the proposed method is simulated and compared with that of two conventional post-FFT methods. Numerical results show that, the proposed SFO estimation method performs better than the conventional methods not only in time-variant channels, but also at low SNRs and large residual carrier frequency offsets (CFOs).     

OFDM系统中同步问题的研究

OFDM系统的实现还存在着一些方面问题,如：同步问题尤其是频偏,峰值功率与平均功率比值过高,器件的非线性化等。OFDM系统对同步偏差敏感是本文讨论的重点,尤其是频偏。因此怎样获得准确的符号定时和载波频偏估计对OFDM系统性能至关重要。本文围绕OFDM同步过程中的符号定时和载波频率偏差问题进行描述,在分别介绍同步过程中捕获和跟踪两个阶段各自一些典型算法基础上,对一种基于保护间隔／循环前缀（PI／CP）的联合实现符号定时和载波同步最大似然估计（MLE）算法进行叙述并仿真。     

基于CAZAC序列的低复杂度抗频偏同步算法

提出了一种基于恒包络零自相关(CAZAC)序列的低复杂度抗频偏同步算法,适用于突发正交频分复用(OFDM)系统.由CAZAC序列构造一种新型前导序列,利用该序列的组合特性,推导其简化的定时度量函数,进而利用时延相关和对称相关特性实现精确稳定的定时同步.进一步地,基于所构造的前导序列,提出了一种联合循环前缀的加权频率同步...     

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.     

利用CAZAC序列共轭对称性的OFDM时频同步算法

符号定时和载波频率偏差将严重影响OFDM系统的性能。基于具有重复结构的CAZAC序列,提出了一种仅利用一个CAZAC符号实现符号定时、整数倍频偏与小数倍频偏估计的同步算法。首先,所提算法在完成粗定时同步的同时得到整数倍频偏估计。然后根据整数倍频偏估计值对定时估计进行修正,最后完成小数倍频偏估计。分析与仿真结果表明,该算法相比于传统PARK算法,不仅完成了定时估计,而且得到了频偏估计;相对于经典SC算法,不但提高了传输效率,还显著改善了OFDM系统的定时与频偏估计性能;相比于利用CAZAC序列的同步算法,提高了算法的估计精度以及对多径信道的适应性。      

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FPGA的OFDM系统同步算法的研究

符号、载波偏移严重影响着正交频分复用(OFDM)接收系统的性能.本文在分析和研究同步估计算法的基础上,设计了一种OFDM同步系统实现方案,并对符号偏移,载波频偏和采样钟偏移的估计及补偿算法和具体电路实现进行了相应优化.并采用VirtexII系列xc2v1000 FPGA进行实现验证,结果表明该同步系统不仅能有效地实现OFDM系统符号和载波同步,而且可降低硬件资源的消耗.