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。     

基于可变遗忘因子RLS在OFDM系统的载波频偏估计

在时变信道下正交频分复用（OFDM）系统中,通过导频辅助,提出基于可变遗忘因子RLS（VFF-RLS）的载波频偏（CFO）估计改进算法。针对传统RLS（CFF-RLS）算法中遗忘因子无法同时满足CFO估计收敛速度和收敛精度的缺陷,本文设计了线性变化遗忘因子(LFF)和非线性变化遗忘因子(NLFF) 两种可变遗忘因子方案来提升CFO估计性能。仿真结果显示：在低信噪比的情形下,基于VFF-RLS算法的CFO估计性能明显优于基于CFF-RLS算法的CFO估计性能。     

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.     

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。      

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。     

Joint carrier frequency offset and channel estimation for OFDM systems with two receive antennas

Carrier frequency offset (CFO) in OFDM systems results in loss of channel orthogonality and hence degrades the system's performance. In this paper, we propose a new method for joint CFO/channel estimation for OFDM systems with two receive antennas. Our method avoids the complexity of full search methods in one or two dimensions. Using one training OFDM symbol and utilizing the knowledge of the structure of the inter‐channel interference that results from CFO, we develop a two‐stage estimation procedure. The first stage derives an initial CFO/channel estimate based on a one shot minimization step. The second stage refines this joint estimate by conducting a small CFO search in the vicinity of the initial estimate. This procedure provides CFO estimates over a full range of −N /2–N /2, (N : number of subcarriers), as well as the channel estimates. Computer simulations show an excellent performance that is very close to the Cramer–Rao lower bound and superior to some existing methods. The effect of antenna correlation on performance is also investigated through computer simulations, showing small performance degradation even at medium correlation coefficients (0.3–0.6). Copyright © 2015 John Wiley & Sons, Ltd.     

Carrier Frequency Offset Estimation for FM and Symbiotic FM Radio Data System Hybrid Signal

The symbiotic FM radio data system(SRDS)is a radio data system that a specially designed OFDM signal co-lives with FM signal,which enables a significantly higher data rate than existing radio data systems.The cyclic prefix of the OFDM symbol has the same length as the OFDM body,which enables the analytic separation of the co-channel OFDM and FM signal at receiver side,utilizing the fact that the OFDM body and prefix is equal.In this work,we show that the OFDM body and prefix cannot be viewed as equal when there is sufficient carrier frequency offset(CFO).Thus,we propose a two-step CFO estimation algorithm for FM and SRDS hybrid signal.The first step estimates the coarse CFO by exploring the characteristics of the FM signal.Once the coarse CFO is removed,the residual CFO is small enough for FM and OFDM separation.The second step fine estimates CFO from the OFDM-only signal using its repeated PN structure after the separation.Detailed mathematical equations are formulated and simulation results are given.The results show that the proposed algorithm works fine with the simulation setup and has a final residual CFO less than 3.9Hz.     

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.      

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.     

Frequency estimators for MIMO-OFDM systems

Orthogonal frequency division multiplexing (OFDM) systems are known to be sensitive to carrier frequency offset (CFO). This paper is concerned with the CFO estimation for multiple input multiple output (MIMO) systems employing OFDM waveforms. We present two approaches to derive maximum likelihood (ML) pilot-assisted frequency estimators that use either two or multiple identical training symbols. It is shown that the resulting ML frequency estimators are similar to maximum ratio combining versions of Moose estimator and Yu–Su solution, respectively. Numerical examples demonstrate that the proposed frequency estimators are robust against spatial Signal-to-noise ratio (SNR) variation and they yield performance superior to that of the corresponding single-antenna system.     

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.     

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。     

高阶QAM调制下OFDM的载波与采样频偏联合纠正

采用高阶QAM (Quadrature Amplitude Modulation)调制的正交频分复用(OFDM,Orthogonal Frequency-Division Multiplexing)系统,相对于低阶调制来说,密集的星座点分布更容易受到载波频偏与采样频偏的影响.本文分析了高阶QAM OFDM系统对频率同步精度的要求,提出了一种载波与采样频偏联合纠正的方法,并分别针对时不变多径信道和块衰落多径Rician信道给出了频偏估计的均方误差解析表达式.均方误差的理论分析与计算机仿真结果吻合;误比特率性能仿真表明,频偏纠正后的误比特率性能与无频偏情况比较,差异小于1dB.     

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).     

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     

一种新的定时和频偏联合估计算法

提出了一种新的正交频分复用(OFDM)系统定时和频率同步联合算法.该算法仅需要一个训练符号就可以完成定时和频偏估计,额外开销量小,定时和频率同步的实现均在时域进行,简化了同步实现的复杂度.由于使用自相关处理,定时同步对频率偏差具有较强的鲁棒性.与以往的定时同步算法相比,该算法避免了定时平台和多峰值的出现,即使在非常低的信噪比下,其定时测度都是一个尖脉冲.仿真结果表明,该算法能快速地实现定时和频率同步.     

Non-data-aided carrier-frequency offset estimation for pulse-shaping OFDM/OQAM systems

In this paper we consider the problem of blind carrier-frequency offset (CFO) estimation for pulse-shaping orthogonal frequency-division multiplexing (OFDM) systems based on offset quadrature amplitude modulation (OQAM). In particular, under the assumption of low signal-to-noise ratio, three maximum-likelihood (ML) CFO estimators for non-dispersive channels are derived. Due to their significant computational complexity a more feasible closed-form CFO synchronization algorithm is considered. The performance of the derived blind estimators is assessed via computer simulations and compared with that of a blind CFO estimator previously proposed in the literature, exploiting the second-order cyclostationarity property of the OFDM/OQAM signal.     

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     

Joint estimation of carrier frequency offset, dc offset and I/Q imbalance for OFDM systems

Orthogonal frequency division multiplexing (OFDM) systems with direct-conversion receiver (DCR) are vulnerable to carrier frequency offset (CFO), dc offset (DCO) and in-phase/quadrature (I/Q) imbalance. In this paper, we propose blind estimator for joint estimation of CFO, DCO and I/Q imbalance in OFDM systems with DCR. Simulation results show that performance of proposed estimator approaches Cramér-Rao lower bound (CRLB) asymptotically, which demonstrates its effectiveness.