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,...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.展开更多
The problem of channel estimation for multiple an- tenna orthogonal frequency division multiplexing （OFDM） systems subject to unknown carrier frequency offset （CFO） is addressed. Multiple signal classification （M...The problem of channel estimation for multiple an- tenna orthogonal frequency division multiplexing （OFDM） systems subject to unknown carrier frequency offset （CFO） is addressed. Multiple signal classification （MUSIC）-Iike algorithm, which generally has been used for direction estimation or frequency estimation, is used for channel estimation in multiple antenna OFDM systems. A reduced dimensional （RD）-MUSIC based algorithm for channel estimation is proposed in multiple antenna OFDM systems with unknown CFO. The Cramer-Rao bound （CRB） of channel estimation in multiple antenna OFDM systems with unknown CFO is derived. The proposed algorithm has a superior performance of channel estimation compared with the Capon method and the least squares method.展开更多
In an orthogonal frequency division multiplexing(OFDM) system,a time and frequency domain least mean square algorithm(TF-LMS) was proposed to cancel the frequency offset(FO).TF-LMS algorithm is composed of two stages....In an orthogonal frequency division multiplexing(OFDM) system,a time and frequency domain least mean square algorithm(TF-LMS) was proposed to cancel the frequency offset(FO).TF-LMS algorithm is composed of two stages.Firstly,time domain least mean square(TD-LMS) scheme was selected to pre-cancel the frequency offset in the time domain,and then the interference induced by residual frequency offset was eliminated by the frequency domain mean square(FD-LMS) scheme in frequency domain.The results of bit error rate(BER) and quadrature phase shift keying(QPSK) constellation figures show that the performance of the proposed suppression algorithm is excellent.展开更多
This paper proposes a scheme to construct time- frequency codes based on protograph low density parity check （LDPC） codes in orthogonal frequency division multiplexing （OFDM） communication systems. This approach s...This paper proposes a scheme to construct time- frequency codes based on protograph low density parity check （LDPC） codes in orthogonal frequency division multiplexing （OFDM） communication systems. This approach synthesizes two techniques： protograph LDPC codes and OFDM. One symbol of encoded information by protograph LDPC codes corresponds to one sub-carrier, namely the length of encoded information equals to the number of sub-carriers. The design of good protograph LDPC codes with short lengths is given, and the proposed proto- graph LDPC codes can be of fast encoding, which can reduce the encoding complexity and simplify encoder hardware implementa- tion. The proposed approach provides a higher coding gain in the Rayleigh fading channel. The simulation results in the Rayleigh fading channel show that the bit error rate （BER） performance of the proposed time-frequency codes is as good as random LDPC- OFDM codes and is better than Tanner LDPC-OFDM codes under the condition of different fading coefficients.展开更多
在探讨正交频分复用(Orthogonal Frequency Division Multiplexing,OFDM)系统的优化中,一个显著挑战在于其信号检测性能的相对不足。同时,针对基于深度神经网络的索引调制(Deep Neural Network Based Index Modulation,DNN-IM)检测算法...在探讨正交频分复用(Orthogonal Frequency Division Multiplexing,OFDM)系统的优化中,一个显著挑战在于其信号检测性能的相对不足。同时,针对基于深度神经网络的索引调制(Deep Neural Network Based Index Modulation,DNN-IM)检测算法,普遍存在着误码率及损失值偏高的问题。为了弥补上述难题,文中提出一种基于多层感知机(Multilayer Perceptron,MLP)的索引调制检测算法,即MLP-IM算法。该算法采用融合两个连接层与一个输出层的架构设计,通过挑选的激活函数实现对OFDM索引调制系统中数据比特的精准还原。首先将OFDM索引调制系统的基础理论巧妙应用于数据的预处理阶段,随后利用仿真数据集对MLP神经网络模型进行全面而深入的离线训练,确保模型的稳健性与准确性。在检测阶段,通过MLP-IM检测算法实现了对OFDM索引调制系统的高效检测。仿真结果表明,所提出的MLP-IM算法在误码率控制和损失值两个方面的性能表现与最大似然检测算法相媲美,甚至在某些场景下超越了现有DNN-IM算法的性能,其性能改善幅度在0.2~6 dB的区间内。展开更多
This paper presents an algorithm that aims to reduce the peak-to-average power ratio（PAPR） of orthogonal frequency division multiplexing（OFDM） communication systems while maintaining frequency tracking.The algorit...This paper presents an algorithm that aims to reduce the peak-to-average power ratio（PAPR） of orthogonal frequency division multiplexing（OFDM） communication systems while maintaining frequency tracking.The algorithm achieves PAPR reduction by applying the complex conjugates of the data symbol obtained from the frequency domain to cancel the phase of the data symbol.A likelihood estimator is used to obtain the sub-carrier phase error due to the residual carrier frequency offset（RCFO） using the same complex conjugates as a pilot signal.Furthermore,a joint time and frequency domain multicarrier phase locked loop（MPLL） is developed to compensate additional frequency offset.Simulation results show that this algorithm is capable of reducing PAPR without impacting the frequency tracking performance.展开更多
For multiple-input multiple-output （MIMO） orthogonal frequency division multiplexing （OFDM） systems, a joint timing synchronization and frequency offset acquisition algorithm based on fractional Fourier transform ...For multiple-input multiple-output （MIMO） orthogonal frequency division multiplexing （OFDM） systems, a joint timing synchronization and frequency offset acquisition algorithm based on fractional Fourier transform （FRFT） is proposed. The linear frequency modulation signals superimposed on the data signals are used as the training signals. By performing FRFT on the received signals and searching the peak value of the FRFT results, the receiver can realize timing synchronization and frequency offset acquisition simultaneously. Compared with the existing methods, the proposed algorithm can provide better timing synchronization performance and larger frequency offset acquisition range even under multi-path channels with low signal to noise ratio. Theoretical analysis and simulation results prove this point.展开更多
与线性调频连续波信号不同,采用正交频分复用调制(orthogonal frequency division multiplexing,OFDM)方案信号的雷达,当每个符号上调制的传输信息完全随机时,其模糊函数具有较高的旁瓣;当探测场景中存在较多较强的杂波时,目标回波对应...与线性调频连续波信号不同,采用正交频分复用调制(orthogonal frequency division multiplexing,OFDM)方案信号的雷达,当每个符号上调制的传输信息完全随机时,其模糊函数具有较高的旁瓣;当探测场景中存在较多较强的杂波时,目标回波对应的距离多普勒峰被淹没在脉压积累后的强杂波的旁瓣之下,导致目标无法被识别。目前已有的杂波抑制方案多面临杂波抑制度不足的问题。基于此,本文提出基于贪心策略的高效杂波处理方案,通过自适应逐点抑制强杂波,使旁瓣基底下降,目标信息浮现出来。并与实测数据进行了对比,结果表明,该方法取得了比常规方法更好的杂波抑制性能。展开更多
The in-phase and quadrature-phase imbalance (IQI) is one of the major radio frequency impairments existing in orthogonal frequency division multiplexing (OFDM) systems with direct-conversion transceivers. During the t...The in-phase and quadrature-phase imbalance (IQI) is one of the major radio frequency impairments existing in orthogonal frequency division multiplexing (OFDM) systems with direct-conversion transceivers. During the transmission of the communication signal, the impact of IQI is coupled with channel impulse responses (CIR), which makes the traditional channel estimation schemes ineffective. A decoupled estimation scheme is proposed to separately estimate the frequency-dependent IQI and wireless channel. Firstly, the generalized channel model is built to separate the parameters of IQI and wireless channel. Then an iterative estimation scheme of frequency-dependent IQI is designed at the initial stage of communication. Finally, based on the estimation result of IQI, the least square algorithm is utilized to estimate the channel-related parameters at each time of channel variation. Compared with the joint estimation schemes of IQI and channel, the proposed decoupled estimation scheme requires much lower training overhead at each time of channel variation. Simulation results demonstrate the good estimation performance of the proposed scheme.展开更多
正交频分复用(Orthogonal Frequency Division Multiplexing,OFDM)调制因其高效的频带利用率和良好的抗多径能力广泛用于合作与非合作通信系统中。合作通信场景下,通常接收机可以利用已知帧结构实现OFDM信号的检测。但在非合作场景下,...正交频分复用(Orthogonal Frequency Division Multiplexing,OFDM)调制因其高效的频带利用率和良好的抗多径能力广泛用于合作与非合作通信系统中。合作通信场景下,通常接收机可以利用已知帧结构实现OFDM信号的检测。但在非合作场景下,接收机没有足够的先验信息,导致帧检测难度加大。针对这一问题,提出了一种适合于非合作通信场景的OFDM数据帧的检测算法。所提算法利用快速小波变换将含噪OFDM信号的功率包络进行小波分解与重构,对重构得到的功率包络进行差分运算后,再通过与阈值比较实现OFDM信号的帧检测。相较于混合能量检测算法,所提算法计算预设参数少,复杂度低。仿真结果表明,所提算法在加性高斯白噪声信道和多径衰落信道下带内信噪比分别取-6 dB和3 dB时即可实现零漏报率,且零漏报率的阈值选取范围比混合能量检测算法扩大了约6 dB。展开更多
Carrier frequency offset （CFO） in MIMO-OFDM systems can be decoupled into two parts： fraction frequency offset （FFO） and integer frequency offset （IFO）. The problem of IFO estimation is addressed and a new IFO ...Carrier frequency offset （CFO） in MIMO-OFDM systems can be decoupled into two parts： fraction frequency offset （FFO） and integer frequency offset （IFO）. The problem of IFO estimation is addressed and a new IFO estimator based on the Bayesian philosophy is proposed. Also, it is shown that the Bayesian IFO estimator is optimal among all the IFO estimators. Furthermore, the Bayesian estimator can take advantage of oversampling so that better performance can be obtained. Finally, numerical results show the optimality of the Bayesian estimator and validate the theoretical analysis.展开更多
基金supported by the National Natural Science Foundation of China(6130110561102069)+2 种基金the China Postdoctoral Science Foundation Funded Project(2013M531351)the Nanjing University of Aeronautics and Astronautics Founding(NN2012022)the Open Fund of Graduate Innovated Base(Laboratory)for the Nanjing University of Aeronautics and Astronautics(KFJJ120219)
文摘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.
基金supported by the National Natural Science Foundation of China(6137116961301108+1 种基金61071164)the Fundamental Research Funds for the Central Universities(NS2013024)
文摘The problem of channel estimation for multiple an- tenna orthogonal frequency division multiplexing （OFDM） systems subject to unknown carrier frequency offset （CFO） is addressed. Multiple signal classification （MUSIC）-Iike algorithm, which generally has been used for direction estimation or frequency estimation, is used for channel estimation in multiple antenna OFDM systems. A reduced dimensional （RD）-MUSIC based algorithm for channel estimation is proposed in multiple antenna OFDM systems with unknown CFO. The Cramer-Rao bound （CRB） of channel estimation in multiple antenna OFDM systems with unknown CFO is derived. The proposed algorithm has a superior performance of channel estimation compared with the Capon method and the least squares method.
基金Project(60532030) supported by the National Natural Science Foundation of China
文摘In an orthogonal frequency division multiplexing(OFDM) system,a time and frequency domain least mean square algorithm(TF-LMS) was proposed to cancel the frequency offset(FO).TF-LMS algorithm is composed of two stages.Firstly,time domain least mean square(TD-LMS) scheme was selected to pre-cancel the frequency offset in the time domain,and then the interference induced by residual frequency offset was eliminated by the frequency domain mean square(FD-LMS) scheme in frequency domain.The results of bit error rate(BER) and quadrature phase shift keying(QPSK) constellation figures show that the performance of the proposed suppression algorithm is excellent.
基金supported by the Beijing Natural Science Foundation of China (4102050)the National Natural Science of Foundation of China (NSFC)-Korea Science and Engineering Foundation (KOSF) Joint Research Project of China and Korea (60811140343)
文摘This paper proposes a scheme to construct time- frequency codes based on protograph low density parity check （LDPC） codes in orthogonal frequency division multiplexing （OFDM） communication systems. This approach synthesizes two techniques： protograph LDPC codes and OFDM. One symbol of encoded information by protograph LDPC codes corresponds to one sub-carrier, namely the length of encoded information equals to the number of sub-carriers. The design of good protograph LDPC codes with short lengths is given, and the proposed proto- graph LDPC codes can be of fast encoding, which can reduce the encoding complexity and simplify encoder hardware implementa- tion. The proposed approach provides a higher coding gain in the Rayleigh fading channel. The simulation results in the Rayleigh fading channel show that the bit error rate （BER） performance of the proposed time-frequency codes is as good as random LDPC- OFDM codes and is better than Tanner LDPC-OFDM codes under the condition of different fading coefficients.
文摘在探讨正交频分复用(Orthogonal Frequency Division Multiplexing,OFDM)系统的优化中,一个显著挑战在于其信号检测性能的相对不足。同时,针对基于深度神经网络的索引调制(Deep Neural Network Based Index Modulation,DNN-IM)检测算法,普遍存在着误码率及损失值偏高的问题。为了弥补上述难题,文中提出一种基于多层感知机(Multilayer Perceptron,MLP)的索引调制检测算法,即MLP-IM算法。该算法采用融合两个连接层与一个输出层的架构设计,通过挑选的激活函数实现对OFDM索引调制系统中数据比特的精准还原。首先将OFDM索引调制系统的基础理论巧妙应用于数据的预处理阶段,随后利用仿真数据集对MLP神经网络模型进行全面而深入的离线训练,确保模型的稳健性与准确性。在检测阶段,通过MLP-IM检测算法实现了对OFDM索引调制系统的高效检测。仿真结果表明,所提出的MLP-IM算法在误码率控制和损失值两个方面的性能表现与最大似然检测算法相媲美,甚至在某些场景下超越了现有DNN-IM算法的性能,其性能改善幅度在0.2~6 dB的区间内。
基金supported by the National Natural Science Foundation of China(60872026)the Natural Science Foundation of Tianjin(09JCZDJC16900)
文摘This paper presents an algorithm that aims to reduce the peak-to-average power ratio（PAPR） of orthogonal frequency division multiplexing（OFDM） communication systems while maintaining frequency tracking.The algorithm achieves PAPR reduction by applying the complex conjugates of the data symbol obtained from the frequency domain to cancel the phase of the data symbol.A likelihood estimator is used to obtain the sub-carrier phase error due to the residual carrier frequency offset（RCFO） using the same complex conjugates as a pilot signal.Furthermore,a joint time and frequency domain multicarrier phase locked loop（MPLL） is developed to compensate additional frequency offset.Simulation results show that this algorithm is capable of reducing PAPR without impacting the frequency tracking performance.
基金supported by the National Natural Science Foundation of China(60672047).
文摘For multiple-input multiple-output （MIMO） orthogonal frequency division multiplexing （OFDM） systems, a joint timing synchronization and frequency offset acquisition algorithm based on fractional Fourier transform （FRFT） is proposed. The linear frequency modulation signals superimposed on the data signals are used as the training signals. By performing FRFT on the received signals and searching the peak value of the FRFT results, the receiver can realize timing synchronization and frequency offset acquisition simultaneously. Compared with the existing methods, the proposed algorithm can provide better timing synchronization performance and larger frequency offset acquisition range even under multi-path channels with low signal to noise ratio. Theoretical analysis and simulation results prove this point.
文摘与线性调频连续波信号不同,采用正交频分复用调制(orthogonal frequency division multiplexing,OFDM)方案信号的雷达,当每个符号上调制的传输信息完全随机时,其模糊函数具有较高的旁瓣;当探测场景中存在较多较强的杂波时,目标回波对应的距离多普勒峰被淹没在脉压积累后的强杂波的旁瓣之下,导致目标无法被识别。目前已有的杂波抑制方案多面临杂波抑制度不足的问题。基于此,本文提出基于贪心策略的高效杂波处理方案,通过自适应逐点抑制强杂波,使旁瓣基底下降,目标信息浮现出来。并与实测数据进行了对比,结果表明,该方法取得了比常规方法更好的杂波抑制性能。
基金supported by the National Natural Science Foundation of China(6140123261471200+4 种基金6150124861501254)the China Postdoctoral Science Foundation(2014M561692)the Jiangsu Province Postdoctoral Science Foundation(1402087C)the NUPTSF(NY213063)
文摘The in-phase and quadrature-phase imbalance (IQI) is one of the major radio frequency impairments existing in orthogonal frequency division multiplexing (OFDM) systems with direct-conversion transceivers. During the transmission of the communication signal, the impact of IQI is coupled with channel impulse responses (CIR), which makes the traditional channel estimation schemes ineffective. A decoupled estimation scheme is proposed to separately estimate the frequency-dependent IQI and wireless channel. Firstly, the generalized channel model is built to separate the parameters of IQI and wireless channel. Then an iterative estimation scheme of frequency-dependent IQI is designed at the initial stage of communication. Finally, based on the estimation result of IQI, the least square algorithm is utilized to estimate the channel-related parameters at each time of channel variation. Compared with the joint estimation schemes of IQI and channel, the proposed decoupled estimation scheme requires much lower training overhead at each time of channel variation. Simulation results demonstrate the good estimation performance of the proposed scheme.
基金supported by the National Science Fund for Distinguished Young Scholars (60725105)National"863"Program of China (2007AA01Z288)+1 种基金the sixth project of the Key Project of National Nature Science Foundation of China (60496316)Teaching Research Award Program for Outstanding Young Teachers in Higher Education Institutions of MOE,the 111 Project (B08038).
文摘Carrier frequency offset （CFO） in MIMO-OFDM systems can be decoupled into two parts： fraction frequency offset （FFO） and integer frequency offset （IFO）. The problem of IFO estimation is addressed and a new IFO estimator based on the Bayesian philosophy is proposed. Also, it is shown that the Bayesian IFO estimator is optimal among all the IFO estimators. Furthermore, the Bayesian estimator can take advantage of oversampling so that better performance can be obtained. Finally, numerical results show the optimality of the Bayesian estimator and validate the theoretical analysis.
