军用突发OFDM系统的符号同步和信道估计技术研究
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摘要
随着军事通信中高速数据传输业务的发展,正交频分复用(OFDM)技术作为一种频谱利用率高、能有效对抗多径衰落信道的调制技术将在军事通信领域发挥重要作用。在军事通信中,OFDM和跳频(FH)的结合,以及OFDM在猝发通信中的应用,可使系统具有抗干扰的能力,但同时也对符号同步和信道估计提出了更高的要求。以上两种系统均属于特殊的突发通信模式,这里统称为军用突发OFDM系统。论文从实用的角度出发,注重提高算法的鲁棒性和高效性,对军用突发OFDM系统的符号同步和信道估计算法进行研究。论文的主要工作有以下4个方面:
1、分析了基于对称相关的定时度量的统计分布,为符号同步算法的改进和性能分析提供了理论依据。
通过分析指出,具有共轭对称结构的前导经过多径信道时,在第1条路径的能量较小的情况下利用定时度量的峰值作为同步估计的依据时会产生定时偏移,从而引起符号间干扰(ISI)和载波间干扰(ICI)。因此,基于共轭对称前导的符号同步算法不适宜采用对称相关定时度量的峰值作为符号同步估计的依据。
2、针对具有重复共轭对称结构的前导,综合利用基于延迟相关的定时度量和基于对称相关的定时度量设计出一种新的符号同步算法,并进一步利用前导的重复特性构造延迟相乘,设计出一种低计算复杂度的符号同步算法。
将基于前导延迟相关特性的定时度量作为搜索窗开关度量,在该度量超过一定门限的范围内对基于对称相关的定时度量进行峰值搜索,并在此基础上向前一定范围通过设置门限搜索出功率可能不是最强的第1条路径。其中,门限是在理论分析的基础上进行设置的,不需要借助实验仿真来确定,也避免了需要对计算出的定时度量值进行统计的办法得到,因此既提高了时域中符号同步估计的性能,又有利于快速建立起较为精确的符号同步。进一步地,综合利用延迟相关定时度量、对称相关定时度量和延迟相乘设计符号同步方法,可以在减小计算量的同时仍保持良好的符号同步估计性能,有利于符号同步估计的快速实现。
3、对于伪噪声(PN)序列加权的前导,分析了现有定时度量的统计分布,结合基于前导循环前缀(CP)的延迟相关定时度量和基于PN序列相关特性的定时度量设计出一种自适应符号同步算法,并进一步提出了一种新的PN序列加权前导结构。
基于CP的延迟相关定时度量可抑制噪声和数据符号对符号同步估计的影响,且自适应的符号同步方案增强了基于PN序列加权前导的符号同步算法对时变信道的鲁棒性。所提出的新前导具有部分重复结构,且仅有部分采样具有PN序列加权因子。因此,基于新前导的符号同步算法既增强了对ISI的抵抗力,同时可以构建出具有类似冲激形状的定时度量,进一步提高了基于PN序列加权前导的符号同步估计性能。
4、对基于离散傅里叶变换(DFT)的信道估计算法进行改进,增强了算法对信道路径延迟分布的鲁棒性。
对于利用补零DFT/IDFT实现信道插值的方法,分析指出传统的时域插值方法和变换域插值方法是等效的,并从重构信道冲激响应(CIR)采样序列这一新的角度出发提出一种对信道路径延迟分布具有较强鲁棒性的补零新方法。对于补零前的数据处理,分析指出采用滤波方法时应进行非对称滤波,并提出一种简单的滤波器截止频率设置方法;当采用在时域抽取较大能量信道抽头的方法时,提出可以采取新的固定检测区间。所提出的新补零方法和两种补零前的数据处理方法均减小了非整数采样间隔信道条件下路径能量泄露的影响,提高了信道估计的性能。
With development of high speed data transmission in military communications, orthogonal frequency division multiplexing (OFDM) will play an important role dut to its high spectral efficiency and robustness to multipath fading channel. In military communications, systems appling OFDM to RF carrier frequency hopping (FH) and OFDM systems in burst mode can be used to achive anti-jamming capacity. These two kinds of systems are collectively called military-burst OFDM systems, which raise high requirement for symbol synchronization and channel estimation. From the utility point, we focus on symbol synchronization and channel estimation for military-burst OFDM system in this dissertation, laying emphasis on the robustness and efficiency of estimation algorithms. The main contributions of the dissertation are shown as follows.
1. The statistic property of timing metric based on symmetric correlation is analyzed, which provides theory basis for algorithm improvement and performance analysis.
Analysis shows that backward motion of timing position occures when the peak of timing metric is used to detect the desired timing position in multipath channel, where the first path has a small gain. The backward motion results in intersymbol interference and interchannel interference. Therefore, the peak of timing metric is not suitable for the synchronization method based on conjugated-symmetric preamble.
2. For the preamble with repeated-conjugated-symmetric structure, a new symbol synchronization method using both delayed correlation and symmetric correlation is proposed. Moreover, a novel low-complexity symbol synchronization method is designed by utilizing the delayed product additionaly.
A searching-window metric using delayed correlation is defined to serach the peak of the timing metric based on symmetric correlation. Then the first path which may not be the strongest one is being detecting with a fixed threshold. The fixed threshold is set by theoretical analysis, which needs neither computer simulation nor derivation from the computed timing metric. Therefore, the proposed symbol synchronization has accurate synchronization performance and fast synchronization procedure. Furthermore, a method using delayed correlation, symmetric correlation and delayed product is presented to reduce the computation complexity and maintain good synchronization performance simultaneously, which contributes to a fast synchronization.
3. For the preamble weighted by pseudo noise (PN) sequence, statistic distribution of the conventional timing metric is analyzed, and an adaptive symbol synchronization method using delayed correlation based on cyclic prefix (CP) and correlation property of PN sequence is proposed. Moreover, a new preamble structure is designed.
The timing metric based on delayed correlation can suppress interference caused by noise and data symbols, and the adaptive synchronization scheme can enhance the robustness to time-variant multipath channel. The proposed preamble has partial repetition structure, and only half samples were weighted by PN sequence. The novel structure is used to enhance the resistance to ISI, and construct an impulse-shaped timing metric. Therefore, the proposed preamble improves the synchronization performance.
4. Channel estimation methods based on discrete Fourier transform (DFT) are improved to enchance the robustness to path delay of multipath channel.
Two conventional methods based on zero-padding DFT/IDFT for channel interpolation, time domain method and transform domain method, are testified to be equivalent. Then a new zero-padding method is proposed to decrease the reconstruction error for channel impulse response samples. For data processing before zero-padding, analysis shows that asymmetric filter rather than conventional filter in the transform domain should be adopted, and then a simple method to set cutoff frequency is proposed. Moreover, to decrease the influence of energy leakage caused by non-sample-spaced path, noise power estimation in frequency domain based on preamble is used to set threshold for detecting the channel tap with large energy in the time domain, and a fixed detecting region is adopted. Simulation results show that the above methods can improve the channel estimation performance.
1、分析了基于对称相关的定时度量的统计分布,为符号同步算法的改进和性能分析提供了理论依据。
通过分析指出,具有共轭对称结构的前导经过多径信道时,在第1条路径的能量较小的情况下利用定时度量的峰值作为同步估计的依据时会产生定时偏移,从而引起符号间干扰(ISI)和载波间干扰(ICI)。因此,基于共轭对称前导的符号同步算法不适宜采用对称相关定时度量的峰值作为符号同步估计的依据。
2、针对具有重复共轭对称结构的前导,综合利用基于延迟相关的定时度量和基于对称相关的定时度量设计出一种新的符号同步算法,并进一步利用前导的重复特性构造延迟相乘,设计出一种低计算复杂度的符号同步算法。
将基于前导延迟相关特性的定时度量作为搜索窗开关度量,在该度量超过一定门限的范围内对基于对称相关的定时度量进行峰值搜索,并在此基础上向前一定范围通过设置门限搜索出功率可能不是最强的第1条路径。其中,门限是在理论分析的基础上进行设置的,不需要借助实验仿真来确定,也避免了需要对计算出的定时度量值进行统计的办法得到,因此既提高了时域中符号同步估计的性能,又有利于快速建立起较为精确的符号同步。进一步地,综合利用延迟相关定时度量、对称相关定时度量和延迟相乘设计符号同步方法,可以在减小计算量的同时仍保持良好的符号同步估计性能,有利于符号同步估计的快速实现。
3、对于伪噪声(PN)序列加权的前导,分析了现有定时度量的统计分布,结合基于前导循环前缀(CP)的延迟相关定时度量和基于PN序列相关特性的定时度量设计出一种自适应符号同步算法,并进一步提出了一种新的PN序列加权前导结构。
基于CP的延迟相关定时度量可抑制噪声和数据符号对符号同步估计的影响,且自适应的符号同步方案增强了基于PN序列加权前导的符号同步算法对时变信道的鲁棒性。所提出的新前导具有部分重复结构,且仅有部分采样具有PN序列加权因子。因此,基于新前导的符号同步算法既增强了对ISI的抵抗力,同时可以构建出具有类似冲激形状的定时度量,进一步提高了基于PN序列加权前导的符号同步估计性能。
4、对基于离散傅里叶变换(DFT)的信道估计算法进行改进,增强了算法对信道路径延迟分布的鲁棒性。
对于利用补零DFT/IDFT实现信道插值的方法,分析指出传统的时域插值方法和变换域插值方法是等效的,并从重构信道冲激响应(CIR)采样序列这一新的角度出发提出一种对信道路径延迟分布具有较强鲁棒性的补零新方法。对于补零前的数据处理,分析指出采用滤波方法时应进行非对称滤波,并提出一种简单的滤波器截止频率设置方法;当采用在时域抽取较大能量信道抽头的方法时,提出可以采取新的固定检测区间。所提出的新补零方法和两种补零前的数据处理方法均减小了非整数采样间隔信道条件下路径能量泄露的影响,提高了信道估计的性能。
With development of high speed data transmission in military communications, orthogonal frequency division multiplexing (OFDM) will play an important role dut to its high spectral efficiency and robustness to multipath fading channel. In military communications, systems appling OFDM to RF carrier frequency hopping (FH) and OFDM systems in burst mode can be used to achive anti-jamming capacity. These two kinds of systems are collectively called military-burst OFDM systems, which raise high requirement for symbol synchronization and channel estimation. From the utility point, we focus on symbol synchronization and channel estimation for military-burst OFDM system in this dissertation, laying emphasis on the robustness and efficiency of estimation algorithms. The main contributions of the dissertation are shown as follows.
1. The statistic property of timing metric based on symmetric correlation is analyzed, which provides theory basis for algorithm improvement and performance analysis.
Analysis shows that backward motion of timing position occures when the peak of timing metric is used to detect the desired timing position in multipath channel, where the first path has a small gain. The backward motion results in intersymbol interference and interchannel interference. Therefore, the peak of timing metric is not suitable for the synchronization method based on conjugated-symmetric preamble.
2. For the preamble with repeated-conjugated-symmetric structure, a new symbol synchronization method using both delayed correlation and symmetric correlation is proposed. Moreover, a novel low-complexity symbol synchronization method is designed by utilizing the delayed product additionaly.
A searching-window metric using delayed correlation is defined to serach the peak of the timing metric based on symmetric correlation. Then the first path which may not be the strongest one is being detecting with a fixed threshold. The fixed threshold is set by theoretical analysis, which needs neither computer simulation nor derivation from the computed timing metric. Therefore, the proposed symbol synchronization has accurate synchronization performance and fast synchronization procedure. Furthermore, a method using delayed correlation, symmetric correlation and delayed product is presented to reduce the computation complexity and maintain good synchronization performance simultaneously, which contributes to a fast synchronization.
3. For the preamble weighted by pseudo noise (PN) sequence, statistic distribution of the conventional timing metric is analyzed, and an adaptive symbol synchronization method using delayed correlation based on cyclic prefix (CP) and correlation property of PN sequence is proposed. Moreover, a new preamble structure is designed.
The timing metric based on delayed correlation can suppress interference caused by noise and data symbols, and the adaptive synchronization scheme can enhance the robustness to time-variant multipath channel. The proposed preamble has partial repetition structure, and only half samples were weighted by PN sequence. The novel structure is used to enhance the resistance to ISI, and construct an impulse-shaped timing metric. Therefore, the proposed preamble improves the synchronization performance.
4. Channel estimation methods based on discrete Fourier transform (DFT) are improved to enchance the robustness to path delay of multipath channel.
Two conventional methods based on zero-padding DFT/IDFT for channel interpolation, time domain method and transform domain method, are testified to be equivalent. Then a new zero-padding method is proposed to decrease the reconstruction error for channel impulse response samples. For data processing before zero-padding, analysis shows that asymmetric filter rather than conventional filter in the transform domain should be adopted, and then a simple method to set cutoff frequency is proposed. Moreover, to decrease the influence of energy leakage caused by non-sample-spaced path, noise power estimation in frequency domain based on preamble is used to set threshold for detecting the channel tap with large energy in the time domain, and a fixed detecting region is adopted. Simulation results show that the above methods can improve the channel estimation performance.
引文
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