OFDM系统中基于导频序列的信道估计研究
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摘要
OFDM系统的接收机需要可靠有效的信道估计结果来实现相干检测。然而实际系统中一些因素例如信道的时变等常常会降低信道估计的精度,因此在各种场景中如何提高信道估计的准确性成为了国内外专家学者研究的热点。本论文以这一技术为主线,从减小信道估计误差、对抗信道时变性两个角度切入,主要以OFDM系统中基于导频训练序列的信道估计技术为主要内容展开研究和讨论。
本论文首先概述了无线信道的衰落特性,并研究了无线信道的线性时变基带等效模型及离散时间基带模型。由于无线多径信道的小尺度衰落与多径信道的冲激响应直接相关,因此无线多径信道通常可建模为一个线性时变信道。接着本论文就OFDM系统的时域模型和频域模型进行了研究,引出OFDM系统中基于导频序列的信道估计方法的大致过程,主要涉及到“发送端导频的设计方法”、“接收端获得导频位置的信道信息的方法”和“接收端根据导频处信道信息重建整个信道全响应的方法”这三个核心问题。
针对发送端导频的设计方法,本论文首先阐述了导频设计的基本准则：决定最小相关时间的最大多普勒频移,和决定最小相关带宽的最大多径时延扩展,它们共同决定了导频图样在时域和频域上的间隔。在这个准则的基础上,本论文对导频的设计方法展开了研究,主要分为重叠的导频设计方法和正交的导频设计方法两种方法进行了阐述。
针对接收端获得导频位置的信道信息的方法,本论文研究了两种典型的信道估计算法：LS算法和MMSE算法,这两种算法对应着算法复杂度和性能的两个极端。LS算法通常与多种插值算法联合使用来重建整个频带上的信道全响应,而MMSE算法由于其算法复杂度太高不易于实现,因此业界围绕其简化算法开展了很多的研究,具有代表性的比如LMMSE算法和基于SVD分解的算法。
针对接收端根据导频处信道信息重建整个信道全响应的方法,本论文主要研究了三种具体场景,就这三种场景下的信道估计算法展开研究,通过理论分析和软件仿真得出了在算法复杂度和算法性能之间进行最优折中的改进算法：
●在包含虚子载波的OFDM系统中,由于虚载波的引入,使得系统在频域虚载波处的信道信息缺失,因而造成时域上的能量弥散。本文研究了含虚载波的系统中两种改进的信道估计算法,一种基于虚载波导频插值,一种基于时域迭代。仿真表明这两种算法减小了虚载波带来的能量弥散,提高了系统性能。
●在快速时变信道的OFDM系统中,由于多普勒频移,信道冲激响应在一个OFDM符号周期内具有时变性,这种信道的时变性影响了OFDM子载波之间的正交性,最终引起了不利的载波间干扰ICI。本文研究了基于线性逼近和分段函数逼近的改进的信道估计算法。仿真表明,改进的信道估计最好地实现了信道估计性能和算法复杂度之间的折衷。
●在多小区TDD-OFDM系统中,由于不同用户使用非正交的导频训练序列引起的导频污染,给系统带来较大的小区间干扰。本文针对导频污染展开分析,研究了改进的带导频污染删除的信道估计算法。仿真表明,通过插入循环移位的导频,然后进行导频干扰删除,系统能获得最大的频谱传输速率。
总结起来,通过全文的研究分析,得知在新一代OFDM通信系统及其他新的通信场景中,信道估计始终是关键的核心技术,对它的优化改进有利于显著改善系统性能,提高传输速率和可靠性。
To implement coherent detection, receivers in OFDM system need reliable and efficient channel estimation. However, due to time variance of channel state information, channel estimation tends to be inaccuracy in real system. Therefore it is vitally important to improve the reliability of channel estimation. This thesis studies the pilot-based channel estimation in OFDM system from two aspects, one is to mitigate the channel estimation error, and the other is to decrease the effect of time-varying channel.
The thesis firstly gives an overview of wireless fading channel, and then studies the time-varying linear baseband equivalent model and discrete-time baseband model of the wireless channel. Due to the direct correlation between small-scale fading and multi-path channel impulse response, the wireless multi-path channel can be built as a linear time-varying channel. Then the thesis discusses both time-domain model and frequency-domain model of OFDM system, and introduces general procedures of the pilot-based channel estimation scheme in OFDM system. This scheme consists of three core issues:1) pilot design at transmitter terminal,2) channel estimation based on the pilots at receiver terminal, and3) the reconstruction of channel impulse response.
In terms of pilot design at transmitter, this paper firstly describes the basic principles of pilot design:the maximum Doppler shift, which determines the minimal coherence time, and the maximum delay spread, which determines the minimal coherence frequency, jointly affect the interval between pilot pattern in both time and frequency domain. Then this thesis further elaborates the overlapping pilot design methodology and orthogonal pilot design methodology.
In the perspective of channel estimation based on the pilots at receiver terminal, this thesis discusses two typical channel estimation algorithm:LS and MMSE. These two algorithms represent the trade-off between computation complexity and performance. Combined with interpolation algorithm, LS is often used to reconstruct the channel impulse response in frequency domain, while MMSE is hard to implement, due to the large computation complexity. Therefore, it is necessary to propose a novel algorithm that can improve the performance and lower the complexity simultaneously, such as LMMSE and SVD-based algorithm.
With regard to the reconstruction of channel impulse response, this thesis focuses on three major scenarios, based on which, the channel estimation is conducted. From the theoretical analysis and simulation result, the conclusion can be obtained that the proposed scheme achieves compromise between computation complexity and performance.
●In OFDM system with virtual subcarriers, the channel information is partially lost in the area of virtual subcarriers. Eventually, it leads to the energy leakage in time domain. This paper mainly discusses two improved channel estimation algorithms in OFDM system with virtual subcarriers, one is based on virtual subcarrier pilot interpolation, and the other is based on iteration in time domain. Simulation results illustrate that both of these two algorithms reduce the energy leakage brought by virtual carrier, and improve the system performance.
●In fast time-varying channel, due to the Doppler shift, the channel impulse response is time-variant during one OFDM symbol. It plays a negative effect on orthogonality between subcarriers in OFDM system, which eventually leads to inter-cell interference (ICI). This thesis proposes an improved channel estimation algorithm on the basis of linear-approximation and piecewise approximation. The simulation results demonstrate that the proposed scheme achieves the better compromise between channel estimation performance and computation complexity.
●In multi-cell TDD-OFDM system, the pilot contamination, resulted from non-orthogonal pilot training sequence used by different users, can cause larger ICI. This paper analyzes pilot contamination, and designs a novel channel estimation algorithm to mitigate the pilot contamination. Simulation results show that after cyclic shift pilot interpolation and pilot interference cancellation, the maximum transmission rate is obtained.
To sum up, through research and analysis, channel estimation is the key technology in the new generation of OFDM communication system. It is beneficial to optimize the channel estimation methods to improve the system performance, to gain higher transmission rate, and to enhance reliability.
本论文首先概述了无线信道的衰落特性,并研究了无线信道的线性时变基带等效模型及离散时间基带模型。由于无线多径信道的小尺度衰落与多径信道的冲激响应直接相关,因此无线多径信道通常可建模为一个线性时变信道。接着本论文就OFDM系统的时域模型和频域模型进行了研究,引出OFDM系统中基于导频序列的信道估计方法的大致过程,主要涉及到“发送端导频的设计方法”、“接收端获得导频位置的信道信息的方法”和“接收端根据导频处信道信息重建整个信道全响应的方法”这三个核心问题。
针对发送端导频的设计方法,本论文首先阐述了导频设计的基本准则：决定最小相关时间的最大多普勒频移,和决定最小相关带宽的最大多径时延扩展,它们共同决定了导频图样在时域和频域上的间隔。在这个准则的基础上,本论文对导频的设计方法展开了研究,主要分为重叠的导频设计方法和正交的导频设计方法两种方法进行了阐述。
针对接收端获得导频位置的信道信息的方法,本论文研究了两种典型的信道估计算法：LS算法和MMSE算法,这两种算法对应着算法复杂度和性能的两个极端。LS算法通常与多种插值算法联合使用来重建整个频带上的信道全响应,而MMSE算法由于其算法复杂度太高不易于实现,因此业界围绕其简化算法开展了很多的研究,具有代表性的比如LMMSE算法和基于SVD分解的算法。
针对接收端根据导频处信道信息重建整个信道全响应的方法,本论文主要研究了三种具体场景,就这三种场景下的信道估计算法展开研究,通过理论分析和软件仿真得出了在算法复杂度和算法性能之间进行最优折中的改进算法：
●在包含虚子载波的OFDM系统中,由于虚载波的引入,使得系统在频域虚载波处的信道信息缺失,因而造成时域上的能量弥散。本文研究了含虚载波的系统中两种改进的信道估计算法,一种基于虚载波导频插值,一种基于时域迭代。仿真表明这两种算法减小了虚载波带来的能量弥散,提高了系统性能。
●在快速时变信道的OFDM系统中,由于多普勒频移,信道冲激响应在一个OFDM符号周期内具有时变性,这种信道的时变性影响了OFDM子载波之间的正交性,最终引起了不利的载波间干扰ICI。本文研究了基于线性逼近和分段函数逼近的改进的信道估计算法。仿真表明,改进的信道估计最好地实现了信道估计性能和算法复杂度之间的折衷。
●在多小区TDD-OFDM系统中,由于不同用户使用非正交的导频训练序列引起的导频污染,给系统带来较大的小区间干扰。本文针对导频污染展开分析,研究了改进的带导频污染删除的信道估计算法。仿真表明,通过插入循环移位的导频,然后进行导频干扰删除,系统能获得最大的频谱传输速率。
总结起来,通过全文的研究分析,得知在新一代OFDM通信系统及其他新的通信场景中,信道估计始终是关键的核心技术,对它的优化改进有利于显著改善系统性能,提高传输速率和可靠性。
To implement coherent detection, receivers in OFDM system need reliable and efficient channel estimation. However, due to time variance of channel state information, channel estimation tends to be inaccuracy in real system. Therefore it is vitally important to improve the reliability of channel estimation. This thesis studies the pilot-based channel estimation in OFDM system from two aspects, one is to mitigate the channel estimation error, and the other is to decrease the effect of time-varying channel.
The thesis firstly gives an overview of wireless fading channel, and then studies the time-varying linear baseband equivalent model and discrete-time baseband model of the wireless channel. Due to the direct correlation between small-scale fading and multi-path channel impulse response, the wireless multi-path channel can be built as a linear time-varying channel. Then the thesis discusses both time-domain model and frequency-domain model of OFDM system, and introduces general procedures of the pilot-based channel estimation scheme in OFDM system. This scheme consists of three core issues:1) pilot design at transmitter terminal,2) channel estimation based on the pilots at receiver terminal, and3) the reconstruction of channel impulse response.
In terms of pilot design at transmitter, this paper firstly describes the basic principles of pilot design:the maximum Doppler shift, which determines the minimal coherence time, and the maximum delay spread, which determines the minimal coherence frequency, jointly affect the interval between pilot pattern in both time and frequency domain. Then this thesis further elaborates the overlapping pilot design methodology and orthogonal pilot design methodology.
In the perspective of channel estimation based on the pilots at receiver terminal, this thesis discusses two typical channel estimation algorithm:LS and MMSE. These two algorithms represent the trade-off between computation complexity and performance. Combined with interpolation algorithm, LS is often used to reconstruct the channel impulse response in frequency domain, while MMSE is hard to implement, due to the large computation complexity. Therefore, it is necessary to propose a novel algorithm that can improve the performance and lower the complexity simultaneously, such as LMMSE and SVD-based algorithm.
With regard to the reconstruction of channel impulse response, this thesis focuses on three major scenarios, based on which, the channel estimation is conducted. From the theoretical analysis and simulation result, the conclusion can be obtained that the proposed scheme achieves compromise between computation complexity and performance.
●In OFDM system with virtual subcarriers, the channel information is partially lost in the area of virtual subcarriers. Eventually, it leads to the energy leakage in time domain. This paper mainly discusses two improved channel estimation algorithms in OFDM system with virtual subcarriers, one is based on virtual subcarrier pilot interpolation, and the other is based on iteration in time domain. Simulation results illustrate that both of these two algorithms reduce the energy leakage brought by virtual carrier, and improve the system performance.
●In fast time-varying channel, due to the Doppler shift, the channel impulse response is time-variant during one OFDM symbol. It plays a negative effect on orthogonality between subcarriers in OFDM system, which eventually leads to inter-cell interference (ICI). This thesis proposes an improved channel estimation algorithm on the basis of linear-approximation and piecewise approximation. The simulation results demonstrate that the proposed scheme achieves the better compromise between channel estimation performance and computation complexity.
●In multi-cell TDD-OFDM system, the pilot contamination, resulted from non-orthogonal pilot training sequence used by different users, can cause larger ICI. This paper analyzes pilot contamination, and designs a novel channel estimation algorithm to mitigate the pilot contamination. Simulation results show that after cyclic shift pilot interpolation and pilot interference cancellation, the maximum transmission rate is obtained.
To sum up, through research and analysis, channel estimation is the key technology in the new generation of OFDM communication system. It is beneficial to optimize the channel estimation methods to improve the system performance, to gain higher transmission rate, and to enhance reliability.
引文
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