多带正交频分复用超宽带系统信道估计方法
摘要
近年来,随着无线通信技术的迅猛发展以及在商业中的成功应用,人们在日常生活受之深刻影响的同时希望在任何时间、任何地点能够接入大量的信息。频谱资源的共享与高效利用成为无线通信领域迫切需要研究解决的问题。超宽带(UWB)因高速率低功耗、低成本的特点,已成为下一代短距离室内无线通信的最佳候选技术之一。它基于共用频段的思想,可以与其它现存的窄带系统共享频带,打破了短距离无线通信频率资源供不应求及不兼容的现状,为解决日趋紧张的频谱资源难题提供新的解决方案。作为超宽带技术实现方式之一,基于正交频分复用的多频带超宽带(MB-OFDM UWB)方案具有频带利用率高、捕捉多径能量强、传输速率灵活等优点,适合高速无线数据传输,在数字化无线家庭网络、数字化办公室、个人便携设备和军事等诸多领域都有着广阔的发展和应用前景。
为了保证传输可靠性和功率效率,超宽带系统一般采用相干检测,然而在相干检测接收中,数据符号不可避免遭受随机相移、幅度波动的影响,必须在接收机必中进行有效的信道估计。本文针对多带正交频分复用超宽带系统特性,从实际应用角度出发,综合考虑信道估计算法实现的复杂度和精度,在如下几个方面进行了研究工作：
与窄带系统相比,超宽带系统接收信号在多径分量的可分辨数量和到达时间方面有着本质的区别。论文首先分析了常规抽头延迟线信道衰落模型、△-K信道模型和S-V信道模型各自的优缺点,研究了IEEE802.15标准室内信道模型。在此基础上考虑超宽带信道“径”和“簇”的多径特性,提出了适用于MB-OFDM UWB系统的延迟抽头信道模型,分析了该模型的多径延迟统计特性和不同子载波上信道频率响应的相关性,从随机过程理论角度证明了在频域将超宽带无线信道建立成低阶AR模型的可行性,为进一步研究适用于MB-OFDM UWB系统的信道估计算法奠定了基础.
传统的数据辅助信道估计通过训练序列或导频符号获得信道状态信息,虽然算法简单、估计速度较快,但是在频谱利用率和功率利用率上付出一定代价,对应用于高速率传输中的功率受限超宽带系统的影响尤为突出。论文在分析基于LS.MMSE等不同准则信道估计算法的计算复杂度和性能的基础上,在准静态信道环境中分别研究了理想同步和存在相位误差条件下的数据辅助信道估计算法。针对理想同步条件,本论文利用MB-OFDM UWB特定前导结构,提出了一种频域加权滤波的数据辅助信道估计方法。该算法首先对信道前导字获得基于LS准则的信道初始估计,通过频率平滑处理提高LS估计的抗噪能力,利用初始估计的加权对帧头前导字进行前向判决,根据判决结果估计数据符号上的信道频率响应,提高了低信噪比条件下信道估计的可靠性。在存在相位噪声时,本论文提出通过对不同符号导频子载波上的信道频率响应的一阶低通滤波来补偿相位噪声对信道估计的影响。该算法计算简单,在低信噪比条件下能达到良好的估计性能。
针对时变信道,在理想同步条件下,论文研究了基于卡尔曼滤波算法的信道自适应估计和跟踪方法。常规的基于卡尔曼滤波的信道估计算法只是应用了信道时域相关性,本论文在频域将超宽带信道模型等效为二阶自适应回归模型,提出了一种基于低维卡尔曼滤波的迭代信道估计算法。该算法同时考虑超宽带信道时域和频域相关特性,利用了不同信道环境下子载波间隔的不同相关性对滤波算法进行降维处理,在一定程度上简化了计算。在此基础上结合导频子载波内插算法,在存在频偏误差的情况下进行对信道估计进行相位补偿,提高了信道估计跟踪性能。
在超宽带系统中应用MIMO技术,能大大提高系统容量,从而扩大超宽带系统的传输速率和通信距离。论文对多天线MB-OFDM UWB系统的信道估计进行有益的探索,分别探讨了采用多天线传输的单用户信道估计和多用户信道估计。为了有效估计和跟踪信道状态,同时考虑带宽利用率及系统复杂度,本论文对两发一收天线的单用户提出了适用于硬判决迭代接收机的ML-EM信道估计算法和适用于软判决迭代接收机的MAP-EM信道状态信息算法。通过EM算法将多输入输出信道估计问题化简为一系列独立的单输入输出问题,避免了大规模矩阵运算。对于多用户情况,提出采用空间交替推广EM算法,通过在每次迭代过程中交替选择接收数据作为隐数据空间,关联噪声方差来加快迭代算法收敛速度。与基于数据辅助的信道估计算法相比,基于EM算法的信道估计不需要周期性的发送训练序列,降低了系统发射功率,同时节约了信道带宽,提高频谱利用率。
In recent years, wireless communication technology has been developed rapidly and applied in commercial successfully, which undoubtedly has a profound influence on our daily life. We hope to exchange a large amount of information at any time, from any location. As a result, sharing and efficiency utilization of spectrum resources has been an important issue in wireless communication field. Ultra wideband (UWB) has become an attractive candidate for future short-range and high date rate wireless communication owing to the potential for high data rates, and the potential for low processing power along with low implementation cost. UWB, which is a shared unlicensed system, coexists with other licensed and unlicensed narrowband systems. In spite of this, UWB will change the status of wireless frequency resource shortage and incompatible, and offer attractive solutions for increasingly scarce spectrum resources. As a strong candidate for UWB, MB-OFDM UWB drew more and more attention because of its advantages such as ability to provide high bandwidth efficiency, capability of capturing the multipath energy efficiently, flexible and high wireless data transmission, which can be applied to digital wireless home or office network, personal handy system or military field.
To ensure the reliability of data and limitation of power, UWB system usually adopts coherent detection. By this way, unfortunately, the data is inevitably affected by random phase shift and amplitude fluctuation, it's needed for receiver to understand the fading propagation effects in frequency selective channels characteristics and carry out channel estimation. Considering the algorithms complexity and precision simultaneously, we present some novel and practical channel estimation schemes tailored to MB-OFDM UWB applications. The main innovations of the dissertation are as follows:
There are important differences between UWB and narrowband channels, especially with respect to the number of resolvable paths and arrival times of multipath components. This paper compares the different transmission characteristics of some classic channel model, such as tap-line delay model,Δ-K model and S-V model, in terms of the statistical characteristics of multi-path delay amplitude and arrival times. Having insight into the in-door channel model of IEEE802.15, this paper probes into multi-path delay time and coefficients statistical characteristics based on tap-line model aiming to MB-OFDM UWB system. In order to analysis the channel estimation method in frequency domain, we demonstrate that the UWB channels can be adequately modeled by low-order autoregressive (AR) models, and discuss the correlation of subcarrier channel frequency response.
The traditional data-aided channel estimation algorithm obtains the channel state information by the training sequences or polite symbols. Although this kind of algorithm is relatively simple and easy to implement, it will waste frequency and power efficiency. Based on comparing the algorithms complexity and performance for different estimation citation such as LS and MMSE, This paper presents new date-aided channel estimation algorithm for MB-OFDM UWB system. To make good trade-off between performance and computational complexity under satisfactory synchronization, we employ a simple LS method together with frequency-domain smoothing operation to estimate the channel frequency response using training preamble, then uses this channel estimate result to detect the frame header and refines the channel estimate by using a decision-directed technique. Furthermore, we developed an efficient phase error mitigation scheme. This method employs inter OFDM symbol smoothing technique for achieving further suppression in low SNR regime.
Aimed to time-varying channel, this paper utilizes the Kalman filter to track the channel state varying on adjacent symbols. The traditional channel estimation based on Kalman filter is only made use of the channel correlation in time domain, this paper refine the estimation performance considering the channel correlation in frequency domain. The algorithm reduces the complexity by reducing dimension according the different correlation of subcarrier in different environment. Based on this, we combine polite interpolate and phase compensating when existing frequency offset, which improve the tracking performance.
To enhance the data rates and transmission ranges of UWB systems, applying the MIMO scheme has attracted considerable interest, which could expand the capacity of UWB system. To estimate the channel state information effectively, this paper proposes channel state information estimation algorithms based on ML-EM with hard-decision receiver and MAP-EM with soft-dicision receiver respectively. The EM algorithm decomposes the MIMO channel estimation problem into a series of single input single output (SISO) problems, avoiding the complex computations. In the situation of multi-users, we adopt Space Alternating Generalized EM to accelerate the algorithm iteration rate. Compared with data-aided channel estimation method, EM channel estimation algorithm needn't transmit training symbols periodically, which reduce the transmission power and improve the bandwidth and spectrum efficiency.
为了保证传输可靠性和功率效率,超宽带系统一般采用相干检测,然而在相干检测接收中,数据符号不可避免遭受随机相移、幅度波动的影响,必须在接收机必中进行有效的信道估计。本文针对多带正交频分复用超宽带系统特性,从实际应用角度出发,综合考虑信道估计算法实现的复杂度和精度,在如下几个方面进行了研究工作：
与窄带系统相比,超宽带系统接收信号在多径分量的可分辨数量和到达时间方面有着本质的区别。论文首先分析了常规抽头延迟线信道衰落模型、△-K信道模型和S-V信道模型各自的优缺点,研究了IEEE802.15标准室内信道模型。在此基础上考虑超宽带信道“径”和“簇”的多径特性,提出了适用于MB-OFDM UWB系统的延迟抽头信道模型,分析了该模型的多径延迟统计特性和不同子载波上信道频率响应的相关性,从随机过程理论角度证明了在频域将超宽带无线信道建立成低阶AR模型的可行性,为进一步研究适用于MB-OFDM UWB系统的信道估计算法奠定了基础.
传统的数据辅助信道估计通过训练序列或导频符号获得信道状态信息,虽然算法简单、估计速度较快,但是在频谱利用率和功率利用率上付出一定代价,对应用于高速率传输中的功率受限超宽带系统的影响尤为突出。论文在分析基于LS.MMSE等不同准则信道估计算法的计算复杂度和性能的基础上,在准静态信道环境中分别研究了理想同步和存在相位误差条件下的数据辅助信道估计算法。针对理想同步条件,本论文利用MB-OFDM UWB特定前导结构,提出了一种频域加权滤波的数据辅助信道估计方法。该算法首先对信道前导字获得基于LS准则的信道初始估计,通过频率平滑处理提高LS估计的抗噪能力,利用初始估计的加权对帧头前导字进行前向判决,根据判决结果估计数据符号上的信道频率响应,提高了低信噪比条件下信道估计的可靠性。在存在相位噪声时,本论文提出通过对不同符号导频子载波上的信道频率响应的一阶低通滤波来补偿相位噪声对信道估计的影响。该算法计算简单,在低信噪比条件下能达到良好的估计性能。
针对时变信道,在理想同步条件下,论文研究了基于卡尔曼滤波算法的信道自适应估计和跟踪方法。常规的基于卡尔曼滤波的信道估计算法只是应用了信道时域相关性,本论文在频域将超宽带信道模型等效为二阶自适应回归模型,提出了一种基于低维卡尔曼滤波的迭代信道估计算法。该算法同时考虑超宽带信道时域和频域相关特性,利用了不同信道环境下子载波间隔的不同相关性对滤波算法进行降维处理,在一定程度上简化了计算。在此基础上结合导频子载波内插算法,在存在频偏误差的情况下进行对信道估计进行相位补偿,提高了信道估计跟踪性能。
在超宽带系统中应用MIMO技术,能大大提高系统容量,从而扩大超宽带系统的传输速率和通信距离。论文对多天线MB-OFDM UWB系统的信道估计进行有益的探索,分别探讨了采用多天线传输的单用户信道估计和多用户信道估计。为了有效估计和跟踪信道状态,同时考虑带宽利用率及系统复杂度,本论文对两发一收天线的单用户提出了适用于硬判决迭代接收机的ML-EM信道估计算法和适用于软判决迭代接收机的MAP-EM信道状态信息算法。通过EM算法将多输入输出信道估计问题化简为一系列独立的单输入输出问题,避免了大规模矩阵运算。对于多用户情况,提出采用空间交替推广EM算法,通过在每次迭代过程中交替选择接收数据作为隐数据空间,关联噪声方差来加快迭代算法收敛速度。与基于数据辅助的信道估计算法相比,基于EM算法的信道估计不需要周期性的发送训练序列,降低了系统发射功率,同时节约了信道带宽,提高频谱利用率。
In recent years, wireless communication technology has been developed rapidly and applied in commercial successfully, which undoubtedly has a profound influence on our daily life. We hope to exchange a large amount of information at any time, from any location. As a result, sharing and efficiency utilization of spectrum resources has been an important issue in wireless communication field. Ultra wideband (UWB) has become an attractive candidate for future short-range and high date rate wireless communication owing to the potential for high data rates, and the potential for low processing power along with low implementation cost. UWB, which is a shared unlicensed system, coexists with other licensed and unlicensed narrowband systems. In spite of this, UWB will change the status of wireless frequency resource shortage and incompatible, and offer attractive solutions for increasingly scarce spectrum resources. As a strong candidate for UWB, MB-OFDM UWB drew more and more attention because of its advantages such as ability to provide high bandwidth efficiency, capability of capturing the multipath energy efficiently, flexible and high wireless data transmission, which can be applied to digital wireless home or office network, personal handy system or military field.
To ensure the reliability of data and limitation of power, UWB system usually adopts coherent detection. By this way, unfortunately, the data is inevitably affected by random phase shift and amplitude fluctuation, it's needed for receiver to understand the fading propagation effects in frequency selective channels characteristics and carry out channel estimation. Considering the algorithms complexity and precision simultaneously, we present some novel and practical channel estimation schemes tailored to MB-OFDM UWB applications. The main innovations of the dissertation are as follows:
There are important differences between UWB and narrowband channels, especially with respect to the number of resolvable paths and arrival times of multipath components. This paper compares the different transmission characteristics of some classic channel model, such as tap-line delay model,Δ-K model and S-V model, in terms of the statistical characteristics of multi-path delay amplitude and arrival times. Having insight into the in-door channel model of IEEE802.15, this paper probes into multi-path delay time and coefficients statistical characteristics based on tap-line model aiming to MB-OFDM UWB system. In order to analysis the channel estimation method in frequency domain, we demonstrate that the UWB channels can be adequately modeled by low-order autoregressive (AR) models, and discuss the correlation of subcarrier channel frequency response.
The traditional data-aided channel estimation algorithm obtains the channel state information by the training sequences or polite symbols. Although this kind of algorithm is relatively simple and easy to implement, it will waste frequency and power efficiency. Based on comparing the algorithms complexity and performance for different estimation citation such as LS and MMSE, This paper presents new date-aided channel estimation algorithm for MB-OFDM UWB system. To make good trade-off between performance and computational complexity under satisfactory synchronization, we employ a simple LS method together with frequency-domain smoothing operation to estimate the channel frequency response using training preamble, then uses this channel estimate result to detect the frame header and refines the channel estimate by using a decision-directed technique. Furthermore, we developed an efficient phase error mitigation scheme. This method employs inter OFDM symbol smoothing technique for achieving further suppression in low SNR regime.
Aimed to time-varying channel, this paper utilizes the Kalman filter to track the channel state varying on adjacent symbols. The traditional channel estimation based on Kalman filter is only made use of the channel correlation in time domain, this paper refine the estimation performance considering the channel correlation in frequency domain. The algorithm reduces the complexity by reducing dimension according the different correlation of subcarrier in different environment. Based on this, we combine polite interpolate and phase compensating when existing frequency offset, which improve the tracking performance.
To enhance the data rates and transmission ranges of UWB systems, applying the MIMO scheme has attracted considerable interest, which could expand the capacity of UWB system. To estimate the channel state information effectively, this paper proposes channel state information estimation algorithms based on ML-EM with hard-decision receiver and MAP-EM with soft-dicision receiver respectively. The EM algorithm decomposes the MIMO channel estimation problem into a series of single input single output (SISO) problems, avoiding the complex computations. In the situation of multi-users, we adopt Space Alternating Generalized EM to accelerate the algorithm iteration rate. Compared with data-aided channel estimation method, EM channel estimation algorithm needn't transmit training symbols periodically, which reduce the transmission power and improve the bandwidth and spectrum efficiency.
引文
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