OFDM基带系统CFOs的估计及校正
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摘要
随着多媒体技术和移动通信的高速发展,人们对信息传输速率的要求日益提高,要求的码元传输速率不断的提高,现今对一路信息的传输速率要求已达若干Mb/s,并且传输信道大多数是多径衰落或有频率选择性衰落的信道。为了克服信道上的衰落,正交频分复用(OFDM, Orthogonal Frequency Division Multiplexing)技术受到了高度重视。
OFDM是一类多(子)载波的并行调制体制。它将数据分为多路,在与子载波对应的多个载波频率上进行传送。在OFDM中各路子载波频谱是可以重叠的,提高了频率利用率,增大了数据传输速率,而且子载波之间是严格正交的,避免了重叠频谱之间的干扰或载波间干扰(ICI, Inter-Carrier Interference),同时接收端能够完全的分离各路已调制的信号。此外,每路子载波可以采用多进制调制,调制制度也可以不同,能够为适应信道的变化而自适应的改变。
但是,如果正交性遭到破坏,载波间干扰将会引起严重的系统性能退化。因此,不同子载波之间正交性的保持是至关重要的,而载波间干扰的主要来源就是载波频率偏移(CFOs, Carrier Frequency Offsets)。
为了保持子载波之间的正交性,避免载波间的干扰引起的系统性能退化,本文提出了基于循环前缀(CP, Cyclic Prefix)的CFOs估计的步长算法。并对步长参数的选定、校正系数的性能、变步长控制、CFOs的估计误差、算法的稳定性及典型平稳状态下的性能等进行了研究。
首先,介绍了OFDM的基本含义、国内外发展现状及主要应用;
其次,阐述了OFDM的原理及实现,并在平均窗口算法的基础上,引入了步长参数,推导得出了CFOs估计步长算法的数学模型;
然后,说明了硬件设计思想及系统各部分是如何实现的,并在此基础上描述了基带操作的VHDL实现过程;
此外,通过MATLAB仿真将步长算法与VSB和LMR估计算法进行了比较,分析了三种算法在典型平稳状态下的性能和步长算法的稳定性,验证了算法中各个系数的性能,比较了可变步长和恒定步长;
最后,设计了步长算法的实现程序,强调了变步长控制,给出并分析了具体的实验结果。
实验结果表明:设计的程序在电路板上能够正确执行;在±3σ估计变化区间内,移动速度为400 km/h时,CFOs估计的最大误差仅为0.00987,符合CFOs估计误差必须精确的在子载波空间1-2%范围内的衡量标准;同时减少了建立时间和稳定时间,提高了码元传输速率。此外,步长算法CP存储器资源的消耗只有其他算法的20%,减少了信息速率损失。
Along with high-speed development of multimedia technique and mobile communication,demand for information transmission velocity is increasingly increased, and required code transmitted velocity is increasingly improved. Nowadays transmission velocity of a path of information is required up to several Mb/s. Moreover, transfers channel are mostly multipath decline or frequency selectivity decline. In order to overcome channel decline, OFDM (Orthogonal Frequency Division Multiplexing) is attached importance.
OFDM is a parallel modulation system using multicarrier, which means that the data are multiplexed onto and transmitted over multiple carrier frequencies called subcarriers. The subcarrier spectra overlap in OFDM which improve frequency availability, and the subcarriers are strictly mutually orthogonal,which make Interference among overlapping spectra, or intercarrier interference (ICI) be avoided. At the same time, receiver can completely separate every path of modulation signal. Further, any path of subcarrier could adopt M-system modulation and modulation mode may be different, which can self-adaptively alter to adapt channel variety.
However, if orthogonality is destroyed, ICI can cause severe performance degradation. Thus, it is critical that orthogonality be maintained among the different subcarriers, and primary source for ICI is Carrier Frequency Offset (CFO).
In order to maintain orthogonality among subcarriers, a CFOs step-size estimation model based on cyclic prefix (CP) is established to avoid severe performance degradation caused by ICI, and some are investigated that includes selection of step-size parameter, performance of correction coefficient, control of variable step-size, the error of CFOs estimation, algorithmic stability and typical steady-state performance and etc.
First, OFDM is presented, including basic signification, development actuality and primary applications.
Then, principle and realization of OFDM are described. After an averaging window algorithm is introduced, the mathematic model of step-size algorithm of CFOs estimation is obtained through step-size parameter.
Third, design idea for hardware is explained, as well as various part in system and VHDL implementation process of baseband operation. Furthermore, comparing with VSB and LMR, step-size algorithm performance is under typical steady-state. Based on the results of simulation referred to MATLAB, stability, coefficients, in- and variable step-size are verified.
Finally, executable program of step-size algorithm is designed, control for variable step-size is stressed, and experimental results are listed and discussed.
Experimental results indicate that self-designed programs can be accurately performed on circuit board. Maximal error of CFOs estimate is 0.0987 in±3σvariance interval, when mobile velocity are 400 km/h, according with verdicted principle that in order for the ICI effects to be negligible, the error of CFOs estimation must be accurate within 1-2% of the subcarrier spacing. At the same time, setup time and steady time are decreased to improve code transmission velocity. Additionally, store resource consumed by CP is 20% of other algorithms’to reduce loss of information velocity.
OFDM是一类多(子)载波的并行调制体制。它将数据分为多路,在与子载波对应的多个载波频率上进行传送。在OFDM中各路子载波频谱是可以重叠的,提高了频率利用率,增大了数据传输速率,而且子载波之间是严格正交的,避免了重叠频谱之间的干扰或载波间干扰(ICI, Inter-Carrier Interference),同时接收端能够完全的分离各路已调制的信号。此外,每路子载波可以采用多进制调制,调制制度也可以不同,能够为适应信道的变化而自适应的改变。
但是,如果正交性遭到破坏,载波间干扰将会引起严重的系统性能退化。因此,不同子载波之间正交性的保持是至关重要的,而载波间干扰的主要来源就是载波频率偏移(CFOs, Carrier Frequency Offsets)。
为了保持子载波之间的正交性,避免载波间的干扰引起的系统性能退化,本文提出了基于循环前缀(CP, Cyclic Prefix)的CFOs估计的步长算法。并对步长参数的选定、校正系数的性能、变步长控制、CFOs的估计误差、算法的稳定性及典型平稳状态下的性能等进行了研究。
首先,介绍了OFDM的基本含义、国内外发展现状及主要应用;
其次,阐述了OFDM的原理及实现,并在平均窗口算法的基础上,引入了步长参数,推导得出了CFOs估计步长算法的数学模型;
然后,说明了硬件设计思想及系统各部分是如何实现的,并在此基础上描述了基带操作的VHDL实现过程;
此外,通过MATLAB仿真将步长算法与VSB和LMR估计算法进行了比较,分析了三种算法在典型平稳状态下的性能和步长算法的稳定性,验证了算法中各个系数的性能,比较了可变步长和恒定步长;
最后,设计了步长算法的实现程序,强调了变步长控制,给出并分析了具体的实验结果。
实验结果表明:设计的程序在电路板上能够正确执行;在±3σ估计变化区间内,移动速度为400 km/h时,CFOs估计的最大误差仅为0.00987,符合CFOs估计误差必须精确的在子载波空间1-2%范围内的衡量标准;同时减少了建立时间和稳定时间,提高了码元传输速率。此外,步长算法CP存储器资源的消耗只有其他算法的20%,减少了信息速率损失。
Along with high-speed development of multimedia technique and mobile communication,demand for information transmission velocity is increasingly increased, and required code transmitted velocity is increasingly improved. Nowadays transmission velocity of a path of information is required up to several Mb/s. Moreover, transfers channel are mostly multipath decline or frequency selectivity decline. In order to overcome channel decline, OFDM (Orthogonal Frequency Division Multiplexing) is attached importance.
OFDM is a parallel modulation system using multicarrier, which means that the data are multiplexed onto and transmitted over multiple carrier frequencies called subcarriers. The subcarrier spectra overlap in OFDM which improve frequency availability, and the subcarriers are strictly mutually orthogonal,which make Interference among overlapping spectra, or intercarrier interference (ICI) be avoided. At the same time, receiver can completely separate every path of modulation signal. Further, any path of subcarrier could adopt M-system modulation and modulation mode may be different, which can self-adaptively alter to adapt channel variety.
However, if orthogonality is destroyed, ICI can cause severe performance degradation. Thus, it is critical that orthogonality be maintained among the different subcarriers, and primary source for ICI is Carrier Frequency Offset (CFO).
In order to maintain orthogonality among subcarriers, a CFOs step-size estimation model based on cyclic prefix (CP) is established to avoid severe performance degradation caused by ICI, and some are investigated that includes selection of step-size parameter, performance of correction coefficient, control of variable step-size, the error of CFOs estimation, algorithmic stability and typical steady-state performance and etc.
First, OFDM is presented, including basic signification, development actuality and primary applications.
Then, principle and realization of OFDM are described. After an averaging window algorithm is introduced, the mathematic model of step-size algorithm of CFOs estimation is obtained through step-size parameter.
Third, design idea for hardware is explained, as well as various part in system and VHDL implementation process of baseband operation. Furthermore, comparing with VSB and LMR, step-size algorithm performance is under typical steady-state. Based on the results of simulation referred to MATLAB, stability, coefficients, in- and variable step-size are verified.
Finally, executable program of step-size algorithm is designed, control for variable step-size is stressed, and experimental results are listed and discussed.
Experimental results indicate that self-designed programs can be accurately performed on circuit board. Maximal error of CFOs estimate is 0.0987 in±3σvariance interval, when mobile velocity are 400 km/h, according with verdicted principle that in order for the ICI effects to be negligible, the error of CFOs estimation must be accurate within 1-2% of the subcarrier spacing. At the same time, setup time and steady time are decreased to improve code transmission velocity. Additionally, store resource consumed by CP is 20% of other algorithms’to reduce loss of information velocity.
引文
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