OFDM传输系统同步方案简介

本文研究数字地面电视DVB-T的OFDM传输系统接收端的同步。根据OFDM的原理特点和DVB-T标准,设计同步方案。该方案采用多载波(OFDM)系统时域插在循环保护间隔携带的冗余信息进行粗符号和分数频率偏移估计。该方案利用OFDM系统时域插入的保护间隔前缀所携带的冗余信息,进行粗符号同步和分数频率偏移估计。FFT后,再利用频域插入的连续导频进行整数频偏估计,及利用分散导频通过估计信道冲击响应,实现精符号同步。仿真表明,同步设计在加性高斯白噪声信道能达到最佳性能,在慢衰落Rayleigh信道下也有较好的性能。     

OFDM传输系统同步方案简介

本文研究数字地面电视DVB-T的OFDM传输系统接收端的同步。根据OFDM的原理特点和DVB-T标准，设计同步方案。该方案采用多载波（OFDM）系统进域插在循环保护间隔携带的冗余信息进行粗符号和分数频率偏移估计。该方案利用OFDM系统时域插入的保护间隔前缀所携带冗余信息，进行粗符号同步和分数频率偏移估计。FFT后，再利用频域插入的连续导频进行整数频偏估计，及利用分数导频通过估计信道冲击响应，实现精符号同步。仿真表明，同步设计在加性高斯白嗓声信道能达到最佳性能，在慢衰落Rayleigh信道下也有较好的性能。     

基于信道分布空间的OFDM信道半盲估计方法

信道分布空间的构建将OFDM系统中发送信号的有限符号特性和信道响应的统计相关特性集中到信道分布平面中.算法使用了一个初始OFDM 导频符号在信道分布平面中建立信道频域响应参考曲线,信道的估计是利用相邻OFDM符号对应的信道响应有相关延续特性,通过在更新的信道分布平面上更新参考曲线来实现.计算机仿真表明,算法的性能和基于最小均方误差的信道估计算法相当,由于避免了矩阵求逆运算,该方法在工程实现中有较大的应用价值.     

基于失配序列的OFDM信道估计

本文提出了一种利用失配序列进行OFDM信道估计的新方法。在发端将失配序列加入到OFDM信号中作为训练序列,接收端将收到的去掉循环前缀的失配序列与失配滤波器系数的循环移位序列做相关运算,利用失配序列的优良自相关特性,估计出信道的冲击响应,进而得到子载波处信道的频率响应。理论分析和仿真结果表明,该方法计算复杂度低,估计速度快、精度高,系统的BER性能好。      

LTE-A物理层下行参考信号的研究

LTE-A（改进的长期演进）OFDM系统接收端采用相干解调技术恢复原始数据信息,相干解调必须利用信道的相关信息,获取信道相关信息接收端需采用先进的信道估计技术实现。LTE-A OFDM系统是基于导频信号的信道估计获取信道的脉冲响应特性。本文主要对LTE-A物理层下行CRS（小区特定参考信号）进行了深入的分析研究。     

基于正交多相序列的OFDM信道估计算法

提出了一种基于正交多相序列的OFDM信道估计算法。该算法采用相关性能良好的正交多相序列作为发送信号的循环前缀,在接收端,提取循环前缀,进行相关计算,最终得到系统频域响应的估计值。通过仿真,验证了在多径衰落信道中,提出的算法比频域梳状导频插入的时域滤波算法及采用小m序列作为循环前缀的信道估计算法具有更高的信道估计精度。     

基于导频辅助的无循环前缀SFBC-OFDM系统

设计了一种基于导频辅助的无循环前缀空频分组编码-OFDM系统(Non-CP SFBC-OFDM).区别于传统的循环前缀空时分组编码-OFDM系统(CP STBC-OFDM),新系统发射端将块状导频序列和由空频分组编码构成的OFDM符号交替排布发射,并且每个OFDM符号前不再附加循环前缀,而接收端针对这一新的数据发射结构设计出一种基于干扰抵消的信号检测和信道估计联合递归算法.理论推导和仿真结果表明:相对于应用最小平方(LS)、迭代最小平方(iterative LS)的信道估计算法的循环前缀STBC-OFDM系统,引入联合递归算法的新系统可以得到更高的信息数据传输效率和更低的系统误比特率.     

一种改进的OFDM系统盲信道估计算法

在OFDM通信系统中,信道参数估计一直是个备受关注的问题。本文利用接收端分集,提出了一种改进的盲信道估计算法,它适用于有无虚拟子载波,有无循环前缀的各种情况,在一定的信道精度要求条件下,可以在一个OFDM 符号内进行信道估计。计算机仿真显示,在积累相同的接收符号时,本算法也比同类的盲信道估计算法的性能好。     

一种低复杂度的MIMO-OFDM信道估计新方法

该文基于m序列设计了一种适合多天线收发-正交频分复用(MIMO-OFDM)系统的时域信道估计新方法.在时域OFDM符号之间插入附加循环前缀(CP)的m序列作为训练序列,在接收端将去掉循环前缀的接收序列与指定的循环移位m序列作相关运算,然后利用m序列的二值自相关特性获得信道的冲激响应估计,从而得到信道的频率响应估计.理论分析和仿真结果都表明:在频率选择性信道中,新方法在系统开销小、计算复杂度低的情况下可以取得比传统算法更好的性能.     

基于子空间分解的OFDM信道盲辨识

该文提出一种基于子空间分解的正交频分复用(OFDM)信道的盲辨识算法,将OFDM信号等效为单输入多输出的过采样信号,采用过采样信号的循环稳态特性和子空间分解方法估计信道参数,算法不需要任何训练序列和周期性的引导信号,实现了0FDM信道的盲辨识。对于宽带OFDM移动通信系统,通常子信道数较大,信道响应持续时间短于0FDM符号周期,因此,可以将整个系统分为若干个子系统,各子系统分别进行信道辨识,能有效地降低信道估算的复杂性。     

Interaction between Symbol Timing Offset and Channel Interpolation in OFDM Systems: BER Analysis and Performance Degradation

In orthogonal frequency division multiplexing (OFDM) systems, since the cyclic prefix (CP) is designed to be longer than the channel impulse response, there exists a certain range within the CP where symbol timing synchronization can be accomplished avoiding adjacent inter symbol interference. However, the appearance of a linear phase term across subcarriers in the frequency-domain due to symbol timing offset (STO) is known to affect the performance of channel interpolation. In this paper, we analyze the performance degradation due to the interaction between STO and channel interpolation in OFDM systems affected by multipath Rayleigh fading. Particularly, simple closed-form expressions for the bit error rate (BER) are obtained for different quadrature amplitude modulation constellations. Results show that there exists an irreducible BER floor due to STO and channel interpolation, which depends on the STO, the subcarrier index, the pilot spacing and the correlation between pilot subcarriers.     

基于粒子滤波的大气激光OFDM时变信道半盲估计

为了有效改善大气激光正交频分复用通信系统接收端的符号检测性能,采用现有的混合粒子滤波算法对大气激光正交频分复用时变信道进行半盲估计,并进行了理论分析与实验验证.与传统的基于导频的时变信道估计方法相比,该方法可有效改善接收端的符号检测性能,并通过MATLAB仿真结果验证了该方法的有效性.结果表明,在相同的信噪比下,所用方...     

基于能量收集的OFDM符号精同步算法

针对OFDM(正交频分复用)符号粗同步定时点会随路径能量变动的缺点,利用信道估计得出的信道脉冲响应,提出一种基于能量收集的精同步算法,不需设定判决门限,得到的最终定时点能有效避免ISI(符号间干扰)和ICI(信道间干扰)的引入.多径信道中的仿真验证了该算法的良好性能.     

Performance enhancement of IM/DD optical wireless systems

Due to the fast development of telecommunications and the great demand of bandwidth, optical wireless communication (OWC) has become a promising trend. OWC offers flexible networks for both indoor and outdoor communications providing high security, high speed and a license free spectrum. Data transmission over indoor OWC systems with diffused channel model at high speed will extend the channel impulse response over several symbol periods with many reflections causing inter-symbol interference. This interference distorts the data symbols being transmitted and affects the signal recovery at the receiver side. This paper studies error control coding for OWC systems over additive white Gaussian noise and ceiling bounce channel models. It considers Hamming coding and channel equalization for unipolar optical orthogonal frequency division multiplexing (OFDM) system. The objective is to mitigate the channel distortion, and hence achieve reliable transmission and correct detection at the receiver side.     

Impulse Noise Correction in OFDM Systems

In orthogonal frequency division multiplexing (OFDM) systems, the impulse noise causes catastrophic accuracy degradation since the impulse noise affects all the subcarriers in a symbol due to the fast Fourier transform (FFT) operations at the receiver. Potential causes of impulse noise include erasure channel, power switching, and circuit failure in integrated circuits. In this paper, from a practical observation, a novel iterative impulse error correction scheme is proposed. This scheme is referred to as the impulse noise location and value search algorithm, which is based on the crucial observation of the relationship of the impulse noise and the symbol constellation. In a 512-FFT OFDM system at 25 dB additive white Gaussian noise signal-to-noise ratio, for quadrature amplitude modulation (QAM)-4 and QAM-8 modulation, simulation results show that our proposed novel scheme can effectively correct impulse errors that corrupt up to 20.7 % and 13.9 % of the received time-domain signal at known locations. In addition, without the knowledge of impulse noise location, the proposed scheme still can correct at least 9.96 % of the received time-domain signal for QAM-4 modulation.     

快衰落信道下OFDM系统的信道估计简化算法

在快衰落时变信道中，正交频分复用（OFDM：Orthogonal Frequency Division Multiplexing）系统子载波间的正交性会遭到破坏，由此引起的载波间干扰（ici：Inter-Carrier Interferellce）会对系统性能带来很大的影响。本文提出了一种利用时域线性模型的信道估计算法，利用随时间线性变化的模型描述信道。仿真结果表明，本算法能够有效地消除载波间干扰，降低系统的BER。与类似算法相比，当系统的BER性能相同时，算法复杂度大大的降低。     

NLOS环境下低复杂度的UWB-SRAKE接收机性能分析

该文提出一种适用于NLOS环境UWB多径信道下低复杂度的选择性RAKE接收机(RC-SRAKE),通过本地参考波形与接收信号的卷积抽样来确定SRAKE的Finger参数,不需要已知信道信息或信道估计过程,降低了复杂度。给出了RC-SRAKE误码率的表达式,分析了Finger数目和信道噪声对RC-SRAKE性能的影响。通过对IEEE.802.15.4a中NLOS信道的仿真实验表明,与能获得准确信道信息的理想SRAKE相比,在Finger数目较少的情况下RC-SRAKE能达到与之相近的性能。     

Rate Optimization for 2D OFDM Relaying

We solve optimal symbol dimensions for a recently introduced two-dimensional orthogonal frequency division multiplexing (2D OFDM) scheme with a two-dimensional cyclic prefix (2D CP). The 2D OFDM technique is shown to improve the rate of a wireless relaying system if the channel coherence time is at least nine times longer than the channel impulse response. Furthermore, feasibility and application scenarios of 2D OFDM are discussed using the parameters of terrestrial digital video broadcasting systems (DVB-T/H) as examples.     

Exploiting Multipath Diversity in Multiple Antenna OFDM Systems With Spatially Correlated Channels

Multiple antennas are useful in orthogonal frequency division multiplexing (OFDM) systems for providing transmit and receive diversity to overcome fading. Typically, these designs require considerable separation between the antennas. Spatial correlation is introduced when antennas are not well separated, and it often leads to performance degradation in a flat fading environment. However, in frequency selective fading channels with rich multipath diversity, OFDM receivers can overcome this performance degradation due to antenna correlation. This is due to transformation of a highly spatially correlated channel impulse response to a less spatially correlated channel frequency response inherently by an OFDM system in the presence of rich multipath diversity. We illustrate this for a simple receive diversity OFDM system and hence introduce the concept of space sampling at the receiver where antennas are placed relatively close to each other. The minimum separation required between the antennas under such circumstances is derived analytically, and it is shown that even with a separation of only$0.44lambda$, the required spatial correlation in the channel frequency response becomes sufficiently low. Simulated performance results with such spacing for various multiple antenna OFDM systems corroborate the analytical results.