多输入多输出变换域通信系统抗干扰性能分析

为实现变换域通信系统(TDCS)与多输入多输出技术(MIMO)的优势互补,将两者结合,使系统的数据传输速率和抗干扰性能得到进一步的提升.在对TDCS和MIMO理论研究和前人研究的基础上,细化了MIMO-TDCS系统收发机结构模型,详细分析了系统抗干扰原理,仿真了系统受到单音干扰、多音干扰以及窄带干扰时系统的误码率性能.实验结果表明,MIMO-TDCS是一种可靠的抗干扰通信系统.     

一种多输入多输出变换域通信系统的设计与仿真

传统的变换域通信系统TDCS(Transform Domain Communication System)在获得强抗干扰能力的同时,存在传输有效性差的问题,这已成为制约其广泛应用的技术瓶颈。通过引入多输入多输出MIMO(Multiple-input Multiple-output)技术改进收发端天线,提出了一种多输入多输出变换域通信系统(MIMO-TDCS)。对其基本原理和收发信机模型作了简要介绍,并进行了信道容量的分析和仿真,结果表明,MIMO-TDCS是一种兼具传输有效性和可靠性的通信手段。     

正交频分复用技术及其在4G移动通信中的应用

正交频分复用（ OFDM）技术是一种特殊的多载波数字调制技术。由于OFDM的各个子载波存在正交性,极大地利用了频谱资源,所以在一定程度上缓解了未来频谱资源紧缺的问题;OFDM有良好的抗码间干扰能力和对抗频率选择性衰落,能够适应未来无线通信无失真高速数据传播的要求;各个子信道的调制和解调能用FFT和IFFT实现,因而能够满足无线通信系统中的业务要求。文中首先介绍了OFDM技术的基本原理,接着阐述了它的优势以及需要克服的缺陷,再分析了4G移动通信技术的特点,最后讨论了OFDM技术在4G移动通信系统中的应用。     

OFDM系统中频率同步算法的研究

OFDM是B3G乃至4G系统中的一种关键技术。它的最大优点是对抗频率选择性衰落,同时又提高了频谱利用率。本文首先介绍了多载波传输与OFDM之间的关系以及OFDM系统的基本原理,接着对OFDM系统中的同步技术和载波同步算法进行了分析,最后对基于训练符号的时域相关算法做了详细的研究。     

OFDM系统中频率同步算法的研究

OFDM是B3G乃至4G系统中的一种关键技术。它的最大优点是对抗频率选择性衰落,同时又提高了频谱利用率。本文首先介绍了多载波传输与OFDM之间的关系以及OFDM系统的基本原理,接着对OFDM系统中的同步技术和载波同步算法进行了分析．最后对基于训练符号的时域相关算法做了详细的研究。     

CDR标准的数字调制系统研究与设计

中国调频频段数字音频广播即CDR标准可提供灵活的频谱模式,本文针对其关键技术——正交频分复用的调制技术进行研究与设计。分析子载波矩阵的构造,完成OFDM符号的有效子载波设计,对FFT设计采用改进基-2蝶形降低乘法器数目,利用块浮点数计算实现高精度,采用流水线方式优化设计流程。用Verilog HDL语言实现OFDM符号生成与FFT的设计,进行FPGA综合仿真。与MATLAB仿真结果对比表明,成帧载波可实现数模同播,FFT变换准确性高,符合CDR调制系统的要求。     

基于特殊导频码的OFDM系统盲帧同步算法

提出了基于特殊导频码的OFDM系统的盲帧同步算法。该算法不需要数据辅助,首先利用FFT窗跨界时产生的能量泄露实现了对符号定时参数的估计,然后对同步后的OFDM信号进行解调得到信息序列,最后利用3阶相关函数(TCF)对序列进行处理从而提取出同步码以实现帧同步。仿真结果表明,该算法在低信噪比下具有较好的同步能力和较高的同步精度。     

基于正交频分复用调制的物理层并行插值加密算法

针对传统链路层安全机制不能从根本上保护无线通信系统信息传输安全的问题,提出了一种基于正交频分复用（OFDM）系统并行调制特性和物理层安全的并行插值加密算法。首先,根据OFDM系统调制的子载波数目确定插入符号个数,并通过密钥控制生成插入符号位置;然后,取出对应插入位置前后的原OFDM符号,并计算其平均值作为插入符号;最后,在反快速傅里叶变换（IFFT）后完成伪随机插值。与传统链路层安全机制相比,所提算法能实现调制符号整体加密,保证了信令、标志及数据信息的安全,并有效降低了算法实现复杂度。仿真实验结果表明,该算法能有效抵抗各种窃听攻击,对通信系统固有性能影响小,能较好地适应高斯信道和多径信道,且表现出一定的抗多径衰落能力。     

基于频谱优化和交织的OFDM-PAPR降低算法

为了降低OFDM系统的PAPR,针对传统算法有损系统性能的缺点,提出了一种优化的算法。该算法基于频谱优化和交织思想,通过确定有效子载波个数来提高OFDM符号的平均功率和系统的频谱效率,并通过预判断,选择性地对OFDM符号进行交织处理。仿真结果表明,该算法在降低OFDM的PAPR的同时,也优化了系统的性能。     

基于大点数FFT的OFDM系统调制解调实现

在实际应用的OFDM系统中,实现在高速信号传输中数据的FFT处理尤为重要。研究OFDM系统在N很大的情况时的调制解调的实现,以N=4096点为例,提出一种大点数FFT硬件实现的设计思想,并通过Matlab仿真验证可行性。     

基于FFT频谱校正的全数字QAM解调法

针对QAM相干解调法需严格载波同步的不足,提出一种新的基于FFT频谱校正的全数字QAM解调方案,此方案采用了软件无线电的处理思想,直接在载波频段对QAM信号进行采样,采用FFT频谱校正估计出各码元周期内的载波幅值和初相位来实现信息比特解码。仿真实验证明,此方案方法简单,不仅降低了同步机制要求,而且在FFT长度为8时即可获得接近于传统相干解调法理论下限的误比特率性能。     

OFDM系统中基于全相位算法的最大似然信道估计器

在OFDM系统中设计了一种基于全相位快速傅里叶变换算法的最大似然信道估计器。全相位傅里叶变换相对于快速傅里叶变换呈现平方的幅度增益性质,在高信噪比情况下可以抑制最大似然估计器中的自带噪声,由此能够更准确地估计出信道冲击响应,并用来均衡信号。同时,OFDM采用全相位傅里叶变换作为解调算法克服了系统晶振不匹配以及信道传输过程中产生的频偏。在3GPP的空间信道模型下,设计了基于全相位最大似然估计器的OFDM系统,并与传统的最大似然估计器系统比较,使用蒙特卡洛方法仿真证明:信道冲击响应估计的均方误差和系统误码率均有所下降。     

The performance of multiwavelets based OFDM system under different channel conditions

In wireless communication reception, the reliability of orthogonal frequency division multiplexing (OFDM) is limited because of the time-varying nature of the channel. This causes inter-carrier interference (ICI) and increases inaccuracies in channel tracking. This can effectively be avoided at the cost of power loss and bandwidth expansion by inserting a cyclic prefix guard interval before each block of parallel data symbols. However, this guard interval decreases the spectral efficiency of the OFDM system as the corresponding amount. Recently, it was found that based on Haar-orthonormal wavelets, discrete wavelet-based OFDM (DWT-OFDM) is capable of reducing the inter symbol interference (ISI) and ICI, which are caused by the loss in orthogonality between the carriers. DWT-OFDM can also support much higher spectrum efficiency than discrete Fourier-based OFDM (DFT-OFDM). In this paper the DFT-OFDM is replaced by Multiwavelets OFDM (DMWT-OFDM) in order to further reduce the level of interference and increase spectral efficiency. It is found that proposed Multiwavelet design achieves much lower bit error rates, increases signal to noise power ratio (SNR), and can be used as an alternative to the conventional OFDM. The proposed OFDM system was modeled tested, and its performance was found under different channel conditions.     

OFDM中基于导频的自适应符号同步算法

提出了一种在OFDM（正交频分复用）系统中基于导频符号的自适应符号同步新算法,该算法利用FFT的特性,采用频域同步的方法对符号定时和载波相位偏差进行联合估计;算法计算量小,导频的占有资源少,并且同步捕获速度、精度高低可以灵活地进行调节。仿真结果表明,提出的算法能够简单而有效地获取符号同步,自适应性强,即使在多径衰落信道中,也能获得较好的性能。     

SER analysis of PTS based techniques for PAPR reduction in OFDM systems

A major drawback of orthogonal frequency division multiplexing (OFDM) is high peak-to-average power ratio (PAPR). An OFDM signal with high PAPR requires power amplifier?s (PAs) with large linear operating ranges but such PAs are difficult to design and costly to manufacture. Therefore, to reduce PAPR various methods have been proposed. One of the existing technique to reduce PAPR is partial transmit sequences (PTS). The major drawback of this technique is that it requires transmission of side information (SI) with each OFDM symbol, which results in low bandwidth efficiency. It is hard to recover the side information from the OFDM signal received at the receiver.The two methods, which do not require SI to decode the OFDM symbol at the receiver, are multi-point square mapping combined with PTS (M-PTS) and concentric circle mapping based PTS (CCM-PTS). In this paper, the SER performance of PTS based methods namely CCM-PTS and M-PTS over AWGN channel is mathematically analyzed. The SER performance of CCM-PTS over AWGN is analyzed using two decoding techniques, namely minimum distance decoding and circular boundary decoding, whereas M-PTS is analyzed using minimum distance decoding. The simulation results for SER performance of CCM-PTS and M-PTS, over fading channel, have been presented using computer simulations and the SER performance of CCM-PTS by both the decoding techniques is compared with M-PTS. Also, a comparison of PAPR reduction capability and computational complexity of CCM-PTS and M-PTS has been presented. CCM-PTS method almost has the same PAPR reduction capability as M-PTS, but its SER performance is better than M-PTS and uses a simpler method to decode the data symbols.     

基于傅里叶系数映射的OFDM系统性能分析

针对正交频分复用(OFDM)系统存在较高峰均比的问题,通过分析OFDM调制系统与周期信号傅里叶级数展开的关系,提出一种基于傅里叶系数映射的OFDM技术方案。该方案将待传输的数据调制为周期性信号,取该信号的部分傅里叶系数进行OFDM调制,替代传统的QPSK或MQAM映射方式。仿真结果表明,该技术方案在和QPSK映射相同频带利用率的情况下,其峰均比可下降3 dB,误码率与QPSK映射基本相当。     

Cross-layer adaptive modulation and coding design for space-time block coded MIMO–OFDM systems

This paper developed a cross-layer adaptive modulation and coding (AMC) design for space-time block coded (STBC) MIMO–OFDM systems, which combines the AMC at the physical layer with an automatic repeat request (ARQ) protocol at the data link layer, in order to maximize spectral efficiency under specified delay and packet error ratio (PER) constraints. The transmission mode for each subcarrier is updated frame-by-frame to match the time-varying channel conditions, in order to take full advantage of the OFDM systems. The proposed cross-layer AMC design is also incorporated with STBC to further improve the system performance. Numerical results have demonstrated that retransmission at the data link layer alleviate rigorous error-control requirements at the physical layer and thereby allows higher data rate transmission. It is shown that this cross-layer design has achieved considerable spectral efficiency gain in comparison with traditional AMC without cross-layer design. Note that increasing the maximum number of retransmissions beyond one has little effect on spectrum efficiency and will cause longer system delay. This suggests that a desirable trade-off between spectral efficiency and delay in the STBC MIMO–OFDM system is achieved by a small number of retransmission.     

OFDM通信中超前多径抑制的交叉频率分组法

正交频分复用(OFDM)技术具有频带利用充分、传输数据率高的优点,通常利用加入保护时间来抑制多径干扰。但是用于浅海远程水声通信时,由于信道恶劣,可能存在多条能量较大且在时间上超前主路径的多径干扰,此时保护时间的作用将被破坏,导致ISI和ICI增大,误码率显著增加。为了解决这一问题,该文针对水声环境和OFDM的特点,提出一种新的OFDM交叉频率分组(CrossFrequencyGrouping,简写为CFG)方法。该方法改变了传统OFDM符号的数据格式,相邻符号采用不同组合的子载波频率来抑制超前多径干扰。仿真结果表明,采用CFG方法的OFDM系统,可显著提高对超前多径干扰的抑制能力,有效提高水声通信系统的可靠性,减小误码率。