一种适用于宽带无线移动通信系统下行链路的TDS-OFDM发送方法

本文将主要介绍宽带无线城域网系统(BRadio)中下行链路的TDS-OFDM(时域同步正交频分复用)发送方法.TDS-OFDM是针对宽带无线传输信道设计的一种多载波调制方式,既适用于具有多径干扰和多普勒频移的地面数字电视广播传输信道,也适用于其他宽带数字传输系统.TDS-OFDM技术已经在地面数字多媒体/电视广播(DMB-T)传输系统中得到了应用,在此基础上,本文结合对已有的宽带无线移动通信系统的研究,对宽带无线移动通信系统的帧结构、OFDM符号组成、子载波分配方案进行了改进,使TDS-OFDM能够适用于宽带无线移动通信系统,满足其需求.     

清华地面数字多媒体/电视广播（DMB-5）传输系统

本介绍一种新型的地面数字多媒体电视广播（Terrestrial Digital Multimedia/TV Broadcasiting,DMB—T）传输系统。经过仔细分析地面数字电视广播的应用需求和传输条件后我们所提出的DMB—T传输系统采用了一种称为时域同步正交频分复用（Time Domain Synchronous Orthogonal Frequency Division Multiplex,TDS-OFDM）的创新的多载波调制方式。这种调制方式是针对地面数字多媒体电视广播传输信道的线性时变的宽带传输信道特性（频域选择性与时域选择性同时存在的传输信道）所设计的。     

地面数字电视广播系统的研究

本文主要介绍了地面数字电视广播（Digital Television Terrestrial Broadcasting,DTTB）系统,在介绍国外现有的DTIB传输标准的基础上说明清华大学数字电视传输技术研发中心提出的DMB—T（Terrestrial Digital Multi—media／Television Broadcasting）标准。DMB—T系统采用了时域同步正交频分复用（TDS-OFDM）的多载波调制方式,适用于具有多径干扰和多普勒频移的地面数字电视广播传输信道。通过比较,DMB—T具有新的关键技术和优越的性能。     

一种VSAT数据通信网络报文传输协议的分析

本文描述并分析了一种适用于多信道ＶＳＡＴ卫星数据通信网的报文传输协议，特别是考虑了无线信道误码率对传输协议的影响，分析表明利用Ｓ－ＲＥ－ＡＬＯＨＡ作为预订信道的多址方式及按报文占用数据信道方法可以减小报文传输延时并增加系统吞吐量。     

移动卫星信道中的视频传输

为向远程便携通信提供低速率的视频服务，提出了一种基于卫星信道的级联编码系统，该系统综合运用了实时信道估计、软判决译码、TCM、深度交织和纠错编码等技术，它不仅可用于语音通信，也适用于MPEG－4标准的图像传输。     

COFDM系统的信道估计算法

本文通过对欧洲数字视频地面广播调制方案的分析,结合计算机仿真和工程应用,深入研究了基于DVB－T标准、适用于编码正交频分复用（COFDM）系统的信道估计算法,Matlab仿真及FPGA电路实现表明该算法具有可靠性高、复杂度低及易于实现的良好性能。     

地面数字多媒体电视传输系统的设计与实现

本文介绍了清华大学提出的地面数字多媒体/电视广播传输标准(DMB-T)方案.该传输方案的调制部分采用了自主创新的时域同步正交频分复用(简称TDS-OFDM)的多载波调制方式,适用于具有多径干扰和多普勒频移的地面数字电视广播传输信道.整个DMB-T接收系统集成在“中视一号“专用芯片上,已经批量生产;在额定8MHz带宽的广播频道中,净载数据传输率在4.813-32.486Mbps范围内可选,实现了单天线移动条件下,高清晰度数字电视的广播与接收.     

移动衰落信道中的自适应OFDM调制

在宽带移动OFDM(正交频分复用)系统中，不同的子信道经受不同的信道衰落，具有不同的传输能力。若采用固定速率调制方案，信道容量和发送功率未能获得充分应用。自适应调制技术能充分利用信道容量和信号功率，满足不同传输速率和服务质量的要求。研究了一种适用于宽带移动OFDM系统的自适应算法。在平均发送功率不变务件下，算法根据子信道的衰落特性，自适应地分配子信道中数据比特，选择不同的信号星座和发送功率，使得系统的功率和频谱效率达到最佳。首先推导了在平均发送功率受限条件下，瑞利衰落信道中最大频谱效率；其次，分析了在给定误比特率(BER)条件下，采用连续功率、连续星座MQAM(多进制正交幅度调制信号)调制的最大功率频谱效率，在此基础上提出适用于实际系统的离散功率、离散星座MQAM调制算法；最后进一步优化系统的功率密度谱。计算机模拟表明：该算法能明显地提高移动OFDM系统的频谱效率。     

基于分段混合调制的导频辅助块传输技术研究

在导频与数据时分复接的块传输系统中,无保护间隔系统具有最佳的数据传输效率,但工作在多径信道下时,该系统接收信号中导频和数据边缘混叠,接收机需要额外的干扰消除操作以精确分离二者。实际干扰消除算法都会存在一定误差,这种误差会降低信道估计算法的精度,进一步影响整个数据块的解调及解码,最后导致系统误码性能下降。该文提出一种分段混合调制方式：数据块中,可能与导频发生混叠的前后两端数据符号采用低阶调制;而中部依然采用高阶调制。仿真结果表明,在典型大延时多径信道下,分段混合调制系统误码性能明显优于传统无保护间隔的时分导频辅助块传输系统。     

关于编码正交频分复用中同步的研究

编码正交频分复用(COFDM)是一种多载波数字通信调制技术,其基本原理是将频域中的一个宽带信道划分成多个重叠的子信道进行窄带传输.在接收端,虽然频谱相互重叠,但是只要保证各子信道上信号的正交性,就可以将各信道上的信号正确分离.文章重点研究了COFDM 通信系统中的同步技术.     

TDS-OFDM系统的设计

介绍了TDS-OFDM系统的调制编码方式和接收端实现技术.与欧洲DVB-T循环前缀OFDM的性能对比中,TDS-OFDM在地面电视广播等领域中表现出显著的优势.     

Effects of nonlinear distortion and imperfect parameters estimation on the performance of OFDM-based DTTB systems

As an effective technique for combating multipath fading and for high-bit-rate transmission over wireless channels, orthogonal frequency-division multiplexing (OFDM) is extensively used in modern digital television terrestrial broadcasting (DTTB) systems, such as the cyclic prefix OFDM (CP-OFDM) based DVB-T DTTB systems and the time domain synchronous OFDM (TDS-OFDM) based DMB-T DTTB systems, to support high performance bandwidth-efficient multimedia services. OFDM-based DTTB systems are very sensitive to the nonlinear distortion at the transmitter and the imperfect parameters estimation at the receiver. In this paper, the combined effects of nonlinear distortion and imperfect parameters estimation on the performance of the OFDM-based DTTB systems are investigated, a closed form of symbol error probability of the OFDM-based DTTB systems with nonlinear distortion and imperfect parameters estimation is derived. Theoretical results show closely matching with those obtained by simulations for both the CP-OFDM based DVB-T DTTB systems and the TDS-OFDM based DMB-T DTTB systems.     

A combined code acquisition and symbol timing recovery method for TDS-OFDM

A novel timing recovery method which combines code acquisition and symbol timing recovery for TDS-OFDM (time domain synchronous OFDM) is developed. The method is based on the searching and tracking of the correlation peaks of the PN sequences embedded in the signals. Simulations show that this algorithm has very good performance for both AWGN and multipath channels, even when a small frequency offset exists in the sampling clock of the receiver.     

DMB-T传输系统中扩频码快速捕获研究

讨论了DMB-T传输系统中基于匹配滤波器的扩频码快速捕获算法,针对传统匹配滤波器占用硬件资源大的缺点,提出了一种新的多停顿捕获方案.新方案在匹配滤波器结构和算法上进行了优化,在原有硬件规模的基础上扩充了系统的捕获验证功能,降低了虚警概率,缩短了捕获时间.结合同步捕获系统框图分析了各个模块的功能和实现方法,并通过Modelsim仿真和FPGA硬件平台测试,验证了方案的可行性和先进性.     

A Simplified Equalization Method for Dual PN-Sequence Padding TDS-OFDM Systems

Time domain synchronous orthogonal frequency division multiplexing (TDS-OFDM) modulation scheme can improve the spectrum efficiency over the traditional Cyclic Prefix (CP) OFDM technology at the cost of computational efforts due to the iterative padding subtraction (IPS) at the receiver to mitigate the inter symbol interference caused by the multipath propagation. In this paper, a simplified equalization scheme is presented for the dual PN-sequence padding (DPNP) TDS-OFDM systems. Although the length of the padded PN sequence needed is double compared to what needed by the IPS, leading to slight decrease of the spectrum efficiency, this method can save about 90% computational effort in practice compared with that of the IPS algorithm. It also outperforms IPS method under the time-varying fading channel in terms of symbol error rate and maximum Doppler frequency, verified by the computer simulation.      

基于PN序列的符号同步算法研究

针对TDS-OFDM系统的工作特点,为了降低频偏估计的误差,采用PN序列作为训练序列来进行信道估计,并提出了双滑动窗口函数进行符号同步。应用MATLAB进行仿真实验,仿真结果证实了提出的方法能较好地提高频偏估计的精度,当采用OFDM符号前后两个前缀序列来进行频偏估计时,可以将频偏的估计精度控制在10-3以下。     

Progressive image transmission over differentially space‐time coded OFDM systems

In this paper, we consider progressive image transmission over differentially space‐time coded orthogonal frequency‐division multiplexing (OFDM) systems and treat the problem as one of optimal joint source‐channel coding (JSCC) in the form of unequal error protection (UEP), as necessitated by embedded source coding (e.g., SPIHT and JPEG 2000). We adopt a product channel code structure that is proven to provide powerful error protection and employ low‐complexity decision‐feedback decoding for differentially space‐time coded OFDM without assuming channel state information. For a given SNR, the BER performance of the differentially space‐time coded OFDM system is treated as the channel condition in the JSCC/UEP design via a fast product code optimization algorithm so that the end‐to‐end quality of reconstructed images is optimized in the average minimum MSE sense. Extensive image transmission experiments show that SNR/BER improvements can be translated into quality gains in reconstructed images. Moreover, compared to another non‐coherent detection algorithm, i.e., the iterative receiver based on expectation‐maximization algorithm for the space‐time coded OFDM systems, differentially space‐time coded OFDM systems suffer some quality loss in reconstructed images. With the efficiency and simplicity of decision‐feedback differential decoding, differentially space‐time coded OFDM is thus a feasible modulation scheme for applications such as wireless image over mobile devices (e.g., cell phones). Copyright © 2006 John Wiley & Sons, Ltd.     

A Novel Method of Frequency-Offset Estimation Using Time Domain PN Sequences in OFDM Systems

In some OFDM systems, time domain cyclic pseudo-random (PN) sequences are inserted between OFDM symbols. This paper proposes a frequency-offset estimation method utilizing the good correlation property of such PN sequences. By combining three levels of estimations, which have different precisions and ranges, the proposed scheme can achieve accurate frequency estimation under different channel environment. Simulation results show that this approach performs well in both AWGN channels and multipath channels.     

Cutoff rate and outage probability performance comparisons between DVB-T and DMB-T systems under mobile multipath channels

As an effective technique for combating multipath fading and for high-bit-rate transmission over wireless channels, orthogonal frequency-division multiplexing (OFDM) is extensively used in modern terrestrial digital television broadcasting systems to support high performance bandwidth-efficient multimedia services. Cutoff rate and outage probability are two important criteria to evaluate the performance of a practical communication system in radio engineering. In this paper, the cutoff rate and outage probability performance are compared between two important terrestrial digital television broadcasting systems, the cyclic prefix OFDM based DVB-T system and the time domain synchronous OFDM based DMB-T system, under different mobile multipath channel conditions. The DVB-T system and the DMB-T system are summarily introduced. The cutoff rate and outage probability expressions are developed, and simulation results are given for both the DVB-T system and the DMB-T system.