Joint low‐complexity equalization and CFO estimation and compensation for UWA‐OFDM communication systems

Frequency synchronization has a great importance in preserving the performance of the underwater acoustic (UWA) orthogonal frequency division multiplexing (OFDM) systems. The carrier frequency offset (CFO) estimation can be blind or data‐aided. In this paper, the Zadoff‐Chu (ZC) sequences are used for OFDM synchronization in UWA communications, and they are compared with different data‐aided algorithms. We propose a low‐complexity algorithm for CFO estimation based on ZC sequences. Also, a joint equalization and CFO compensation scheme for UWA‐OFDM communication systems is presented. Simulation results demonstrate that the proposed CFO estimation algorithm allows estimation of the CFO accurately with a simple implementation in comparison with the traditional schemes. Also, the performance of the UWA‐OFDM system can be preserved in the presence of frequency offsets.     

基于LDPC码的OFDM系统性能分析

正交频分复用(OFDM)是一种多载波调制技术,已成为实现高速数据传输的主流技术,在无线局域网、HDTV中得到广泛应用;低密度奇偶校验(LDPC)码是一种译码复杂度低的纠错码,可获得接近香农限的优异性能。本文提出了结合LDPC码的COFDM系统方案,阐述了相关原理,对其性能进行了仿真分析,比较了在不同调制方法下系统的性能差异,并将基于LDPC码和Turbo码的COFDM系统在不同信道中的误码性能进行了对比。仿真结果表明,基于LDPC码的COFDM系统在提供较高比特传输速率的同时也具有良好的纠错性能。     

正交频分复用技术在协作通信下的应用

协作分集技术利用其他用户进行协作传输,可以获得一定的分集增益,但是在频率选择性衰落信道下性能不很乐观。为了对抗频率选择性衰落,采用正交频分复用(ODFM)技术与协作分集技术相结合的方法,研究了基于OFDM技术的协作通信,对在不同调制技术、不同信道估计方法下系统的性能进行了比较,为了进一步提高系统性能,对传统S&C算法进行了改进并应用于该系统中,仿真实验结果表明,改进的定时同步算法性能优于传统的S&C算法。     

Performance of degree distribution based HARQ scheme for LDPC-coded OFDM system

This article studies the degree distribution property of low density parity check (LDPC) codes by Gaussian approximation (GA) and presents an efficient hybrid automatic repeat quest (HARQ) scheme for LDPC-coded orthogonal frequency division multiplexing (OFDM) system. In the scheme, the important bits with large degrees have high retransmission priorities and are mapped to the sub-carriers with better channel quality indicator (CQI) levels in the OFDM system. The new scheme provides more protection to the bits with large degrees and thus contributes more to the decoding process by offering more transmission power. In this way the system performance would be improved. The statistics and simulation results also prove the new scheme.     

Performance of robust symbol‐timing and carrier‐frequency estimation for OFDM systems

In recent years, many maximum likelihood (ML) blind estimators have been proposed to estimate timing and frequency offsets for orthogonal frequency division multiplexing (OFDM) systems. However, the previously proposed ML blind estimators utilizing cyclic prefix do not fully characterize the random observation vector over the entire range of the timing offset and will significantly degrade the estimation performance. In this paper, we present a global ML blind estimator to compensate the estimation error. Moreover, we extend the global ML blind estimator by accumulating the ML function of the estimation parameters to achieve a better accuracy without increasing the hardware or computational complexity. The simulation results show that the proposed algorithm can significantly improve the estimation performance in both additional white Gaussian noise and ITU‐R M.1225 multipath channels. Copyright © 2008 John Wiley & Sons, Ltd.     

一种OFDM调制解调器的FPGA实现

OFDM技术在高速数据传输中得到了广泛的应用，根据OFDM的特点，提出了一种基于IFFT/FFT的OFDM调制解调器的低成本FPGA实现方法，最后给出了FPGA上的15点FFT运算单元的计算结果。实践证明，该方法具有设计简单、快速、高效和实时性好等特点。     

基于OFDM的短波电台改进方案设计

短波电台作为远距离传输的重要载体,有着非常重要的作用。然而传统短波电台受短波信道固有缺点的限制,传输速率较低,频带利用率也不高。正交频分复用(OFDM)技术的各子载波之间的正交性使得其频谱可重叠从而提高了频带利用率,同时能有效对抗频率选择性衰落。为了有效提高短波电台的性能,可以采用OFDM技术。分析了OFDM的基本原理,提出了基于OFDM的短波电台性能增强方案。     

OFDM技术及仿真性能分析

正交频分复用（OFDM）是一种多载波宽带数字调制技术，它能有效地克服传输中的多径干扰和消除码间串扰，所以在高速数据传输中得到了广泛的应用。这里就OFDM技术进行了较为详尽的分析；OFDM的传输特性进行了基于FFT算法实现的MATLAB软件模拟及模拟结果分析。     

一种基于自适应调制的LDPC-COFDM系统性能研究

本文研究了一种基于自适应调制和软解调算法的LDPC-COFDM(LDPC Coded OFDM)系统模型，该自适应调制方案根据各个了信道的信道增益来自适应地分配比特。文中对自适应调制和软解调算法进行了讨论和分析，最后给出了仿真结果。仿真结果表明，基于自适应调制的LDPC-COFDM能够有效提高系统的误码性能，适应多径衰落信道的时变特性。     

Performance analysis of broadband satellite communication system based on OFDM/TDM

Orthogonal frequency division multiplexing （OFDM） is attracting more attention for its capability of high speed transmission. However, the OFDM possesses an obvious shortage in its high ratio of the peak power to the average power （PAPR）, which has become the main issue holding it back to be applied to the broadband satellite communication system. OFDM combined with time division multiplexing （TDM）, dividing the subcarriers of OFDM into some blocks in time tune, can decrease the high PAPR of OFDM. Meanwhile, the advantages of OFDM can be preserved. In this paper, OFDM/TDM is applied to the broadband satellite communication system. This paper theoretically analyses OFDM/TDM system model as well as its restraining effect on PAPR, and proposes frequency domain multiplexing-pilot （FDM-Pilot） channel estimation algorithm. Simulation results show OFDM/TDM in broadband satellite communication system has approving performance and decreased the PAPR.     

Equalization and blind CFO estimation for performance enhancement of OFDM communication systems using discrete cosine transform

In wireless communication systems, equalization is one of the most important schemes to improve the system performance. This paper consists of two main parts. The first part presents a blind carrier frequency offset (CFO) estimation scheme based on discrete cosine transform (DCT). The second part presents a joint low‐complexity equalization, and CFO compensation in orthogonal frequency division multiplexing (OFDM) systems. Moreover, in the second part, we present a joint low‐complexity regularized zero‐forcing (JLRZF) equalizer based on the proposed CFO estimation scheme. Simulation results show that the proposed configuration has the ability to perform blind CFO estimation and enhance the system performance in the presence of estimation errors.     

基于自适应OFDM的室内可见光通信系统性能研究

为了提高室内可见光通信(VLC)系统性能并充分利用信道带宽资源,将OFDM与自适应技术结合应用于VLC系统中.讨论了系统反馈途径的实现方法,在室内空间环境建模的基础上,对自适应OFDM通信系统的吞吐量和误码率进行了Matlab仿真和分析.     

光正交频分复用(OFDM)光纤通信系统综述

最近的一些研究表明,OFDM是一种很有希望在光纤通信领域应用的技术.本文回顾了光正交频分复用(OFDM)技术进展,介绍了其基本原理:简述了多模和单模光纤OFDM通信系统技术的发展概况,综述了光OFDM-PON在接入网中的应用情况,最后指出光OFDM系统的优缺点并展望了其发展前景.     

OFDM调制的FPGA实现

提出了一种切实可行的OFDM调制方式的FPGA实现方法与结构，并介绍了其中IFFT运算与保护间隙加入的具体电路。     

OFDM移动通信信道估计与仿真

正交频分复用技术（OFDM）可以在条件恶劣信道环境中进行高速数据传播。OFDM是一种多载波传输系统．相对于单载波传输系统具有很多有利条件。移动信号在无线信道中传输时会产生多径时延和多普勒频移,为正确接收信号需要对信遘作出估计并进行补偿。本文重点分析了信道分析与补偿的方法,对信号时域和频域的信道补偿进行详细论述,构建系统进行仿真分析,仿真结果符合理论分析。     

SLM‐based data position permutation method for PAPR reduction in OFDM systems

The high peak‐to‐average power ratio (PAPR) in orthogonal frequency division multiplexing (OFDM) systems not only increases the complexity of the analog‐to‐digital (A/D) and digital‐to‐analog (D/A) converters but also reduces the efficiency of the radio frequency (RF) power amplifier. In this paper, we present a data position permutation (DPP) method, which is based on a selected mapping (SLM) scheme, for reducing the PAPR in OFDM systems. The candidate signal on each branch of the SLM scheme is generated by permuting the position and rotating the phase of the original data. In addition, a modified DPP method with lower bit error rate (BER) is proposed. The simulation results show that the proposed method provides better performance with regard to complexity, spectrum efficiency, and BER as compared to that of the SLM‐based dummy sequence insertion (SLM‐DSI) method. Copyright © 2008 John Wiley & Sons, Ltd.     

基于FPGA的OFDM系统采样钟同步

利用频域离散导频估计采样钟频偏,采用Farrow结构的数字内插滤波器完成采样钟频偏补偿,并且在充分考虑硬件可实现性下作了简化,从而实现了基于FPGA的全数字域采样钟同步模块,基于实测数据的分析表明,该模块不仅有稳定的性能,而且其模块化设计也有利于更高阶算法或多通道系统的实现。     

基于双环反馈的OFDM系统数字AGC设计算法

针对正交频分复用系统中接收信号因受传输环境的大、小尺度衰落、多普勒衰落影响,使接收信号均值变化较大而导致ADC过载和解调器溢出的问题,提出了一种基于双环设计理念的数字AGC设计算法;该算法通过在解调器的设计中,引入前、后环反馈的双环AGC,使接收系统解调时动态地调节信号均值,防止均值发生过大跳变;实验仿真结果显示,接收信号时,该方法可以降低系统的均方差,有效地防止ADC过载和OFDM解调器溢出;当信号经历信道为动态信道,调节效果更佳。     

基于802.11a的OFDM系统基带处理器的FPGA实现

探讨基于802．11a的OFDM系统的硬件构架，给出了适应于OFDM系统的并行存储的高速FFT处理器电路结构，以及实现导频插入、循环前缀的硬件结构。经FPCA验证，在系统时钟频率为20MHz时，64点FFT计算时间为2．55us。     

Design and implementation of channel estimation for low-voltage power line communication systems based on OFDM

An optimized channel estimation algorithm based on a time-spread structure in OFDM low-voltage power line communication (PLC) systems is proposed to achieve a lower bit error rate (BER). This paper optimizes the best maximum multi-path delay of the linear minimum mean square error (LMMSE) algorithm in time-domain spread OFDM systems. Simulation results indicate that the BER of the improved method is lower than that of conventional LMMSE algorithm, especially when the signal-to-noise ratio (SNR) is lower than 0 dB. Both the LMMSE algorithm and the proposed algorithm are implemented and fabricated in CSMC 0.18 μm technology. This paper analyzes and compares the hardware complexity and performance of the two algorithms. Measurements indicate that the proposed channel estimator has better performance than the conventional estimator.