SER performance evaluation and optimization of OFDM system with residual frequency and timing offsets from imperfect synchronization

This paper deals with the effects of residual timing and frequency offsets on the symbol error rate (SER) performance of an orthogonal frequency division multiplexing (OFDM) system. The synchronization of an OFDM system generally consists of a coarse frequency and timing acquisition stage and a refine stage. Due to the presence of Gaussian noise, channel distortions and implementation losses of synchronization and equalization algorithms, residual frequency and timing offsets always exist for an OFDM receiver. The residual frequency and timing offsets are proven to be Gaussian distributed, with their corresponding variances determined. The reception process of an OFDM signal with frequency and timing offsets is analyzed. A closed-form analytical result on the SER of an OFDM system with residual synchronization errors is derived. Computer simulations and analyses show that the frequency and timing offsets affect the OFDM subcarriers differently. With this observation, a new technique is proposed to minimize the SER of the OFDM systems by adjusting the distribution of transmission power among the subcarriers.     

基于前导字的WLAN OFDM系统载波频偏估计

载波同步是正交频分复用(OFDM)系统的一项关键技术.利用帧前导字中的重复性训练序列进行频偏估计是一种简便有效的处理方法.本文根据Morelli和Mengali[3]算法,提出适用于无线局域网系统IEEE802.11a OFDM[1]的载波频偏估计算法和性能界.在此基础上,提出将短和长训练序列的估计结果综合考虑的两种估计算法,进一步改善估计性能.     

OFDM系统时频同步的ML算法研究

OFDM系统被广泛应用于新一代无线通信网络,由于本身的抗码间干扰能力,使其设计的复杂度减低,但一般OFDM系统采用了绝对相位调制,其对时频同步有较高的要求。论文研究了最大似然估计(ML)算法如何应用于OFDM系统,并进行了MATLAB仿真分析,验证了ML算法在时频同步上的良好性能。     

OFDM系统中一种基于PN序列的频率同步算法及FPGA设计

OFDM（正交频分复用）以它在抗多径和窄带干扰方面的良好性能以及高频谱利用率得到广泛应用，但是OFDM对频率偏移非常敏感，所以OFDM系统中的频率同步十分重要。介绍OFDM系统中一种基于PN序列的频率同步的FPGA设计方法。     

基于分数阶傅里叶变换的正交频分复用系统同步分析

在基于分数阶傅里叶变换的正交频分复用系统中,分析并讨论了载波频率误差、采样频率误差以及时间同步误差对系统性能的影响,并与传统的基于普通傅里叶变换的正交频分复用系统的同步性能进行了比较.仿真结果表明:当系统参数选择适当时,基于分数阶傅里叶变换的正交频分复用系统在载波频率误差方面具有较好的同步性能.     

多载波CDMA系统仿真与性能分析

多载波CDMA是OFDM和CDMA结合的综合技术,兼有OFDM和CDMA的优点,可以在无线信道中传送高速数据流,在频谱效率、频率分集、抗多径干扰等方面都能获得较大好处。首先介绍了OFDM的基本原理,给出MC-CDMA的系统模型,然后通过仿真,比较了OFDM系统和MC-CDMA系统的误码性能。结果表明,MC-CDMA系统性能优于OFDM系统,说明了OFDM和CDMA技术结合的必要性。     

ZP-CI/OFDM: A Power Efficient Wireless Transmission Technology

Low power efficiency is a deficiency in traditional Orthogonal Frequency Division Multiplexing (OFDM) systems. To counter this problem, a new wireless transmission technology based on Zero-Padding Carrier Interferometry OFDM (ZP-CI/OFDM) is proposed. In a ZP-CI/OFDM system, transmission symbols are spread to all OFDM subcarriers via carrier interferometry codes. This reduces the Peak-to-Average Power Ratio (PAPR) that traditional OFDM suffers and also exploits frequency diversity gain. By zero-padding at the transmitter, advanced receiver technologies can be adopted for ZP-CI/OFDM so that frequency diversity gain can be further utilized and the power efficiency of the system is improved.     

一种基于Kalman滤波的OFDM系统频偏估计方法

文章基于正交频分复用（OFDM）系统对频偏的敏感性特点,介绍和分析了在OFDM接收机中的一种快速采样和精确跟踪技术。这种技术基于Kalman滤波算法和它快速的收敛速度,通过改变OFDM系统和接收端的设计参数来研究载波频率及估计器在接收端错误率中的影响。通过研究将得到一个最佳的频率偏移估计。该方法应用良好,有快速收敛能力,能在极短信号间歇中获取频偏,并拥有良好的跟踪能力。     

一种适用于快衰落信道的ICI抑制方案

针对高速移动环境下多普勒频偏造成信道的快衰落和正交频分复用(OFDM)系统中子载波间干扰(ICI)的问题,提出了一种适合快衰落环境的OFDM系统子载波间干扰抑制算法。此算法用线性变化模型来近似一个OFDM符号周期内的信道冲激响应,并以此为基础采用迭代MMSE均衡方法抑制载波间干扰。分析和仿真结果表明,此方法能有效地保证载波间的正交性,从而改善了OFDM系统的误码率(BER)性能。     

SINR analysis of FFH/OFDM over frequency selective Rayleigh fading channel

Fast frequency hopping/orthogonal frequency division multiplexing (FFH/OFDM) has been previously proposed to achieve frequency diversity over frequency selective channels. However, the performances of the FFH/OFDM scheme have been usually calculated using simulations over empirical channel models in earlier works. The main aim of this paper is to theoretically expose the diversity of the FFH/OFDM signal over statistical models, specifically Rayleigh distributed, for frequency selective fading channels. In order to achieve this aim, we first derive the decision variables and the signal‐to‐interference noise ratios of the FFH/OFDM signal at the output of minimum mean square error and zero forcing receivers. For various levels of quadrature amplitude modulation used in FFH/OFDM, the performances are calculated and validated via simulations. The results show that FFH/OFDM in case of zero forcing equalization does not provide any diversity gain even over a frequency selective channel. However, in case of minimum mean square error equalization, it brings significant diversity gain at high signal‐to‐noise ratio (SNR) values with the rise of number of sub‐carriers. Moreover, the FFH/OFDM with four quadrature amplitude modulation scheme provides better performance than that of the conventional OFDM even at low SNR values, as well as a significant diversity gain at high SNR values. Copyright © 2016 John Wiley & Sons, Ltd.     

高速移动环境下OFDM系统载波同步的混合结构算法

在分析载波频偏OFDM系统信号模型基础上，提出了基于前导符号循环前缀相关和前导符号相关的混合结构载波同步的算法，将基于循环前缀相关的方法和前导符号相关的方法融合，从链路级研究了易于实现的载波同步算法，在高速无线多径Rayleigh信道下，车速120km／h时，比较了两种不同的载波同步算法仿真结果。     

Fast frequency hopping OFDM concept

A system concept, combining orthogonal frequency division multiplexing (OFDM) with fast frequency hopping (FFH), termed FFH/OFDM concept, is illustrated. Data symbols in the frequency domain are mapped to time domain samples by a unitary transformation based on a shuffled version of the well-known inverse discrete Fourier transform. The FFH/OFDM concept shows a promising performance.     

Selected mapping with symbol re‐mapping for OFDM/TDM using MMSE‐FDE

Orthogonal frequency division multiplexing (OFDM) signals have a problem with a high peak‐to‐average power ratio (PAPR). A distortionless selected mapping (SLM) has been proposed to reduce the PAPR, but a high computational complexity prohibits its application to an OFDM system with a large number of subcarriers. Recently, we proposed OFDM combined with time division multiplexing (OFDM/TDM) using minimum mean square error frequency‐domain equalization (MMSE‐FDE) to improve the bit error rate (BER) performance of conventional OFDM with a lower PAPR. The PAPR problem, however, cannot be completely eliminated. In this paper, we present an SLM combined with symbol re‐mapping for OFDM/TDM using MMSE‐FDE. Unlike the conventional OFDM, where SLM is applied over subcarriers in the frequency domain, we exploit both time and frequency dimensions of OFDM/TDM signal to improve the performance with respect to PAPR and BER. A mathematical model for PAPR distribution of OFDM/TDM with SLM is presented to complement the computer simulation results. It is shown that proposed SLM can further reduce the PAPR without sacrificing the BER performance with the same or reduced computational complexity. Copyright © 2010 John Wiley & Sons, Ltd.     

OFDM系统中信道编码对系统性能的影响

正交频分复用（OFDM）技术是下一代移动通信的核心技术之一。其最突出的优点是频谱利用率高和抗多径时延能力强,适合未来高速宽带无线通信的应用。OFDM技术通过将频率选择性多径衰落信道在频域内转换为平坦信道,减小了多径衰落的影响。根据OFDM基本原理特构建了一个OFDM通信系统仿真模型,分析了在不同调制方式、不同信道下信道编码对系统性能的影响。仿真结果表明,采用信道编码可有效的降低OFDM系统的误码率。     

OFDM信号频率偏移盲估计算法

该算法通过联合多个正交频分复用（OFDM,Orthogonal Frequency Division Multiplexing）符号对频率偏移进行盲估计。该算法不需要数据辅助,利用接收信号循环前缀的相关性,通过计算相同子信道所传数据的相位差,来实现信号的小数倍频偏估计。利用相邻OFDM符号中频域导频的延迟相关运算来实现整数倍频偏估计。仿真结果表明该算法估计精度高,适合实际信号的应用。     

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.     

OFDM系统仿真与性能分析

正交频分复用（OFDM）是一种特殊的多载波方案,它既是一种调制技术,也是一种复用技术。由于其具有良好的抗频率选择性衰落或窄带干扰、频谱利用率高和成本低的优点,应用越来越广泛。采用Simulink搭建的OFDM仿真系统,既能有效测试OFDM的性能指标,也能研究和理解OFDM特点和具体应用。     

OFDM系统中同步问题的研究

OFDM系统的实现还存在着一些方面问题,如：同步问题尤其是频偏,峰值功率与平均功率比值过高,器件的非线性化等。OFDM系统对同步偏差敏感是本文讨论的重点,尤其是频偏。因此怎样获得准确的符号定时和载波频偏估计对OFDM系统性能至关重要。本文围绕OFDM同步过程中的符号定时和载波频率偏差问题进行描述,在分别介绍同步过程中捕获和跟踪两个阶段各自一些典型算法基础上,对一种基于保护间隔／循环前缀（PI／CP）的联合实现符号定时和载波同步最大似然估计（MLE）算法进行叙述并仿真。     

Low Complexity Discrete Hartley Transform Precoded OFDM System over Frequency‐Selective Fading Channel

Orthogonal frequency‐division multiplexing (OFDM) suffers from spectral nulls of frequency‐selective fading channels. Linear precoded (LP‐) OFDM is an effective method that guarantees symbol detectability by spreading the frequency‐domain symbols over the whole spectrum. This paper proposes a computationally efficient and low‐cost implementation for discrete Hartley transform (DHT) precoded OFDM systems. Compared to conventional DHT‐OFDM systems, at the transmitter, both the DHT and the inverse discrete Fourier transform are replaced by a one‐level butterfly structure that involves only one addition per symbol to generate the time‐domain DHT‐OFDM signal. At the receiver, only the DHT is required to recover the distorted signal with a single‐tap equalizer in contrast to both the DHT and the DFT in the conventional DHT‐OFDM. Theoretical analysis of DHT‐OFDM with linear equalizers is presented and confirmed by numerical simulation. It is shown that the proposed DHT‐OFDM system achieves similar performance when compared to other LP‐OFDMs but exhibits a lower implementation complexity and peak‐to‐average power ratio.     

无线局域网中OFDM技术

首先介绍了正交多载波调制(OFDM)的基本原理，并分析OFDM基于FFT算法在基带实现多载波调制的原理，着重讨论了其定时同步最大似然估计(MLE)算法，最后对OFDM在无线局域网的应用作了介绍。