MIMO OFDM无线局域网的性能分析及其实现

叙述了无线局域网的多输入多输出天线系统的技术特点，分析了OFDM无线局域网中的多输入多输出天线系统容量，探讨了多输入多输出OFDM无线局域网的实现。     

OFDM技术应用现状分析

以其频谱利用率高、有效抗多径干扰等优势成为第四代移动通信系统关键技术的OFDM技术,目前已得到广泛研究与发展。通过对OFDM技术的基本原理及特性进行阐述,分析了数字广播系统、蜂窝移动通信系统、电力线载波通信系统、光纤通信系统等领域OFDM技术的实际应用情况,能够有效为未来OFDM技术在通信领域的应用发展提供参考借鉴。     

探讨OFDM系统中PAPR问题的解决

对目前在无线领域最被看好的OFDM技术进行了介绍，归纳了OFDM技术的优缺点，着重探讨了OFDM系统实现中PAPR减小问题的解决方案。     

OFDM系统的抗多径性能的仿真研究

OFDM正交多载波传输技术具有抗无线传输多径效应影响的能力,正逐步成为各种宽带无线数字传输系统的首先技术.本文介绍了OFDM系统的基本组成原理,给出了OFDM仿真模型,对系统的各个环节用MATLAB语言进行编程,并得出OFDM系统抗多径性能的仿真结果.     

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。     

采用不同STBC分集模式的OFDM系统性能分析

由于空时编码技术和OFDM技术各自的特点，两者能很好地结合在一起，而影响空时编码获得最大分集增益的主要因素是系统的发射、接收天线数目和多径数目。该文详细分析了采用空时分组编码的OFDM系统的模型，通过改变发射分集和接收分集来仿真不同传输环境下OFDM系统的性能，并对不同分集模式性能进行对比分析，仿真结果表明，多普勒频移对系统性能有较大影响，增加发射天线个数能有效克服系统错误平台效应，接收端采用分集技术比发射端增加天线个数能更有效提高系统性能。在进行空时分组编码设计时，可以首先考虑增加接收天线数目以改善系统性能。     

多径瑞利衰落信道下OFDM系统仿真

正交频分复用(OFDM)技术是下一代移动通信的核心技术之一.重点研究了多径瑞利衰落信道下最大多径时延对基于OFDM技术的通信系统性能的影响.根据OFDM基本原理构建了一个OFDM无线通信基带系统仿真模型,信道带宽为15MHz.调制方式为16-QAM,编码方式采用码率为1/2的卷积,模型采用了60个子载波,并插入了循环前缀.分析了系统抗多径干扰的性能,比较了不同信道下系统的误码率,以及不同的保护间隔长度下系统的误码率.表明在瑞利衰落信道下OFDM技术具有良好的抗多径干扰的性能.     

OFDM系统抗多径衰落性能及仿真分析

OFDM技术在现代通信技术中的应用越来越广泛,因为该系统具备了多种有利于信号高效传输的优点,特别是其抗多径效应特别突出.作为OFDM系统的显著特点之一,抗多径效应使得系统传输信号的误码率大大降低.为了验证其抗衰落性能,文中分析了多径衰落信道接收信号的特点,建立了多径传播的仿真模型,利用改进的IDFT和DFT的方法,分别在两径及六径瑞利衰落信道进行传输,通过对有无OFDM系统时误差的仿真,说明了OFDM系统具有较强抗多径衰落性能的结论,并提出了一些改善系统性能的方法.     

OFDM系统降低峰值平均功率比研究

正交频分复用（OFDM）是一种利用相互正交多个子载波传输信息的方案,具有较好的频谱利用率和抗多径干扰能力,但是OFDM系统具有较高的峰值平均功率比（PAPR）,造成应用成本加大和运行效率降低。本文分析了了OFDM实现原理,讨论了如何降低峰均比的问题,横向对比分析了几种OFDM系统中降低峰均比的技术。最后完成仿真实验进行效果验证,进一步讨论了不同技术中不同参数设置对仿真结果的影响。     

Error rate performance of Hybrid QAM-FSK in OFDM systems exhibiting low PAPR

Multicarrier transmission systems like orthogonal frequency division multiplexing (OFDM) support high data rate and generally require no equalization at the receiver, making them simple and efficient. This paper studies the design and performance analysis of a hybrid modulation system derived from multifrequency and MQAM signals, employed in OFDM. This modulation scheme has better bit error rate (BER) performance and exhibits low PAPR. The proposed hybrid modulator reduces PAPR while keeping the OFDM transc...     

A new method of speech transmission over space time block coded co-operative MIMO–OFDM networks using time and space diversity

Reliable and good quality of service for speech transmission over wireless network has been a major challenge for the communication engineers and researchers. In this paper a new technique of speech compression and transmission using different Daubechies wavelets in a space time block coded co-corporative MIMO–OFDM networks using time and space diversity has been proposed. The main focus has been laid on design and development of wavelet based compression of multimedia signals for cooperative MIMO–OFDM system. We tried to find out various major issues regarding the wavelet compression of a speech signal. These issues include choice of a wavelet, decomposition level and thresholding criteria suitable for speech compression and transmission in co-operative MIMO–OFDM systems. A wavelet based speech compression technique using hard and soft thresholding algorithm has been proposed. The work shows that wavelet compression with QPSK modulation is a promising compression technique in a cooperative MIMO–OFDM system which makes use of the elegant theory of wavelets. The performance has been evaluated using mean square error, peak signal to noise ratio, compression ratio, bit error rate, and retained signal energy. It has been found that the transmitted speech signal is retrieved well under noisy conditions at higher order Daubechies wavelets. From the results it is clear that proposed technique aims at a radio access technology that can provide service performance comparable to that of current fixed Line accesses. To evaluate the performance of the proposed method, various performance parameters have been compared with previously implemented techniques and it has been found that the proposed work shows better performance as compared to the already existing techniques.     

利用自适应调制进行OFDM系统PAPR抑制

根据相同的信息比特采用不同调制样式时所得的正交频分复用（OFDM）信号的峰均功率比（PAPR）不同,提出了一种利用自适应调制进行OFDM系统PAPR抑制的算法,并设计了利用该算法的OFDM系统。算法通过设定一个可选调制样式集合,改变发端信息比特的调制样式,选择具有最小PAPR值的OFDM信号作为传输的OFDM信号,从而使整个系统的PAPR得到抑制。在接收端,通过对OFDM符号的调制样式进行识别,完成对调制信息解调。由于算法不需要传输需严格保护的边带信息,系统具有较强的可靠性和抗干扰能力。仿真结果表明,利用自适应调制可有效地进行OFDM系统PAPR抑制;同已有算法相比,在多径信道下,利用该算法的OFDM系统的误码率性能总体上较好。     

OFDM测距误差分析——第一部分

OFDM作为一种有效和便捷的数字调制传输方式已经可以被广泛应用在宽带数字传输系统中.由于OFDM接收系统需要具备很高的频率和定时精度,因此可以构建基于测量OFDM信号传播时间的测距定位系统.重点建立了用于精确OFDM广播测距的信号模型,并分析影响测距精度的各种因素和推导给出了测距误差下界.第一部分在给出OFDM帧同步测距原理和基本OFDM系统描述基础上,建立了系统测距时间模型,包括信号发射和接收时序、时间参考系和信号传播伪距观测方程,最后建立了用于测距的OFDM信号发射模型和信道传输模型.用于测距的OFDM信号接收模型以及测距误差分析在第二部分研究.     

OFDM测距误差分析——第二部分

OFDM作为一种有效和便捷的数字调制传输方式已经可以被广泛应用在宽带数字传输系统中.由于OFDM接收系统需要具备很高的频率和定时精度,因此可以构建基于测量OFDM信号传播时间的测距定位系统.本文在第一部分的基础上首先研究了用于测距的OFDM信号接收模型,重点分析了影响测距精度的各种因素,并推导了基于ODFM帧同步伪距观测误差的Cramer-Rao下界,总结了在进行OFDM测距时选用系统时域和频域同步方法的标准与策略.     

APSK调制在基于OFDM可见光通信系统中的性能分析

在基于正交频分复用(Orthogonal frequency division multiplexing,OFDM)的可见光通信(Visible light communication, VLC)系统中,发光二极管(Light emitting diode,LED)的非线性特性是影响系统性能的一个重要因素。为了减小LED非线性的影响,将幅度-相移联合键控(Amplitude-phase shift keying, APSK)调制与OFDM技术相结合的传输方式引入可见光通信系统中。本文介绍了APSK调制的原理及其在基于OFDM调制的可见光通信系统中的应用,建立了基于APSK-OFDM调制的可见光通信系统模型,并与常用的基于正交幅度调制（Quadrature amplitude modulation, QAM）的OFDM系统性能进行了对比。仿真结果表明,在OFDM系统中,如果信道是理想的,QAM调制的性能要比APSK调制的性能好;但当考虑到LED的非线性特性时,后者的性能要明显优于前者。     

两种LDPC译码算法在OFDM系统应用中的改进

低密度校验码是一种基于稀疏校验矩阵的线性码，此码性能接近Shannon限，并具有简单有效的迭代译码算法。大部分文献提到的LDPC迭代译码算法均基于BPSK调制，相对于MQAM／MPSK调制，BPSK调制频谱利用率太低。在未来的3G＋高速无线通信中，最广泛运用的物理层技术OFDM必然采用频谱利用率较高的多电平调制。因此。如何把LDPC码用于多电平调制下的OFDM是一个关键，将目前较为简化的基于二进制调制的两种译码算法作改进，也即扩展，使之能够应用到基于多进制调制的OFDM系统中．     

Enhancing IEEE 802.11a/n with dynamic single-user OFDM adaptation

Earlier paper have demonstrated that the achievable throughput of OFDM systems can benefit significantly from individual modulation/transmit power selection on a per sub-carrier basis according to the actual gain of individual sub-carriers (so called dynamic OFDM scheme). Usage of such an approach requires, however, providing support for additional functionalities such as: acquisition of the sub-carrier gains, signaling of the modulation types used between sender and receiver, etc. Therefore dynamic OFDM is actively pursued for future radio interfaces, rather than considered as extension of existing OFDM based standards. In this paper we introduce a proposal on how the widely accepted IEEE 802.11a/g systems as well as the emerging IEEE 802.11n system might be extended to support the dynamic OFDM in a single-user (point-to-point) setting. The presented approach guarantees backward compatibility to legacy devices. We address these issues by presenting (a) a set of protocol modifications required to incorporate dynamic OFDM in 802.11a/g/n; and (b) a performance evaluation of the suggested extension (referred to further on as single-user 802.11 DYN mode). Although 802.11n already includes advanced MAC and PHY features, i.e., frame aggregation and MIMO transmissions, our performance evaluation demonstrates that a further improvement is achievable by incorporating dynamic OFDM.     

A high performance parallel Radon based OFDM transceiver design and simulation

A major goal of the next-generation wireless communication systems is the development of a reliable high-speed wireless communication system that supports high user mobility. They must focus on increasing the link throughput and the network capacity. In this paper a novel, spectral efficient system is proposed for generating and transmitting two-dimensional (2-D) orthogonal frequency division multiplexing (OFDM) symbols through 2-D inter-symbol interference (ISI) channel. Instead of conventional data mapping techniques, discrete finite Radon transform (FRAT) is used as a data mapping technique due to the increased orthogonality offered. As a result, the proposed structure gives a significant improvement in bit error rate (BER) performance. The new structure was tested and a comparison of performance for serial one-dimensional (1-D) Radon based OFDM and parallel 2-D Radon based OFDM is made under additive white Gaussian noise (AWGN), flat, and multi-path selective fading channels conditions. It is found that Radon based parallel 2-D OFDM has better speed and performance than serial 1-D Radon based OFDM.     

分数阶傅里叶变换对OFDM系统峰均比的影响

分析并讨论了分数阶傅里叶变换对OFDM系统峰均功率比性能的影响,并与传统的采用离散傅里叶变换的OFDM系统的峰均功率比性能进行了比较。结果表明在子载波数较少的情况下,采用分数阶傅里叶变换的OFDM系统的峰均功率比性能要优于采用离散傅里叶变换的OFDM系统的峰均功率比性能,而随着系统子载波数量的逐渐增加,二者性能趋向一致。