基于Simulink的窄带OFDM系统设计与仿真

为有效提高短波通信系统的数据传输速率,可采用正交频分复用（OFDM）技术。OFDM是一种特殊的多载波数字调制方案,各子载波之间的正交性使得频谱可重叠从而提高了频带的利用率,同时能有效对抗频率选择性衰落或窄带干扰。采用Simulink建模对窄带OFDM系统进行设计并仿真,仿真结果表明,在3kHz的带宽内,短波通信系统的数据传输速率能够达到12800b／s,而且具有良好的误码性能。     

短波正交频分复用(OFDM)传输性能研究

短波通信作为一种重要的通信方式一直受到信道带宽的限制,加之传输信道的不稳定性致使其传输效果不佳,如何有效的提高频带利用率、改善传输性能是短波通信的重要研究内容。正交频分复用(OFDM)是一种多载波数字调制技术,在提高频谱利用率、克服多径方面发挥了重要作用。文章以Simulink为平台构建短波OFDM通信系统仿真模型,并在频偏和多径条件下进行仿真,分析仿真结果可以看到,频偏和多径对系统的传输效果都有影响,其中频偏对通信系统的影响较为严重。     

基于压缩感知的OFDM宽带短波信道估计方法研究

为了提高OFDM宽带短波信道估计的精确性,针对短波信道固有的低稀疏性,在将压缩感知理论应用于OFDM宽带短波信道估计的基础上进行OFDM短波信道的稀疏建模,接着提出需要解决的问题,进而提出采用正交匹配追踪(OMP)算法进行短波信道的重构。通过仿真实验证实,与传统信道估计算法中的最小二乘(LS)算法比较,可以达到在使用更少导频的情况下提供更好的短波信道估计性能的效果,从而提高短波系统的频带利用率。     

短波FH/OFDM通信系统抗跟踪式干扰性能分析

对于短波多载波跳频通信系统,跟踪式干扰是一种有效的干扰模式。分析了正交频分复用( OFDM)部分子信道干扰与符号误码率的关系,通过仿真获得了最佳跟踪干扰的部分时间参数,结合OFDM频谱结构与干扰频谱关系,推导了高斯信道条件下部分频带干扰和多音干扰时系统误码率,分析了短波FH/OFDM通信系统抗跟踪式干扰的误码率性能,仿真结果表明：跟踪干扰时间窗口对误码率的影响与部分频带干扰因子和信干比有关;在部分频带干扰与部分时间干扰之间,存在等效的干扰效果区域;多音干扰因子越大,系统所受影响越大,跟踪干扰时间窗口对误码率的影响与多音干扰因子和信干比有关,针对OFDM符号的多音干扰影响要远大于部分频带干扰的影响;跳频与OFDM技术的结合、提高载波跳速、减小跟踪式干扰对OFDM符号的影响,也是消除多音干扰的重要手段。     

基于OFDM技术的短波通信试验研究

短波通信是远距离信息传输的重要手段,OFDM(正交多载波调制)是一种在无线环境下具有较高频谱利用率的多载波调制技术。将OFDM技术应用于短波通信中,搭建了OFDM的试验平台,建立了短波通信链路,对接收端采集的基带OFDM信号进行了处理。通过编码和信道估计分别对信号进行纠错和信道补偿,处理结果表明,在传输速率600 b/s下系统的误比特率达到10-5数量级,系统性能良好。     

OFDM自适应调制技术综述

OFDM(正交频分复用)技术可以有效对抗频率选择性衰落克服窄带干扰,提高频谱利用率,适用于多径环境和衰落信道中的高速数据传输,在通信领域得到了广泛的应用,但是固定调制解调技术并不能有效实现频带资源的充分利用。OFDM自适应调制技术是根据各子信道的状况对比特与功率进行动态分配以实现系统整体性能的提高。本文对OFDM自适应调制解调的基本原理及传统OFDM自适应调制算法进行了较为全面系统的综述,并介绍了一些国内学者在OFDM自适应调制算法改进发面的研究成果。     

多天线系统及其在OFDM系统中的应用

利用配置多个天线实现高速可靠的通信是当今无线通信领域研究的热点之一。本文介绍了具有多个天线系统的几种不同构成方式，从系统误码率和频带利用率两个方面讨论了不同构成方式对系统的改善。同时，对多天线技术应用于正交频分复用(OFDM)系统的性能进行了分析，并给出了频率选择性瑞利衰落信道中最优译码算法下的系统性能比较。仿真结果表明，多天线OFDM系统既保持了OFDM抗频率选择性衰落的特点，又能够改善系统的误码率性能，提高频带利用率。     

基于Vogler模型的短波OFDM性能仿真

OFDM(正交多载波调制)是一种在无线环境下具有较高频谱利用率的多载波调制技术,它具有较强的对抗多径干扰和消除符号间干扰的能力.基于Vogler模型出发,根据实测的短波信道参数数据进行了OFDM系统仿真,同时,用导频辅助的方法对信道进行估计,并进行信道补偿,克服了短波信道的多径衰落和多普勒频移等因素的影响,有效降低了系统的误码率,提高了整个系统的性能.     

基于MATLAB的OFDM系统仿真分析

正交频分复用（OFDM）是一种多载波宽带数字调制技术，它具有频带利用率高和抗多径干扰能力强等优点，因而适合于高速率的无线通信系统。分析了OFDM技术的实现原理，用MATLAB软件对OFDM的传输性能进行了仿真模拟并对结论进行了分析。     

基于OFDM技术的短波电台系统研究

OFDM（正交频分复用）技术是一种多载波数字调制技术,在无线多径衰落信道下具有较好的传输特性,文中设计短波电台系统采用了OFDM技术来对克服多径衰落带来的干扰,并且给出了系统的设计方案。利用Simulink建模,并在瑞利衰落信道和高斯信道下进行了仿真,结果显示出性能上有较大的改进。     

Corona Based Optimal Node Deployment Distribution in Wireless Sensor Networks

This paper shows the trade off between different modulation techniques such as multi level quadrature amplitude modulation, multi level phase shift keying, and multi level differential phase shift keying for upgrading direct detection optical orthogonal frequency division multiplexing systems with possible transmission distance up to 15,000 km and total bit rate of 2.56 Tb/s. The 2.56 Tb/s signal is generated by multiplexing 64 OFDM signals with 40 Gb/s for each OFDM. Variations of optical signal to noise ratio (OSNR), signal to noise ratio (SNR), and bit error rate (BER) are studied with the variations of transmission distance. Maximum radio frequency power spectrum, and output electrical power after decoder are measured for different multi level modulation techniques with carrier frequency. It is observed that multi level QAM has presented better performance than multi level PSK and finally multi level DPSK in optical OFDM systems. Maximum output power after decoder is enhanced with both 32-PSK, and 64-QAM. Quadrature signal amplitude level at encoder is upgraded with 64-QAM. It is noticed that OSNR, SNR, and BER are improved using 4-QAM OFDM system than either QPSK or 4-DPSK.     

用于认知无线电的自适应双模跳频OFDM系统

认知无线电感知的空闲频谱具有非连续、非均匀特性,对认知无线电物理层实现提出挑战。跳频正交频分复用技术(Orthogonal Frequency Division Multiplexing,OFDM)兼具跳频与OFDM二者优势,适用于宽带无线传输。提出一种自适应双模跳频OFDM系统,即把射频跳频OFDM与基带跳频OFDM相结合,根据频谱感知信息和基带处理能力自适应地在2种模式间切换,给出系统的设计规则和实现流程。分析表明该系统可提高非连续、非均匀空闲频谱的利用率,是一种可行的认知无线电物理层实现机制。     

Broad-band OFDM radio transmission for multimedia applications

Future multimedia services will require the transmission of very high data rates over broad-band radio channels. In order to provide these services to mobile users, an appropriate transmission technique has to cope with frequency-selective and time-variant radio channels. The computation complexity for an equalizer increases in a frequency-selective radio channel for high data rate applications. Furthermore, the overhead for channel estimation procedures increases in time-variant channels. To overcome these drawbacks orthogonal frequency division multiplexing (OFDM) has been considered for broadband applications in many publications. The objective of this paper is to describe the large potential, the flexibility and adaptivity of the OFDM transmission technique in frequency-selective and time variant radio channels. Several technical aspects of OFDM transmission systems are discussed, especially the topics of differential modulation for which we compare different demodulation methods and channel coding with soft decision decoding. For higher level differential modulation, multilevel coding is taken into consideration     

OFDM混合调制信号在认知网络中的应用

对OFDM混合调制信号进行了谱相关分析,并将其应用到认知网络中。在采用OFDM混合调制信号传输的认知网络中,利用信号的循环平稳特性可以快速识别出信道空闲、认知信号传输、主用户信号传输和两种信号混叠这四种状态,为认知网络中认知信号的快速可靠接入和退出提供了保障。进一步利用混合调制的位置来标记不同的认知网络,认知节点可以区分不同认知网络信号。在提出的认知网络应用模型中,传输和接收设备较简单,而且不需专门的信道来传输控制命令,节约了带宽。仿真结果显示,在认知网络中,该模型可以提高频谱利用率,使通信系统的性能得到改善。     

OFDM关键技术及应用

与传统单载波技术相比,正交频分复用（OFDM）技术具有频谱利用率高和抗频率选择性衰落能力强等优点,是提高系统传输速率和可靠性的有效手段,并且随着对OFDM技术的研究逐渐完善和成熟,OFDM技术已经得到广泛应用。在此主要介绍了OFDM技术原理和关键技术,分析了OFDM技术的应用现状,展望了在下一代移动通信中的应用和研究方向。     

Outage Analysis of OFDM‐Based Cognitive AF Relay Network in the Presence of Narrowband Interference

Orthogonal frequency‐division multiplexing (OFDM) is one of the most widely used technologies in current wireless communication systems and standards. Cognitive radio (CR) provides a robust solution to the problem of spectrum congestion as it offers opportunistic usage of frequency bands that are not occupied by primary users. Due to the underlying sensing, spectrum shaping, scaling, and interoperable capabilities of OFDM, it has been adapted as a best transmission technology for CR wireless systems. However, the performance of an OFDM‐based CR wireless system is affected by the existence of narrowband interference (NBI) from other users. Further, due to carrier frequency offset in NBI sources, NBI energy may spread over all subcarriers of an OFDM signal. In this paper, a fixed Amplify‐and‐Forward (AF) relay that operates at a frequency band that is different from that of direct mode is introduced to suppress the effect of NBI. Analytical expressions are derived for outage probability in direct, AF‐relay, and incremental relaying modes. The outage performance of the proposed AF relay–based CR network is proven to be better than that of direct mode.     

Limiting ratios between symbol rate and minimal carrier frequency in ultra-wideband digital information transmission systems

Limiting ratios between symbol rate and minimal carrier frequency f ₀ in ultra-wideband digital information transmission systems, when using rectangular pulse as the message symbol and transmission efficiencies of radio signal’s energy of 90 and 99% in the frequency band [0–f ₀], are obtained. It is shown that modulation of sine and cosine oscillations by a rectangular pulse, in case of small number of carrier’s periods within the message symbol, gives different efficiency results of radio signal’s energy transmission in the provided frequency band. It is stated that, when using a rectangular pulse as message symbol and a requirement to transmit 99% of the radio signal’s energy in the frequency band[0 ? f ₀], the maximal symbol rate of information transmission should be 12 times less than the carrier frequency. However, if transmission of 90% of the rectangular radio signal’s energy in the frequency band[0 ? 2 f ₀] is acceptable, the symbol rate of information transmission may exceed the minimal carrier frequency f ₀, but no more than 1.45 times.     

OFDM技术及在认知无线电中的应用

OFDM技术是一种无线环境下的高速多载波传输技术,抗频率选择性衰落或窄带干扰能力较强,同时又有很高的频谱利用率,适合在多径传播的无线移动信道中传输高速数据。论文分析了OFDM的基本原理,论述了其关键技术及优缺点。OFDM技术将成为未来无线通信中的核心技术,在认知无线电、4G移动通信和无线宽带接入等领域得到广泛的应用。     

Robust Cognitive‐Radio‐Based OFDM Architecture with Adaptive Traffic Allocation in Time and Frequency

Cognitive radio (CR) has been proposed as an effective technology for flexible use of the radio spectrum. The interference between primary users and CR users, however, becomes a critical problem when they are using adjacent frequency channels with different transmission power levels. In this paper, a robust CR orthogonal frequency division multiplexing (OFDM) architecture, which can effectively suppress interference to nearby primary users and overcome adjacent channel interference (ACI) to the CR user, is proposed. This new approach is characterized by adaptive data repetition for subcarriers under heavy ACI, and adaptive time spreading for subcarriers near the borders of the CR user's spectrum. The data repetition scheme provides extra power gain against the ACI coming from primary users. Time spreading guarantees an acceptable interference level to nearby primary users. By computer simulation, we demonstrate that, under a CR environment, the proposed CR OFDM architecture outperforms conventional OFDM systems in terms of throughput and BER performance.