高速移动环境下基于多天线角域分辨力接收算法

在高速移动环境下,OFDM系统由于信道的时变性破坏了子载波间的正交性,而使系统性能急剧下降。针对此问题,该文分析了高速移动产生的信号多普勒扩展的数学模型,提出了一种基于多天线角域分辨力接收算法。该算法采用正交角域子空间投影,将多普勒扩展简化为各个正交角域空间上的多普勒频移,并通过多普勒参数估计来抑制时变信道对系统性能的影响。仿真结果表明,在高速移动环境下,该文提出的算法有效地抑制了OFDM系统中子载波间干扰,获得了较好的系统性能。     

Performance analysis of synchronous MC-CDMA in mobile Rayleighchannel with both delay and Doppler spreads

Rapid time variations of the mobile communication channel have a dramatic effect on the performance of multicarrier modulation. This paper models the Doppler spread and computes its effect on the bit error rate (BER) for multicarrier code division multiple access (MC-CDMA) transmission and compares it to orthogonal frequency division multiplexing (OFDM). Also, we evaluate the transmission capacity per subcarrier to quantify the potential of MC-CDMA and (coded-) OFDM. We focus on linear receivers, in particular those using the minimum mean-square error (MMSE) criterion. Our channel and system models allow the computation of analytical performance results. Simulations verify some commonly used, yet critical assumptions     

Routeing and filtering in the ETS-V transponder for mobile communication experiments

The Japanese spacecraft ETS-V implemented an experimental mobile satellite communication system via a transponder called AMEX (aeronautical maritime experimental transponder). L-band was used for the mobile link and C-band for the feeder link. For the L-band two-beam system routeing and filtering were performed through 12 SAW filters and four switches in the IF part. The effective frequency allocation plan was obtained considering the features of the links used and the characteristics of SAW filters. The filtering to prevent EMC problems was also studied for AMEX, which handles high- and low-level signals simultaneously. AMEX shows good performance in orbit and is being used for a variety of mobile communication experiments by many organizations including the Communications Research Laboratory.     

On the Performance of a Constellation-rotated Time-spreaded Space-frequency Coding Scheme for 4 × 1 MISO OFDM Transceivers

In this article, we propose a time-spreaded quasi-orthogonal space-frequency coded OFDM system with constellation rotation. A constellation rotated quasi-orthogonal OFDM system could offer full rate and full diversity in a frequency selective fading channel. Time spreading can give additional time diversity gain in a fast fading channel. Assuming that complex channel gains between adjacent subcarriers are approximately equal, we develop a coding scheme for 4 × 1 MISO transceiver and its BER performance is evaluated for different Doppler frequencies in an OFDM system. The simulation results show that 2 dB gain can be achieved at BER of 10⁻³ using the proposed scheme compared to a scheme without time spreading and constellation rotation when 512 subcarriers are used at maximum Doppler frequency of 300 Hz. Also, the proposed system is analyzed for different delay spread of the channel and the results show that if adjacent subcarriers are correlated, it is better in SF-OFDM decoding.     

Resource Management Techniques for OFDM Systems with the Presence of Inter-Carrier Interference

The aim of the paper is to study the problems of resource management in the presence of inter-carrier interference (ICI) and multipath fading channel for orthogonal frequency division multiplexing (OFDM) systems. OFDM is a promising technique for the broadband wireless communication systems. However, the OFDM communication system is sensitive to ICI which arises because of Doppler spread and carrier frequency offset (CFO). To solve these problems, an optimization method has been exploited, and a computationally efficient method using numerical optimization techniques is proposed. The simulation results show that these derived optimal solutions and proposed suboptimal algorithms as compared with the uniform power allocation algorithm or conventional water-filling algorithm can significantly improve the performance of the OFDM systems.     

Channel Modeling and Inter-Carrier Interference Analysis for V2V Communication Systems in Frequency-Dispersive Channels

Vehicle-to-vehicle (V2V) communication has attracted much attention recently. In V2V communications mobility plays a major role in yielding frequency dispersion of the channels, and thus accurately modeling of Doppler effect becomes a challenging issue as the two fundamental assumptions of the Jack’s model may not be valid due to time-varying channel characteristics in V2V communication systems. In this paper, we present a practical model to characterize V2V communication channel and its corresponding Doppler spread spectrum is derived. In addition, we will study the impact of inter-carrier interference (ICI) generated in an orthogonal frequency division multiplexing (OFDM) based V2V communication system. Compared with the classical Jake’s channel model, our proposed new channel model is more accurate and fits in particular well for the performance assessment of vehicle-to-vehicle communication systems.     

Analysis and characterization of intercarrier and interblock interferences for wireless mobile OFDM systems

OFDM has been applied in a wide variety of wireless communications in recent years since it has the advantage over the conventional single-carrier modulation schemes when enduring the frequency-selective fading. However, intercarrier-interference (ICI) and interblock interference (IBI) due to the Doppler effect, carrier frequency drift of local oscillators and multipath fading limit the capability of OFDM systems. In this paper, a new generalized mathematical model for intercarrier and interblock interferences is derived for wireless mobile OFDM systems, in which Doppler frequency drift, local carrier frequency offset, multipath fading, and cyclic prefix coding are all present in reality. Such a new ICI/IBI model can be applied for OFDM performance evaluation in different environments. This new formulation of IBI and ICI provides a generalized framework which includes special ICI models appearing in the previous literature. Besides, the derived OFDM performance evaluation analysis in this paper can greatly benefit OFDM designers for wireless multimedia networks and digital video broadcasting technologies.     

OFDM系统中载波间干扰抑制方法

在高速移动通信环境下,OFDM系统在传输过程中出现的多普勒频移和收发两端本地振荡器之间的频率偏差,形成子载波间干扰(ICI)并造成系统性能降低。在分析子载波间干扰机制的基础上,讨论了自消除方法和分段均衡方法,并提出一种利用加窗技术改进的分段均衡方法。仿真结果表明,采用该改进的分段均衡方法能更好地改善系统的性能,有约2 dB的信道估计增益。     

Synchronization techniques for underwater acoustic communications

Underwater acoustic channel is a challenging medium for communication due to the presence of significant multipath, high noise, frequency-dependent propagation loss, and high and non-uniform Doppler spread. Doppler shift is non-negligible in underwater communication due to the low velocity of underwater signals. Synchronization and Doppler estimation are important requirements for achieving good performance in this channel. Synchronization algorithms that give good performance in radio communication do not work well in underwater communication. Hence, this area has received a lot of attention from researchers. This paper surveys important works in the area. The techniques proposed in the literature for frame synchronization, frequency and phase synchronization, and timing synchronization in single carrier communications are reviewed here. The synchronization techniques proposed for OFDM, MIMO OFDM, and spread spectrum communication are also surveyed. Doppler estimation methods proposed in the literature are also reviewed. It is found that most of the recent works in underwater acoustic communication focus on OFDM synchronization. Deep learning-based methods proposed in the literature are also reviewed. Key open problems and areas that require future research attention in the field of synchronization and Doppler estimation in underwater communications are highlighted in this paper. The area needing most attention of underwater communication researchers was found to be MIMO OFDM due to the difficulty in synchronization in such systems while used in underwater communication. Reducing the computational complexity of the algorithms used is also important for future work. Schemes that work with Doppler due to relative velocity over 10 m/s also need to be developed.     

基于数据样本方差的正交频分复用水声通信多普勒频移估计方法

针对正交频分复用(OFDM)水声移动通信易受时变多普勒频移影响的缺点,该文提出一种基于数据样本方差的多普勒频移估计方法。利用前序符号的信道估计值恢复当前符号的有效数据序列及其频域分集副本,计算分集副本与数据序列的比值并搜索该比值序列在不同多普勒补偿因子下的方差,选取方差最小时对应的补偿因子作为多普勒频移估计值,利用稀疏贝叶斯学习和判决反馈信道估计算法获得修正后的信道频域响应并传递给后序符号,实现对多普勒频移的实时跟踪。数值仿真验证了该方法的可行性和优越性,海上试验证明,该方法实现了基于无人水下航行器的OFDM水声移动通信,能够对时变多普勒频移进行有效估计。     

Terrestrial digital video broadcasting for mobile reception using OFDM

Performance of a system design for digital video broadcasting is examined with emphasis on mobile reception. Orthogonal frequency division multiplexing (OFDM) is used to achieve good bandwidth efficiency and to mitigate the intersymbol interference resulting from the channel delay spread. The resulting equivalent channel including OFDM can be modeled as a flat Rayleigh fading channel plus an interchannel interference (ICI) term due to the channel Doppler spread. This ICI term is analyzed and shown to result in an error floor. Performance improvements due to antenna diversity and trellis-coded modulation (TCM) are given. Finally, multiresolution modulation is discussed as a means of achieving graceful degradation and giving degrees of freedom for further performance improvement.This research was supported by the Multimedia Systems R & D Laboratory, Hitachi, Ltd.     

A bandwidth-efficient method for cancellation of ICI in OFDM systems

Orthogonal frequency division multiplexing (OFDM) is a very important modulation technique in wideband wireless communication and multimedia communication systems. While, it can effectively deal with multipath delay spread produced by frequency fading channels, its main drawback is the effect of frequency offset (FO) produced by the receiver local oscillator or by motion-induced Doppler. The FO breaks the orthogonality among the subcarriers and hence causes intercarrier interference (ICI). In this paper, ICI caused by frequency drift is eliminated by equalizing the complex weighting coefficients of interference. In most of the commonly used ICI cancellation schemes, bandwidth efficiency suffers because of the requirement of redundancy in the transmission. In the proposed scheme, repetition of data symbols or transmission of training sequence is not required. Thus, the bandwidth efficiency of normal OFDM system is maintained. The improved performance of the present scheme is confirmed through extensive simulations.     

基于正交频分复用(OFDM)的航空移动通信多普勒效应研究

OFDM由于其自身的技术优势使其在未来的航空移动通信中将占有重要的地位，但其中存在的多普勒(Doppler)频移将会对系统的性能产生极大的影响．本文在介绍OFDM基本原理的基础上，分析了OFDM技术在现代航空移动通信中应用时产生的Doppler，并提出了采用相邻的子载波共同传输同一符号的方法，力图减小Doppler的影响．经过计算仿真表明，通过采用上述新方法，可以有效的克服Doppler的影响，提高OFDM系统抵抗Doppler频移的性能．     

BER performance analysis of an OFDM-based visible light communication system using DHT post-coding

The theoretical analysis of the bit error rate(BER) performance in a discrete Hartley transform(DHT) post-coded orthogonal frequency division multiplexing(OFDM) visible light communication(VLC) system over an additive white Gaussian noise(AWGN) channel is presented. The theoretical analysis results show that the proposed DHT post-coding scheme does not degrade the BER performance of the post-coded OFDM VLC systems. The analysis result is confirmed by our simulation results for random bit source....     

LEO卫星通信OFDM子载波间隔优化

LEO卫星移动通信系统中,卫星与终端的运动会造成可观的多普勒效应。当系统采用OFDM传输体制时,多普勒效应会造成OFDM数据点的旋转及子载波间干扰(ICI)。通过分析LEO卫星的运动规律,得到由卫星造成的多普勒频偏的规律性,该特性保证了多普勒频偏可被估算并消除;接着通过地面终端的移动规律,计算得到系统的多普勒频展;最后计算得到铱星系统和全球星系统的多普勒频展,在最优系统容量的准则下,得到在该系统中适用的OFDM子载波间隔。     

Hartley变换OFDM调制的可见光通信研究

针对正交频分复用(OFDM)可见光通信系统中存在高峰均功率比的问题，将预编码与迭代限幅滤波技术相结合应用于Hartley变换OFDM调制的可见光通信系统中，并对传统的采用FFT/IFFT限幅滤波的OFDM系统进行改进。根据建立的FHT的可见光OFDM系统模型，分别比较了不同方案下系统的频谱利用率、PAPR和误码率等性能，并分析了离散余弦变换和Hadamard矩阵预编码对PAPR的抑制作用以及FFT/IFFT、DCT/IDCT和FHT/IFHT三种不同变换方案的滤波性能。结果表明:FHT的可见光OFDM系统比FFT的OFDM系统的计算复杂度低、频谱利用率高；DCT预编码技术在克服系统PAPR及提升系统误码性能上更具优势；FHT/IFHT迭代限幅滤波的误码性能优于FFT/IFFT和DCT/IDCT。      

Understanding the Time and Frequency Varying Characteristics of a Multipath Wireless Channel

Designing a spectrally efficient wireless channel requires a comprehensive understanding of its time and frequency varying characteristics, making it a stochastic medium of communication. These characteristics become more challenging to cater at the receiving terminal in a multipath transmission. This is because of the fading effect and Doppler shift of the transmitted frequency, specifically in cellular mobile radio systems and vehicle to vehicle communications. This paper presents the modeling, simulation, and then characterization of a cellular mobile radio multipath channel over its time and frequency varying fading gain. For this purpose, a discrete-time Finite Impulse Response (FIR) type filter of such a channel is designed, modeled, and simulated using time and frequency varying characteristics of the received signal. The simulated channel response is further analyzed in terms of coherence bandwidth, coherence time, delay spread, Doppler spread, and symbol time.

     

基于Watterson模型的短波航空移动信道建模与仿真

信道建模与仿真是短波航空通信系统性能分析评估的基础,该文通过分析研究飞行器最大移动速度、加速度、机动频率、运动轨迹等机动状态参数对短波航空移动信道时变多普勒频移的影响,提出一种基于Watter-son模型的短波航空移动信道模型.该模型能够充分描述由收发两端相对运动引入的信道多普勒频移与扩展,适用于短波超视距航空移动通信...     

The generalized method of equal areas for the design of sum‐of‐cisoids simulators for mobile Rayleigh fading channels with arbitrary Doppler spectra

We present in this paper a generalized version of the method of equal areas (MEA) that is well suited for the design of sum‐of‐cisoids (SOC) simulators for narrowband mobile Rayleigh fading channels characterized by any type of Doppler power spectral densities (DPSDs). Unlike the original MEA, the generalized MEA (GMEA) can be applied to the simulation of fading channels with asymmetrical DPSDs. This is an important feature because the simulation of such channels is of great interest for the laboratory analysis of mobile communication systems under non‐isotropic scattering conditions. We show that irrespective of the underlying DPSD, the GMEA results in a very good approximation to the autocorrelation function, average Doppler shift, Doppler spread, and envelope distribution of the channel. We compare the performance of the GMEA with that of the L_p‐norm method (LPNM), which is the method that has been most widely used for designing SOC simulators for fading channels with arbitrary DPSDs. The obtained results demonstrate that the performance of the GMEA measures up to that of the LPNM. In addition, the results show that the determination of the model parameters is easier and less time‐consuming when applying the GMEA. Copyright © 2011 John Wiley & Sons, Ltd.     

基于差分编码的OFDM系统ICI消除方法的研究

在高速移动通信环境下,OFDM 系统在传输过程中出现的多普勒频移和收发两端本地振荡器之间的频率偏差,形成子载波间干扰(ICI)并造成系统性能降低。该文在分析子载波间干扰机制的基础上,从信道估计的角度提出了一种高效的ICI自消除差分编码方案。该方案提高了传统ICI自消除方案频谱利用率。仿真表明,在系统归一化频率偏差大于0.1时,该方案具有4 的信道估计增益,消除了因ICI带来的地板效应。