无线视频通信中信道特性的研究

在无线视频通信中，无线信道环境的好坏将直接影响到通信质量，信号传输中的带宽．波动、高误码率和接收的异种性为视频通信系统的设计带来了极大的困难。讨论了无线信道电波传播的特性，给出了无线时变衰落信道的模型并分析了视频传输的信道失真，为复杂无线环境下的视频通信系统的设计提供了理论依据。     

混沌扩频通信系统在多径Rayleigh衰落信道中的性能分析

本文研究了混沌直接扩频信号在多径Rayleigh衰落信道中传输时的抗多径干扰和多址干扰的特性。对于所给出的混沌扩频通信系统,当加入多径干扰和信道噪声时,给出了理论误码率与实际误码率的比较,数值结果表明在多径衰落信道中,混沌扩频通信系统抗多径干扰的性能很好;同时对混沌直接扩频信号的奇、偶互相关函数进行了计算,结果表明混沌扩频通信系统具有良好的抗多址干扰能力。     

移动通信中的无线信道技术

从移动无线信道的时变冲激响应的功率密度谱函数和相关函数出发,研究广义平稳非相关散射(WSSUS)无线信道特性的统计参数,对无线移动衰落信道的特性进行了分类。分析了无线通信中影响移动无线通信信号传输质量的原因,对提高移动通信质量的可行性提供参考。     

基于Simulink的OFDM系统仿真分析

无线衰落信道使得通信系统在传输过程中出现符号间干扰和信道间干扰,严重影响通信质量。正交频率分割复用(OFDM)是一种多载波数字调制技术,他在无线多径衰落信道下却具有较好的传输特性,对其抗多径衰落的原理进行了研究。在Simulink下建立了系统的仿真模型,并对所建模型进行了分析。同时在两径和六径瑞利衰落信道以及系统中有无交织下对其进行仿真,给出了仿真结果。     

OFDM系统仿真与分析

正交频分复用(OFDM)技术在高速数据传输中得到了广泛的应用,尤其在无线接入和移动通信中应用前景非常广泛.无线衰落信道使得通信系统在传输过程中出现符号间干扰和信道干扰,严重影响信道质量,而OFDM是一种多载波教字调制技术,在无线衰落信道下具有较好的传输特性.运用Matlab构建OFDM系统,并加入自适应功率和比特分配算法.通过具有高斯白噪声和频率选择性信道,观察在不同信噪比条件下系统误码率是如何变化的.给出仿真结果,从而分析出系统在不同调制方式下的性能.仿真结果表明,OFDM系统能抗多径衰落,而且具有良好的误码性能.     

一种新的MIMO信道模型

实际无线通信环境中发送天线之间以及接收天线之间存在相关性.针对以上特点,从多径MIMO信道的特性出发,首先建立发射天线相关系数矩阵和接收天线相关系数矩阵,并将它们引入无线信道的莱斯MIMO信道模型中.最后通过分析LOS MIMO信道相关模型和瑞利衰落MIMO信道相关模型,给出了具体的建模步骤.仿真结果表明采用本文方法产生的信道模型的MIMO系统误码率更低,从而验证了该信道模型能够较好地模拟MIMO系统的空间信道.     

协同通信中分集合并性能分析

无线通信信道时变多径的传输特性严重影响了传输系统的性能,必须采取有效的措施来对抗信道衰落。分集接收技术成本低廉,性能优越,在无线通信技术中广泛使用,是对抗无线信道衰落最有效的技术之一。文中详细地叙述了分集接收合并的原理,给出了严密的数学公式,通过理论推导,研究了三种合并技术(最大合并比MRC、选择合并SC、等增益合并EGC)的误码率和合并增益,仿真并分析了其在瑞利衰落信道下的性能,为实际应用提供参考。     

探空信号的信道建模及仿真

从分析探空信号的传输、传播路径及其信道的特点开始,建立了探空仪信号传输的两径信道模型,并根据历史探空数据给出适当的信道衰减参数和信号传输函数。利用Matlab对建立的模型进行仿真并分析结果,得到的信道衰减参数曲线基本符合实际情况。结果还表明地面反射信号引起的快衰落对探空信号传输系统造成了很大的影响,是系统误码率高的主要原因,这为探空信号的抗干扰性能分析提供依据。     

一种自适应调制方案

阐述一种星座图可调的自适应调制方案，并在Beyond ITM-2000的相关参数下进行了性能仿真。该方案能克服移动无线信道时变衰落特性对传输性能的影响，在信道的传输质量满足一定误码率要求的前提下，根据无线信道衰落情况，使调制方式在四相移相键控调制（Quaternary Phase Shift Keying，QPSK）、正交幅度调制（Quadrature Amplitude Modulation，QAM）包括8QAM，16QAM，32QAM，64QAM，128QAM，256QAM之间自如切换。仿真结果表明，该系统方案可以有效地适应无线信道的时变衰落，不但能增加系统平均吞吐量，而且能提高频谱利用率。     

无线光自适应副载波MDPSK调制系统特性分析

无线光通信中的自适应传输技术可有效抑制大气湍流引起的信道衰落.根据接收信噪比和预定目标误比特率要求,本文对通过瞬时湍流特性改变调制阶数的无线光自适应传输的通信系统进行了研究.给出采用Malaga分布湍流模型分别描述Gamma-Gamma、对数正态以及K分布湍流信道的相关参数设置;推导了Malaga湍流信道下无线光自适应多进制差分相移键控（MDPSK,M-ary Differential Phase Shift Keying）调制系统的平均误比特率、频谱效率以及中断概率表达式,以及基于MeijerG函数的渐近表达式.MDPSK自适应和非自适应两种调制系统特性分析结果表明：自适应调制系统不但在相同电信噪比下可获得更低的平均误比特率,还可以大大提高频谱效率,而不需要增加额外传输功率或牺牲误比特率.     

无线局域网信道特性的研究

无线信道是整个现代数字无线移动通信研究的基础,从电磁波传播方式、无线信道对传输信号的影响以及等效低通信道的时变冲激响应等几个方面对无线信道的特性进行了研究。结果表明,信号通过无线信道传播时,会产生各种形式的衰落,衰落对无线移动通信系统传输性能的影响是很大的,在实际中应设法消除。     

Approximate equation of average bit-error rate in DS-CDMA systems over fading channels

The transmission quality in mobile wireless communications is affected not only by additive white Gaussian noise, but also by multipath fading, which drastically changes the amplitudes and phases of wireless signals. It is one of the key themes in direct-sequence code-division multiple-access systems to evaluate average bit error rate (BER) performances as parameters of the Doppler frequency, the delay profile, the number of simultaneous access users, and so on. This paper proposes the approximate equation for easily calculating the average BER over fading channels (multiray fading models). The performance evaluations are carried out in the two-ray model, the IMT2000 model, and the COST207 model. It is confirmed from the coincidence of approximate results with computer simulation results that the proposed approach is applicable to a variety of parameters.     

移动平台对无线信道特性的影响分析

针对移动平台对无线信道特性的影响,在分析移动多径信道特性的基础上,建立了不同信道模型下多普勒效应的计算机仿真模型。不仅针对不同信道衰落条件下平台移动引起的多普勒效应进行仿真,而且为进一步阐述多普勒效应的影响,还对平台移动引起的数字通信误码率进行了仿真。信真结果表明,平台移动不但会扩展多普勒频谱,而且会加快信道衰落,造成误码率上升。     

BER performance of linear STBC from orthogonal designs over MIMO correlated Nakagami-m fading channels

This paper presents the evaluation of the average bit error rate (BER) performance of linear space-time block codes (STBC) from orthogonal designs over correlated identically distributed Nakagami-m fading channels. Starting from the moment-generating function (MGF) of the multipath component signals at the antenna array elements, analytical expressions of the BER performance for both integral and nonintegral Nakagami-m fading parameters are derived. Closed-form expressions of the spatial cross-correlation function for mobile nonfrequency selective Nakagami-m fading multiple-input-multiple-output (MIMO) channels are obtained, which are valid for small angle-of-arrival (AOA) spread. In this expressions, various parameters of interest, such as the mean AOA of the signal, AOA spread, and array configurations, are all taken into account. The effects of antenna array configuration and the operating environment (mean AOA, AOA spread, Nakagami fading parameter) on the BER performance of the system are illustrated by several numerical examples.     

Bit error simulation for π/4 DQPSK mobile radio communicationsusing two-ray and measurement-based impulse response models

An accurate software/hardware bit-by-bit error simulator for mobile radio communications is described. Simulation results in indoor and outdoor channels are compared with theoretical results. Bit error rate (BER) results in simulated frequency-selective fading channels generated by several channel models such as two-ray, constant amplitude, and simulated indoor radio channel impulse models (SIRCIMs) are presented. It is shown that BER is not only dependent on the RMS delay spread, but also on the distribution of temporal and spatial multipath components in local areas. An important result is that a two-ray Rayleigh fading model is a poor fit for indoor wireless channels and, if used, can underestimate the BER by orders of magnitude. A real-time bit error simulation of video transmission using the bit-by-bit error simulator is described. The simulator, called BERSIM, is shown to be a useful tool for evaluating emerging data transmission products for digital mobile communications     

Performance analysis of multi‐input multi‐output systems with maximum likelihood detection over shadowed fading channels

Multiple‐input multiple‐output (MIMO) transmission techniques constitute an important technology in modern wireless communication. Hence, performance analysis methods for such systems are of considerable interest. This paper considers first the average pairwise error probability for uncoded MIMO systems employing maximum likelihood detection over a composite Rayleigh‐Lognormal fading channel with spatial correlation. It provides general results, applicable also to a wider class of shadowing models, concerning asymptotical diversity gains and shows that they are not changed by such shadowing. Then, analytical evaluation techniques for bit‐error‐rate (BER) over composite Rayleigh‐Lognormal fading channels, based on the truncated union bound and the transfer function, are considered. Furthermore, these techniques are modified for applications over spatially correlated channels. This paper shows that such performance evaluation techniques provide good approximations to BER of spatially uncorrelated MIMO systems and also in the presence of moderate spatial correlation, over Rayleigh‐Lognormal fading channels. Copyright © 2013 John Wiley & Sons, Ltd.     

An Improved Switched Diversity Combining Using Fuzzy Adaptive Control

Diversity combining techniques, due to their extreme effectiveness incounteracting fading effects in wireless environment, have been extensivelystudied in recent years. In this paper, we propose an improved switchedcombining scheme using fuzzy adaptive control, namely, fuzzy adaptive switcheddiversity (FASD), which is applicable for implementation at mobile units.Specifically, the fuzzy adaptive control dynamically adjusts the thresholdlevel, at which switching is performed, based on the present varying channelconditions. By incorporating the statistics of mobile fading channels andextensive simulations, we have achieved an effective system design of FASDwhich has a significant improvement in system performance in terms of both thediversity gain and BER (bit error rate), while maintaining reasonablesimplicity in implementation.     

Unified BER and optimum threshold analysis of binary modulations in simple and cascaded Rayleigh fading channels with switched combining

Following a unified analytical framework, the bit error rate (BER) of several coherent and non‐coherent binary modulation schemes is derived for a switched diversity system. The two variants of switched combining that have been investigated are switch and stay combining and switch and examine combining. For channel modelling, at first a simple slow flat fading channel is assumed, where the amplitude attenuation obeys the Rayleigh distribution. Later the BER calculations are repeated for cascaded Rayleigh fading channel case. Rayleigh fading is the most popular model for electromagnetic signal propagation in wireless media when both or either of the transmitter/receiver is fixed. On the other hand, when both the transmitter and the receiver are mobile, a cascaded (or double) Rayleigh fading model is better suited. The applicability of these two models, namely simple and cascaded Rayleigh model, has been indicated by several theoretical studies and their suitability is established by various field measurements. In our paper, simple closed‐form BER expressions as a function of switching threshold have been found and optimum switching thresholds have been computed for both these models as well as for both types of diversity combining described earlier. The results presented in this paper can be very useful for communication system designers to analyze link quality of switched diversity assisted systems in various wireless environments. Copyright © 2010 John Wiley & Sons, Ltd.     

Performance Analysis of Land Mobile Satellite Communication Systems with Equal-Gain Combining over Shadowed-Rice Fading Channels

An accurate model for performance analysis of land mobile satellite (LMS) channels is the Shadowed-Rice (SR) fading model. Recently, an approximate SR model has been proposed by the author for LMS channels. Compared with the existing models, the proposed model is analytically tractable and makes it possible to approximate very accurately the probability density function (PDF) of the SR random variable (RV) by a finite weighted sum of Rice PDFs. In this paper and based on the proposed model, the statistics of the sum of independent SR RVs is studied. While this problem is well understood for common fading models such as the Rayleigh, Rice, and Nakagami, a minor attention has been devoted to the SR case. In this paper and by using the well-known Beaulieu series approach, the PDF and the Cumulative distribution function (CDF) of the sum of independent SR RVs are derived in the form of infinite convergent series. In this regard, analytical, closed-form expressions are derived for the coefficients of the Beaulieu series. To give an application of the presented results, the performance of LMS communication system employing equal-gain combining (EGC) diversity techniques over SR fading channel is investigated and novel closed-form expression is derived for the outage probability of the system.     

Mobile-to-mobile fading channels in amplify-and-forward relay systems under line-of-sight conditions: statistical modeling and analysis

This paper deals with the modeling and analysis of narrowband mobile-to-mobile (M2M) fading channels for amplify-and-forward relay links under line-of-sight (LOS) conditions. It is assumed that a LOS component exists in the direct link between the source mobile station (SMS) and the destination mobile station (DMS), as well as in the links via the mobile relay (MR). The proposed channel model is referred to as the multiple-LOS second-order scattering (MLSS) channel model. The MLSS channel model is derived from a second-order scattering process, where the received signal is modeled in the complex baseband as the sum of a single and a double scattered component. Analytical expressions are derived for the mean value, variance, probability density function (PDF), cumulative distribution function (CDF), level-crossing rate (LCR), and average duration of fades (ADF) of the received envelope of MLSS channels. The PDF of the channel phase is also investigated. It is observed that the LOS components and the relay gain have a significant influence on the statistics of MLSS channels. It is also shown that MLSS channels include various other channel models as special cases, e.g., double Rayleigh channels, double Rice channels, single-LOS double-scattering (SLDS) channels, non-line-of-sight (NLOS) second-order scattering (NLSS) channels, and single-LOS second-order scattering (SLSS) channels. The correctness of all analytical results is confirmed by simulations using a high performance channel simulator. Our novel MLSS channel model is of significant importance for the system level performance evaluation of M2M communication systems in different M2M propagation scenarios. Furthermore, our studies pertaining to the fading behavior of MLSS channels are useful for the design and development of relay-based cooperative wireless networks.