An Additive Model as a Physical Basis for Shadow Fading

Received signal power in mobile wireless communications is typically modeled as a product of three factors: distance-dependent average path loss law, variation in the local mean power (shadow fading), and small-scale fading. Of these three factors, the least investigated is the shadow fading, which is usually explained as a result of multiplication of large number of random attenuating factors in the radio channel. In this paper, the authors propose an additive model as an alternative physical basis for shadow fading within an "extended local area" where path loss is constant. Starting from a sum-of-sinusoids signal model, they show that under mild statistical assumptions on the powers of the sinusoids, the resulting signal power will have approximately Gaussian distribution in logarithmic scale. A cluster-based model for shadow fading emerges as a special instance of the general result. They present simulation and measurement results that support their theoretical findings. The new physical basis for shadow fading also provides insights into simulation and modeling of radio channels     

Performance analysis of a variable‐gain amplify‐and‐forward relayed mixed RF‐FSO system

In this work, an amplify‐and‐forward variable‐gain relayed mixed RF‐FSO system is studied. The considered dual‐hop system consists of a radio frequency (RF) link followed by a free space optical (FSO) channel. The RF link is affected by short‐term multipath fading and long‐term shadowing effects and is assumed to follow the generalized‐K fading distribution that approximates accurately several important distributions often used to model communication channels. The FSO channel experiences fading caused by atmospheric turbulence that is modeled by the gamma‐gamma distribution characterizing moderate and strong turbulence conditions. The FSO channel also suffers path loss and pointing error induced misalignment fading. The performance of the considered system is analyzed under the collective influence of distribution shaping parameters, pointing errors that result in misalignment fading, atmospheric turbulence, and path loss. The moment‐generating function of the Signal power to noise power ratio measured end‐to‐end for this system is derived. The cumulative distribution function for the Signal power to noise power ratio present between the source and destination receiver is also evaluated. Further, we investigate the error and outage performance and the average channel capacity for this system. The analytical expressions in closed form for the outage probability, symbol and bit error rate considering different modulation schemes and channel capacity are also derived. The mathematical expressions obtained are also demonstrated by numerical plots.     

阴影Nakagami-m衰落信道条件下分布式天线系统中断分析

This paper presents an outage analysis of distributed antennas system (DAS) suffering from shadowed Nakagami-m fading environment where the desired signal also suffers from cochannel interference. The desired signal and interfering signal are subjected to path loss, multipath and shadowing fading. Based on Wilkinson's method, the signal to interference ratio (SIR) probability density function ( PDF) of fixed DAS is obtained. Some numerical results of outage probability with different parameters are analyzed. The analysis results can provide sufficient precision for evaluating the outage performance of DAS     

噪声对消射频光子链路的噪声系数研究

噪声系数是宽带射频光子链路的重要参数,影响 射频信号传输和处理系统的信噪比。 提出了一种噪声对消射频光子链路,采用平衡光探测器对直流光和调制光转换的光电流相减 处理,得到链路的噪声系数改善。从光信号调制、探测及噪声功率变化特性出发,建立了噪 声对消射频光子链路的理论模型,并通过理论分析研究了噪声系数与激光功率、相对强度噪 声、光调制器损耗、光耦合器分光比等参数的关系。构建了噪声对消射频光子链路实验,对 频率6~18 GHz的噪声系数进行对比研究。实验研究结果表明,在相同 条件下,噪声 对消射频光子链路的噪声系数减小2～4 dB。理论和实验结果充分证 明提出的方法正确性。     

Bilateral averaging over receiving and transmitting areas foraccurate measurements of sector average signal strength inside buildings

In defining path loss for narrowband systems, it is common to introduce the concept of the sector or local average in order to remove the rapid (Rayleigh or Rician) fading. As originally developed for cellular mobile radio (CMR) systems, the sector average is obtained from the spatial average of the received signal as the mobile traverses a path of 20 or so wavelengths. This approach has also been applied indoors with the assumption that a unique average could be obtained by moving either end of the radio link. Unlike the CMR path for indoor propagation, however, both ends of the radio link are in a rich multipath environment. It is shown both theoretically and experimentally that moving both ends of the link is required to achieve a unique average. When averaging is accomplished by moving only one end, the result can experience variations of as much as ±2 dB when the other end is moved by a fraction of a wavelength     

Spectrum efficiency analysis for microcellular mobile radio systems

Spectrum efficiency is analysed in a microcellular mobile radio environment considering an appropriate model for UHF ground-wave path loss. The influence of cluster size and reuse distance on the spectrum efficiency is investigated by obtaining the probability of cochannel interference for uncorrelated Rayleigh fading interferers and Rician fading desired signal. The effect of traffic intensity is also considered in evaluating the spectrum efficiency.<>     

Propagation analysis of signal fading for basic exchange radios

Although empirical multipath fading models are available for microwave links above 2 GHz, these models are not directly applicable to basic exchange radio (BEXR) links because of the substantial differences in frequency, antenna beamwidth, and radio path clearance. A method for obtaining a scaling factor which accounts for the differences between BEXR and microwave links is presented. First, the terrain scattering is studied using a rough surface model, and the atmospheric refraction is studied using a ray tracing approach. Then, the received signal powers of a microwave link and two BEXR links on the same path under the same propagation condition are calculated. The signal characteristics are investigated and used to simulate the fading distributions for all three links. From the simulation results, a scaling factor is derived and used to modify the existing microwave multipath fading models for BEXR application. The predictions of the modified model agree well with measured BEXR data. This study shows that the probability distribution of signal fading on BEXR links is a strong function of antenna height and beamwidth     

UHF fading in factories

The authors detail the results of narrowband propagation measurements performed at five factories. The extensive empirical data indicate that path loss is dependent upon local surroundings and is log-normally distributed, temporal fading is Rician, and small-scale signal fluctuations due to receiver motion are primarily Rayleigh, although Rician and log-normal distributions fit some of the data. Shadowing effects of common factory equipment likely to obstruct indoor radio paths are also examined     

星载SAR双程信道建模与仿真实现

目前卫星移动通信的信道研究主要集中在星-地单程链路对无线电波传播的影响,而在星载SAR回波信号的无线传播环境中,信号需要经过星-地和地-星的双程传播,2次穿越大气层,且要经过地面目标的反射。因此,有必要针对星载SAR的这个特点,建立星载SAR的信道模型,为回波信号的模拟提供参考。在经典的卫星移动通信信道模型C.Loo模型的基础上,考虑星-地和地-星链路损耗及地面后向散射系数,推导了星载SAR星-地-星双程链路的理论信道模型,并利用正弦和方法设计了与理论模型相对应的仿真模型,通过仿真验证了理论模型和仿真模型的一致性。     

HFC双向网络回传通道的调试技术

文中给出回传通道调试的相关研究成果,介绍回传通道的前端信号处理、光纤链路调节、电缆分配网设置的方法,提出采用等效数字调制信号源、通过测试回传通道NPR性能评价回传通道质量的实用测试方法.     

相位调制微波光传输链路特性分析（特邀）

常规的微波光子系统采用强度调制方式实现微波信号的电光转换,由于调制器采用马赫-曾德尔干涉结构(MZI),系统性能不仅受到自身正弦响应特性的制约,而且需要进行偏置点控制,因此存在动态范围受限、系统控制复杂以及3 dB固有损耗带来的效率不足的问题,而采用相位调制可避免该问题。围绕相位调制光传输链路,为了完成相位调制信号的光电解调,文章提出采用薄膜滤波器通过边带抑制与边带选通两种方式实现相位调制到强度调制的转换,并分析了链路射频性能与器件参数之间的映射关系。实测对比了相位调制与常规强度调制链路之间的传输特性,通过分析可知,在相同链路配置条件下,相位调制链路具有更高的传输效率,而且光滤波带来的均衡作用,使得相位调制链路的3 dB带宽比强度调制链路大两倍。     

无线射频注入的物理层分析

通过物理层进行无线射频注入是实现更高层次网络对抗的基础。建立了一个无线射频注入的模型,在物理层实现注入必须满足足够的信道容量、匹配的信号形式和严格的时序关系。分析了实现物理层注入需要确定的几个参数:估计路径传播损耗、收发天线增益、噪声基底等指标以确定发射功率。对被攻击网络的信号进行精确参数估计,以控制产生相匹配的信号。要遵循合适的通信规程,以实现无线链路的建立和欺骗信息的传送。给出了实现无线注入的工作流程,将欺骗信息通过物理层向高层渗透。     

射频信号光纤延时的幅相特性研究

光纤传输具有大带宽、低损耗的特点,射频信号的光延时系统在雷达等电子信息系统中得到了广泛 应用。采用射频光纤延时技术来延时射频信号时,射频信号的幅度和相位会发生改变。频率越高、延时时间越长, 幅相变化就越大,其中相位变化尤为突出。文中阐述了这些变化产生的原因和导致的效果。幅度的变化,主要是光 纤传输损耗和色散导致的射频信号功率衰落而引起的;相位的变化,主要是激光器和光纤所处的环境温度变化以及 光纤自身损耗发热而引起的。在外部温度环境变化时,射频信号的相位会发生很大的飘移;即使在恒温条件下,相 位漂移依然较大,这会造成雷达相参等下降,降低雷达对微弱信号的检测能力,应该引起足够重视。     

Experimental Study of a Fixed WiMAX Network at 2.62 GHz

Field measurements of signal strength is one of the basic techniques for coverage determination of a cellular network. In this paper, we report on the experimental study of the signal strength measurements in a typical worldwide interoperability for microwave access network operating at 2.62 GHz installed for fixed wireless access in the district of Greater Noida, Uttar Pradesh, India. The experimental data are analyzed for deriving characteristic parameters of the radio propagation environment. The characteristic parameters include path loss exponent and standard deviation of shadow fading. Having derived the characteristic parameters, we propose path loss models for the cellular network. The study also aims at the determination of network coverage for the given system parameters. Results and their analysis are utilized for network planning and optimization of the system parameters for the installed cellular network.     

Performance of cellular CDMA with cell site antenna arrays,Rayleigh fading, and power control error

The performance of code-division multiple-access (CDMA) systems is affected by multiple factors such as large-scale fading, small-scale fading, and cochannel interference (CCI). Most of the published research on the performance analysis of CDMA systems usually accounts for subsets of these factors. In this work, it is attempted to provide a comprehensive analysis which joins several of the most important factors affecting the performance of CDMA systems. In particular, new analytical expressions are developed for the outage and bit-error probability of CDMA systems. These expressions account for adverse effects such as path loss, large-scale fading (shadowing), small-scale fading (Rayleigh fading), and CCI, as well as for correcting mechanisms such as power control (compensates for path loss and shadowing), spatial diversity (mitigates against Rayleigh fading), and voice activity gating (reduces CCI). The new expressions may be used as convenient analysis tools that complement computer simulations. Of particular interest are tradeoffs revealed among system parameters, such as maximum allowed power control error versus the number of antennas used for spatial diversity     

Space and frequency diversity measurements of the 1.7 GHz indoorradio channel using a four-branch receiver

The practical space and frequency diversity performance achievable inside a building at 1.75 GHz under fading conditions due to the motion of a portable terminal and due to the movement of people are investigated. Data are collected using a four-branch dual-frequency envelope receiver positioned throughout one floor of a university building of common construction type. The measurement environment is characterized for large-scale path loss and wall transmission loss. Envelope cross correlations are calculated, and performance of diversity is measured for various frequency separations and antenna spacings which would be applicable given the physical size of portable telephones and data terminals. Two-branch space diversity is directly compared to two-branch frequency diversity and to four-branch hybrid diversity based on simultaneous measurements of each using selection combining. The distributions of correlations and diversity gain at different locations are also investigated. Results indicate that two- and four-branch diversity can be a very effective way to combat signal fading for portable terminals in an indoor radio environment     

A new power control function for multirate DS-CDMA systems

This paper proposes a closed-form power control function for the reverse link of a multirate single chip-rate variable processing gain DS-CDMA system in a mobile radio environment that assumes a Rayleigh fading channel with log-normal shadowing and path loss. A closed-form open-loop power control function based on a newly defined traffic exponent is proposed, and nonlinear programming is used to perform the optimization. In addition, a user model that allows users to dynamically switch traffic rates for different connection applications is implemented. Results obtained using random chip sequences demonstrate improvement in the system capacity with the new power control function compared to the conventional power control function. Furthermore, the proposed function also simplifies the power control processing     

On the performance of interference cancellation in wireless ad hoc networks

This letter presents a novel approach for deriving a lower bound on the outage probability of interference cancellation receiver in ad hoc networks for a class of channel fading. Our approach is based on the following observation: under the power decay law model for large-scale fading with an exponent α strictly greater than 2, the sum of the interferences at a receiver has the same order as the strongest signal. This feature still holds when the small-scale fading has a finite 2/α-fractional moment. This fact is used to develop an approximation which is also a compact lower bound on the outage probability.     

具有能量收集的双向RF/FSO中继系统性能分析

设计了一种在大气湍流和指向误差影响下的带有能量收集的混合双向RF/FSO中继传输系统。RF信号部分采用能量分割的方式进行能量收集,FSO信号部分则将其信号中的直流电分量所携带的能量收集起来存储在中继器中。其中射频RF链路采用Nakagamim信道衰落模型,自由空间光FSO链路采用Gamma Gamma信道衰落模型。并且计算出了两条链路各自的累积分布函数CDF。采用DF的中继协作方式,并利用Meijei G函数推导出系统中断概率的闭合表达式,最终通过仿真来验证结果的正确性。     

Fade margin calculation for channels impaired by Rician fading

Excess path loss due to multipath severely restricts the performance of power limited mobile networks such as those using satellite-aided links. To reduce multipath related losses, the higher elevation angle of the spacecraft can be exploited by utilizing mobile antennas which reduce the strength of the multipath reflections in favor of the line-of-sight signal. The presence of a strong and stable path in a fading link will change the envelope statistics of the received waveform from Rayleigh to a more favorable Rician distribution. It is determined that the excess path loss, or fade margin, of a Rician channel when coherent detection of binary phase shift keying (BPSK) or quaternary phase shift keying (QPSK) signals is considered. The results are presented parametrically such that they can be applied to a wide range of propagation characteristics from heavy fading to nonfading situations. Furthermore, similar results are also given for the case where only limited coverage is provided.