Towards a more accurate method of predicting the distribution of multipath fading on terrestrial microwave links

Initial work is described towards the development of a more accurate asymptotic prediction method for the high fade depth range of the fading distribution on line-of-sight terrestrial microwave links. The influence of parameters such as path length, frequency, terrain roughness, path inclination, antenna beamwidth and path clearance is analysed using multipath fading data from a large number of links in the UK.     

A study of the relationship between multipath distortion and wavenumber spectrum of refractive index in radio links

The study reported here is concerned with relating the multipath and frequency selective fading characteristics of radio links caused by scatter phenomena to the statistical spatial characteristics of the refractive index. Two specific antenna configurations are studied in detail: the LOS (line-of-sight) case where the two antennas are pointed concentrically and the OTH (over-the-horizon) case where the antennas point along a great circle path. For each of these antenna configurations two scatter situations are considered: isotropic scatter with the propagation medium controlling the focusing of energy and locally isotropic scatter with narrow-beam antennas. In all cases single scattering theory is used and narrow-band signal transmission is assumed. It is found that when multipath is of any importance in causing signal distortion for a LOS link, the scatter portion of the channel may be modeled as a continuum of uncorrelated scatterers. Also OTH links are found to be characterizable in the same way. Single integrals and specific analytic and numerical examples are given relating frequency correlation functions and delay power spectra to the wavenumber spectra of assumed refractive index fluctuations. It is shown that a measurement of the frequency correlation function or delay power spectrum can yield an estimate of the wavenumber spectrum of the refractive index fluctuations.     

Systematic development of new multivariable techniques forpredicting the distribution of multipath fading on terrestrial microwavelinks

Multipath fading data obtained from 47 terrestrial microwave line-of-sight links in France and the United Kingdom are analyzed to derive narrowband prediction equations for the deep fading range of the cumulative distribution for the average worst month. A large number of possible predictor variables based on typical radio link parameters are investigated, and equations which reduce the standard error of prediction to less than half that of previous techniques for this part of Europe are developed. The improvement is achieved by increasing the number of prediction variables, using the most statistically significant variables, introducing a zonally varying geoclimatic factor, and employing more accurate analysis techniques. A reasonable choice of variables for system design includes path length, radio frequency, path inclination, and the grazing angle of specular reflection from the average terrain profile. A physical interpretation of the results suggests that a large amount of multipath fading is caused by ground reflection in combination with fading of the direct wave through the atmosphere     

A statistical characterization of point-to-point microwave linksusing biased Rayleigh distributions

A multipath fading model has been used to examine the characteristics of a point-to-point microwave radio channel. The new attributes of this model that differ from other multipath models enable the incorporation of the ideas of fading severity and time variability. This is achieved by the use of a biased Rayleigh-distributed second (interfering) ray. The biasing constant determines the fading severity, while the Rayleigh cutoff frequency determines the rate-of-change of the channel. For point-to-point microwave radio channels, this model provides narrow-band fading signal-level distributions that completely replicate the Norton or Nakagami-Rice distributions while at the same time providing signal level distributions that replicate those obtained from broad-band measurements. Thus, this model unifies the narrow-band and broadband multipath fading models. Using this model, nondiversity and diversity system operation is investigated and the concept of a broad-band correlation coefficient introduced. The distributions of notch speed are also investigated. Software simulation results are compared with some hardware simulation results and field measurements     

Using two ray multipath model for microwave link budget analysis

A two-ray ground multipath deterministic model for worst-case fading-depth prediction in microwave link budget analysis is presented. Simple formulas, providing insight into fading depth as a function of geometrical and electrical parameters, are derived. A detailed analysis shows that, in many cases, the fading depth depends mainly on the path-clearance angle. Comparison with the Olsen-Segal model shows that both models have roughly the same path-clearance-angle factor. To the best of the authors' knowledge, this can be considered the first theoretical justification of the path elevation factor in the Olsen-Segal model     

Diversity protections for digital radio-summary of ten-yearexperiments and studies

A summary is presented of a series of critical experiments that led to recent discoveries of large improvement factors for digital radio performance by antenna pattern diversity, antenna angle diversity, frequency diversity, and space diversity. The measured diversity improvement factors for digital radio against multipath dispersive fading are larger than those predicted by the existing analog radio model by at least one order of magnitude. Applications of these findings will lead to substantial savings in the cost for diversity protections for digital radio routes. These discoveries stimulated the development of new models of diversity improvement factors for digital radio and the development of the DRDIV computer program for engineering digital radio routes. Background information is given on multipath fading and diversity concepts, and a glossary of terms is included     

Measurement, modeling, and simulation of digital LOS microwavechannels with applications to outage prediction

A summary of prior work in the field of microwave line-of-sight (LOS) channel propagation measurements, channel models, and channel simulators is presented. The objective of these efforts is the reliable prediction of digital microwave radio performance on any specified link. Many digital radio outage prediction techniques are predicted upon the use of m-curves, which characterize a digital radio's performance in a simulated fading environment. These m-curves are generated by subjecting the radio under test to simulated multipath fading. This requires the use of a channel simulator that accurately emulates the fading conditions found on real LOS links. All parameters of an ideal channel simulator should be based on a model that has been validated through propagation measurements on a number of different paths. Although much progress has been made, some issues remain for further investigation. New channel propagation measurement, modeling, and simulation results that are directly applicable to these issues are given     

Gigabit/s indoor wireless systems with directional antennas

The design of a high speed (>150 Mb/s) wireless local area network (WLAN), requires that many factors be considered, including technical, economic, and regulatory. A major technical factor is the channel response behavior (multipath) in the indoor environment as a function of the frequency band, building type and radio system architecture. The consequences of designing indoor wireless systems with directional antennas at one or both ends of a line-of-sight (LOS) link are investigated. We determine how narrow the beamwidth must be so that the maximum data rate is not limited by multipath effects. For such beamwidths, simple unequalized two-level frequency shift keying (FSK) or phase shift keying (PSK) modems can be used in place of the more costly and complex “anti-multipath” modems, and data rates above 1 Gb/s may be achieved. The channel impulse response in an empty room is estimated using geometrical optics, observing that with directional antennas, multipath rays must arrive from the same direction as the LOS ray. The link outage probability is then estimated as a function of the antenna beamwidth, and guidelines are established for the selection of the frequency band and antenna placement. Experiments using a 19-GHz 622-Mb/s binary phase shift keying (BPSK) link and 15° beamwidth horn antennas in an office building with plaster walls and large metallized windows have demonstrated error-free performance on both LOS and non-LOS (NLOS) links     

一种DS-CDMA无线反向链路空间分集方案研究

本文利用自适应天线阵列分离出多径信号,利用RAKE消除多径衰落。导频位符号辅助(PSA)信道估计用来对RAKE的输入端进行加权,天线阵列的加权系数的自适应更新是采用RLS算法而得。结果表明,该方案具有很强的抗多径干扰和多址干扰的能力。     

基于MATLAB的两径模型仿真分析

针对无线电波远距离空—地传播,为了得到其在球面反射两径模型下的多径衰落特性,介绍了自由空间路径损耗和平面反射两径传播的数据模型。分析了球面反射两径传播的几何模型,并提出了用二分法确定几何反射点的位置,进而得到等效天线高度和扩散因子后用平面反射模型计算球面反射接收信号功率的方法。给出了基于矩阵实验室(MATLAB)编程的方法和仿真结果分析。     

Altitude Measurement Based on Terrain Matching in VHF Array Radar

The multipath signal and direct signal lying within a beamwidth of a receiving antenna are highly correlated, which degrades the performance of DOA in a VHF array radar. Especially on rough terrain, the reflection centers of elements are not at the same horizontal plane; thus the variation of indirect wave path difference from each element with respect to the target’s depression angle is not linear. In this paper, we assume the two-path-component data model, with one direct component and one indirect component related to the ground reflection. This paper builds a highly deterministic multipath signal model which takes the curvature of the signal path and the terrain parameters of the reflection region into account based on the sphere model, gives calculation methods of the reflection coefficient and lengths of the direct path and indirect path, and proposes a synthetic steering vector super-resolution algorithm. The results for both simulated and measured data show that this method provides excellent performance in resolving the DOA problem in a multipath environment compared to the data model that only considers the flat-earth model.     

Adaptive beam-space nulling of multipath signals

A beam-space adaptive nulling technique that has potential for applications in communication circuits degraded by multipath signals is described. The technique is based on a well-known vector relationship in beam space. A linear transformation is used to prevent the degradation of the desired signal reception. Results are presented from tests of the technique that were conducted using real multipath data. The data used in these tests were recorded using a 32-element sampled aperture antenna (SAMPAR). These measurements were carried out on an over-water path. The direct signal, i.e. the signal that propagates via the shortest path from the transmitter to the receiver antenna, is selected as the desired signal; its reception is enhanced by suppressing the unwanted signals, i.e. the multipath signals that propagate via a reflection from the surface of the water. Examples are given in which the wanted and unwanted signals are separated by less than a beamwidth     

多径衰落的数学模型及分析

多径效应是影响低仰角测控系统正常工作的重要因素。介绍了多径效应产生的机理,建立了多径衰落的数学模型。无线电信号在收发天线间传播时,存在直射路径和多条反射路径,当直射信号和反射信号相互抵消时就会产生多径效应。依据建立的数学模型,分析了多径衰落产生的条件和对系统影响的程度。多径衰落产生的位置由地面站天线高度、飞行器飞行高度和二者之间的距离决定,多径衰落深度主要由地面对电波的反射系数决定。在上述分析的基础上,提出了避免和减小多径衰落对系统影响的几种措施。     

Multipath Delay Characteristics on Line-of-Sight Microwave Radio System

Multipath properties on microwave line-of-sight links for the purpose of applying to microwave systems design are discussed. The theoretical analyses of the fading mechanism and the relation between multipath delay and radio duct parameters are carried out using the ray-optical method. Multipath delay have been measured with the frequency-sweep method and the three-frequency method on several paths in Japan since 1954. These paths are distributed in length from 20 km to 80 km, and include horizontal and slant paths. Through these theoretical and experimental analyses, a number of wave components in multipath propagation, the probability density function of multipath delay, the maximum delay and the relation between multipath delay and radio duct parameters are examined. The estimation method for path-length difference statistics, needed for evaluation of propagation distortion, is quantitatively given.     

Finding the approximate angular probability density function of wave arrival by using a directional antenna

The first-order approximate angular probability density function of wave arrival can be obtained by plotting the mean-square value of signal level versus the pointing angle of a mobile radio directional antenna during an entire test run. The accuracy of this first-order approximation is dependent on the beamwidth of the antenna. As the beamwidth of the antenna becomes narrower, the approximate density function becomes more accurate. There are also higher order approximations for obtaining a more accurate angular probability density function without narrowing the directional antenna beamwidth. These higher order approximations are related to the moments of the signal strength received by the same directional antenna used in obtaining the first-order approximation. All of these are theoretical predictions. An experimental verification using a first-order approximate angular probability density function in finding theoretical level crossing rates of a mobile radio signal is given. The theoretical level crossing rates are in fairly good agreement with the experimental ones as long as the angular probability density function is not far from a uniform distribution.     

Tropospheric microwave propagation--An X band diagnostic system

A microwave system for direct measurement of angles of arrival and multipath delay times is described. Based on the utilization of interferometer and very wide frequency sweeping (1- GHz range) techniques, high accuracy measurements on these parameters are possible. Some experimental results are presented which suggest that movements in the angle of arrival of a single ray path (as opposed to multipath) may be responsible for much of the fading experienced on light-of-sight microwave links in southwestern Ontario.     

Refractive Multipath Model for Line-of-Sight Microwave Relay Links

A major source of propagation outages on microwave line-of-sight (LOS) links is multipath fading caused by steep negative refractive index gradients. This paper develops a complete transfer function characterization of refractive multipath on LOS links. The results clearly show the dependence of discrete path amplitudes and delays on receiver height, transmitter height, refractive index gradient, layer height and ground range.     

An investigation of space-path hybrid diversity scheme for basestation reception in CDMA mobile radio

In CDMA mobile radio communication systems, degradation of instantaneous signal to interference power ratio (C/I), which causes impairment of frequency utilization factor, will be significant in multipath fading environments. In this paper, a hybrid diversity scheme for fading reduction combining effects of space diversity and path diversity (represented by the RAKE method) is investigated. A quantitative evaluation of fading reduction effects of the hybrid diversity is performed, comparing them with those of space diversity only and path diversity only. The hybrid diversity scheme is promising in environments where the delay spread of a transmission path is 1 μs or less     

小尺度时变多径衰落信道的仿真研究

从理论上分析了多径效应和多普勒效应对无线信道的影响,并分别从时域和频域给出了信号经历信道后的仿真图形,分析了典型时变多径衰落信道理论模型,并在此基础上建立了抽头延迟线（TDL）的确定性仿真模型;仿真分析表明,通过改变时延功率谱和确定性模型的参数计算方法,此模型可以进一步扩展并用于多种传播环境下的小尺度衰落仿真。     

The implementation of channel diversity in mobile software radio receivers

A novel channel diversity concept is proposed and demonstrated, which avoids receiving signal deterioration due to multipath fading in mobile receivers. The system is based on coherent superposition of the signals received from several transmitters supplying the same information at different frequencies. Based on a software radio architecture this concept may increase the quality of mobile reception in modern car receivers considerably. Compared with multiantenna-receivers which overcome the multipath fading problem by simultaneously receiving the same program with several antennas, the proposed solution is advantageous, since it requires only a single antenna.