Origin of multipath fading on a terrestrial microwave link

The effects of surface and atmospheric mechanisms involved in the generation of frequency-selective multipath fading have to be investigated to help improve the performance of wide-band digital radio systems on line-of-sight microwave links. In this paper, three typical mechanisms of super-refractive layer formation are simulated: a layer which develops and grows close to the ground level (ground-based layer), a layer which forms close to the ground and then weakens in elevating (rising or elevating layer), and a layer which appears above the antenna heights and grows at this altitude (elevated layer). A ray-tracing program is then used to compute the time response of the propagation channel for each of these three layers. The results are compared with experimental observations and space diversity considerations are also presented.     

Steerable reflector-assisted communication in obstructed line-of-sight FSO link with optimal mitigation of dust-induced signal fading

Photonic Network Communications - This paper introduces an effective technique for bridging free-space optical (FSO) communication between end-to-end nodes with obstructed direct line-of sight...     

Multipath fading characteristics of a digital microwave link in Western India

In the present paper multipath fading affecting the performance of an oversea digital microwave link situated in Western India is investigated. The results show that the highest number of severely error seconds are observed in the early morning and late evening hours. The investigations reveal that ground based layers are responsible for the high bit error rate observed and, elevated layers have no effect. The observed probability of outage is compared with that deduced from Serizawa’s and Lavergnat’s prediction methods.     

A method to predict multipath effects on a line-of-sight link

A method that computes the quality of a digital microwave line-of-sight link affected by multipath propagation is presented. The use of a statistical propagation model and of the signature concept allows the propagation effects to be separated from the characteristics of the equipment. Since the variations of the statistical mode coefficients with respect to the characteristics of the link under consideration are not known, experimental results have been achieved using assumptions based on physical considerations. A single channel is considered and the same principles, although more speculative because of the lack of data, are applied to a diversity channel     

Frequency division multiplexed microwave and baseband digitaloptical fiber link for phased array antennas

A frequency-division multiplexed optical fiber link is described in which microwave (1-8 GHz) and baseband digital (1-10 Mb/s) signals are combined electrically and transmitted through a direct-modulation microwave optical link. The microwave signal does not affect bit error rate (BER) performance of the Manchester-coded baseband digital data link. The baseband digital signal affects microwave signal quality by generating second-order intermodulation noise. The intermodulation noise power density is found to be proportional to both the microwave input power and the digital input power, enabling the system to be modeled as a mixer (AM modulator). The conversion loss for the digital signal is approximately 68 dB for a 1-GHz microwave signal and is highly dependent on the microwave frequency, reaching a minimum value of 41 dB at 4.5 GHz corresponding to the laser diode relaxation oscillation frequency. It is shown that Manchester coding on the digital link places the intermodulation noise peak away from microwave signal, preventing degradation of close-carrier phase noise (<1 kHz offset). A direct trade-off between intermodulation noise and digital link margin is developed to project system performance     

Extension of the normalized two-ray transfer-function model to aspace-diversity line-of-sight link

Many methods for the prediction of multipath effects on the availability of a digital microwave line-of-sight link use a mathematical and statistical modeling of the propagation channel transfer function. Such a method, making use of the normalized two-ray model and adapted to a nonprotected hop, has been presented in a previous paper. In this paper we investigate the possibility to extend this model to a diversity channel, while keeping the statistical representation of the channel as simple as possible. Two theoretical possibilities, each leading to only one new statistical coefficient, are presented and validated on experimental data. From this result, a simple extension of the availability prediction method to a diversity channel seems to be feasible. Some problems remain to be solved however, which are discussed in the concluding section     

Effect of thunderstorm on microwave line-of-sight propagation

Thunderstorms frequently occur during the months of April and May over Calcutta. The behavior of microwave signal propagation during such storms has been studied, utilizing a terrestrial microwave line-of-sight link. Storm period is characterized by an increase in signal level with deep fluctuations.     

Case study of line-of-sight microwave multipath propagation

A case study of multipath fading on a microwave line-of-sight link is presented. Concurrent measurements by an aircraft-mounted refractometer are shown. Strong vertical and horizontal refractivity gradients are observed due to ground-based layering. Ray tracing shows three rays with subnanosecond separation and an angle-of-arrival spread of 0-4°. The fade is non-minimum-phase.     

Characterization of multipath parameters for line-of-sight microwave propagation

Ray equations are derived for microwave multipath propagation in a two-layer atmosphere, and the connection between the different solutions is demonstrated by numerical examples. Analytical expressions are developed to characterize the condition for multiple rays, the variation in angle-of-arrival (AOA), and the delay spread. The results suggest that more than three rays will be rare in practice and that the three-ray model (obtained from the zero-order rays defined in the paper) may provide a good representation of selective fading on microwave links.     

Optical simulation of microwave antennas

It has recently been suggested that microwave antennas may be simulated at optical frequencies using the neon-helium continuous gas laser. The present paper discusses various aspects of this problem and presents a simulation example using as a model the 96-element Australian array used to scan the surface of the sun at 80 megacycles. An optical system can be used for simulation in which an objective lens obtains the Fourier transform of the antenna aperture field which may be photographed. An additional lens produces an image of the antenna aperture field in the same air space as the far field pattern, thus permitting photographs of near field as well as far field patterns. Antenna masters are made as optical transparencies using photographic plates or film. Phasing may be obtained by using a variable thickness optical material evaporated on glass. Various combinations of array distribution, antenna weighting, and phasing are possible to simulate. Atmospheric perturbations are introduced on the antenna pattern by the use of two-dimensional phase delay patterns. Photographs are presented of the antenna aperture field, the far field pattern and several intermediate near field patterns.     

Angles-of-arrival and delay times on terrestrial line-of-sight microwave links

A ray-tracing approach is used to generate quantitative information on the ranges of angles-of-arrival, delay times, and amplitudes associated with individual paths in a multipath propagation situation as expected on typical terrestrial microwave links. Some experimental evidence relating to, and confirming, the predicted values is considered.     

The theory of microwave line-of-sight propagation through a turbulent atmosphere

In this paper the covariance and spectra for amplitude and phase fluctuations for a microwave signal propagated over a line-of-sight path through a turbulent medium are calculated. It is shown that the results derived under the assumption that the wavelength is much smaller than the inner scale of turbulencel_{0}are also valid when the wavelength is equal to or greater thanl_{0}.     

一种可用于卫星导航抗干扰天线的降耦方法

针对基于功率倒置算法的空时自适应抗干扰技术中天线阵元间互耦误差严重恶化算法性能这一问题,分析比较了目前常见的应对解决方案,并提出一种新的可用于抗干扰卫星导航终端的天线阵列降耦方法。该方法通过在卫星导航抗干扰天线阵列中加载电磁谐振吸波器,降低天线阵元间互耦。实验数据表明,利用该方法可以使阵元间互耦降低10 dB,使抗干扰接收机最大抑制干信比提升5 dB。     

A practical optical modulator and link for antennas

This paper describes a practical application of a technique for coupling an antenna to a receiver using a passive fiber-optic link. This technique should avoid pickup and electromagnetic perturbations normally associated with the use of electrically conductive cables. Laser light (632.8 nm) is modulated at the antenna by an electrooptic lithium-tantalate crystal and is then transmitted with a fiber-optic cable to the receiver electronics. Using an avalanche photodiode, the amplitude modulated optical signal is converted to an electrical signal. The crystal is mounted directly on an antenna without amplifiers or other electrically powered components. Using a broad-band antenna with fields generated in an anechoic chamber and a standard TEM cell, the frequency response as measured dropped 3 dB per 1.0 GHz from 100 MHz to at least 2.0 GHz, with a signal-to-noise ratio of 5 dB with a 1.0-V/m field and a 1.0-kHz bandwidth. A dynamic range of at least 60 dB is shown.     

High resolution three-dimensional microwave imaging of antennas

A procedure for imaging antenna currents that uses a relationship between the radiated far-field hemisphere and the Fourier transform domain of the source current density distribution is presented. The technique is applied to an array of two orthogonal waveguides, a slotted waveguide array and a reflector antenna. In each case the radiated far-field hemisphere is inverted to produce a high-resolution volumetric image of the antenna currents. Polarization discrimination is demonstrated as is the ability of the technique to `see behind' blockages by defocusing the foreground. It is shown that accurate distribution is available from the reconstructed image. Selective editing of the Fourier domain of the current distribution is performed to suppress unwanted artifacts in the reconstruction     

Scattering by spherically stratified microwave lens antennas

A comprehensive treatment of scattering by sources in the region of a spherically stratified object composed of discrete shells is presented and compared with measured results. The work encompasses the developments necessary for a thorough understanding of the focusing mechanism of a spherical lens. The method used is a modal technique that employs separation of variables in order to determine an eigenfunction representation of the incident and scattered fields. This formulation provides the Green's function required for the calculation of the scattering from a multiple layered sphere excited by an infinitesimal current element. The algorithm allows both near- and far-field predictions. The scattering by more complicated source field distributions is modeled by applying superposition with the appropriate coordinate transformations. From a practical standpoint, the present analysis allows the limitations of spherical lenses to be determined while providing the information required to optimize the lens design. A system was built based on the new design data, and its measured radiation characteristics are compared to the theoretical predictions. Excellent agreement is observed in most cases