Microwave Communication Links Research Articles

VERTICAL PROFILE OF RADIO REFRACTIVITY GRADIENT IN AKURE SOUTH-WEST NIGERIA.

In the planning and design of microwave communication links, the structure of the radio refractive index in the lower part of the atmospheric boundary layer is very important. In this work, measurements of atmospheric pressure, temperature and relative humidity were made in Akure (7.15 ◦ N, 5.12 ◦ E), South Western Nigeria. Wireless weather stations (Integrated Sensor Suite, ISS) were'positioned at five different height levels beginning from the ground surface and at intervals of 50 m from the ground to a height of 200 m (0, 50, 100, 150 and 200 m) on a 220 m Nigeria Television Authority TV tower at Iju in Akure North Local Government area of Ondo State. The measurement of the atmospheric variables was made every 30 minutes everyday. The study utilized the data for the first year of measurement (January-December 2007) to compute the radio refractivity and its refractivity gradient in Akure. From these parameters, the vertical distributions of radio refractivity are then determined. The results obtained show that the propagation conditions have varying degree of occurrence with sub-refractive conditions observed to be prevalent between January-July while Super-refraction and Ducting were observed mostly between August-December.

Outage Performance of Single/dual Polarized Fixed Wireless Access Links in Heavy Rain Climatic Regions

The outage performance of line-of-sight fixed wireless access channels operating above 10 GHz is predominantly controlled by rain attenuation. In this paper, a prediction model, appropriate for heavy rain climatic regions based on gamma distribution assumption for the rainfall rate and the rain attenuation is being reviewed to incorporate the recently released ITU-R rainmaps. The model exhibits a very good performance, as validated by tests against the ITU-R databank Moreover, the above model is extended to evaluate the outage performance of a dual-polarized mm-wave fixed wireless access link located in heavy rain climatic regions. The obtained numerical results focus on the investigation of the effects of various operational parameters upon the outage performance of the microwave communication link.

A microwave communication link with self-heterodyne direct down conversion and system predistortion

A novel design of a microwave communications link operating at 5.8 GHz is presented based on self-heterodyne direct downconversion (SHDDC) and system predistortion. Utilizing the SHDDC scheme eliminates the need of any local oscillators at the receive ends and minimizes spectrum usage; however, the transmitter power efficiency is low, and there exist high mixer intermodulation levels in the receiver. To overcome these drawbacks, a system predistortion approach is proposed. A two-tone measurement is performed to validate the idea. It shows 12.67 dBc of overall improvement in the signal-to-intermodulation ratio by applying a fourth-order predistortion technique. Further, successful transmission of a digitally modulated signal is also demonstrated.

Cryogenic high-Q microwave resonators for stable oscillators

Cryogenic microwave resonators have a strong potential as frequency stabilising elements for oscillators to be used in advanced radar systems and high-bitrate microwave communication links. Depending on frequency, either 2D planar HTS-resonators, HTS-shielded sapphire TE/sub 011/ resonators or cryogenic sapphire whispering-gallery mode resonators represent the best compromise between resonator quality factor and size. We have built and tested an all-cryogenic oscillator based on a WG-resonator at f=23 GHz. Phase noise measurements indicate values superior to quartz stabilized oscillators. A two-step electric frequency tuning consisting of an integrated varactor phase shifter and a dielectric plunger moved by a piezomechanical transducer is introduced to compensate frequency drifting with temperature. For further improvement of long-time frequency stability we have developed rutile-sapphire composite dielectric resonators. Due to the opposite sign of the temperature slope of the dielectric constant of sapphire and rutile a turning point appears in the temperature dependence of the resonance frequency. Employing a moderate temperature stabilization as good as a few millikelvin around the turning point at T=78 K, we have demonstrated a long time frequency stability at least as good as for oven controlled quartz oscillators.

Cumulative distributions of rainfall rate and microwave attenuation in Singapore's tropical region

The microwave attenuation due to rainfall in tropical regions has not been very widely studied yet. In Singapore's tropical environment, line‐of‐sight microwave communication links were set up and have been operated for several years to study the microwave attenuation characteristics due to tropical rainfall. In this paper the experimental results are presented, including the cumulative distributions of rainfall rate, the cumulative distributions of microwave attenuation, the relationship between specific attenuation and rainfall rate, and frequency scaling formulae. The results show that the rainfall rate and the microwave propagation characteristics in Singapore over a 10‐month experiment period are out of accord with International Radio Consultative Committee predictions.

Fading Phenomena in Line of Sight Microwave Communication Links

Scintillation and multipath fadings are observed in microwave LOS links established at different locations in India. Scintillation fading dominates over the southern plains while the multipath fading prevails for more time in east coast. Both types of fading occur for significant percentage of time in a year over northern plains. The association of meteorological parameters with fading phenomenon is also investigated.

Correlated humidity and temperature measurements in the urban atmospheric boundary layer

To investigate the effect of atmospheric turbulence on microwave communication links, temperature and water vapor pressure have been measured and radio refractivity has been computed, during different meteorological conditions, in the atmospheric boundary layer of an urban site. The cospectra between temperature (T) and water vapor pressure (e) have been found to be either negative over the whole range of frequencies, or the low-frequency end of the cospectrum is of opposite sign relative to higher frequency end. In both cases cospectra follow a−5/3 law in the inertial subrange, in agreement with the theoretical predictions. The coherence spectra clearly show that the temperature and humidity fluctuations are highly coherent within the inertial subrange under both convective and stable conditions. The relative contribution ofC ,C eT andC to the real refractive index structure parameterC is examined and discussed.

Propagation theory in adaptive cancellation of cross‐polarization

AbstractThe application of propagation theory to the structure and control of adaptive networks designed to cancel hydrometeor‐induced cross‐polarization in microwave communication links is summarized. When adaptive cross‐coupling networks are realized at radio frequency as variable waveguide polarizers, desirable simplifications are to make them lossless devices with at most two free parameters. Performance calculations for such simplified forms are fully reviewed.Several new approaches to the design of controllers for the cancelling networks are also made possible. New results are presented on the performance of systems which use only one pilot tone, or even dispense with cross‐polar measurement completely. It is also shown that the reciprocity of the hydrometeor medium, an apparently safe assumption, makes possible an extract precorrection of a transmitted polarization if control is based on a beacon signal of the orthogonal polarization sent back from the receiving point.

Long Path Line of Sight Microwave Communication Links

Long Path Line of Sight Microwave Communication Links

30-Second and One-Minute Rainfall Rate Modelling and Conversion for Millimetric Wave Propagation in South Africa

The knowledge of adequate rainfall statistics will contribute greatly to the roll-out of emerging wireless technologies on the platform of Long-Term Evolution (LTE) and WiMax (IEEE 802.16). Therefore, it is important that rainfall measurements at shorter integration time are incorporated as useful inputs in the planning of Line-of-Sight (LOS) microwave and millimetre communication links for hosting these technologies. As compared to the use of one-minute rainfall data as suggested by International Telecommunication Union (ITU), the equivalent data measured at 30-second interval gives more information of temporal rain rates in the time domain. Therefore, in this study, rainfall rate measurements of 5-minute integration time representative of 10 locations in South Africa are evaluated to obtain their cumulative distributions. Results from these analyses are compared with rainfall data of one-minute and 30-second integration time data obtained over Durban (29°52'E, 30°55'S), South Africa. Consequently, rainfall rate models for conversion to one-minute and 30-second integration times were obtained over 10 locations in South Africa using the power-law regression functions. Our results obtained over these locations were used to estimate specific attenuation values in selected microwave and millimetric wave bands at 12 GHz, 30 GHz and 60 GHz for 10 locations under study. It is confirmed that the 30-second integration time provides more information needed for estimation of specific attenuation on microwave and millimeter-wave radio links in South Africa.

Pipeline Location: Special Surveys—I

The specialized surveying procedures used in submarine and coastal-marsh pipeline locations, as well as major pipeline bridges and microwave communication links for pipeline operation are examined....

Circuit technique in frequency-modulated microwave links

The paper describes the circuit problems in wide-band microwave communication links and is illustrated by reference to the development of links operating at frequencies around 4 000 Mc/s for 180-channel telephony and for television.Modulating and demodulating circuits with very linear characteristics, and i.f. amplifiers with uniform time-delay are described, together with the necessary techniques required to assess their performances.The optimum design for a broad-band low-noise i.f. amplifier is shown to consist of cascaded earthed-grid triodes.Automatic-frequency-control systems, the special video problems of television, and high-level intermediate-frequency/microwave mixers are described.