Copolar Attenuation Research Articles

Estimating the Effect of Copolar Attenuation Caused by Rain Events on Radio Wave Propagation

Estimating the Effect of Copolar Attenuation Caused by Rain Events on Radio Wave Propagation

Reflective Switchable Polarization Rotator Based on Metasurface With PIN Diodes

A planar metasurface that rotates the polarization of reflected signals by 90° is presented in this article. The surface also permits the relative phase of the rotated, reflected signal to be controlled. The metasurface rotator uses a square printed patch element mounted above a conducting ground plane. The addition of PIN diode controlled vertical interconnect access (VIA) at the corners of each square patch provides the ability to modify the scattering properties of the surface. In particular, the surface can be programed to shift the phase of the rotated waveform by ±90°. The polarization rotator was manufactured, measured, and compared against numerical simulations, and the maximum measured bandwidth was 25% (4.8-6.2 GHz) for co-polarized attenuation of 10 dB. The proposed surface can be adopted for practical applications, such as circularly polarized antennas.

Assessment of dual-polarized interference due to precipitation for high-capacity satellite communication systems in Nigeria

High-capacity satellite systems are specially designed to provide several gigahertz-allotted spectra for multiple spot beams. The reliability of the communication links at high frequencies is often subjected to rain attenuation. In addition to rain attenuation, rain-induced cross-polarization is an important factor for dual-polarization microwave radio communication systems operating at such frequencies. Dual-polarized communication systems adopting frequency reuse usually encounter interference due to a change of transmission energy from one polarization state to another. Signal degradation as a result of some degree of noise causes reduction in the quality of signal between the two orthogonal channels—cross-polarization discrimination (XPD). Therefore, this study assesses the level of degradation due to interference on dual-polarized communication systems for millimeter wave’s band over some selected locations in Nigeria using 5 years (2008–2012) data collected at tropical data acquisition network of the Centre for Atmospheric Research Anyigba. Assessment of dual-polarized interference is based on cumulative distribution of XPD at different frequencies, effect of Earth station receiver frequencies on XPD, XPD–CPA (Co-polar attenuation) relationship and XPD–rain rate relations. Influence of polarization tilt angle for linear and circular polarization on XPD was also assessed. The specific result shows that XPD is extremely low at high rain rates for frequency up to 20 GHz. Depolarization effect is better at the lowest availability with higher value of XPD, and its performance contributes to system availability at the low exceedance probabilities. Port Harcourt, Nsukka, Yola and Sokoto experiences increased interference at receiver stations than other locations considered in this study, especially at Q/V band frequencies. Additional fade margin and appropriate mitigation methods for link availability and good quality of service will be required for sustainability.

English

Rain-induced depolarization impairments have been a constraint on the effective use of satellite communication systems at millimetre wave band especially in the tropical regions like Nigeria. This region often experiences high and varying degree of rainfall intensity accompanied with large raindrops due to distortion of raindrops when dual orthogonal polarization is employed to double the channel capacity without increasing the bandwidth. This paper examines the performance of five cross polarization discrimination (XPD) models at millimetre wavelengths over ten (10) different locations in Nigeria using five years climatological data acquired from Tropospheric Observatory Data Acquisition Network (TRODAN) of the Centre for Atmospheric Research (CAR). The performance of five different XPD models namely: recent ITU-R 618-15, old ITU-R-722, SIM, CHU and DHW were tested based on the level of estimated interference due to tropospheric effects. The results revealed that at Ku-band uplink/downlink frequencies (10/14 GHz), the unwanted signals will completely overshadow the co-polarized signal between 0.001 and 0.018% of time for the new ITU-R model at Jos, Yola, Sokoto and Nsukka. However, at fade level between 8.93 and 11.08 dB, the crosstalk is prevalent at the receiving station. Though, the recent ITU-R model received wider acceptance, the performance in this study is ranked the lowest while SIM model displayed the best performance. The study therefore recommends, SIM model as the most preferred model to be adopted in estimating depolarization in this region. However, further experimental data for validation is suggested to substantiate this assertion. Key words: Millimeter waves, cross polarization discrimination (XPD) models, co-polar attenuation, earth-space paths, tropical region.

Cross‐polarisation discrimination models assessment and improvement on earth‐space propagation paths at Ka and V‐bands

The performance of the satellite communication systems employing polarisation diversity or frequency-reuse schemes to improve the spectral efficiency is degraded due to the depolarisation-induced interference originated by raindrops and ice particles present along the Earth-Space propagation path. Two models account for both rain and ice contributions. One predicts the long-term cumulative distribution function (CDF) of cross-polarisation discrimination (XPD). The other predicts the relationship between XPD and co-polar attenuation (CPA) and it was derived considering exclusively data at the V-band. In this study, the former model is improved by considering the individual ice and rain contributions and their combined effects, while the latter is validated against new measurements at the Ka band. New models are then proposed for the XPD-CPA relationships at the Ka-band taking into account both rain and ice contributions and also their combined effects. Finally, the predictions provided by the first model are usually converted to the corresponding XPD-CPA relationship using the long-term first-order statistics of rain attenuation, (incorrectly) considering that the equiprobability hypothesis applies. A new approach for the conversion of the XPD CDF into the corresponding XPD-CPA relationship is presented.

Statistical Studies of Cross-Polarization Due to Subtropical Rain on SHF Radio Propagation Paths Over Some Stations in South Africa

The employment of frequency re-use in orthogonal polarization continues to gain recognition due to the congestion in the lower frequency bands and the demand for more satellite service applications. However, communication signals operating above 10 GHz and adopting frequency re-use in orthogonal polarization, are often hampered by cross-polarization among others. In this paper, an attempt has been made to estimate cross-polarization discrimination (XPD) due to subtropical rain at super high frequency ranges in order to estimate accurately the signal total availability needed for satellite services in this region. Cross-polarization has been estimated based on the ITU model in the recommendation 618-9 over the Intelsat 17 (IS-17) Satellite (Geostationary at 66°E) with its service footprint links, at the respective elevation angle, to each of the nine locations within the provinces of South Africa. The estimation is based on modification using log-normal distributions of raindrop sizes at 20 °C and assuming circular polarization for Earth-space propagation paths. Some results of the influence of co-polar attenuation, frequency and subtropical rainfall rates are investigated. The overall information will enable the system provider to locate repeater stations for optimum performances within the provinces of South Africa.

Four Years of Total Attenuation Statistics of Earth-Space Propagation Experiments at Ka-Band in Toulouse

The Ka-band propagation experiments conducted by ONERA in Toulouse (43.57°E, 1.47°N) in the southwest of France started in 2009 and is still on-going. The equipment comprises a beacon Earth station, a profiling radiometer, and a rain gauge. The ground station measures the received beacon signal using a 10-Hz sampling rate. The profiling radiometer measures the sky brightness temperatures at five Ka-band and seven V-band channels, surface temperature, surface humidity, and surface pressure. From July 2009 to March 2011, the beacon receiver recorded the 19.7-GHz (horizontal polarization) HotBird 6 beacon signal along a slant path of 38.6° of elevation angle. Since April 2011, the beacon receiver has been recording the 20.2-GHz (vertical polarization) Astra 3B beacon signal along a slant path of 35.1° of elevation angle. This paper aims at providing a complete description of the ONERA Data Processing Tool (in particular the methodology followed to retrieve total attenuation) used to compute 4 years (from July 2009 to June 2013) of copolar attenuation statistics. The experimental setup and the characteristics of the Earth-space links are briefly described. The complementary cumulative distribution function (CCDF) of total attenuation for the whole period is presented and compared with ITU-R recommendations. The measured CCDF of the rainfall rate is computed and compared with ITU-R Rec. P.837 and will also be used as input for the rain attenuation model given in ITU-R Rec. P.618. The measured CCDFs of total attenuation duration and total attenuation slope are also presented.

Effects of melting layer on Ku-band signal depolarization

Propagation effects on Ku-band over an earth-space path is carried out at Kolkata, India, a tropical location, by receiving a Ku-band signal with horizontal plane polarization transmitted from the geostationary satellite NSS-6 (at 95°E). The amplitude of co-polar attenuation has been monitored along with the measurements of rain rate, rain drop size distribution and height profile of rain rate. The cross-polar enhancement of the signal is also monitored by receiving the same signal in orthogonal direction with another identical receiver. The experimental observations are used to study the effect of melting layer on both co-polar attenuation and cross-polar enhancement for the rain events observed during 2012–2013. Melting layer is indicated by the bright band signature in vertical profile of rain rate. The ground based drop size measurements indicate that the stratiform rain has more number of small drops whereas convective rain composed of large rain drops. The results indicate that the depolarization due to melting layer is more dominant compared to that due to the drop deformation mechanism at low rain rates.

Estimation of cross-polarization due to rain over some stations in Nigeria

The statistical morphology of cross-polarization due to rain is of considerable interest to system designers in order to estimate accurately the signal total availability of time. In this paper, an attempt has been made to compute cross-polarization of millimeters waves in the Ku- and Ka-band frequencies range due to rain. ITU-R P.618-9 model has been used to predict cross-polarization discrimination (XPD) over three different elevation angles in Nigeria with modification using log-normal distributions of raindrop sizes at 20°C and assuming circular polarization for Earth–satellite propagation paths. The study includes a number of earth stations in Nigeria, namely Akure, Ikeja, Calabar, Minna, and Kano. Variations of co-polar attenuation with XPD as well as cumulative distribution of XPD have also been presented. The result of the estimation of XPD at various elevations angles and at the frequencies considered shows the dependence of XPD on frequency, co-polar attenuation, and rainfall rates. The orthogonal channels at the same frequency situated at the coastal region is prone to higher amount of unwanted signal mostly at the elevation angle of 23° (for communication links over the Indian Ocean) than at other elevation angles considered in this report.

Calculations of cross‐polar discrimination spread for 20 GHz fixed satellite systems using rain microstructure information

Individual drop images in natural rain from a two‐dimensional video disdrometer (2DVD) have been processed to obtain their shapes, sizes, and orientation angles, which, in turn, have been used to derive their complex forward scattering amplitudes at 19.701 GHz. Using these calculations, the specific attenuation for horizontal and vertical polarizations and the specific differential attenuation and the specific differential phase of all drops over 1 min are determined. A point‐to‐path scaling is used to derive the total rain attenuation for both polarizations as well as the total differential attenuation and the total differential phase. A 19.701 GHz experimental Earth‐space link is simulated to determine the variation of cross‐polar discrimination versus copolar attenuation (XPD‐CPA) and the calculations are compared with 2 years of joint probability XPD‐CPA beacon measurements. The comparisons indicate clearly that the variations in drop shape and drop orientation can give rise to significant spread in XPD.

Specific attenuation and depolarisation in rain from 2-dimensional video disdrometer data

Data from 2-dimensional video disdrometers (2DVD) taken in different locations are used to derive the specific attenuation for horizontal and vertical polarisations at 20 GHz. The data include both drop-size distribution and their axis ratio distributions. T-matrix calculations using data over 1 min integration time show a power-law dependence on rain rate. The best-fit coefficients are compared with those given in the ITU-R Recommendation P. 838, both versions 2 and 3. Closest agreement is seen for vertical polarisation for version 2 and the largest discrepancies are seen for horizontal polarisation for version 3. The discrepancies are attributed to the different assumptions in the drop-size distributions, as well as the upper limit of integration for deriving the specific attenuation. The variation of the crosspolar discrimination (XPD) against copolar attenuation (CPA) is also examined. The 2DVD data are used to simulate a beacon experimental scenario at 20 GHz, the actual locations of the two instruments being different but climatically similar. Good agreement is seen with one year of beacon measurements, but the latter shows more spread in the variation, partly attributed to drop oscillations. Calculations also show that XPD-CPA variation is sensitive to the assumed drop shapes.

Eight years of ITALSAT copolar attenuation statistics at Spino d'Adda

The ITALSAT propagation experiment started in early 1991 and ended in January 2001. A receiving station for the radio-wave propagation experiment at 18.7, 39.6 and 49.5 GHz was installed in 1992 at...

Computer simulation of multipath depolarization due to clear air

Considering the different heights of transmitting and receiving antennas and the height variation of the superrefractive layer, the multipath depolarization statistical distributions due to clear air, the equiprobability linear relationship between cross-polarization (XPD) and copolarized attenuation (CPA) is studied based on computer simulation at a 7.6 GHz frequency. The computer simulation results show that the slope values of the equiprobability linear relationship between XPD and CPA vary in the interval from −1 to 0, which is different from earlier theoretical results. ©1999 John Wiley & Sons, Inc. Microwave Opt Technol Lett 23: 4–7, 1999.

Cross polarization on line-of-sight links in a tropical location: effects of the variation in canting angle and rain dropsize distributions

The effects of the variation in canting angle of falling raindrops and the distribution of raindrop sizes in different types of rain on cross polarization discrimination (XPD) on line-of-sight propagation paths in a tropical location in the frequency range 1-50 GHz are investigated. The dropsize distribution (DSD) model of Adimula and Ajayi (see Ann. Telecomm., vol.51, no.1/2, p.87-93, 1996) has been used. Although, some previous studies of XPD assumed equi-orientation of the raindrops along the propagation paths, the present study employs the more realistic distribution of canting angles along the path. The results obtained show that the XPD improves by about 4-7 dB over those based on the equi-orientation model. It is also shown that for the same copolar fade and for frequencies greater than about 10 GHz, the variation of the XPD with copolar attenuation (CPA) is relatively insensitive to the assumed DSD in rain and that the deterioration in signal quality or outage will be influenced more by the signal attenuation rather than by the cross-polarization interference.

Slant-path attenuation and depolarisation statistics at the Ka-band in Belgium

Experimental results of long term attenuation and depolarisation statistics at 20 and 30 GHz using ESA's Olympus satellite are discussed and compared with current prediction methods. While copolar attenuation (CPA) is accurately estimated, crosspolar attenuation (XPD) is poorly predicted, mainly owing to an inadequate account of the ice depolarisation effect.

Rain induced cross polarization at millimeter waves in a Xi'an environment

The rainfall intensity accumulative distribution, raindrop size and canting angle distributions in Xi'an(108.9 E, 34.3 N) have been obtained from data measured over a long period of time. Rain induced crose polarization in this environment has been studied. The crose polarization discrimination, XPD, was compu ted over a frequency range of 19.3 to 300 GHz for non spherical drops. The variations of XPD with frequency, rainfall rate and copolar attenuation, CPA, were investigated. The computational resulte are in good agreement with experimental data. A mathematical relationship was established between the XPD and CPA, raindrop canting angle, which result is in good agreement with experimental data. The cumulative distribution of rain induced cross polarization for eleven years and in month have been ob tained with the cumulative distribution of the rain rate from the data measured from 1975 to 1985. The role of multiple scattering also has been discussed, it is shown that the multiple scattering play an important part in short millimeter wave.

Ice depolarizations on Ka band (20 GHz) satellite‐to‐ground path and correlation with radar observations

The ice depolarization events without appreciable copolarized attenuation are investigated using the CS‐2 beacon signal (19.45 GHz) and the simultaneous radar observations. In these events, increase of instantaneous depolarizations of the beacon signal closely corresponds to appearance of intense radar echoes at the cloud top. Correlation of the time variations between the depolarizations and the radar echoes is, in general, found in the upper half of the ice region. In addition, the large depolarization events are primarily related to the radar echo intensities at 2.0–2.5 km above the bright band. In these heights the ice crystals may have the fastest growth rate around the temperature of −15°C. The impact of ice depolarizations on the satellite‐to‐ground path is shown to depend strongly upon the height and the background temperature at which ice crystals are first produced near the cloud top. This characteristic feature of ice depolarization is also confirmed in the stratus type events with considerable rain attenuation by theoretically subtracting the effects of raindrops from the total observed depolarization.

Distinction between rain and ice depolarisation by calculation of differential attenuation and phase shift

System-induced depolarisation of single-polarised satellite beacon signals can be cancelled with vector cancellation. It is demonstrated that the residual error after application of this bias removal technique depends only on the polarisation tilt angle and the XPD of the satellite antenna. It appears that for the calculation of the atmospheric-induced differential attenuation A and phase shift β, correction of the system-induced differential phase shift for the copolar and crosspolar signals is necessary. An experiment is described to measure this system-induced differential phase shift. It is demonstrated that it is possible to clearly distinguish between rain and ice depolarisation by looking at the scatterplots of the calculated A and β against measured copolar attenuation.

815 MHz radio attenuation measured within two commercial buildings

Copolarized attenuation measurements were made at 815 MHz inside two office/laboratory buildings. For the propagation paths along a straight corridor and into a adjoining room received signal power decayed with distance as d/sup -3.6/. Signal powers higher than their free-space values were routinely measured for line-of-sight paths along a corridor. As expected, isolation between floors and between parallel corridors is heavily dependent on building construction techniques. Energy propagates selectively in directions not blocked by large amounts of metal. Cross-floor isolations of 26 dB were measured between floors separated by solid sheet metal. Coupling between otherwise isolated corridors was produced by an open central cavity in the larger building.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Distributions of the cross‐polarization discrimination and the copolarized attenuation due to multipath in clear air

For multipath propagation in spherically stratified clear air, the formulas are derived for the phase difference between the direct ray and the multipath ray reflected from an atmospheric layer, and for their arrival angles. With the earth's curvature set to zero, the above formulas will reduce to those currently in use for a plane‐stratified atmosphere. With the formulas here derived and the empirical cumulative distribution of refractive index gradients, theoretical expressions for the statistical distributions of the cross‐polarization discrimination (XPD) and copolarized attenuation (CPA) are obtained. Furthermore, the relationship between equiprobable values of XPD and CPA is found to be linear. All the above formulas are related to the frequency, the path length, meteorological parameters and cross‐polar antenna patterns. The theoretically predicted results for XPD and CPA agree well with experimental data for several paths.