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Microwaves are a form of electromagnetic radiation with wavelengths ranging from about one meter to one millimeter; with frequencies between 300 MHz (1 m) and 300 GHz (1 mm). Different sources define different frequency ranges as microwaves; the above broad definition includes both UHF and EHF bands. A more common definition in radio engineering is the range between 1 and 100 GHz. In all cases, microwaves include the entire SHF band at minimum. Frequencies in the microwave range are often referred to by their IEEE radar band designations: S, C, X, Ku, K, or Ka band, or by similar NATO or EU designations.

Radio frequency (RF) is the oscillation rate of an alternating electric current or voltage or of a magnetic, electric or electromagnetic field or mechanical system in the frequency range from around twenty thousand times per second to around three hundred billion times per second. This is roughly between the upper limit of audio frequencies and the lower limit of infrared frequencies; these are the frequencies at which energy from an oscillating current can radiate off a conductor into space as radio waves. Different sources specify different upper and lower bounds for the frequency range.

Rain is liquid water in the form of droplets that have condensed from atmospheric water vapor and then become heavy enough to fall under gravity. Rain is a major component of the water cycle and is responsible for depositing most of the fresh water on the Earth. It provides suitable conditions for many types of ecosystems, as well as water for hydroelectric power plants and crop irrigation.

## Contents

Rain fade is usually estimated experimentally and also can be calculated theoretically using scattering theory of rain drops. Raindrop size distribution (DSD) is an important consideration for studying rain fade characteristics. [1] Various mathematical forms such as Gamma function, lognormal or exponential forms are usually used to model the DSD. Mie or Rayleigh scattering theory with point matching or t-matrix approach is used to calculate the scattering cross section, and specific rain attenuation. Since rain is a non-homogeneous process in both time and space, specific attenuation varies with location, time and rain type.

The raindrop size distribution (DSD), or granulometry of rain, is the distribution of the number of raindrops according to their diameter (D). Three processes account for the formation of drops: the accumulation of small drops on large drops and collisions between sizes. According to the time spent in the cloud, the vertical movement in it and the ambient temperature, the drops that have a very varied history and a distribution of diameters from a few micrometers to a few millimeters.

Total rain attenuation is also dependent upon the spatial structure of rain field. Horizontal, as well as vertical, extension of rain again varies for different rain type and location. Limit of the vertical rain region is usually assumed to coincide with 0˚ isotherm and called rain height. Melting layer height is also used as the limits of rain region and can be estimated from the bright band signature of radar reflectivity. [2] The horizontal rain structure is assumed to have a cellular form, called rain cell. Rain cell sizes can vary from a few hundred meters to several kilometers and dependent upon the rain type and location. Existence of very small size rain cells are recently observed in tropical rain. [3]

Possible ways to overcome the effects of rain fade are site diversity, uplink power control, variable rate encoding, and receiving antennas larger than the requested size for normal weather conditions.

Site diversity is one of six techniques used to improve the reliability of satellite communications by limit atmospheric effects, particularly those caused by rain fade. A diversity scheme is typically required when using frequencies in the Ka, V, or W-band. The downlink transmissions of satellites cover very large areas, that will have different weather. The site diversity technique consists of linking two or more ground stations receiving the same signal: this way, if the signal is heavily attenuated in one area, another ground stations can compensate it. These intense rain areas, for example, supercells, often have a horizontal length of no more than a few kilometers: putting the ground stations at a sufficient distance the possibility of rain fade in the downlink signal will be reduced.

The simplest way to compensate the rain fade effect in satellite communications is to increase the transmission power: this dynamic fade countermeasure is called uplink power control (UPC). Until more recently, uplink power control had limited use, since it required more powerful transmitters - ones that could normally run at lower levels and could be increased in power level on command (i.e. automatically). Also uplink power control could not provide very large signal margins without compressing the transmitting amplifier. Modern amplifiers coupled with advanced uplink power control systems that offer automatic controls to prevent transponder saturation make uplink power control systems an effective, affordable and easy solution to rain fade in satellite signals.

In terrestrial point to point microwave systems ranging from 11 GHz to 80 GHz, a parallel backup link can be installed alongside a rain fade prone higher bandwidth connection. In this arrangement, a primary link such as an 80 GHz 1 Gbit/s full duplex microwave bridge may be calculated to have a 99.9% availability rate over the period of one year. The calculated 99.9% availability rate means that the link may be down for a cumulative total of ten or more hours per year as the peaks of rain storms pass over the area. A secondary lower bandwidth link such as a 5.8 GHz based 100 Mbit/s bridge may be installed parallel to the primary link, with routers on both ends controlling automatic failover to the 100 Mbit/s bridge when the primary 1 Gbit/s link is down due to rain fade. Using this arrangement, high frequency point to point links (23 GHz+) may be installed to service locations many kilometers farther than could be served with a single link requiring 99.99% uptime over the course of one year.

## CCIR interpolation formula

It is possible to extrapolate the cumulative attenuation distribution at a given location by using the CCIR interpolation formula: [4]

Ap = A001 0.12 p(0.546 0.0043 log10p).

where Ap is the attenuation in dB exceeded for a p percentage of the time and A001 is the attenuation exceeded for 0.01% of the time.

## ITU-R frequency scaling formula

According to the ITU-R, [5] rain attenuation statistics can be scaled in frequency in the range 7 to 55 GHz by the formula

${\displaystyle {\frac {A_{2}}{A_{1}}}=\left({\frac {b_{2}}{b_{1}}}\right)^{1-1.12\cdot 10^{-3}{\sqrt {b_{2}/b_{1}}}(b_{1}A_{1})^{0.55}}}$

where

${\displaystyle b_{i}={\frac {f_{i}^{2}}{1+10^{-4}f_{i}^{2}}}}$

and f is the frequency in GHz.

## Related Research Articles

The Ku band is the portion of the electromagnetic spectrum in the microwave range of frequencies from 12 to 18 gigahertz (GHz). The symbol is short for "K-under", because it is the lower part of the original NATO K band, which was split into three bands because of the presence of the atmospheric water vapor resonance peak at 22.24 GHz, (1.35 cm) which made the center unusable for long range transmission. In radar applications, it ranges from 12-18 GHz according to the formal definition of radar frequency band nomenclature in IEEE Standard 521-2002.

In communications and electronic engineering, an intermediate frequency (IF) is a frequency to which a carrier wave is shifted as an intermediate step in transmission or reception. The intermediate frequency is created by mixing the carrier signal with a local oscillator signal in a process called heterodyning, resulting in a signal at the difference or beat frequency. Intermediate frequencies are used in superheterodyne radio receivers, in which an incoming signal is shifted to an IF for amplification before final detection is done.

A communication channel or simply channel refers either to a physical transmission medium such as a wire, or to a logical connection over a multiplexed medium such as a radio channel in telecommunications and computer networking. A channel is used to convey an information signal, for example a digital bit stream, from one or several senders to one or several receivers. A channel has a certain capacity for transmitting information, often measured by its bandwidth in Hz or its data rate in bits per second.

Ultra high frequency (UHF) is the ITU designation for radio frequencies in the range between 300 megahertz (MHz) and 3 gigahertz (GHz), also known as the decimetre band as the wavelengths range from one meter to one tenth of a meter. Radio waves with frequencies above the UHF band fall into the super-high frequency (SHF) or microwave frequency range. Lower frequency signals fall into the VHF or lower bands. UHF radio waves propagate mainly by line of sight; they are blocked by hills and large buildings although the transmission through building walls is strong enough for indoor reception. They are used for television broadcasting, cell phones, satellite communication including GPS, personal radio services including Wi-Fi and Bluetooth, walkie-talkies, cordless phones, and numerous other applications.

A satellite dish is a dish-shaped type of parabolic antenna designed to receive or transmit information by radio waves to or from a communication satellite. The term most commonly means a dish used by consumers to receive direct-broadcast satellite television from a direct broadcast satellite in geostationary orbit.

Radio propagation is the behavior of radio waves as they travel, or are propagated, from one point to another, or into various parts of the atmosphere. As a form of electromagnetic radiation, like light waves, radio waves are affected by the phenomena of reflection, refraction, diffraction, absorption, polarization, and scattering. Understanding the effects of varying conditions on radio propagation has many practical applications, from choosing frequencies for international shortwave broadcasters, to designing reliable mobile telephone systems, to radio navigation, to operation of radar systems.

The Ka band is a portion of the microwave part of the electromagnetic spectrum defined as frequencies in the range 26.5–40 gigahertz (GHz), i.e. wavelengths from slightly over one centimeter down to 7.5 millimeters. The band is called Ka, short for "K-above" because it is the upper part of the original NATO K band, which was split into three bands because of the presence of the atmospheric water vapor resonance peak at 22.24 GHz, (1.35 cm) which made the center unusable for long range transmission. The 30/20 GHz band is used in communications satellite uplinks in either the 27.5 GHz and 31 GHz bands, and high-resolution, close-range targeting radars aboard military airplanes. Some frequencies in this radio band are used for vehicle speed detection by law enforcement. The Kepler Mission used this frequency range to downlink the scientific data collected by the space telescope.

Extremely high frequency (EHF) is the International Telecommunication Union (ITU) designation for the band of radio frequencies in the electromagnetic spectrum from 30 to 300 gigahertz (GHz). It lies between the super high frequency band, and the far infrared band, the lower part of which is also referred to as the terahertz gap. Radio waves in this band have wavelengths from ten to one millimetre, so it is also called the millimetre band and radiation in this band is called millimetre waves, sometimes abbreviated MMW or mmW or mmWave. Millimetre-length electromagnetic waves were first investigated in the 1890s by Indian scientist Jagadish Chandra Bose.

Satellite Internet access is Internet access provided through communications satellites. Modern consumer grade satellite Internet service is typically provided to individual users through geostationary satellites that can offer relatively high data speeds, with newer satellites using Ku band to achieve downstream data speeds up to 506 Mbit/s.

NSS-6 is a communications satellite owned by SES WORLD SKIES.

Microwave transmission is the transmission of information by microwave radio waves. Although an experimental 40-mile (64 km) microwave telecommunication link across the English Channel was demonstrated in 1931, the development of radar in World War II provided the technology for practical exploitation of microwave communication. In the 1950s, large transcontinental microwave relay networks, consisting of chains of repeater stations linked by line-of-sight beams of microwaves were built in Europe and America to relay long distance telephone traffic and television programs between cities. Communication satellites which transferred data between ground stations by microwaves took over much long distance traffic in the 1960s. In recent years, there has been an explosive increase in use of the microwave spectrum by new telecommunication technologies such as wireless networks, and direct-broadcast satellites which broadcast television and radio directly into consumers' homes.

Satellite television is a service that delivers television programming to viewers by relaying it from a communications satellite orbiting the Earth directly to the viewer's location. The signals are received via an outdoor parabolic antenna commonly referred to as a satellite dish and a low-noise block downconverter.

A satellite truck is a mobile communications satellite earth station, typically mounted on a truck chassis as a platform. Employed in remote television broadcasts, satellite trucks transmit video signals back to studios or production facilities for editing and broadcast. Satellite trucks usually travel with a production truck, which contains video cameras, sound equipment and a crew. A satellite truck has a large satellite dish antenna which is pointed at a communications satellite, which then relays the signal back down to the studio. Satellite communication allows transmission from any location that the production truck can reach, provided a line of sight to the desired satellite is available.

Advanced Extremely High Frequency (AEHF) is a series of communications satellites operated by the United States Air Force Space Command. They will be used to relay secure communications for the Armed Forces of the United States, the British Armed Forces, the Canadian Armed Forces and the Royal Netherlands Armed Forces. The system will consist of six satellites in geostationary orbits, four of which have been launched. AEHF is backward compatible with, and will replace, the older Milstar system and will operate at 44 GHz Uplink and 20 GHz Downlink. AEHF systems is a joint service communications system that will provide survivable, global, secure, protected, and jam-resistant communications for high-priority military ground, sea and air assets. It is the follow-on to the Milstar system. AEHF systems' uplinks and crosslinks will operate in the extremely high frequency (EHF) range and downlinks in the super high frequency (SHF) range.

The Intelsat VI series of satellites were the 8th generation of geostationary communications satellites for the Intelsat Corporation. Designed and built by Hughes Aircraft Company (HAC) in 1983-1991, there were five VI-series satellites built: 601, 602, 603, 604, and 605.

The Unified S-band (USB) system was a tracking and communication system developed for the Apollo program by NASA and the Jet Propulsion Laboratory (JPL). It operated in the S band portion of the microwave spectrum, unifying voice communications, television, telemetry, command, tracking and ranging into a single system to save size and weight and simplify operations. The USB ground network was managed by the Goddard Space Flight Center (GSFC). Commercial contractors included Collins Radio, Blaw-Knox, Motorola and Energy Systems.

Intelsat 901 (IS-901) was the first of 9 new Intelsat satellites launched in June 2001 at 342°E, providing Ku-band spot beam coverage for Europe, as well as C-band coverage for the Atlantic Ocean region, and provides features such as selectable split uplink for SNG, tailored for increased communications demands such as DTH and Internet.

The C band is a designation by the Institute of Electrical and Electronics Engineers (IEEE) for a portion of the electromagnetic spectrum in the microwave range of frequencies ranging from 4.0 to 8.0 gigahertz (GHz); however, this definition is the one used by radar manufacturers and users, not necessarily by microwave radio telecommunications users. The C band is used for many satellite communications transmissions, some Wi-Fi devices, some cordless telephones as well as some surveillance and weather radar systems.

## References

1. Das, Saurabh; Maitra, Animesh; Shukla, Ashish K. (2010). "PIER B Online - Rain Attenuation Modeling in the 10-100 GHz Frequency Using Drop Size Distributions for Different Climatic Zones in Tropical India". Progress in Electromagnetics Research B. 25: 211–224. doi:10.2528/PIERB10072707.
2. Das, Saurabh; Maitra, Animesh; Shukla, Ashish K. (2011-07-01). "Melting layer characteristics at different climatic conditions in the Indian region: Ground based measurements and satellite observations". Atmospheric Research. 101 (1–2): 78–83. doi:10.1016/j.atmosres.2011.01.013.
3. Shukla, Ashish K.; Roy, Bijoy; Das, Saurabh; Charania, A. R.; Kavaiya, K. S.; Bandyopadhyay, Kalyan; Dasgupta, K. S. (2010-02-01). "Micro rain cell measurements in tropical India for site diversity fade mitigation estimation". Radio Science. 45 (1): RS1002. doi:10.1029/2008RS004093. ISSN   1944-799X.
4. CCIR [1990] Report 564-4 "Propagation data and prediction methods required for earth-space telecommunication systems"
5. “Propagation Data and Prediction Methods Required for the Design of Earth-Space Telecommunication Systems,” Recommendations of the ITU-R, Rec. P.618-10, 2009.