LINK BUDGET CALCULATIONS FOR A SATELLITE LINK WITH AN .
LINK BUDGET CALCULATIONS FOR A SATELLITE LINK WITHAN ELECTRONICALLY STEERABLE ANTENNA TERMINAL1 JUNE 2019793-00004-000-REV01 2019 Kymeta Corporation and its affiliates. KYMETA, CONNECTED BY KYMETA, MTENNA, KĀLO image, and KĀLO are trademarks of Kymeta Corporation, with registrations or pending applications forthese marks in Brazil, the European Union, Japan, Norway, Singapore, South Korea, and the United States. All other trademarks are the property of their respective owners.
TABLE OF CONTENTS1 Introduction .12 The ESA compared to the parabolic dish.13 The three types of link budgets.33.1 FWD link: hub to terminal.43.2 Simple RTN link: terminal to satellite.43.3 Complex RTN link: terminal to satellite and satellite to hub.44 Link components and their parameters.54.1 Terminal .54.1.1Antenna scan angle and cosine roll-off.54.1.2Gain, noise temperature, and G/T.64.1.2.14.1.3Terminal Noise Temperature.6EIRP and EIRP density.84.2 Transmission medium.84.2.1Free-Space Path Loss (FSPL).94.2.2Atmospheric loss.104.2.3Rain fade and availability.114.3 Satellite .134.3.1Transponder EIRP.134.3.2Saturation Flux Density (SFD), HPA back-off, and satellite G/T.144.3.2.1Additional Factors Affecting.154.4 Hub .154.4.1G/T.155 Calculating SNR .165.1 FWD equation.165.2 Simple RTN equation.165.3 Complex RTN equation.175.4 If satellite and hub parameters are not available.176 Converting SNR to spectral efficiency and throughput.176.1 MODCOD .186.2 Spectral efficiency (SE).186.3 Throughput (data rate).197 Example link budget SNR calculations.207.1FWD link.207.2 Simple RTN link.217.3 Complex RTN link.218 Conclusion .229 References .23 2019 Kymeta Corporation and its affiliates. KYMETA, CONNECTED BY KYMETA, MTENNA, KĀLO image, and KĀLO are trademarks of Kymeta Corporation, with registrations or pending applications forthese marks in Brazil, the European Union, Japan, Norway, Singapore, South Korea, and the United States. All other trademarks are the property of their respective owners.
1 INTRODUCTIONWhen designing a telecommunications system, one of the first andmost important steps is to calculate a link budget (also called a linkanalysis). The link budget is a theoretical calculation of the end-to-endperformance of the communications link. It accounts for all the gainsand losses of the link under a specific set of conditions, and the result ofthe analysis is a set of figures of merit that characterizes the quality ofthe link. The most common figures of merit are: SNR (signal-to-noise ratio) Spectral efficiency (bits per second/Hz) Throughput (bits per second)This paper shows you how to calculate the link budget for a satellite linkthat uses a flat panel, electronically steerable antenna (ESA) at the userterminal in mobility and fixed configurations. Before performing someexample calculations, we’ll first look briefly at the differences between atraditional parabolic dish antenna and an ESA, define three basic typesof link budgets, and then identify the basic components of a satellite linkand their associated gains and losses.2 THE ESA COMPARED TO THE PARABOLIC DISHThe conventional method of communicating with a geostationarysatellite uses a parabolic dish antenna, which consists of a feed systemand a passive parabolic reflector (Figure 1). The feed antenna ispositioned in front of the reflector and illuminates it with radiation. Theparabolic shape of the reflector ensures that the paths of the radiationfrom the feed antenna to the reflector and outwardsfrom the reflector are all the same length, so thatthey combine in phase to produce a plane wave.Also, the paths from the reflector outward are all inparallel. The result is an accurate, highly directionalbeam. The directivity of the dish antenna also appliesto incoming signals, greatly reducing the receivedpower of signals coming from satellites or othersources that the antenna is not pointed at.The combination of high directivity and high radiationefficiency results in a high gain for the dish antenna.The gain can be further increased by increasing thesize of the dish.FIGURE 1. PARABOLIC DISH ANTENNAIn stationary installations, the dish antenna canbe set up, pointed at the satellite, and calibratedjust once. But if used in mobility applications, such as on a vehicle, itmust be repositioned whenever the vehicle moves to keep it pointedat the satellite. This requires a motorized mount, which adds bulk 2019 Kymeta Corporation and its affiliates. KYMETA, CONNECTED BY KYMETA, MTENNA, KĀLO image, and KĀLO are trademarks of Kymeta Corporation, with registrations or pending applications forthese marks in Brazil, the European Union, Japan, Norway, Singapore, South Korea, and the United States. All other trademarks are the property of their respective owners.1
and weight, often making the dish antenna slow and cumbersome, orsimply impractical for use, especially on smaller vehicles. The costs ofmanufacturing an accurate paraboloid reflector and a stable, precisegimbal mount can be high. Furthermore, themount is subject to wearand mechanicalfailure frommovement andfrom dirt intrusion.FIGURE 2. ELECTRONICALLY SCANNEDANTENNA (KYMETA)Flat panel antennas thatelectronically steer (scan) their beams to track satellitesare an increasingly popular alternative to dish antennas (Figure2). Although there are various technologies in use, most ESAs useelectronic control of the relative phase of the signals emitted from upto thousands of small antenna elements in a planar array to produceconstructive and destructive interference, forming a beam that can besteered to track a satellite much more quickly than mechanical systems.Since they have no external feed antenna or moving parts, ESAs canhave a low profile of only a few inches and a relatively low weight. Theycan be simply mounted on the roof or other external surface of a vehicle(Figure 3).The accuracy and speed of electronic beam steering means that ESAs,in comparison with dish antennas, can more easily stay connected to asatellite while the vehicle is moving. They can also be more easily sealedagainst weather and contamination than dish antenna assemblies. Theseadvantages make them ideal for use on mobile platforms of almostany size, including buses, first responder vehicles, cars, constructionvehicles, airplanes, and ships. Also, an ESA-based satelliteterminal can be built into a rugged, transportable case forquick setup and use anywhere there is satellite coverage.FIGURE 3. ESA MOUNTED ON THE ROOFOF A COMPACT SUVThe main disadvantages of ESAs compared to dish antennasare lower gain, and the variation in gain as the location andorientation are changed. For example, at high latitudes undergeostationary satellites, a horizontal ESA will have lessaperture area facing the satellite, resulting in lower efficiencythan a dish, which can be mechanically pointed at the satellitefor maximum gain. However, as non-geostationary LEO(Low Earth Orbit) and MEO (Medium Earth Orbit) satelliteconstellations become available, this problem will diminish,since the satellites are moving, so that there will always be asatellite overhead. Even with a fixed terminal, an ESA will havean advantage over a dish when using non-geostationary satellites, sinceit can track them without a gimballed tracking pedestal. 2019 Kymeta Corporation and its affiliates. KYMETA, CONNECTED BY KYMETA, MTENNA, KĀLO image, and KĀLO are trademarks of Kymeta Corporation, with registrations or pending applications forthese marks in Brazil, the European Union, Japan, Norway, Singapore, South Korea, and the United States. All other trademarks are the property of their respective owners.2
3 THE THREE TYPES OF LINK BUDGETSSatellite links are named according to the flow of information on them.On the forward (FWD) link, information flows from the hub (earthstation) to the user terminal. On the return (RTN) link, information flowsfrom the user to the hub. Figure 4 shows the hub, satellite, and userterminal and the direction of data flow for the FWD and RTN paths. TheFWD link budget is usually easiest to calculate, since good assumptionscan be made to simplify the calculations. For example, the output powerof the satellite can be assumed to be constant for the downlink to theterminal, since the hub has sufficient power to keep the satellite’s highpower amplifiers (HPAs) operating at saturation (or at the maximumpower set by the operator). However, with a small aperture antenna suchas an ESA, the uplink power from the terminal received by the satellitevaries with antenna orientation and is not typically high enough to drivethe HPAs to saturation, resulting in variable output power from thesatellite. This means the RTN link budget calculations are usually morecomplicated.SATELLITEFORWARD (FWD) LINKRETURN (RTN) LINKHUBTERMINALINTERNETKYMETAESAMODEMFIGURE 4. FORWARD (FWD) AND RETURN (RTN) LINKS 2019 Kymeta Corporation and its affiliates. KYMETA, CONNECTED BY KYMETA, MTENNA, KĀLO image, and KĀLO are trademarks of Kymeta Corporation, with registrations or pending applications forthese marks in Brazil, the European Union, Japan, Norway, Singapore, South Korea, and the United States. All other trademarks are the property of their respective owners.3
FWD LINK: HUB TO TERMINALAs explained above, the FWD link budget is the simplest to calculate,because the hub can be assumed to have sufficient uplink power to drivethe FWD carrier up to the limits of the satellite transponder, ensuringconstant output power. This means that our FWD analysis need onlyconsider the portion of the link that connects the satellite and the userterminal. SIMPLE RTN LINK: TERMINAL TO SATELLITEThe simple version of the RTN link budget considers only the uplink fromthe user terminal to the satellite, and so does not require knowledge ofcertain satellite and hub parameters, which can be difficult to obtain. COMPLEX RTN LINK: TERMINAL TO SATELLITE ANDSATELLITE TO HUBSince the power from the user terminal received by the satellite varieswith antenna orientation, and because this power is not enough to drivethe satellite output power to the maximum level, additional noise isintroduced into the link from satellite to hub. Therefore, an accuratecalculation of the SNR for the entire RTN link must consider:1.the SNR of the terminal-to-satellite link2. the SNR of the satellite-to-hub linkWhen the output power of the satellite is at a maximum, SNR #2 istypically at least an order of magnitude greater than SNR #1, so it can bedisregarded. But since the power density produced by the ESA terminalat the satellite is not high enough to yield maximum transponder outputpower, SNR #2 must be calculated and combined with SNR #1 to find theSNR of the complete RTN link.In the section below, we’ll identify the key components of the FWDand RTN links, and define the associated parameters and assumptionsused for the calculation of SNR. This paper does not cover all potentialsources of loss in satellite links, but it does include those that arecommon to all links. 2019 Kymeta Corporation and its affiliates. KYMETA, CONNECTED BY KYMETA, MTENNA, KĀLO image, and KĀLO are trademarks of Kymeta Corporation, with registrations or pending applications forthese marks in Brazil, the European Union, Japan, Norway, Singapore, South Korea, and the United States. All other trademarks are the property of their respective owners.4
4 LINK COMPONENTS AND THEIR PARAMETERS TERMINAL ANTENNA SCAN ANGLE AND COSINE ROLL-OFFTo determine the gain of an ESA terminal, we must know its scan angle.The scan angle is one of two angles that define the direction of thesatellite beam (known as the bores-ight vector) relative to the antennain a spherical coordinate system (Figure 5):Broadside Vector(perpendicular toplane of antenna)ScanAngleBoresight Vector(direction of beam)Plane ofAntennaFIGURE 5. ESA BEAM POINTING COORDINATES1. Theta (θ) is the scan angle. This is the angle between the boresightvector and the broadside vector (the axis that is perpendicular to theplane of the antenna). If the plane of the antenna panel is horizontal,the broadside vector is vertical.2. Phi (φ) is the second angle coordinate that determines the directionof the boresight vector. It is measured in the plane of the antennafrom a reference direction, which can be different for variousantennas. For the Kymeta u7 antenna, the reference is a lineconnecting the center with the midpoint of the right side (the y-axisin Figure 5), and phi is measured.For our calculations of SNR, we only need to consider theta, the scanangle. 2019 Kymeta Corporation and its affiliates. KYMETA, CONNECTED BY KYMETA, MTENNA, KĀLO image, and KĀLO are trademarks of Kymeta Corporation, with registrations or pending applications forthese marks in Brazil, the European Union, Japan, Norway, Singapore, South Korea, and the United States. All other trademarks are the property of their respective owners.5
In fixed applications, the broadside vector is normally pointed at thesatellite, so the angle between the broadside and boresight vectors(theta) is zero. In mobility scenarios, however, the antenna is typicallymounted horizontally, so that the broadside vector is always vertical,but since the boresight vector must always point at the satellite, thetais positive (unless the satellite is directly overhead). Since less antennaarea (aperture) is available to the beam in the case of a positive scanangle, the effective gain of the antenna is reduced. The reduction in gainwith increasing theta is called the cosine (or scan) roll-off, discussedbelow. GAIN, NOISE TEMPERATURE, AND G/TThe gain (G) of the terminal is easily calculated for both fixed andmobility installations:Where:Peak Gain (dBi) Peak gain of the antenna when the broadside andboresight vectors are aligned (θ 0), so that there is no loss of gaincaused by a reduction in the effective area of the antenna as seen by thesatellite. This is typically specified by the antenna manufacturer.Cosine roll-off An antenna coefficient for the reduction in gain causedby scan angle (θ). Also called scan roll-off. This is also provided by themanufacturer.However, the calculation of SNR requires that we determine a quantitycalled the gain-to-noise-temperature ratio, G/T, of the terminal. Thisratio is a critical figure of merit for the terminal, and a peak value (for θ 0) is usually provided by the manufacturer. T is the equivalent noisetemperature (in kelvins) of the receiving system, which includes theantenna itself and the RF chain, ending at the output of the LNB (lownoise block downconverter), assuming there is sufficient gain to makeany noise contribution after the LNB negligible. The next section showshow terminal noise temperature is determined. TERMINAL NOISE TEMPERATUREThere are three main contributors to the noise
the satellite output power to the maximum level, additional noise is introduced into the link from satellite to hub. Therefore, an accurate calculation of the SNR for the entire RTN link must consider: 1. the SNR of the terminal-to-satellite link 2. the SNR of the satellite-to-hub link When the output power of the satellite is at a maximum, SNR .
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