ECE 5233 Satellite Communications Prepared by: Dr. Ivica Kostanic Lecture 13: Propagation effect and link margin calculation (Section 8.1-8.3) Spring 2014

Outline Design for reliability Components of the atmospheric losses Abortion losses Cloud losses Losses associated with rain Important note: Slides present summary of the results. Detailed derivations are given in notes.

Link performance and availability Key link budget equation Pr – received power EiRP – effective radiated power FSPL – free space path loss La – atmospheric losses Note: La is a random variable that changes due to condition of the atmosphere between TX and RX Two thresholds are defined Performance threshold – link’s performance above target Availability threshold – link is not available due to bad performance

Components of atmospherics losses Many components of loss (green – attenuation, blue – depolarization and refraction) Atmospheric absorption (gaseous effects) Cloud attenuation (aerosol and ice particles) Rain attenuation Tropospheric scintillation (refractive effects) Ionospheric scintillation Faraday rotation (polarization loss) Rain and ice crystal depolarization Losses are frequency dependent – affects different bands in different manner All losses are random variables with spatial and temporal distribution Many years of careful measurements have established spatial and temporal distribution of the loss contributing components The most significant attenuation comes from rain effects (C, Ku and Ka bands) Note: Link design is performed with a margin that ensures that the system performance and availability targets are met for desired time. The margin is referred to as the “fade margin”

Atmospheric absorption Atmospheric attenuation due to oxygen and water Oxygen absorption peaks: 60GHz and 120GHs Water vapor absorption peaks: 22GHz and 185GHz Attenuation is relatively small for all frequencies below 50GHz ITU graph provides zenith attenuation. For other elevation angles Note: typically margin of about 1dB is used for atmospheric losses Range of interest for commercial satellites

Cloud attenuation Become important for frequencies above 10GHz Difficult to predict due to wide variety of cloud types On the order of 0.1 to 0.2 dB/km Increases with frequency and temperature Increases with lower elevation angles Typical margin 1-2dB at frequencies around 30GHz (smaller for frequencies below) Reference: G. W. Stimson, Airborne Radar, SciTech Publishing, 1998 Note: the length of the satellite link path through the clouds is usually quite small except for very low elevation angles

Rain attenuation Most significant source of attenuation in C, Ku and Ka satellite bands Random attenuation – needs to be dealt with using probability tools Three basic steps in calculating rain attenuation Step 1: determine the rain rate threshold exceeded at a given link reliability threshold Step 2: determine specific attenuation in dB/km corresponding to the rain rate Step 3: estimate the effective length of the path and calculate the overall rain attenuation There are two broad categories Stratiform rain Convective rain Over large geographic areas Relatively low rain rate Over small geographical areas High intensity thunderstorm Irregular profile of the rainfall Satellite links reliability depends mostly on convective rains. There is usually enough margin in the link design for the low rates associated with the stratiform rains Note 1: only small portion of the satellite path goes through rain Note 2: rain intensity along the path may vary

Rain characterization Principle tool – cumulative distribution function (CDF) of rain rate Curves accumulated over many years and are accurate on average Significant variability from year to year – especially at low time percentages of interest to satellite design Different sources provide different averaging time (time resolution) ITU recommends using 1min resolution Different sources provide different spatial averaging ITU recommends interpolation methods Example CCDF curve for rain rate Note: CCDF curves are frequently given in tabular format

Rain climate maps Developed by ITU World divided into 15 different regions (A-Q) Rain rate CDF tabulated for each climate region The CDF based on long term average and are within +/- 10% Not very accurate, but simple and widely used for basic calculations ITU climate map for the US Climate map rain rate CCDF in mm/h Percentage of time (%) A B C D E F G H J K L M N P Q 10 0.1 0.5 0.7 2.1 0.6 1.7 3 2 8 1.5 4 5 12 24 0.3 0.8 2.8 4.5 2.4 7 13 4.2 11 15 34 49 6 20 22 35 65 72 0.03 9 18 28 33 40 105 96 0.01 19 30 32 60 63 95 145 115 0.003 14 21 26 29 41 54 45 55 140 200 142 0.001 42 70 78 83 150 120 180 250 170

Rainfall exceedance contour maps Developed by ITU Provide 1 min temporal resolution ITU REC P.837-x provides method for calculating CCDF of rain rates Recommendation provides 0.01 % maps Note: ITU REC P.837-5 is uploaded to the website Example of ITU rainfall exceedance map (0.01%)

Example Determine rain rate exceeded in 0.1% of time for Melbourne, FL Using ITU climate maps Using ITU exceedance curves Answer: 95 mm/hour 80 mm/hour Note: Melbourne is close to boundary between regions M and N. Using average between the two: (65+95)/2=80mm/hour