1. ECE 5233 Satellite Communications Prepared by: Dr. Ivica Kostanic Lecture 15: Secondary atmospheric losses effects (Section 8.5-8.7) Spring 2011

2. Florida Institute of technologies Page 2  Tropospheric scintillation (refractive effects)  Ionospheric scintillation  Faraday rotation (polarization loss)  Rain and ice crystal depolarization  Propagation impairment counter measures Outline Important note: Slides present summary of the results. Detailed derivations are given in notes.

3. Florida Institute of technologies Tropospheric scintillation  Losses associated with variations of the atmosphere close to the ground  Due to weather conditions (heating and cooling), the refractive index of the atmosphere changes  Change of refractive index changes the direction of signal propagation  Change of direction of arrival is “modulated” by antenna pattern -> causes signal fluctuation  Scintillation is more pronounced for higher frequencies  Scintillation does not cause depolarization  At low elevation angles (< 10 deg), scintillation may cause path loss behavior similar to terrestrial multipath fading Page 3 Physical explanation of atmospheric scintillation

4. Florida Institute of technologies Tropospheric scintillation - modeling  Scintillation losses depend on oOperating frequency oClimate oSatellite elevation oAntenna beam  Modeled as additional random path loss  Mitigation approaches oFade margin oError control coding Page 4 Example. Scintilation losses may be modeled as a random variable with a PDF given by: Where  is 1.2 dB. Estimate required design margin to guarantee reliability of 90% with respect to the scintillation losses. Answer: 2dB Example of scintilation losses

5. Florida Institute of technologies Ionospheric scintillation  Energy from the sun causes variations to total electron content in the ionosphere  Typical range 10 18 during day, 10 16 during night  At the local sunsets/sunrises there are rapid changes of concentration that cause changes of magnitude and phase of radio waves  The changes are further modulated by the antenna pattern  The net result are variations of the RSL at sunset and down  Magnitude of the ionosphere scintillation varies with sun activity Page 5

6. Florida Institute of technologies Faraday rotation – polarization loss  Radio waves propagate through Earths magnetic field  Magnetic field changes the polarization of the wave  Two negative effects: oIncreased losses due to polarization mismatch between RX antenna and radio wave oIncreased adjacent channel interference  The rotation angle depends on oLength of the path through ionosphere oConcentration of ionosphere charges oOperating frequency  The effects becomes smaller with frequency increase Page 6 Illustration of Faraday’s rotation Magnetic field of the Earth Estimation of losses  – Faraday’s rotation angle

7. Florida Institute of technologies Depolarization losses  Rain affects two polarizations in a different way  Rain attenuates horizontal component more than the vertical one  If a linearly polarized wave has a general orientation w.r.t. rainfall, the wave tilts towards vertical polarization  In a non-wind condition, raindrops have elliptical shape with minor axis in the vertical direction  In wind-conditions, the orientation of the raindrop ellipse changes – canting angle Page 7 Definition of canting angle

8. Florida Institute of technologies Tilt angle  Due to geometry – vertically polarized transmission from the satellite is received at a tilted angle  Tilt depends on the earth station location  May be estimated using Page 8 L e – latitude of earth station l e – longitude of earth station l s – longituide of su-satellite point

9. Florida Institute of technologies Prediction of XPD losses (ITU-R P.618-6)  Algorithm provided in the text book  Consists of eight steps  Review with students Page 9

10. Florida Institute of technologies Propagation impairment counter measures  Adaptive power control  Diversity reception/transmission  Signal processing (on-board processing)  Adaptive modulation and coding  Adaptive power control oTX power adjusted to compensate for losses oPower control usually operates in closed loop  Measurement at the RX compared against threshold  If the signal falls below threshold – feedback is sent to TX Page 10  Diversity reception/transmission  Used in high capacity FSS hubs  The signal is received/transmitted from multiple location on the ground  Probability of simultaneous fades is reduced with separation between earth stations  Signal (on-board processing)  Used in VSAT systems  Uplink demodulated to the baseband and rerouted towards different antenna beams  Each beam examined independently where rate, power, coding and modulation may be varied depending in the path loss

11. Florida Institute of technologies Propagation impairment counter measures  Adaptive modulation and coding oIdea: Modulation and coding changes as a function of SNR oThe lower SNR – more robust modulation and coding oThe lower SNR – lower data rate oLink designed for availability at the worst conditions (at the lowest rate) oIf the conditions are better than worst case – higher throughput is achieved Page 11 AMC example for DVBS-2 standard