ECE 5233 Satellite Communications

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

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

3. 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 Physical explanation of atmospheric scintillation

4. Tropospheric scintillation - modeling Example.Scintilation losses may be modeled as a random variable with a PDF given by: • Scintillation losses depend on • Operating frequency • Climate • Satellite elevation • Antenna beam • Modeled as additional random path loss • Mitigation approaches • Fade margin • Error control coding Where s is 1.2dB. Estimate required design margin to guarantee reliability of 90% with respect to the scintillation losses. Answer: 2dB Example of scintilation losses

5. Ionospheric scintillation • Energy from the sun causes variations to total electron content in the ionosphere • Typical range 1018 during day, 1016 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

6. Faraday rotation – polarization loss • Radio waves propagate through Earths magnetic field • Magnetic field changes the polarization of the wave • Two negative effects: • Increased losses due to polarization mismatch between RX antenna and radio wave • Increased adjacent channel interference • The rotation angle depends on • Length of the path through ionosphere • Concentration of ionosphere charges • Operating frequency • The effects becomes smaller with frequency increase Illustration of Faraday’s rotation Estimation of losses Magnetic field of the Earth b – Faraday’s rotation angle

7. 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 Definition of canting angle

8. 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 Le – latitude of earth station le – longitude of earth station ls – longituide of su-satellite point

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

10. Propagation impairment counter measures • 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 • Adaptive power control • Diversity reception/transmission • Signal processing (on-board processing) • Adaptive modulation and coding • Adaptive power control • TX power adjusted to compensate for losses • Power control usually operates in closed loop • Measurement at the RX compared against threshold • If the signal falls below threshold – feedback is sent to TX • 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. Propagation impairment counter measures • Adaptive modulation and coding • Idea: Modulation and coding changes as a function of SNR • The lower SNR – more robust modulation and coding • The lower SNR – lower data rate • Link designed for availability at the worst conditions (at the lowest rate) • If the conditions are better than worst case – higher throughput is achieved AMC example for DVBS-2 standard