Satellite Communication: Radio Wave Propagation Challenges

1. Chapter 4. Radio Wave Propagation 第四章 無線電波之傳播 亞洲大學 資訊工程學系碩士班 呂克明教授 二○○六年十月九日

2. Chapter 4. Radio Wave Propagation(第四章無線電波之傳播) • Introduction (緒言) • Propagation concern for satellite communication systems (衛星通訊系統的傳播顧慮) • Atmospheric Losses (大氣層的耗損) • Ionospheric Effects (電離層的效應) • Rain Attenuation (雨衰) • Other Propagation Impairments (其他傳播的損耗) • Homework problems (習題)

3. Introduction (緒言) • A signal traveling between an earth station and a satellite must pass through the earth’s atmosphere, including the ionosphere, and this can introduce certain impairments (losses). • Layers in the earth’s atmosphere: • Troposphere (對流層, 同溫層下之大氣層, 在此層中雲形成, 對流擾亂發生,溫度隨高度而減低): 0-90 km (D layer 50-90 km, disappear after sunset) • Ionosphere (電離層): 90-400 km (E layer, or Kennelly-Heaviside [discovered in 1902 explained Italian G. Marconi {1874-1937}’s experiment in 1901] Layer, 90-150 km & F layer 150-400 km) • Free space: above 600 km

4. Attenuation and sky noise increase (衰減與天空雜訊之增加) Signal depolarization (訊號去極化) Refraction, atmospheric multi-path (折射, 大氣層多路徑折射) Signal scintillations (訊號閃爍) Atmospheric gases, cloud, rain, frequencies above about 10 GHz Rain, ice crystals, dual-polarization systems at C and Ku Bands Atmospheric gases, communication and tracking at low elevation angles Tropospheric (對流層) at frequencies above 10 GHz and low elevation angles; ionospheric (電離層) at frequencies below 10 GHz Propagation concern for satellite communication systems-I: (衛星通訊系統的傳播顧慮)

5. Propagation concern for satellite communication systems-II: (衛星通訊系統的傳播顧慮) • Reflection multi-path, blockage (多路徑反射, 阻隔) • Propagation delays, variation (傳播延遲, 變數) • Inter-systems interference (衛星系統的干擾) • Mobile satellite services (移動衛星通訊) • Precise timing and location systems (精準時間與位置) ; time-division multiple access (TDMA, 時分多址) systems • Mainly C band at present; rain scatter may be significant at higher frequencies

6. Atmospheric Losses(大氣層的耗損) • Atmospheric absorption (大氣層的吸收): energy absorbed by the atmospheric gases. • Atmospheric attenuation (大氣層的衰減): weather-related loss is atmospheric attenuation. • Two absorption frequencies (兩個吸收頻率): • 22.3 GHz (water vapor 水蒸氣H2O) and • 60 GHZ (oxygen 氧分子O2). • K-band is between 18 and 27 GHz that covers the absorption frequency (吸收頻率) of H2O, 22.3 GHz. • V-band is between 40 and 75 GHz that covers the absorption frequency (吸收頻率) of O2, 60 GHz.

7. Ionospheric Effects-I(電離層的效應) • The ionosphere(電離層), is the upper region of the earth’s atmosphere, which has been ionized, mainly by solar radiation (太陽的輻射), . • Ionospheric effects decrease as frequency increases (電離層的效應和頻率成反比), include: • Polarization rotation (極化角之旋轉) • Scintillations (閃爍, light pulses) • Absorption (吸收) • Variation in the direction of arrival (入射角的變化) • Propagation delay (傳播的延遲) • Dispersion (分散) • Frequency change (頻率的改變)

8. Ionospheric Effects-II(電離層的效應) • Only the polarization rotation (極化角之旋轉) and scintillation (閃爍) effects are of major concern for satellite communication.

9. Rain Attenuation & Other Propagation Impairment(雨衰與其他傳播的損耗) • Rain attenuation(雨衰) is a function of rain rate. • Rain rate (降雨量): By rain rate is meant the rate at which rainwater would accumulate in a rain guage situated at the ground in the region of interest, for example at an earth station. • Hail (冰雹), ice, and snow have little effect on attenuation because of the low water content. • Ice (碎冰) can cause depolarization (去極化).

10. Homework Problems(習題) • 4.1 With reference to Table 4.1, identify the propagation impairments which most affect transmission in the C-band. • 4.2 Repeat Prob. 4.1 for Ku-band transmission.