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06/28/22 Analysis of Satellite Link Budgets (S band downlink)

Satellite Downlink Budget Analysis

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Text of Satellite Downlink Budget Analysis

The Digital Video Broadcast (DVB) Project* *
Hata, COST231, Walfisch-Ikegami, SUI
O2, H2O, Precipitation effects
F layer scintillations
Inter-Satellite Interference
Non-linear effects (Saleh Modeling)
OFDM vs TDM for SC
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ICO
TerraStar
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OFDM not suitable for satellite downlinks!!!
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CGC downlink – 800 MHz (UHF)
GEO orbit at 35788.925 km AMSL
CGC employs a TR(b) class transmitter
Uplink frequency and power is irrelevant
The orbital plane aligns with the equatorial plane
No Doppler shifts and TDM sync loss
The earth is round!!!
Pmax – Bo,o
Bo,o = Back Off at transponder output
GR = Gain at the receiver antenna
T = System Temperature
LTj = Total losses in the link received at receiver j
Free Space loss, Ls
Is not a practical construction
Isotropic Radiator distributes power evenly in a 360° steradian solid angle
Amount of power radiated by an “Isotropic Radiator” to produce the required amount of power in the direction of interest
Measured in dBW
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Broadband RF channel
Pre-Amp and Mixers
Non-linear characteristic
Efficiency at saturation is higher
Ill effects of saturation
AM/AM and AM/PM effects
Operating point needs to pushed back to the linear region of the characteristic
Typical value of OBO is 3 – 6 dB
Desired operating point
QPSK = 4QAM
Non-constant modulus
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Output is of the form
Saleh model parameters are used (ar, af, br, bf)
AM/AM Conversion
AM/PM Conversion
Rotation along the primary axis
Rounding along the edges
Constellations with circular symmetry are not susceptible to rotation or rounding!!!
APSK class modulation schemes are preferred over QAM class constellations
Additional back off of 1.5 dB
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Largest contributor to signal attenuation
Direct function of slant height, r
LoS distance from receiver location to satellite
Typical value ranges from 180 to 200 dB
Mean radius of the earth (6378.1 km)
Mean orbital height of GEOS (35786 km)
Latitude of receiver location
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Heavily frequency dependent
Contributes to log-normal attenuation
Assumed to be <2 dB overall for S band
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O2/H2O and F Layer
O2 and H2O attenuation is approx. 0.1 dB in the S band
More prevalent in Ku/Ka bands
Ionosphere is the uppermost active layer of earth’s atmosphere
D (50 to 90 km), E/Es (90 to 120 km) and F (120 to 400 km)
Ionized by solar radiation
Frequency dependent EM propagation characteristics
F layer splits into 2 sub-layers (F1 and F2) in the absence of sunlight
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F Layer
Short term variations in refractive index cause alternate signal fading and enhancement
Scintillation Index modeled as a ÖN process
About 2 dB in the S band
Wavelength
In F layer (about 600 km)
Irregularity autocorrelation distance
(about 1 km)
Gain of Rx antenna, GR is offset by system noise
Noise is introduced by thermal processes within silicon devices, metallic connects, cables (Johnson noise)
Antenna efficiency (60%)
Antenna temperature, Ta
Effective temperature of receiver (with cooled pre-amp) (about 100 K)
Ts is computed using Friis’ Equation
Typical values of GR is 100 to 120 dB (119 dB for a 2.1 GHz channel)
Ts is typically taken as 114 K
Cable and other losses may be assumed to be 4 dB
G/T values range from 20 to 26 dB
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More prevalent in urban environments where there are more obstacles
Multiple (and delayed) copies of the signal reach the same receiver
Superposition causes constructive and/or destructive interference
Slow vs. Fast fading
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No dominant carrier
Suitable to model terrestrial (CGC) links (gap filler to mobile receiver)
Rician Fading Channels
Follow Rician distribution
One dominant carrier
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Another generalization of Rayleigh fading
Follows a 1/kth power law, rather than a square root law
Is effective for both indoor as well as outdoor scenarios
Nakagami fading
k = 1 gives a Rayleigh fading characteristic
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Various empirical models have been developed
Okumura-Hata (Tokyo)
COST231
CCIR
COST231-Walfisch-Ikegami
SUI
These models account for height of cellular Tx towers, diffraction and scattering effects
Hata and COST231-WI models are the most commonly used in the L and S bands
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Works best for UHF and L band (<2 GHz) carriers
Extended Hata and Hata-Davidson are variants
Contain additional parameters
COST231-WI
Distinguishes between LoS and NLoS situations
Max. cell size of 5 km
Min. cell size of 200 m
COST231-WI is best suited for CGC
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Kepler’s Laws
Velocities differ at apogee and perigee and everywhere in between
Typical Doppler shifts of 75 Hz observed in simulations
May be mitigated by increasing the bandwidth of each subcarrier in an OFDM symbol
Difference in slant height, r at apogee and perigee positions mean that the signal take longer time to reach the earth
Effects the sync/timing system of TDM
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Peak to Average Power Ratio (PAPR)
In rare cases, all subcarriers of an OFDM symbol are transmitted at equal and peak power
Eg. For a 2K mode (2048 subcarriers per OFDM symbol), the PAPR is 33 dB
More likely (real) scenario gives a PAPR of 16 dB
Throws the operating point well into saturation
Intermodulation products increase system bandwidth
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