1 05/02/23

Analysis of Satellite Link Budgets (S band downlink)

2 05/02/23

Agenda

Coverage and Service Areas DVB-SH system architecture SC and CGC Orbits and Orbital Mechanics

Geosynchronous Earth Orbit (GEO) Inclined Orbits and its effects

Path Losses (Free Space, Propagation) Hata, COST231, Walfisch-Ikegami, SUI

O2, H2O, Precipitation effects Noise temperatures, G/T, sun outage

effects F layer scintillations

3 05/02/23

Agenda

Multipath Rayleigh, Rician, Log-Normal channel modeling

Inter-Satellite Interference Doppler Effects and Gap fillers Non-linear effects (Saleh Modeling) Choice of carrier frequency and

Modulation schemes OFDM vs TDM for SC

4 05/02/23

Initiative

DVB-SH system architecture solutions for 2 potential customers ICO TerraStar

Pre-sales “capabilities demo” for Motorola

5 05/02/23

The System

TDM

/OFD

M

TDM/

OFDM

OFDMOFD

MOFDM

BDN

DVB-SH broadcast head end

CP

Services

TR(a)

TR(c)

TR(b)

DVB-SH satellite

TDM mode influenced by DVB-S2

OFDM not suitable for satellite downlinks!!!

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Assmptions

Sub-satellite point

Longitudinal extremities of CONUS

Median Longitude of CONUS

CONUS

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Assumptions

SC downlink – 2.1 GHz (lower S band) 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!!! Geoid shape ignored

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SC – Satellite EIRP

All values are in dB

Pmax – Bo,o

AiPi

GR/T

LTj

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Satellite EIRP EIRP Pmax = total output power of satellite transponder Bo,o = Back Off at transponder output GR = Gain at the receiver antenna T = System Temperature

GR/T = Figure of Merit AiPi = Isotropic Power of the ith carrier LTj = Total losses in the link received at receiver j

Free Space loss, Ls Antenna Pointing losses Sun Outage loss Precipitation (Rain/Snow/Hail) loss Radome loss

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EIRP Effective/Equivalent Isotropic Radiated Power

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 dB over 1 W

Typical values range from 30 to 40 dBW

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Back Off Traveling Wave Tube Amplifiers (TWTA)

Broadband RF channel Acts as a simple amplifier

Pre-Amp and Mixers Converts from uplink to downlink frequency

Non-linear characteristic

Non-linear portion of characteristic

Linear portion of characteristic

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Back Off Maximum drive power of the TWTA leads to saturation

Efficiency at saturation is higher Ill effects of saturation

Intermodulation components 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 pointOBO

IBO

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Choice of modulation scheme Two popular schemes are:

APSK QPSK/QAM

QAM has a rectangular constellation map QPSK = 4QAM Non-constant modulus

APSK has constant modulus constellation map

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Input to amplifier is of the empirical form

Output is of the form

Saleh model parameters are used (ar, a, br, b)

Choice of modulation scheme

AM/AM Conversion

AM/PM Conversion

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Choice of modulation scheme A(.) and (.) cause distortion in the constellation map

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

16 05/02/23

Free Space Loss

Follows 1/r2 law of signal attenuation 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

Long. diff. btw receiver location and sub-satellite point

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Precipitation Loss Rain, Hail, Snow

Rain is the major contributor Heavily frequency dependent

More prevalent in the C, Ku and Ka bands Contributes to log-normal attenuation Raises the effective temperature and G/T Modeled using Mie Extinction Rate tables 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

Acts as a refractive medium for L band and above

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F Layer

Wavelength

Zenith angle at ionospheric intersectionpoint

Slant height to ionospheric irregularityIn F layer (about 600 km)

Irregularity autocorrelation distance(about 1 km)

Short term variations in refractive index cause alternate signal fading and enhancement

Scintillation Index modeled as a N process

About 2 dB in the S band

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Antenna Figure of Merit Defining characteristic of a Rx antenna 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%)

Carrier frequency (2.1 GHz)

Diameter of antenna dish

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Antenna Figure of Merit System Temperature, Ts

Generates noise equivalent to Johnson noise at that temperature

Antenna temperature, Ta

Ambient temperature, T0 (290 K) 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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Fading Occurs due to multipath effects

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 Shadowing

Flat vs. Frequency selective fading

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Fading Various models

Rayleigh Rician Weibull Log-Normal

Rayleigh Fading Channels Follow Rayleigh distribution Multiple scattered copies of the signal No dominant carrier Suitable to model terrestrial (CGC) links (gap filler to

mobile receiver) Rician Fading Channels

Follow Rician distribution Multiple scattered copies of the signal One dominant carrier Suitable to model satellite to ground links (SC)

24 05/02/23

Fading Weibull fading

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 Assumes an isotropic (360 degree) coverage of fading

environment k = 1 gives a Rayleigh fading characteristic

25 05/02/23

Terrestrial propagation loss CGC faces different propagation loss

characteristics compared to SC 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

SUI assumes mobile receivers rather than fixed gap fillers (TR(c) receivers)

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Terrestrial propagation loss Okumura-Hata

Originally modeled for urban areas (Tokyo) Works best for UHF and L band (<2 GHz) carriers Extended Hata and Hata-Davidson are variants

Contain additional parameters Slight variants for urban and suburban regions

COST231-WI European model (Stockholm) Works well for UHF, L and S bands 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

27 05/02/23

Doppler effects - CGC Similar to DVB-H

Shinkansen/Shanghai MaglevTGV

28 05/02/23

Doppler effects - SC Orbital drift Orbital plane at a non-zero angle w.r.t

equatorial plane Kepler’s Laws

Orbit becomes elliptical rather than circular 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

29 05/02/23

SC and OFDM 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 TDM (from DVB-S and DVB-S2) preferred over

OFDM in DVB-SH

30 05/02/23

Members

31 05/02/23

Thank You