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!!!
6 05/02/23
Assmptions
Sub-satellite point
Longitudinal extremities of CONUS
Median Longitude of CONUS
CONUS
7 05/02/23
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
8 05/02/23
SC – Satellite EIRP
All values are in dB
Pmax – Bo,o
AiPi
GR/T
LTj
9 05/02/23
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
10 05/02/23
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
11 05/02/23
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
12 05/02/23
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
13 05/02/23
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
14 05/02/23
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
15 05/02/23
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
17 05/02/23
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
18 05/02/23
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
19 05/02/23
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
20 05/02/23
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
21 05/02/23
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
22 05/02/23
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
23 05/02/23
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)
26 05/02/23
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