ECE 5233 Satellite Communications
Prepared by:
Dr. Ivica Kostanic
Lecture 8: Satellite link design
(Section 4.1, 4.2)
Spring 2014
Outline
Objectives of link design
Elements of satellite link
Free space path loss equation
Signal to noise ratio and link capacity
Examples
Important note: Slides present summary of the results. Detailed
derivations are given in notes.
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Objective of a link analysis
 Link analysis determines properties of
satellite equipment (antennas, amplifiers,
data rate, etc.)
 Two links need to be planned
o Uplink – from ground to satellite
o Downlink – from satellite to ground
 Two way communication – 4 links (two
way maritime communications)
 One way communication – 2 links
(example – TV broadcast)
One way
communication
 Two links are not at the same frequency
 Two links may or may not be in the same
band
o Fixed / broadcast satellite services
– usually same band
o Mobile satellite services may use
different bands
 In some systems satellite links may be
combined with terrestrial returns
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Two way
communication
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Elements of a satellite link
 Transmit power
 TX antenna gain
 Path losses
o Free space
o TX/RX antenna losses
o Environmental losses
 RX antenna gain
 RX properties
o Noise temperature
o Sensitivity (S/N and ROC)
 Design margins required to guarantee
certain reliability
Note: satellite signals are usually very weak
– requires careful link budget planning
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Free space path loss – transmit side
 Free Space Path Losses (FSPL) due to
dispersion of EM wave energy
Power flux in the direction of
maximum radiation
 Antenna used to focus the energy of the wave in
the direction of the receiver
 Note: antenna gain is usually quoted in the
direction of radiation maximum. For other
direction need to use the actual radiation pattern
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PT GT
W
4R 2
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Free space path loss – receive side
Received power
PR  W  Ae 
PT GT
 Ae
2
4R
Using
2
Ae 
 GR
4
One obtains
PT GT GR
PR 
4R /  2
Effective antenna gain (effective aperture)
Ae   A A
A – aperture efficiency of the antenna (50-90%)
FSPL equation
FSPL  4R /  
2
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Free Space Path Loss (FSPL)
Equation for FSPL (linear)
FSPL  4R /  
2
R = distance between TX and RX
 = wavelength of the RF wave
Equation for FSPL (logarithmic) – Friis’ equations
FSPL  96.5  20 log d miles   20 log  f GHz 
FSPL  92.44  20 log d km  20 log  f GHz 
Notes:
FSPL grow 20dB/dec as a function of distance
FSPL grows 20dB/dec as a function of frequency
FSPL curves 1-32GHz
range
FSPL curves are straight lines in log-log coordinate system
For Geo-Stationary satellites – loss may be above 200dB!
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Additional losses
 Additional losses
o Misalignment of the antennas
o Atmospheric losses
o Radome losses
o Component mismatch losses
 The additional losses are taken into
account through appropriate design
margins
 Typical design margin 5-10dB
o Component accuracy
o Operating frequency
o Required reliability
Link equation
PR  EiRP  GR  FSPL  AL
AL – additional losses
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Shannon capacity formula
 Shannon capacity formula – establishes
fundamental limits on communication
 In the case of AWGN satellite channel
S

C  B  log 2 1  
 N
C – capacity of the channel in bits/sec
B – bandwidth of the channel in Hz
S/N – signal to noise ratio (linear)
Define g = R/B - bandwidth utilization in bps/Hz,
where R is the information rate in bps.
g
 E R
C
 log 2 1  b 
B
 N0 B 

g  log 2 1 

Eb 
 g 
N0 
Minimum energy per bit normalized to
noise power density that is required for a
given spectrum utilization
 Eb  2g  1
Eb
 min   
N0
g
 N0 
Note: g is the fundamental
measure of spectrum utilization.
Ultimate goal of every wireless
communication system is to
provide largest g for a given set
of constraints.
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Bandwidth utilization vs. power trade-off
 Bandwidth utilization increases with an
increase of available power
7
6
 In power limited regions small increase
of power produce significant increase in
bandwidth utilization
Spectral efficiency [bps/Hz]
Bandwidth limited
5
 In bandwidth limited region large power
increase is required for increase in
bandwidth utilization
4
 Eb  2g  1
Eb
 min   
N0
g
 N0 
3
 For systems that are in bandwidth
limited region – capacity is increased
through frequency reuse
2
Power limited
 By combining power and reuse methods,
contemporary systems reach spectrum
utilization of 3-7bps/Hz
1
0
0
2
4
6
8
10
12
14
EbNo ~ Power (linear ratio)
16
18
20
Note: most of contemporary satellite systems are bandwidth limited – lot of
efforts invested in means for spectrum reuse
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Examples
Example 4.2.1. A satellite at a distance of
40000km from a point on Earth surface
radiates power of 10W into antenna gain of
17dB. Find the flux density on the Earth
surface and the power received using
antenna with effective aperture of 10
square meters.
Example 4.2.2. The satellite in Example 4.2.1
operates at a frequency of 11GHz (Ku band).
The gain of the receiving antenna is 52.3dB.
Find the received power.
Answer:
Received power: -126dBW
Answers:
Flux density: 2.49e-14 W/m2
Received power: -126dBW (-96dBm)
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