Title: The Space Link
1The Space Link
- Paul Yang
- March 31, 2005
- EE 381k.3 Satellite Communications
- University of Texas at Austin
2Overview of Presentation
- Introduction
- EIRP
- Transmission Losses
- The Link Equation
- System Noise
- Carrier-to-Noise Ratio
- The Uplink
- The Downlink
- Effects of Rain
- Combined Uplink and Downlink C/N Ratio
- Intermodulation Noise
- System Design Example
- Satellite Communication Link Design Procedure
- Conclusion
3Introduction
- An engineer is one who can do for fifty cents
what any fool can do for a dollar. - Links between a satellite and an earth station
are designed to deliver information while meeting
a number of performance objectives - Tradeoffs quantity and quality of messages, cost
of equipment, development of technology at the
time
4EIRP
- EIRP Equivalent Isotropic Radiated Power
(above Flux density for isotropic source
radiating total power watts)
Figure ref. 2
(above Flux density for isotropic source
radiating total power watts)
5Transmission Losses
- Losses for clear weather conditions losses
which dont vary significantly with time and
losses which are calculated statistically - Free-space loss
- Antenna misalignment losses
- Fixed atmospheric and ionospheric losses
6Transmission Losses Free-space loss
- Free-space loss power loss that comes from the
spreading of the signal in space - Most significant type of transmission loss
where
and
Basic form of link equation
7Transmission Losses Feeder losses
- Basic form of link equation only accounts for
free-space loss - Other types of losses need to be accounted for
- Feeder losses Losses that occur between the
receiver antenna the receiver proper - Ex Losses in the connecting waveguides, filters,
and couplers - Receiver feeder losses RFL dB
8Transmission losses Antenna misalignment losses
Figure ref. 1
- Ideal situation earth station and satellite
antenna aligned for maximum gain - Two possible sources of off-axis loss satellite
and earth station - AML dB
9Transmission Losses Fixed atmospheric and
ionospheric losses
- Atmospheric gases result in losses by absorption
- atmospheric absorption loss AA dB
- Polarization loss
- angle of mismatch
- polarization loss PL dB
-
10The Link Equation
- Cumulative effect of losses for clear-sky
conditions - LOSSES FSL RFL AML AA PL
- FSL free-space loss
- RFL receiver feeder loss
- AML antenna misalignment loss
- AA atmospheric absorption loss
- PL polarization mismatch loss
(link equation with all losses taken into account)
11System Noise
- Random thermal motion in the resistive and active
devices in the receiver - Lossy components of antennas
- Available noise power
- Flat frequency spectrum
- Noise power spectral density
12System Noise Antenna Noise
- Two types of antenna noise
- 1. Noise originating from antenna losses
- 2. Sky noise microwave radiation present
throughout the universe
Figure ref. 1
- Figure Equivalent noise temperature of the sky
as seen from earth station antenna - Equivalent noise temperature of the earth as
seen from the satellite antenna is about 290 K
13System Noise Amplifiers
- Single stage amplifier
- Amplifiers in cascade
Figures ref. 1
For amplifiers in cascade, its important to have
a low noise, high gain amplifier for the first
stage.
14System Noise Noise Figure
- F (actual noise power present in the output of
a two port system) / (noise power present in the
output of a perfect two port system) - Room temperature 290 K
(relationship between equivalent noise
temperature and noise figure)
Figure ref. 1
(Noise figure of a lossy device)
(System noise temperature referred to the input
for above figure)
15Carrier-to-noise Ratio
- Answers the question why space link?
- Space link calculations give us C/N ratio
- C/N ratio gives us
- gives us Pr(error) and capacity
B bandwidth bit rate
R data rate
16Carrier-to-Noise Ratio
Noise power
Definition of C/N ratio
Received power with all losses taken into account
C/N ratio in product form
C/N ratio in dB form
Definition of carrier-to-noise density ratio
17Carrier-to-Noise Ratio Uplink
- Uplink
- Earth station EIRP, satellite receiver feeder
losses, satellite receiver G/T, - Frequency dependent calculations calculated for
the uplink frequency
subscript U stands for uplink
18Traveling wave tube amplifiers (TWTAs)
- Widely used in transponders to provide the final
output power required to the transmit antenna - Provides amplification over a very wide bandwidth
- Nonlinear transfer characteristic
- Input power of the TWTA needs to be carefully
controlled to minimize distortion
Figure ref. 1
Low input powers input-output relationship is
linear Higher input powers output power saturates
19Uplink Saturation Flux Density
- In the uplink, the TWTA will be at the receiving
end - Received signal from earth will be input to the
TWTA - Saturation flux density flux density required at
the satellites receiving antenna to produce
saturation of the TWTA - Using the saturation flux density, one can
calculate the required EIRP at the earth station
to produce saturation of the TWTA at the
satellite
Effective area of an isotropic antenna
subscript S stands for saturation
20Uplink Input Back-off
- A number of simultaneous carriers present in the
TWTA requires back-off of the operating point to
reduce intermodulation distortion - Its the input back-off because the received
signal will be input to the TWTA - Earth station EIRP has to be reduced by this
specified back-off
input back-off
21Uplink Earth Station HPA
- Earth station high power amplifier is at the
transmitting end of the uplink - Supplies the radiated power plus the transmitter
feeder losses - Earth station HPA transfer characteristic can
also be nonlinear, requiring output back-off - Output back-off since HPAs output is the
transmitted signal - An HPA with a high saturation point has larger
physical size and higher power consumption, but
penalty for this is not as large since its on
the earth
TFL transmitter feeder losses
22Carrier-to-Noise Ratio Dowlink
- Downlink
- Satellite EIRP, earth station receiver feeder
losses, earth station receiver G/T - Frequency dependent calculations calculated for
the downlink frequency
subscript D stands for downlink
23Downlink Output Back-off, Satellite TWTA Output
- Output back-off
- Satellite TWTA Output
- TWTA supplies radiated power and transmit feeder
losses -
A rule of thumb
The output of the TWTA is being transmitted, so
its the output back-off
Figure ref. 1
24Effects of Rain
- So far, calculations have been made for clear-sky
conditions - Rainfall is a significant cause of fading in the
C band and especially in the Ku band - Rainfall causes attenuation by scattering and
absorption of the radio waves - Rain attenuation for horizontal polarization is
greater than for vertical polarization
25Rain Attenuation
- Rain attenuation data is usually available as
curves or tables - Tables gives percentage of time over a year that
the attenuation exceeds the dB values
Radomes are truncated spherical shells composed
of panels to protect the earth station antenna
from the environment. Radome transmission loss
ordinary insertion loss, scattering loss Layer
of water caused by rain introduces attenuation by
absorption and reflection
Figure ref. 1
Figure ref. 4
26Rain-fade margins Uplink and Downlink
- Uplink (satellite is receiving)
- Increase in noise due to rain usually not a major
factor since satellite antenna is pointed toward
a hot earth - With uplink power control, power output from the
earth station may be increased to compensate for
fading - Downlink (earth station is receiving)
- No power control since user doesnt have control
of satellite EIRP - A rain attenuation caused by absorption
- Equivalent noise temperature for the rain
27Combined Uplink and Downlink C/N Ratio
- overall C/N ratio is less than the lower of the
uplink and downlink C/N ratios
Figure ref. 1
28Intermodulation Noise
- Occurs whenever multiple carriers pass through a
device with nonlinear characteristics, such as
TWTAs
Figure ref. 5
Figure ref. 1
C/N ratio for intermodulation noise is a function
of the number of carriers and their modulation
characteristics, and the amplitude and phase
characteristics of the high-power amplifier
29Intermodulation Noise
Figure ref. 1
- To reduce intermodulation noise, we can operate
the traveling wave tube in a back off condition - Increasing back off decreases uplink and downlink
C/N ratios - There is an optimum operating point that gives
maximum overall C/N ratio as a function of
back-off
Figure ref. 5
30System Design Example
- Ku-band geostationary satellite with bent pipe
transponders to distribute digital TV signals
from an earth station to many receiving stations - Bent pipe transponder transponder that amplifies
the received signal and retransmits it at a
different frequency
Figure ref. 2
Need a minimum overall C/N ratio of about 9.5 dB
in the TV receiver
31Table of specifications
Figure ref. 2
32Ku-Band Uplink Design
Minimum required receive power P_r C/N
P_n 30 -125.3 -95.2 dBW
P_r P_t 123.5 dB -95.2 dBW gt P_t
28.3 dBW gt P_t 675 W
33Ku-Band Downlink Design
P_t, sat 80 W gt P_t, sat 19 dBW Output
back-off 1 dB P_t 19 1 18 dBW
P_r G_r 160.2 -113.5 dBW gt G_r
46.7 dB gt G_r 46,774
Minimum required receive power P_r
C/NP_n 17.2 -130.7 -113.5 dBW
34Satellite Communication Link Design Procedure
- Determine the frequency band in which the system
must operate. Comparative designs may be
required to help make the selection. - Determine the communications parameters of the
satellite. Estimate any values that are not
known. - Determine the parameters of the transmitting and
receiving earth stations. - Start at the transmitting earth station.
Establish an uplink budget and a transponder
noise power budget to find uplink C/N in the
transponder. - Find the output power of the transponder based on
transponder gain or output back-off. - Establish a downlink power and noise budget for
the receiving earth station. Calculate downlink
C/N and overall C/N for a station at the edge of
the coverage zone (worst case). - Calculate S/N or BER in the baseband channel.
Find the link margins. - Evaluate the result and compare with the
specification requirements. Change parameters of
the system as required to obtain acceptable
overall C/N or S/N or BER values. This may
require several trial designs. - Determine the propagation conditions under which
the link must operate. Calculate outage times
for the uplinks and downlinks. - Redesign the system by changing some parameters
if the link margins are inadequate. Check that
all parameters are reasonable, and that the
design can be implemented within the expected
budget.
The above can be found in ref. 2
35Summary
- Transmission losses include free-space loss,
feeder losses, antenna misalignment losses and
fixed atmospheric and ionospheric losses - To reduce system noise for amplifiers in cascade,
have a low noise, high gain amplifier in the
first stage - C/N ratio gives error probability and capacity
- Multiple carriers present means back-off must be
accounted for - Rain attenuation can be overcome with uplink
power control, increasing the antenna diameter,
or using an amplifier with higher gain and lower
noise - C/N ratios add as reciprocals
- Space link calculations are an iterative process
since its hard to get it all right on the first
try
36References
- 1) D. Roddy, Satellite Communications, 3rd Ed.,
2001, The McGraw Hill Companies, ISBN
0-07-137176-1 main reference, gives good
overall description of the space link - 2) T. Pratt, C. Bostian, and J. Allnutt,
Satellite Communications, 2nd Ed., 2003, John
Wiley Sons, ISBN 0-471-42912-0 very clear
description of EIRP, good link design examples - 3) Class notes, EE 381K.3 System noise and
noise figure equations - 4) http//www.radome.net/tl.html lengthy
description and pictures of radomes - 5) W. Pritchard, Satellite Communication Systems
Engineering, 1986, Prentice-Hall Inc., ISBN
0-13-791245-5 algorithm flow diagram for
calculating maximum C/N ratio as a function of
back-off