The Space Link

1
The Space Link

• Paul Yang
• March 31, 2005
• EE 381k.3 Satellite Communications
• University of Texas at Austin

2
Overview 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

3
Introduction

• 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

4
EIRP

• 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)
5
Transmission 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

6
Transmission 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
7
Transmission 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

8
Transmission 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

9
Transmission 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

10
The 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)

• received power

• receiving antenna gain

11
System 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

12
System 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

13
System 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.
14
System 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)
15
Carrier-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
16
Carrier-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
17
Carrier-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
18
Traveling 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
19
Uplink 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
20
Uplink 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
21
Uplink 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
22
Carrier-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
23
Downlink 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
24
Effects 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

25
Rain 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
26
Rain-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

27
Combined 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
28
Intermodulation 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
29
Intermodulation 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
30
System 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
31
Table of specifications
Figure ref. 2
32
Ku-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
33
Ku-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
34
Satellite 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
35
Summary

• 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

36
References

• 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