Transmission fundamentals

1. system noise temperature in satellite communication
galaxy
sun
ionosphere
troposphere
SATELLITE TRACKING, TELEMETRY AND COMMAND
1

2. Receiving station architecture
LNA block and first conversion
Receiver station diagram block

3. Receiving station architecture
Second conversion and IF2 amplifier block

4. Noise


Noise is an electronic signal that gets added
to a radio or information signal as it is
transmitted from one place to another.
It is not the same as interference from other
information signals.

5. Noise


Noise is the static you hear in the speaker
when you tune any AM or FM receiver to any
position between stations. It is also the
“snow” or “confetti” that is visible on a TV
screen.
The noise level in a system is proportional to
temperature and bandwidth, the amount of
current flowing in a component, the gain of
the circuit, and the resistance of the circuit.

6. Signal-to-Noise Ratio



The signal-to-noise (S/N) ratio indicates the
relative strengths of the signal and the noise in a
communication system.
The stronger the signal and the weaker the noise,
the higher the S/N ratio.
The S/N ratio is a power ratio.

7. External Noise

External noise comes from sources over which we
have little or no control, such as:

Industrial sources


Atmospheric sources


motors, generators, manufactured equipment
The naturally occurring electrical disturbances in the earth’s
atmosphere; atmospheric noise is also called static.
Space

The sun radiates a wide range of signals in a broad noise
spectrum.

8. Internal Noise

Electronic components in a receiver such as
resistors, diodes, and transistors are major
sources of internal noise. Types of internal noise
include:



Thermal noise
Semiconductor noise
Intermodulation distortion

9. Expressing Noise Levels

The noise quality of a receiver can be expressed
in the following terms:




The noise factor is the ratio of the S/N power at the
input to the S/N power at the output.
When the noise factor is expressed in decibels, it is
called the noise figure.
Most of the noise produced in a device is thermal,
which is directly proportional to temperature.
Therefore, the term noise temperature (TN) is used.
SINAD is the composite signal plus noise and
distortion divided by noise and distortion contributed
by the receiver.

10. Noise in Cascaded Stages


Noise has its greatest effect at the input to a
receiver because that is the point at which the
signal level is lowest.
The noise performance of a receiver is
determined in the first stage of the receiver,
usually an RF amplifier or mixer.

11. The Earth is Curved
!
•
•
•
•
•
Radio waves above 30 MHz travel in straight lines
Ways must be found to get signals beyond horizon
Ionospheric reflection uses hf band, 2 – 30 MHz
Microwave link uses line of sight between towers
Chain of repeaters can take the signal thousands of
miles
• Satellite communications uses a repeater in the sky
• Single link via GEO satellite can reach round one
third of the earth’s surface.
11

12. Ionospheric layers
multipath
Tx
Rx
Earth
Fig. HF Radio Communication
12

13. Tx
Rx
Earth
Fig. LOS Microwave Communications
13

14. GEO satellite
Altitude 35,680 km
Tx
Rx
Earth
Fig. Satellite Communications
14

15. noise @ Satellites
1.
2.
3.
4.
Thermal Noise
Intermodulation noise
Crosstalk
Impulse Noise

16. Thermal Noise
 Thermal noise due to agitation of
electrons
 Present in all electronic devices and
transmission media
 Cannot be eliminated
 Function of temperature
 Particularly significant for satellite
communication

17. Thermal Noise
 Amount of thermal noise to be found in a
bandwidth of 1Hz in any device or
conductor is:
N 0 = kT ( W/Hz )
• N0 = noise power density in watts per 1 Hz of
bandwidth
• k = Boltzmann's constant = 1.3803 x 10-23 J/K
• T = temperature, in Kelvin's (absolute temperature)

18. Thermal Noise
 Noise is assumed to be independent of frequency
 Thermal noise present in a bandwidth of B Hertz
(in watts):
N = kTB
or, in decibel-watts
N = 10 log k + 10 log T + 10 log B
= −228.6 dBW + 10 log T + 10 log B

19. Noise Terminology
 Intermodulation noise – occurs if signals with
different frequencies share the same medium
o Interference caused by a signal produced at a
frequency that is the sum or difference of original
frequencies
 Crosstalk – unwanted coupling between signal
paths
 Impulse noise – irregular pulses or noise
spikes
o Short duration and of relatively high amplitude
o Caused by external electromagnetic disturbances, or
faults and flaws in the communications system
o Primary source of error for digital data transmission

20. Comm. Subsystem—Design
Typical System Noise Temperatures

21. Expression Eb/N0
 Ratio of signal energy per bit to noise power
density per Hertz
Eb S / R
S
=
=
N0
N0
kTR
 The bit error rate for digital data is a function of
Eb/N0
o Given a value for Eb/N0 to achieve a desired error rate,
parameters of this formula can be selected
o As bit rate R increases, transmitted signal power must
increase to maintain required Eb/N0

22. Other Impairments
 Atmospheric absorption – water vapor
and oxygen contribute to attenuation
 Multipath – obstacles reflect signals so
that multiple copies with varying delays
are received
 Refraction – bending of radio waves as
they propagate through the atmosphere

23. Multipath Propagation
 Reflection - occurs when signal encounters a
surface that is large relative to the wavelength of
the signal
 Diffraction - occurs at the edge of an impenetrable
body that is large compared to wavelength of
radio wave
 Scattering – occurs when incoming signal hits an
object whose size is in the order of the
wavelength of the signal or less

24. R= Reflection D= Diffraction S= Scattering

25. Effects of Multipath Propagation
 Multiple copies of a signal may arrive at
different phases
o If phases add destructively, the signal level
relative to noise declines, making detection
more difficult
 Intersymbol interference (ISI)
o One or more delayed copies of a pulse may
arrive at the same time as the primary pulse
for a subsequent bit

26. Fading
 Time variation of received signal power
caused by changes in the transmission
medium or path(s)
 In a fixed environment:
o Changes in atmospheric conditions
 In a mobile environment:
o Multipath propagation

27. Types of Fading
 Fast fading
 Slow fading
 Flat fading
 Selective fading
 Rayleigh fading
 Racian fading
Transmit
beam
θτ
Receive
beam
θρ

28. Error Compensation Mechanisms
1. Forward error correction
2. Adaptive equalization
3. Diversity techniques

29. 1.Forward Error Correction
 Transmitter adds error-correcting code to
data block
o Code is a function of the data bits
 Receiver calculates error-correcting code
from incoming data bits
o If calculated code matches incoming code, no error
occurred
o If error-correcting codes don’t match, receiver attempts
to determine bits in error and correct

30. 2.Adaptive Equalization
 Can be applied to transmissions that carry analog
or digital information
o Analog voice or video
o Digital data, digitized voice or video
 Used to combat intersymbol interference
 Involves gathering dispersed symbol energy
back into its original time interval
 Techniques
o Lumped analog circuits
o Sophisticated digital signal processing algorithms

31. 3.Diversity Techniques
 Space diversity:
o Use multiple nearby antennas and combine received
signals to obtain the desired signal
o Use collocated multiple directional antennas
 Frequency diversity:
o Spreading out signal over a larger frequency bandwidth
o Spread spectrum
 Time diversity:
o Noise often occurs in bursts
o Spreading the data out over time spreads the errors and
hence allows FEC techniques to work well
o TDM
o Interleaving

32. GLIMPSES

33. ECHO 1

34. TELSTAR

35. SYNCOM 2

36. Major problems for satellites
1. Positioning in orbit
2. Stability
3. Power
4. Communications
5. Harsh environment

37. 1.Positioning
• This can be achieved by several methods
• One method is to use small rocket motors
• These use fuel - over half of the weight of
most satellites is made up of fuel
• Often it is the fuel availability which
determines the lifetime of a satellite
• Commercial life of a satellite typically 1015 years

38. 2.Stability
• It is vital that satellites are stabilised
– to ensure that solar panels are aligned properly
– to ensure that communications antennae are
aligned properly
• Early satellites used spin stabilisation
– Either this required an inefficient omnidirectional aerial
– Or antennae were precisely counter-rotated in
order to provide stable communications

39. Stability (2)
• Modern satellites use reaction wheel
stabilisation - a form of gyroscopic
stabilisation Other methods of stabilisation
are also possible
• including:
– eddy current stabilisation
– (forces act on the satellite as it moves through
the earth’s magnetic field)

40. Reaction wheel stabilisation
• Heavy wheels which rotate at high speed often in groups of 4.
• 3 are orthogonal, and the 4th (spare) is a
backup at an angle to the others
• Driven by electric motors - as they speed up
or slow down the satellite rotates
• If the speed of the wheels is inappropriate,
rocket motors must be used to stabilise the
satellite - which uses fuel

41. 3.Power
• Modern satellites use a variety of power
means
• Solar panels are now quite efficient, so solar
power is used to generate electricity
• Batteries are needed as sometimes the
satellites are behind the earth - this happens
about half the time for a LEO satellite
• Nuclear power has been used - but not
recommended

42. 5.Harsh Environment
• Satellite components need to be specially
“hardened”
• Circuits which work on the ground will fail
very rapidly in space
• Temperature is also a problem - so satellites
use electric heaters to keep circuits and
other vital parts warmed up - they also need
to control the temperature carefully

43. Alignment
• There are a number of components which
need alignment
– Solar panels
– Antennae
• These have to point at different parts of the
sky at different times, so the problem is not
trivial

44. Antenna alignment
• A parabolic dish can be used which is
pointing in the correct general direction
• Different feeder “horns” can be used to
direct outgoing beams more precisely
• Similarly for incoming beams
• A modern satellite should be capable of at
least 50 differently directed beams