1. ECE 5233 Satellite Communications
Prepared by:
Dr. Ivica Kostanic
Lecture 10: Satellite link design
(Section 4.4)
Spring 2014

2. Outline Example of noise temperature calculations Heterodyning
Downlink link budget
Example link budget in C-band
Important note: Slides present summary of the results. Detailed derivations are given in notes.

3. Single conversion receiver
Block diagram of SC super heterodyne RX
LNA – Low Noise Amplifier – first signal amplifying stage
BPF1 – band filter, selection of the band of operation
Mixer – down conversion to IF stage
BPF2 – channel filter, section of the appropriate satellite channel
IF amplifier – main signal amplification on RX chain
Demod – demodulation of the signal
BB – base band processing
Note 1: LNA noise temperature is the most dominant term
Note 2. band selection filter insertion losses need to be small to minimize noise in mixer
Noise power at the output of the IF stage

4. Single conversion receiver –Ex 4.3.1
Suppose we have 4GHz receiver with following gains and temperatures: Ta = 25K, Trf = 50K, Tif = 1000K, Tm = 500K, Grf = 23dB, Gif = 30dB . Calculate system noise temperature assuming no losses in the mixer. Recalculate the system noise temperature assuming 10dB mixer losses. How can the system temperature be minimized in the 10dB mixer lose case.
Answers:
Ts = 82.5K
Ts = 127.5K
Increase LNA gain
Note: spreadsheet allows for insertion losses and noise temperatures of the filtering components.

5. RX with antenna feed losses (Ex 4.3.2)
The system in Ex has an LNA with a 50dB gain. A section of lossy wave-guide with attenuation of 2dB is inserted between antenna and LNA. Find the new noise temperature if the physical temperature of the waveguide is 300K.
Answers:
Ts = K
𝑡 𝑐
Note: To increase sensitivity of the front end, the receiver may be cryogenically-cooled. For example using liquid Nitrogen, the physical temperature may be brought down to 77K.

6. Super-heterodyne receiver
Heterodyning – translation of signal from RF to IF frequency
Reason – to facilitate filtering and demodulation
May be single conversion or double conversion
Opposite to heterodyning – direct conversion receivers
Note: double conversion RX has two IF stages
Block diagram of a SH RX

7. Downlink link budget Downlink – satellite to ground
Downlink link budget – usually limiting budget in fixed satcomm
Link budget components
Satellite TX (PA power, backoff, antenna, pointing)
Path losses (FSPL, additional losses due to elements, fade margin)
Ground RX (antenna, G/T ratio, system noise figure)
Link budget – evaluates various system design trade-offs
Date rate / capacity
Reliability
Component size and cost

8. Components of the link budget table
Transmitter side
TX components
Principle result: EiRP
Receiver side
RX components
Principle results
RxSensitivity
System temperature
RSLA
Propagation modeling
Free space loss
Environmental losses
Other system losses
Link reliability and margin calculations
Elements of the radio link

9. Satellite transmission – link budget components
Transponder output power
Transponder backoff
Dependent on Peak to Average Power Ratio (PAPR) of the modulated signal
Backoff ensures that transponder PA works in linear region of power amplification
Satellite antenna gain at the edge of coverage region
Usually the coverage region is defined through 3dB beamwidth – gain to the edge is 3dB lower than the maximum gain of the antenna
EiRP – effective isotropic radiated power [dBm]
PT – transponder power [dBm]
Gt – transmit antenna gain on axis [dB]
Bo – backoff [dB]
Lant – edge of beam loss of antenna gain [dB]
PAPR for a given modulation scheme is usually specified through CCDF curves

10. Propagation losses Note: All losses are frequency dependent.
Components of propagation losses
Free Space Path Loss (path length)
Atmospheric losses
Design margin (misalignment, severe weather, drift in component parameters)
Propagation losses - calculated for worst case
Satellite
Note: All losses are frequency dependent.
Earth station

11. Earth station – link budget components
Antenna gain
Function of efficiency and size
System noise temperature
Determined from the Rx front end
Receiver sensitivity
Minimum required RX power for proper link operation
Determined from noise power and required S/N ratio
Required S/N ratio
Determined from the modulation type
Ultimately driven by bandwidth efficiency requirement
Obtained from receiver operating curves
Signal to noise ratio requirement
R – information rate
B – bandwidth of the channel
IM – implementation margin

12. Downlink link budget example
Typical downlink LB spreadsheet
Note: Link budget as shown here has four distinct sections
Transmitter
Receiver
Propagation
Budget results