1. CT-474: Satellite Communications
Yash Vasavada
Autumn 2016
DA-IICT
Lecture 8
Satellite Link Design
29th August 2016

2. Review and Preview Review of the prior lecture:
An example design of SAT link
Preview of this lecture:
Continuation of the SAT link design
Syllabus for the first mid-term exam:
Chapter 1
Chapter 4: sections 4.1, 4.2, 4.3, 4.4, 4.7, 4.8 and 4.9
Chapter 5: sections 5.1, 5.2 (omit 5.2.3), 5.3, 5.4, 5.5 and 5.6

3. Spectral Efficiencies and Required Es/No for Different Modulation and Coding Schemes

4. DVB-S2 versus DVB-S2X

5. DVB-S2 versus DVB-S2X

6. An Example Link Calculation

7. Self Study Exercises
Compare DVB-S2 suite of MODCODs (MODulation and CODing options) with that in the first- generation DVB standard whose MODCODs are provided in Table 4.9 of the text
Repeat the prior exercise for Digital Transmission of Telephony described in Section of the text
Table 4.11 has 𝐶/𝑁 values for different code rates 𝑟, for a Bandwidth 𝐵=36 MHz
Convert 𝐵 to 𝑅 𝑠 . Use 𝛼=0.333…
Convert 𝐶/𝑁 values in Table 4.11 to 𝐸 𝑆 / 𝑁 0
Obtain overall spectral efficiency for the four different code rates by multiplying code rate 𝑟 with the spectral efficiency of QPSK modulation mentioned as 1.5 bps/Hz
Note that the latter is simply 1/(1+𝛼). Why?
Repeat the prior exercise for Digital Broadcasting of Television described in Section of the text

8. Simplified Versus Actual Transmitter
Symbol Rate:
𝑅 𝑆 symbols/sec
Bit Rate:
𝑅 𝑏 bits/sec
Detailed
(for DVB-S2 system):

9. A Model of Downlink From SAT to the Ground Station
𝑃 𝑇𝑋
𝐺 𝑇𝑋
𝑃 𝐸𝐼𝑅𝑃 𝐿
𝐺 𝑅𝑋
𝑇 𝑎 𝑁𝐹
𝑇 𝑒 𝑇
𝐺 𝑅𝑋 /𝑇

10. EIRP, Antenna Gain 𝐺, and Path Loss 𝐿
Equivalent Isotropically Radiated Power or EIRP is given as EIRP = 𝑃 𝑇𝑋 × 𝐺 𝑇𝑋
Gain of the antenna 𝐺 is a function of the area (effective aperture) of the antenna and the transmission frequency
Free-space Path Loss is given as
Therefore, the ratio 𝐺/𝐿 is given as
𝜂: antenna efficiency, typically in the range of 0.5 to 0.7
𝐷 𝐴𝑛𝑡 : antenna diameter in meters
𝑓: carrier frequency in Hertz
𝑐: speed of light in vacuum, m / s
𝑑: path distance in meters

11. Antenna Gain Antenna gain (maximum) is given as 𝐺= 4𝜋 𝜆 2 × 𝐴 𝑒𝑓𝑓
Effective aperture (area) of an antenna with circular aperture of diameter 𝐷 𝐴𝑛𝑡 is 𝐴 𝑒𝑓𝑓 = 𝜋 𝐷 𝐴𝑛𝑡 2 4
Therefore, the max gain is given as 𝐺= 𝜋 𝐷 𝐴𝑛𝑡 𝜆 2
Overall efficiency 𝜼 of the antenna is determined by several factors:
Uniform versus tapered illumination
Amount of energy radiated by the primary source that is captured by the reflector
Difference between the ideal parabolic shape of the antenna versus the actual mechanical shape
Losses due to ohmic and impedance mismatch

12. Antenna Beamwidth
Beamwidth of the antenna (3 dB) can be approximated as κ× 𝑐 𝑓×𝐷 , whereκ is a constant that depends on the antenna shape. For parabolic antenna, κ is usually taken as 70 𝑜
See

13. Power Flux Density and Path Loss
For an isotropic antenna, the portion of transmit power 𝑃 𝑇𝑋 radiated per unit solid angle is given as 𝑃 𝑇 4𝜋
For an actual antenna with a gain of 𝐺 𝑇𝑋 , the power per unit angle is 𝑃 𝑇 𝐺 𝑇 4𝜋
A receiving antenna with an aperture of 𝐴 𝑅𝑋 at a distance of 𝑑 from the transmit antenna subtends a solid angle of 𝐴 𝑅𝑋 / 𝑑 2 at the transmitter
Thus, it receives a power of 𝑃 𝑇 𝐺 𝑇 4𝜋 𝑑 2 × 𝐴 𝑅 = 𝑃 𝑇 𝐺 𝑇 4𝜋 𝑑 2 × 𝐺 𝑅 4𝜋 𝜆 2
Ratio 𝑃 𝑇 𝐺 𝑇 4𝜋 𝑑 2 is called the power flux density Φ in Watts/ 𝑚 2
Ratio 4𝜋𝑑 𝜆 2 is called the path loss and it is the ratio of the received to transmit powers for isotripic antennas at the transmitter and the receiver
Additional losses: atmospheric losses, depointing losses, tx/rx hardware losses, and polarization mismatch losses (see section 5.4.4)

14. Path Length 𝑑
SAT
Three angles and three lengths define the geometry of SATGS link
Angles:
Elevation of the SAT: 𝛽
Latitude of the GS from the SAT relative to EC: 𝛼
Latitude of the GS from the EC relative to the SAT: 𝜃
Distances:
SAT to GS distance: 𝑑
Earth Radius: 𝑅 𝑒
SAT height (altitude) above the Earth Surface: 𝐻
SAT orbit radius: 𝑅= 𝑅 𝑒 +𝐻
Tangent at the location of ground station 𝛼
SAT to User Distance, 𝑑
Orbit Height 𝛽
Orbit Radius
Earth Radius 𝑅 𝑒 𝜃
Earth Center

15. Path Length 𝑑 Use Law of Cosines to determine the path length 𝑑
Therefore, 𝑑 𝛽 = 𝑅 𝑒 sin 𝛽 ± 𝑅 𝑒 2 sin 2 𝛽 − 𝑅 𝑒 2 + 𝑅 2
Use Law of Sines to determine the angles
sin 𝛼 = 𝑅 𝑒 × sin 𝜋 2 +𝛽 𝑅  𝛼= sin −1 𝑅 𝑒 × sin 𝜋 2 +𝛽 𝑅 sin −1 𝑅 𝑒 × sin 𝜋 2 +𝛽 𝑅
sin 𝜃 =𝑑× sin 𝜋 2 +𝛽 𝑅  𝛼= sin −1 𝑑× sin 𝜋 2 +𝛽 𝑅 sin −1 𝑑× sin 𝜋 2 +𝛽 𝑅

16. Transmit Antenna Gain Calculation
Color legend: green refers to input, red refers to output, yellow is a constant

17. Path Loss Calculation

18. Evaluation of SNR
We have seen that the SNR = 𝑃 𝑆 𝑃 𝑁 = 𝐶 𝑁 is a critical parameter that determines the BER and the achievable data rate
We have also seen earlier that 𝑃 𝑁 = 𝑁 0 𝐵
Here 𝑁 0 is the power spectral density
When the source of the noise is thermal
agitations of the electrons in the transceiver circuitry, the noise power spectral density 𝑁 0 is given
as 𝑘×𝑇
Here, 𝑘 is Boltzmann’s Constant with a value of × 10-23 m2 kg s-2 K-1
𝑇 is the system noise temperature in deg K
Power Spectral Density 𝑁 0
Filter Bandwidth B
Ideal (Brickwall) Filter with Bandwidth 𝐵

19. Noise Figure
Signal with Power 𝑃 𝑆 𝐺
Noise with Power 𝑃 𝑁
A device 𝐷 with a gain of 𝐺
Consider an additive noise model, where the noise is added to the
signal before the sum is fed to a device 𝐷 (e.g., an amplifier) with a gain of 𝐺
If the device 𝐷 is ideal, it won’t introduce any additional noise: SNR at input = 𝑃 𝑆 𝑃 𝑁 equals SNR at the output of the device which is 𝐺× 𝑃 𝑆 𝐺× 𝑃 𝑁 = 𝑃 𝑆 𝑃 𝑁
Actual devices add some extra noise with power 𝑃 𝑁,𝐷 such that the SNR at the output is 𝐺× 𝑃 𝑆 𝑃 𝑁,𝐷 +𝐺× 𝑃 𝑁
Ratio of the Input SNR to the Output SNR is called the Noise Factor 𝐹: 𝑃 𝑁,𝐷 +𝐺× 𝑃 𝑁 𝐺× 𝑃 𝑁
This ratio depends on three terms: device noise power 𝑃 𝑁,𝐷 , device gain 𝐺 and input noise power 𝑃 𝑁 .
For a given device, the first two terms are fixed. Input noise power 𝑃 𝑁 is fixed, by convention, to 𝑘 𝑇 0 𝐵, where 𝑇 0 =273 degK (which is the typical room temperature of 𝑜 𝑐)
Noise Figure NF = 10× log 10 𝐹 dB

20. Effective Noise Temperature
Effective noise temperature of the device is called 𝑇 𝑒 = 𝑃 𝑁,𝐷 𝑘𝐺𝐵
It can be shown that 𝑇 𝑒 = 𝑇 0 𝐹−1
Total noise temperature at the output of the device is called the system noise temperature
𝑇 𝑆𝑌𝑆 = 𝑇 𝑖𝑛 + 𝑇 𝑒
where 𝑇 𝑖𝑛 is the noise temperature at the device input. This can be different from 𝑇 0 = 273 𝑜 𝐾
If there are two devices 𝐷 1 and 𝐷 2 in cascade with noise factors of 𝐹 1 and 𝐹 2 , effective noise temperatures of 𝑇 𝑒1 and 𝑇 𝑒2 and gains of 𝐺 1 and 𝐺 2 , the cascaded system’s overall noise factor and overall effective temperature are given as follows:
𝐹= 𝐹 1 + 𝐹 2 −1 𝐺 1 , and
𝑇 𝑒 = 𝑇 𝑒1 + 𝑇 𝑒2 𝐺 1

21. Receiver Antenna Gain and Figure of Merit Calculation Worksheet

22. Link Equation (linear scale)
Definition of 𝐶 𝑁 0
Received power 𝑃 𝑅𝑋 = 𝐺 𝑅𝑋 ×𝑅𝐼𝑃
Received noise power N 0 =𝑘𝑇
𝑅𝐼𝑃= 𝑃 𝐸𝐼𝑅𝑃 𝐿
𝑃 𝐸𝐼𝑅𝑃 = 𝑃 𝑇𝑋 × 𝐺 𝑇𝑋 ; 𝑇= 𝑇 𝑎 + 𝑇 𝑒
𝑇 𝑒 = 𝑇 0 (𝐹−1)

23. Link Equation (decibel scale)
Link equation in dB scale: multiplications and divisions turn into additions and subtractions
All the values are in dB, including Receiver Figure of Merit ( 𝐺 𝑅𝑋 /𝑇) which is a ratio that is evaluated in linear scale (using Noise Figure, 𝑇 𝑒 and 𝑇 𝑎 ) and converted in dB

24. An Example Link Calculation (Continued)