QUESTION 1

(i) Show the satellite orbital constants with illustrated diagrams. Why are these needed ?

(ii) Show the orbital elements using a well illustrated diagram. Why are these needed?

(iii) By means of well labeled diagrams explain look angles and subsatellite point

(iv) What elements are needed to calculate the look angle

(v) List the various orbital perturbations

(vi) Interpret the formula below for a GEO

γ≤ arccos(re=rs)

where γ=Central angle

r_s=Distance of satellite from earth centre

r_e=Radius of earth

QUESTION 2

(i) Draw a labeled diagram of a typical earth station receiver

(ii) Draw a labeled diagram of its noise model

(iii) Write down its Equivalent noise temperature

(iv) Write down the natural sources of earth station antenna noise contributors

(v) How is C/N related to the G/T of a receive earth station antenna

(vi) Write down the formula for the noise temperature of a lossy element

QUESTION 3

(i) Find Pe for a BPSK signal given that the probability of error of any binary data set is given by Pe= q
d2=2N0 where d is the distance between the two constellation points in the signal space diagram

(ii) Compare QPSK with BPSK

(iii) Relate C/No ,Eb/No and Rb

(iv) When do we get ISI in digital communication

(v) What are the e_ects of ISI

(vi) Given the following:

1. Transponder bandwidth = 36 MHz
   
2. Preamble = 240 bits
   
3. Symbol rate (QPSK)=30 Msps
   
4. Four stations share transponder in TDMA, each using 125 _s frames
   
5. Preamble = 240 bits
   
6. Guard time = 1.6 _s
   
7. nd:
   
8. Transponder capacity in terms of 64 kbps speech channels?
   
9. How many channels can each earth station transmit?
   

(vii) Explain the FM improvement ratio

QUESTION 4

(i) List the various Forward Error Protection Coding Types briey describing each

(ii) De_ne the following

_ The Hamming weight

_ The Hamming distance

_ The minimum Hamming distance

(iii) Describe the features of convolutional codes

(iv) What are the advantages of concatenated codes in satellite communication?

(v) How rain e_ects are alleviated in satellite communication

QUESTION 5

(i) What elements are taken into considerations when designing an Earth Station?

(ii) List elements of an earth station RF Subsystems

(iii) List the types of satellite antenna tracking briey enumerating their advantages and disadvantages

(iv) What are the design considerations in VSAT design

(v) What are the advantages and disadvantages of Vsat Mesh Network

QUESTION 6

A Direct Broadcast Television (DBS-TV) satellite is in geostationary orbit at 100 degrees west longitude. It carries 16 transponders, each with a saturated output power of 200 W and a bandwidth of 25 MHz. The antenna on the satellite has a gain (on axis) of 34 dB. The receiving terminals all use antennas with a circular aperture with a diameter of 18 inches and an aperture e_ciency of 65%. The noise bandwidth of the digital TV receiver is 20 MHz. Use a distance to the GEO satellite of 38,500 km in your calculations

(a) Calculate the free space path loss and the receiving terminal antenna gain at 12.2 GHz

(b) Draw up a link budget for the downlink from the satellite to an earth station on the 3 dB contour of the satellite antenna beam. Assume that the satellite transmits at a power level of 180 W. Include a clear air atmospheric loss of 0.5 dB and miscellaneous losses of 0.2 dB in your downlink power budget

(c) The receiving terminal has a system noise temperature of 110 K in clear air. Draw up a noise power budget for the receiver using the receiver's noise band- width

(d) Calculate the clear air C/N ratio for the receiver with a noise bandwidth of 20 MHz. The minimum permissible C/N ratio is 10.0 dB. What is the clear air link margin?

(e) For 0.3% of the time at the receiving location, heavy rain causes 2 dB excess path attenuation and the system noise temperature of the receiver increases to 210 K. Calculate the C/N under these rain conditions, and the link margin above the C/N threshold of 10.0 dB

QUESTION 7

A Ku-band VSAT station receives a TDM data stream at 1.544 Mbps from a GEO satellite. The modulation is QPSK and under clear air conditions the downlink C/N in the VSAT receiver is 20 dB (ignoring noise from the satellite). The C/N in the satellite transponder is 30 dB. A nearby terrestrial LOS link causes interference with the VSAT such that the carrier to interference ratio C/I in the VSAT receiver is 19.6 dB. All C/N and C/I values are quoted for the optimum noise bandwidth of the VSAT receiver. The receiver uses ideal RRC _lters with _=0.4 and its QPSK demodulator has an implementation margin of 1 dB

(a) What is the clear air overall C/N ratio in the VSAT receiver, assuming that the interference can be considered AWGN? What BER would you expect at the data output of the VSAT receiver assuming no FEC is applied to the signal?

(b) The system is redesigned and half rate FEC is added to the signal so that the bit rate at the transmitter is doubled, but transmitter power is not increased. In the receiver, the FEC decoder has a coding gain of 6 dB. For the case when FEC is used, determine the overall C/N ratio and the expected BER during a rain fade that causes the C/N ratio of the received signal to fall by 5 dB but which does not attenuate the interfering signal

(c) If the extra bandwidth to implement half rate FEC is available at the satellite, would you recommend that FEC be used in this case? Give reasons for your answer

(d) What are the advantages and disadvantages of using forward error correction in satellite links? Illustrate your answer using the above example of a high data rate signal sent to a small earth terminal