SBCA Digital Transmission 5 — Questions and Answers
Question 1: What is the purpose of channel bonding in satellite broadband systems?
- Combining multiple satellite transponders to increase single-user throughput (Correct answer)
- Reducing the number of satellites needed
- Encrypting data across separate channels
- Splitting uplink from downlink frequencies
Correct answer: Combining multiple satellite transponders to increase single-user throughput
Channel bonding aggregates multiple satellite transponder channels to deliver higher throughput to a single user or hub terminal.
Question 2: In VSAT networks using MF-TDMA, what happens during a rain fade event affecting a terminal's uplink?
- The terminal switches to a lower frequency band automatically
- The network may assign more time slots or a more robust coding to compensate (Correct answer)
- The hub permanently deactivates the terminal
- The satellite repositions to maintain the link
Correct answer: The network may assign more time slots or a more robust coding to compensate
MF-TDMA networks can compensate for rain fade by dynamically allocating more burst capacity or switching to a more robust modcod for the affected terminal.
Question 3: What is the function of a PCR (Program Clock Reference) in an MPEG-2 transport stream?
- Identifying the video PID
- Providing a time reference to allow receivers to reconstruct the encoder's 27 MHz clock (Correct answer)
- Defining the conditional access system
- Indicating the start of a new GOP
Correct answer: Providing a time reference to allow receivers to reconstruct the encoder's 27 MHz clock
PCR stamps allow the decoder to recover the encoder's 27 MHz system time clock, enabling accurate audio-video synchronization and buffer management.
Question 4: Which technique allows multiple uplink stations to share a single satellite transponder by transmitting at different frequencies simultaneously?
- TDMA
- FDMA (Correct answer)
- CDMA
- SDMA
Correct answer: FDMA
FDMA (Frequency Division Multiple Access) assigns each uplink station a unique frequency slot within the transponder bandwidth for simultaneous access.
Question 5: In satellite digital transmission, what does the term 'occupied bandwidth' refer to?
- The total allocated transponder bandwidth in the satellite plan
- The frequency range containing 99% of the total signal power (Correct answer)
- The bandwidth consumed by the FEC overhead only
- The guard band between adjacent carriers
Correct answer: The frequency range containing 99% of the total signal power
Occupied bandwidth is the frequency range that contains 99% of the total transmitted signal power, as defined by ITU regulations.
Question 6: What is the purpose of a baseband filter with raised cosine (Nyquist) shaping in a satellite modem?
- Amplifying the signal before upconversion
- Limiting bandwidth while minimizing inter-symbol interference at sampling instants (Correct answer)
- Performing analog-to-digital conversion
- Providing automatic gain control
Correct answer: Limiting bandwidth while minimizing inter-symbol interference at sampling instants
Raised cosine filtering shapes the signal to occupy minimum bandwidth while ensuring zero ISI at the correct sampling instants in the receiver.
Question 7: What advantage does APSK (Amplitude and Phase Shift Keying) offer over QAM for satellite links operating through nonlinear transponder amplifiers?
- APSK requires no FEC coding
- APSK has a more uniform amplitude envelope, reducing distortion from amplifier nonlinearity (Correct answer)
- APSK uses less bandwidth than QAM
- APSK eliminates the need for LNB frequency conversion
Correct answer: APSK has a more uniform amplitude envelope, reducing distortion from amplifier nonlinearity
APSK constellations have a more circular, amplitude-efficient structure that is less susceptible to nonlinear distortion from saturating TWTAs in satellite transponders.
SBCA Digital Transmission 5
What is the purpose of channel bonding in satellite broadband systems?