SBCA Digital Transmission 3 — Questions and Answers
Question 1: What is the Nyquist sampling theorem's requirement for lossless digital sampling of an analog signal?
- Sample at the signal's fundamental frequency
- Sample at least twice the highest frequency component (Correct answer)
- Sample at exactly the signal bandwidth
- Sample once per wavelength
Correct answer: Sample at least twice the highest frequency component
The Nyquist theorem states the sampling rate must be at least twice the highest frequency in the signal to allow perfect reconstruction.
Question 2: Which parameter directly determines the symbol rate (baud rate) of a satellite carrier?
- Transponder EIRP
- Occupied bandwidth and roll-off factor (Correct answer)
- LNB noise figure
- Satellite orbital position
Correct answer: Occupied bandwidth and roll-off factor
Symbol rate is set by the transponder's occupied bandwidth and the roll-off factor of the Nyquist filter shaping the signal.
Question 3: What does a BER (Bit Error Rate) of 10⁻⁶ mean in a digital satellite link?
- One error per 100 bits
- One error per one million bits (Correct answer)
- One million errors per second
- The link is unusable
Correct answer: One error per one million bits
BER of 10⁻⁶ means one bit error occurs for every one million bits transmitted, which is an acceptable threshold for many satellite services.
Question 4: In satellite broadcasting, what is the advantage of using statistical multiplexing over fixed bit-rate multiplexing?
- It eliminates the need for FEC
- It dynamically allocates bandwidth to maximize overall picture quality (Correct answer)
- It reduces the number of required transponders to one
- It removes the need for conditional access
Correct answer: It dynamically allocates bandwidth to maximize overall picture quality
Statistical multiplexing allocates more bits to complex, fast-moving scenes and fewer to static scenes, improving overall video quality within a fixed total bit rate.
Question 5: What is the role of an LDPC (Low-Density Parity-Check) code in DVB-S2?
- Compressing the transport stream payload
- Providing powerful inner forward error correction near Shannon limit (Correct answer)
- Scrambling data for conditional access
- Defining the framing structure of the baseband signal
Correct answer: Providing powerful inner forward error correction near Shannon limit
LDPC codes in DVB-S2 provide near-Shannon-limit error correction performance, enabling operation at very low Eb/No with high reliability.
Question 6: Which measure describes the minimum Eb/No required to achieve error-free communication as defined by information theory?
- Johnson noise floor
- Shannon limit (Correct answer)
- Friis formula
- Boltzmann constant
Correct answer: Shannon limit
The Shannon limit defines the theoretical minimum Eb/No (-1.6 dB) below which reliable communication is impossible regardless of coding complexity.
Question 7: What causes inter-symbol interference (ISI) in a digital satellite transmission system?
- Excessive uplink power
- Multipath or bandwidth limiting causing symbols to overlap in time (Correct answer)
- Incorrect polarization alignment
- High satellite orbital inclination
Correct answer: Multipath or bandwidth limiting causing symbols to overlap in time
ISI occurs when a signal's bandwidth is excessively limited or when multipath causes delayed copies of symbols to interfere with adjacent symbols.
SBCA Digital Transmission 3
What is the Nyquist sampling theorem's requirement for lossless digital sampling of an analog signal?