3. Quality of Service

Question 1

How is the Shannon rate defined?

Answer 1

Defines the minimum Eb/N0 that can, in theory, provide error-free transmission for a given code rate in an AWGN channel

i.e.error free communication is limited by rate of data transmission

Question 2

What are the BER performance objectives for telephony (CCIR Rec. 522)?

Answer 2

BER ≤ 10⁻⁶ for 10-minute mean, ≤ 20% of any month

BER ≤ 10⁻⁴ for 1-minute mean, ≤ 0.3% of any month

BER ≤ 10⁻³ for 1-second mean, ≤ 0.01% of any year

Question 3

What are the BER objectives for 64 kbps ISDN data transfer (CCIR Rec. 614)?

Answer 3

BER ≤ 10⁻⁷ for ≤ 10% of any month

BER ≤ 10⁻⁶ for ≤ 2% of any month

BER ≤ 10⁻³ for ≤ 0.03% of any month

Question 4

What is the purpose of scrambling in digital communications?

Answer 4

Ensures symbol transition probability ≈ 0.5 to maintain timing synchronization

Eliminates long sequences of 0s or 1s

Acts as energy dispersal, removing discrete spectrum lines to meet PFD limits

Question 5

What is the difference between stream and block ciphering?

Answer 5

Stream cipher: encrypts bit-by-bit with keystream

Block cipher: encrypts block-by-block, often stronger against errors

Question 6

What are common types of channel coders and their characteristics?

Answer 6

Block codes (e.g., Hamming, RS): Fix errors in blocks

Convolutional codes: Good for random errors, decoded with Viterbi

Concatenated codes: Combine RS + Conv with interleaving

Turbo codes: Iterative decoding, near Shannon limit, used in 3G

LDPC codes: Long codes with sparse parity-check matrices, used in DVB-S2

Question 7

Which digital modulation schemes are used in satellite links?

Answer 7

BPSK, QPSK: Common for power-limited satellite links

MSK, OQPSK: Used in mobile to minimize amplifier distortion

16QAM, 32APSK: Used in bandwidth-limited channels

Question 8

What is the main tradeoff when choosing modulation schemes?

Answer 8

Power vs. Bandwidth
Higher-order modulations (e.g., 16QAM) = more bandwidth-efficient, but require higher SNR

Question 9

How does BPSK compare to QPSK in terms of performance?

Answer 9

BPSK: Better error performance (lower BER)

QPSK: Half the bandwidth of BPSK

BER:
𝐵𝐸𝑅 = 1/2erfc (SQRT(𝐸_𝑏 / 𝑁_0)) (for AWGN)

Question 10

What is the difference between coherent and differential modulation?

Answer 10

Coherent (BPSK/QPSK): Best BER performance, requires carrier recovery

Differential (DBPSK/DQPSK): No carrier recovery needed, but worse BER

Question 11

How is required C/No calculated from Eb/No and bit rate (Rb)?

Answer 11

C/No = Eb/No (dB) + 10log(Rb)

Question 12

What is spectral efficiency?

Answer 12

η= R/B​ (bit/s/Hz)

Where R = data rate, B = bandwidth

Example: 54 Mbps in 36 MHz → 1.5 bit/s/Hz

Question 13

What filters minimize ISI in satellite systems?

Answer 13

Matched filters at Tx and Rx: typically root raised cosine

Splitting symmetric filtering → minimizes ISI at symbol decision points

Filter roll-off α typically 0.2–0.35

Question 14

How does coding help manage power and bandwidth trade-offs?

Answer 14

Coding increases required bandwidth but reduces required Eb/No

Enables operation closer to Shannon limit, improving throughput

In power-limited systems: prioritize coding

In bandwidth-limited systems: use spectral-efficient modulations

Question 15

What were the key results of the hub-to-VSAT link budget example?

Answer 15

Service rate (Rb): 2 Mbps

C/No required: 69.5 dB-Hz

Uplink C/No (C/Nou): 86.4 dB-Hz

Downlink G/Tv in rain: 17.6 dB/K

EIRP per carrier: 39.5 dBW

Power-limited link (4 users vs 15 possible in bandwidth)

Margin: 2 dB

Question 16

What method is used for complete satellite service design?

Answer 16

Define BER and availability requirements

Choose modulation & coding scheme

Translate BER → Eb/No → C/No

Apply margin: C/No_total = C/No_required + M

Factor in satellite specs: EIRP, G/T

Add propagation impairments (rain, FSL, etc.)

Finalize link budgets and design terminals

Question 17

What are the key modulation/coding standards used in DVB-S and DVB-S2?

Answer 17

DVB-S (RCS): QPSK + RS + convolutional coding

DVB-S2 (RCS2):

Forward: QPSK, 8PSK, 16APSK, 32APSK + LDPC + BCH

Return: QPSK, 8PSK, 16QAM with Turbo coding

Operates in Adaptive Coding and Modulation (ACM) mode for link adaptation

Question 18

What is Adaptive Coding and Modulation (ACM)?

Answer 18

ACM dynamically adjusts modulation and coding based on current channel conditions (e.g., rain fade) to optimize throughput while maintaining link quality.

Question 19

What is the significance of ‘root raised cosine (√Rc)’ filters?

Answer 19

They are split between transmitter and receiver to ensure ISI-free detection.

α (roll-off) determines excess bandwidth.

α = 0 → no excess

α = 1 → doubles bandwidth

Question 20

How is total bandwidth calculated from symbol rate and roll-off?

Answer 20

Bandwidth = Rs × (1 + α)
Where:

Rs = symbol rate

α = roll-off factor (commonly 0.2–0.35)

Question 21

What is ‘symbol rate (Rs)’ and how is it related to Rc and modulation?

Answer 21

Rs = Rc / m

Rc = coded bit rate

m = bits per symbol (e.g., QPSK → m = 2)

Question 22

What is the coded bit rate (Rc) in terms of information bit rate (Rb)?

Answer 22

Rc = Rb / ρ

ρ = code rate (e.g., ½, 2/3).
For concatenated codes, ρ = ρ₁ × ρ₂.

Question 23

Why is QPSK often preferred over BPSK in satellite links?

Answer 23

QPSK requires half the bandwidth for same bit rate.

Though BER slightly worse, spectral efficiency is 2x greater than BPSK.

Question 24

What does phase ambiguity mean in coherent systems like BPSK/QPSK?

Answer 24

It’s uncertainty in determining the absolute carrier phase without a reference. Requires carrier recovery circuits at the receiver.

Question 25

What modulation schemes are used in power-limited vs bandwidth-limited systems?

Answer 25

Power-limited: BPSK, QPSK Bandwidth-limited: 16QAM, 32APSK (higher-order modulations)

Question 26

What are typical roll-off factors (α) in satellite filters, and what do they mean?

Answer 26

Common: 0.2, 0.35 Lower α = narrower bandwidth but sharper filters Some systems use α = 0.05 for high efficiency (e.g., DVB-S2X)

Question 27

What are the advantages of differential modulation (e.g., DQPSK)?

Answer 27

Eliminates need for carrier recovery Robust to phase slips Drawback: Worse BER compared to coherent detection

Question 28

What are key elements of a satellite QoS-based design flow?

Answer 28

BER/availability requirements Select Mod/Cod Convert BER → Eb/No Convert Eb/No → C/No via Rb Add margin Factor satellite parameters (EIRP, G/T) Compute link budget & propagation impairments

Question 29

What does a noise figure (NF) of 1 dB in an LNB correspond to in noise temperature?

Answer 29

NF = 10log(1 + T/290) → For NF = 1 dB, T ≈ 75 K Contributes directly to system noise temperature (Ts)

Question 30

What is the role of Forward Error Correction (FEC) in satellite communications, and how does it influence system design?

Answer 30

FEC adds redundancy to transmitted data to enable error detection and correction at the receiver without retransmission. Improves link robustness against noise, fading, and interference Reduces required Eb/No to achieve a given BER Increases bandwidth usage (lower code rate ρ → more redundancy) Examples: Convolutional + RS (DVB-S) LDPC + BCH (DVB-S2) Turbo codes (3G, mobile systems) In system design, FEC helps optimize the trade-off between transmit power, bandwidth, and link reliability.