Modulation, Coding and Signal Processing

Introduction and Context

A satellite communication link must transport information through a channel characterized by severe path loss, limited spectrum, variable interference, nonlinear satellite amplifiers, and—particularly in higher-frequency systems—rain attenuation. Modulation, coding, and signal processing determine how efficiently the available bandwidth and transmit power are converted into useful information throughput.

The design problem is inherently multidimensional. Increasing constellation order can improve spectral efficiency, but it reduces the distance between signal points and therefore requires a higher signal-to-noise ratio (SNR). Stronger forward error correction (FEC) can reduce the required SNR, but adds redundancy, processing delay, and sometimes bandwidth overhead. Pulse shaping controls occupied bandwidth and intersymbol interference (ISI), while synchronization algorithms ensure that the receiver samples and demodulates the waveform correctly. Modern systems combine these functions through adaptive coding and modulation (ACM), as exemplified by DVB-S2 and DVB-S2X.

This chapter develops the signal-chain concepts required to evaluate satellite waveforms, compare power and bandwidth efficiency, and design robust links under changing propagation conditions.

Learning Objectives

After studying this chapter, the reader should be able to:

1. Explain analog and digital modulation methods used in satellite links.

2. Relate constellation size, SNR, spectral efficiency, and implementation complexity.

3. Analyze pulse shaping, raised-cosine filtering, and ISI.

4. Explain carrier, timing, and frame synchronization.

5. Evaluate FEC, interleaving, and iterative decoding.

6. Describe adaptive coding and modulation and the operating principles of DVB-S2/S2X-class waveforms.

7. Distinguish power efficiency from spectral efficiency and identify engineering trade-offs.

8. Assess a practical waveform design under bandwidth, power, and availability constraints.

1. Satellite Link Constraints and Performance Measures

The received signal power is usually much lower than the transmitted power because of free-space spreading and atmospheric losses. A useful link-quality measure is the ratio of energy per information bit to noise spectral density, denoted by $E_b/N_0$. If the information bit rate is $R_b$ and the received carrier power is $C$, then, under consistent units,

$$\frac{E_b}{N_0}=\frac{C}{N_0R_b}.$$

Here, $N_0$ is the one-sided noise power spectral density in W/Hz. The expression assumes that $C$ represents average received signal power and that $R_b$ refers to uncoded information bits. For coded systems, engineers may instead use energy per transmitted coded bit or define the coding rate explicitly; the convention must be stated.

The ratio $C/N$ measured over receiver bandwidth $B$ is related to $E_b/N_0$ by

$$\frac{E_b}{N_0}=\frac{C}{N}\frac{B}{R_b},$$

provided $N=N_0B$ and the same bandwidth convention is used. In decibels,

$$\left(\frac{E_b}{N_0}\right)_{\mathrm{dB}}=\left(\frac{C}{N}\right)_{\mathrm{dB}}+10\log_{10}\left(\frac{B}{R_b}\right).$$

Two central measures are:

• Spectral efficiency, $\eta=R_b/B$ in bit/s/Hz. For coded transmission, it is more informative to use information rate rather than coded bit rate.
• Power efficiency, commonly expressed as the required $E_b/N_0$ for a specified bit-error rate (BER), packet-error rate (PER), or availability.

No waveform simultaneously maximizes both measures. A low-order modulation with powerful coding is power efficient but may consume more bandwidth. A high-order modulation is spectrally efficient but requires a cleaner link and more linear hardware.

2. Analog Modulation in Satellite Systems

Early satellite systems commonly used frequency modulation (FM), especially for analog television and voice. In FM, the instantaneous frequency varies with the message signal. FM has an important amplitude-limiting property: amplitude noise can be suppressed before demodulation. However, its bandwidth expands with message deviation. Carson's approximation for a single-channel FM signal is

$$B_{FM}\approx 2(\Delta f+f_m),$$

where $\Delta f$ is peak frequency deviation and $f_m$ is the highest message frequency. The approximation applies to conventional wideband FM with a single dominant baseband bandwidth.

Analog FM can provide graceful degradation: as noise increases, quality gradually deteriorates until a threshold region is reached. Digital systems, by contrast, often show a sharper transition between acceptable and unacceptable performance. Analog satellite broadcasting has largely been replaced by digital transmission because digital systems support compression, encryption, statistical multiplexing, powerful FEC, and flexible service rates.

Amplitude modulation and single-sideband methods are relevant historically and in specialized low-rate systems, but their sensitivity to amplitude noise and limited integration with modern coding make them uncommon for high-capacity satellite payloads.

3. Digital Modulation and Constellation Design

A digital modulator maps groups of bits to complex symbols. A passband signal may be represented as

$$s(t)=\Re\{x(t)e^{j2\pi f_ct}\},$$

where $x(t)$ is the complex baseband waveform and $f_c$ is the carrier frequency. A symbol $a_k$ is transmitted using a pulse $p(t)$:

$$x(t)=\sum_k a_kp(t-kT_s),$$

where $T_s$ is the symbol interval. The constellation determines the possible values of $a_k$.

3.1 PSK and APSK

In binary phase-shift keying (BPSK), two phases represent two symbols. Quadrature phase-shift keying (QPSK) uses four phases and carries two bits per symbol. With ideal coherent detection in additive white Gaussian noise (AWGN), QPSK has the same BER versus $E_b/N_0$ as BPSK, while doubling the bit rate for the same symbol rate.

M-ary PSK increases the number of phases, but adjacent points become close and performance degrades. Quadrature amplitude modulation (QAM) varies both amplitude and phase and is highly spectrally efficient, but amplitude variations make it sensitive to nonlinear amplification.

Amplitude phase-shift keying (APSK) places constellation points on several amplitude rings. APSK can offer a useful compromise for satellite nonlinear channels: its geometry can be selected to tolerate amplitude and phase distortion better than rectangular QAM. DVB-S2 uses QPSK, 8PSK, 16APSK, and 32APSK modes, with higher-order modes generally intended for higher SNR conditions.

3.2 Gray labeling and symbol errors

Gray labeling assigns adjacent constellation points bit patterns differing in one bit. At moderate SNR, most symbol errors occur between nearest neighbors, so Gray labeling tends to minimize BER for a given symbol error rate. It does not eliminate errors, nor is it universally optimal when iterative decoding, nonuniform signaling, or non-AWGN channels are considered.

3.3 The spectral-efficiency trade-off

If a modulation has $M$ symbols and coding rate $R_c$, the ideal number of information bits per symbol is

$$b_i=R_c\log_2M.$$

If the occupied bandwidth is approximately $B\approx (1+\alpha)R_s$, where $R_s$ is symbol rate and $\alpha$ is roll-off factor, then an approximate information spectral efficiency is

$$\eta\approx\frac{R_c\log_2M}{1+\alpha}.$$

This expression neglects pilots, physical-layer headers, framing, and implementation losses. It is therefore a design estimate rather than an exact throughput formula.

4. Pulse Shaping and Intersymbol Interference

A rectangular symbol pulse has a sinc-shaped spectrum with slowly decaying sidelobes. Practical satellite systems use band-limited pulse shaping, most commonly raised-cosine (RC) or root-raised-cosine (RRC) filtering.

The roll-off factor $\alpha$ satisfies $0\leq\alpha\leq1$ in the usual formulation. For symbol rate $R_s$, the nominal two-sided occupied bandwidth of an RC-shaped signal is approximately

$$B=(1+\alpha)R_s.$$

A smaller $\alpha$ improves spectral efficiency but produces a sharper transition band, increasing sensitivity to timing errors, filter imperfections, and adjacent-channel interference. A larger $\alpha$ eases filtering and timing recovery but consumes more bandwidth.

In a transmitter–channel–receiver cascade, the transmitter and receiver often use matched RRC filters. Their combined response is RC. At the ideal sampling instants, the RC impulse response has zero crossings at all other symbol intervals, satisfying the Nyquist zero-ISI condition. This statement assumes ideal synchronization, linear time-invariant filtering, and a channel that does not introduce uncorrected distortion.

Satellite channels may violate these assumptions through transponder filtering, group-delay variation, Doppler, oscillator phase noise, nonlinear amplification, and multipath in some land-mobile or aeronautical scenarios. Equalization, adaptive filtering, or more robust waveform margins may then be required.

5. Synchronization and Receiver Signal Processing

Synchronization is the process of estimating the timing, frequency, phase, and frame position needed for reliable demodulation and decoding.

5.1 Carrier synchronization

Carrier frequency offset arises from oscillator mismatch, Doppler shift, and frequency conversion errors. A residual offset rotates the received constellation continuously. Carrier phase error rotates all points and can cause severe errors in phase-based modulation.

Common methods include:

• Pilot-aided estimation, which uses known symbols and is robust but introduces overhead.
• Decision-directed loops, which use detected symbols after acquisition and are efficient at adequate SNR but can suffer cycle slips or error propagation.
• Non-data-aided methods, such as squaring or fourth-power techniques, which exploit constellation symmetry but may have ambiguities.
• Digital phase-locked loops and Kalman-type estimators, useful for tracking time-varying phase and oscillator noise.

A phase-locked loop must balance noise bandwidth against tracking capability. A narrow loop suppresses estimation noise but responds slowly; a wide loop tracks dynamics but admits more noise.

5.2 Timing recovery

The receiver must determine when to sample the matched-filter output. Timing error causes samples to move away from the zero-ISI points. Algorithms include early-late detection, Gardner timing recovery, and Mueller–Müller detection. Gardner recovery is widely used for approximately two samples per symbol and does not require known data, although its performance depends on signal conditions and excess bandwidth.

5.3 Frame and burst synchronization

A receiver searches for a known synchronization word or physical-layer header. False detection and missed detection are competing risks. In a coded system, frame synchronization must also identify modulation, coding, pilot configuration, and sometimes the physical-layer scrambling state.

6. Forward Error Correction and Interleaving

FEC adds structured redundancy so that the receiver can detect and correct errors without retransmission. The coding rate is

$$R_c=\frac{k}{n},$$

where $k$ information bits are represented by $n$ coded bits. Lower $R_c$ generally provides more redundancy and improved error performance, but reduces information throughput for a fixed symbol rate.

6.1 Coding families

Convolutional codes process a continuous bit stream using shift-register memory. They are commonly decoded using the Viterbi algorithm, whose complexity grows with the number of trellis states.

Turbo codes concatenate constituent convolutional codes and use iterative soft-information exchange. They approach the Shannon limit at moderate block lengths but can have significant decoding latency and an error floor under some conditions.

Low-density parity-check (LDPC) codes use a sparse parity-check matrix. Iterative belief-propagation decoding passes log-likelihood information between variable and check nodes. LDPC codes are central to DVB-S2 because they provide excellent performance and parallelizable decoding.

BCH codes are often concatenated with LDPC codes as an outer code. The outer code helps correct residual errors and suppress error floors after inner decoding.

6.2 Soft decisions and log-likelihood ratios

A soft decoder receives reliability information rather than only hard bit decisions. For bit $b$, a log-likelihood ratio may be written as

$$L(b)=\ln\frac{P(b=0\mid y)}{P(b=1\mid y)},$$

where $y$ is the received observation. The sign indicates the more likely bit and the magnitude indicates confidence. Accurate soft metrics substantially improve coding gain, particularly near the decoding threshold.

6.3 Interleaving

Interleaving rearranges coded bits so that a burst of channel errors becomes approximately distributed errors at the decoder input. This is valuable against rain-induced fades, interference bursts, and frequency-selective fading. The cost is memory and latency. An interleaver cannot create information or recover a signal during a complete deep fade; it only makes the error pattern more suitable for the code.

7. DVB-S2 and DVB-S2X-Class Waveforms

DVB-S2 is a widely adopted second-generation digital satellite broadcasting standard. Its physical layer combines powerful FEC, selectable modulation, pilot options, physical-layer framing, and adaptive coding and modulation. A typical chain includes baseband filtering, BCH outer coding, LDPC inner coding, bit interleaving, constellation mapping, physical-layer scrambling, pilot insertion when enabled, and RRC pulse shaping.

DVB-S2 supports QPSK, 8PSK, 16APSK, and 32APSK. Coding rates and frame sizes are selected according to service and channel conditions. Normal and short FEC frames provide a trade-off between coding performance, latency, and implementation requirements.

DVB-S2X extends the family for broader professional and high-throughput applications. It introduces additional modulation and coding options, finer operating points, improved spectral efficiency, and mechanisms suited to wideband carriers, high-throughput satellites, beam hopping, and specialized professional services. Exact modes and operational limits depend on the applicable specification profile; system designers must consult the normative standard rather than relying on a simplified list of modes.

A key design principle is that a waveform is not characterized by modulation alone. The same QPSK constellation can have substantially different performance depending on FEC rate, frame length, pilots, roll-off, implementation loss, and channel impairment.

8. Adaptive Coding and Modulation

ACM selects a modulation and coding mode according to measured or predicted channel quality. A gateway may estimate received signal quality from return-channel reports, beacon measurements, pilot quality, or decoder statistics. It then assigns a mode such as robust QPSK with strong coding during rain attenuation and a higher-order APSK mode with weaker coding during clear-sky conditions.

The mode decision is a control problem involving:

• channel-estimation accuracy;
• reporting delay and feedback reliability;
• hysteresis to prevent rapid mode oscillation;
• target packet-error rate and service availability;
• beam, terminal, and transponder power constraints;
• fairness among users.

A practical ACM table contains thresholds with margins. Up-switching normally requires a better threshold than down-switching, creating hysteresis. The margin accounts for channel prediction error and the time required to apply a new mode.

ACM differs from simple adaptive modulation because coding rate changes as well as constellation order. It is especially valuable on satellite links with slow or moderate rain fading, where the system can trade throughput for availability without changing the physical infrastructure.

9. Nonlinearity, Predistortion and Spectral Integrity

Satellite high-power amplifiers often operate near saturation to maximize DC-to-RF efficiency. Traveling-wave tube amplifiers and solid-state power amplifiers introduce amplitude compression and AM/PM conversion. Constant-envelope or low-amplitude-variation signals are comparatively tolerant. APSK and QAM require greater output back-off, reducing amplifier efficiency.

Important distortion metrics include adjacent-channel power ratio, error vector magnitude, and nonlinear degradation in required $E_b/N_0$. Digital predistortion can compensate for predictable amplifier characteristics, but its performance depends on memory effects, temperature, bandwidth, and model accuracy. Filtering must also control spectral regrowth so that neighboring carriers and protected services are not harmed.

10. Worked Design Example: Selecting a Clear-Sky and Rain Mode

Consider an illustrative forward link with symbol rate $R_s=30$ Msymbol/s and roll-off factor $\alpha=0.20$. Assume physical-layer overhead is ignored initially.

For QPSK with coding rate $R_c=3/4$:

$$R_b=R_s\log_2(4)R_c

=30\times2\times0.75=45\text{ Mbit/s}.$$

The approximate occupied bandwidth is

$$B=(1+0.20)30=36\text{ MHz},$$

so the information spectral efficiency is

$$\eta=45/36=1.25\text{ bit/s/Hz}.$$

Now consider 16APSK with coding rate $R_c=3/4$. Since $\log_2(16)=4$,

$$R_b=30\times4\times0.75=90\text{ Mbit/s},$$

and

$$\eta=90/36=2.50\text{ bit/s/Hz}.$$

The second mode doubles throughput in the idealized calculation, but it requires substantially higher link quality and is more sensitive to nonlinear distortion. During rain attenuation, the system may return to QPSK with a stronger code, for example $R_c=1/2$:

$$R_b=30\times2\times0.5=30\text{ Mbit/s}.$$

This mode sacrifices throughput to preserve service continuity. In an operational system, pilot and header overhead, framing efficiency, implementation loss, and actual occupied-spectrum masks would reduce the net rate.

11. Applied Case: ACM for a Ka-Band Broadband Gateway

Background

This illustrative case concerns a Ka-band gateway serving broadband terminals through a geostationary high-throughput satellite. The gateway has high clear-sky capacity but experiences periodic rain attenuation. Service policy requires high availability for basic connectivity and permits reduced throughput during severe weather.

Problem

A fixed high-order waveform provides excellent clear-sky throughput but produces frequent outages during rain. A permanently robust waveform preserves availability but wastes capacity during clear weather. The gateway must select modes while avoiding unstable switching.

Analysis

The gateway monitors pilot quality, decoder frame-error statistics, and reported terminal measurements. A propagation model predicts short-term attenuation, while mode thresholds are calibrated using measured implementation loss. The analysis identifies three operating regions:

1. Clear sky: 16APSK or 32APSK with moderate coding redundancy.

2. Moderate attenuation: 8PSK or QPSK with intermediate code rates.

3. Severe attenuation: QPSK with strong coding and increased interleaving.

The design also evaluates amplifier back-off. A high-order APSK mode is rejected if nonlinear distortion increases the required margin more than its throughput benefit justifies.

Approach and solution

The gateway uses hysteretic ACM thresholds, a minimum dwell time, and a conservative down-switch rule. It reserves a robust control channel so that mode commands remain reliable during degradation. Interleaving depth is increased only where latency requirements permit. Terminals use soft-decision LDPC decoding and report both signal quality and decoder outcomes.

Outcome

The resulting system maintains a robust low-rate service during heavy rain while exploiting high-order modulation in clear conditions. Average throughput increases compared with a permanently robust design, and outage probability decreases compared with a fixed high-order design. The gain depends on climate, beam loading, feedback latency, and the accuracy of the propagation model.

Learning points

The case demonstrates that ACM is a system-level control strategy, not merely a modulation-selection table. Reliable operation requires measurement, prediction, coding, interleaving, amplifier management, feedback design, and service prioritization.

12. Design Challenges, Governance and Best Practices

Designers must account for regulatory emission masks, coordination with neighboring satellites, spectrum licensing, and protection of terrestrial services. Waveform changes may affect lawful interception, encryption interfaces, emergency communications, and service-level commitments. Security controls should protect ACM feedback and management channels from spoofing or denial-of-service attacks.

Best practices include:

• define whether rates are gross, coded, physical-layer, or net application rates;
• use a complete link budget with rain, pointing, polarization, implementation, and nonlinear margins;
• validate BER and PER using realistic channel models rather than AWGN alone;
• test synchronization under Doppler, phase noise, frequency offset, and low SNR;
• measure amplifier behavior at the intended operating point;
• include hysteresis and feedback delay in ACM simulations;
• document standard versions and waveform profiles;
• verify spectral masks and adjacent-channel performance in hardware;
• use soft decoding and calibrated reliability metrics;
• assess latency and memory costs of interleaving and long FEC frames.

Machine-learning methods are increasingly studied for channel prediction, demodulator impairment compensation, and ACM policy optimization. They can be useful when propagation or hardware behavior is difficult to model, but they require representative training data, robustness testing, explainable operating bounds, and protection against distribution shift. A learned controller should not replace deterministic safety limits for spectrum compliance and link availability.

Contemporary Developments

Current research addresses very high-throughput multi-beam satellites, optical feeder links, non-geostationary constellations, integrated terrestrial–satellite networks, and joint communication and sensing. Research topics include short-blocklength coding for low-latency services, nonbinary and spatially coupled LDPC codes, probabilistic constellation shaping, nonlinear compensation, faster-than-Nyquist signaling, and AI-assisted receiver design.

Probabilistic shaping changes the probability of transmitted constellation points so that the signal distribution can approach favorable channel input distributions. It may improve the spectral-efficiency versus power-efficiency trade-off, but it complicates framing, rate adaptation, synchronization, and nonlinear analysis. Such techniques must therefore be evaluated as complete waveforms rather than isolated coding innovations.

Chapter Summary

Satellite waveform design balances bandwidth, power, complexity, latency, and availability. Analog FM remains historically important, but modern systems rely mainly on coded digital modulation. PSK, QAM, and APSK provide different compromises between spectral efficiency and sensitivity to noise and amplifier nonlinearity. Raised-cosine pulse shaping limits bandwidth while satisfying the Nyquist zero-ISI condition under ideal sampling.

Carrier, timing, and frame synchronization are essential for correct demodulation. FEC codes—including LDPC, BCH, convolutional, and turbo codes—provide coding gain, while soft decisions and interleaving improve performance against random and burst errors. DVB-S2 and DVB-S2X-class systems integrate modulation, coding, framing, pilots, scrambling, and filtering into configurable physical layers.

ACM converts varying channel quality into a controlled throughput–availability trade-off. Effective implementation requires reliable measurement, hysteresis, feedback management, nonlinear amplifier analysis, and regulatory compliance. The most appropriate waveform is therefore selected by system-level evaluation rather than by modulation order alone.

Key Terms

Modulation: Mapping information onto a carrier waveform.

Constellation: Set of complex signal points used by a digital modulator.

Symbol rate: Number of transmitted modulation symbols per second.

Spectral efficiency: Information rate per unit bandwidth, measured in bit/s/Hz.

Power efficiency: Ability to achieve a target error performance with low required signal energy.

Roll-off factor: Excess-bandwidth parameter of a raised-cosine filter.

Intersymbol interference: Distortion caused when symbols overlap at sampling instants.

Matched filter: Receiver filter optimized for detection in additive white Gaussian noise.

Synchronization: Estimation of timing, carrier frequency, phase, and frame position.

FEC: Addition of structured redundancy for error correction.

Code rate: Ratio of information bits to transmitted coded bits.

LDPC code: Sparse-graph block code decoded iteratively using message passing.

Interleaving: Reordering of bits or symbols to disperse burst errors.

APSK: Modulation using multiple amplitude rings and phase positions.

ACM: Adaptive selection of coding and modulation according to channel quality.

E_b/N_0: Energy per information bit divided by noise spectral density.

Physical-layer frame: Structured transmission unit containing headers, pilots, and coded data.

Amplifier back-off: Reduction from saturation used to limit nonlinear distortion.

Error vector magnitude: Measure of deviation between received and ideal constellation points.

DVB-S2X: Extended DVB-S2 family supporting additional efficiency and application profiles.

Review Questions

Short Answer Questions

1. Define spectral efficiency and explain why it is not the same as symbol rate.

2. What is the function of a root-raised-cosine filter?

3. Why are soft decisions useful to an LDPC decoder?

4. State two purposes of interleaving in a satellite receiver.

5. What is the principal objective of adaptive coding and modulation?

Long Answer Questions

1. Compare QPSK, 16APSK, and high-order QAM with respect to spectral efficiency, power requirement, and amplifier nonlinearity.

2. Explain carrier, timing, and frame synchronization and discuss the consequences of errors in each.

3. Describe the processing chain of a DVB-S2-class physical layer.

4. Explain the relationship between coding rate, throughput, coding gain, and occupied bandwidth.

5. Discuss how raised-cosine filtering creates a trade-off between spectral containment and implementation tolerance.

Analytical / Application Questions

1. A link uses 20 Msymbol/s, QPSK, coding rate 2/3, and roll-off factor 0.25. Estimate its ideal information rate and spectral efficiency, ignoring framing overhead.

2. Design an ACM strategy for a satellite terminal that experiences slow rain fades but has a strict latency limit. Justify the modulation, coding, interleaving, and threshold decisions.

3. A proposed 32APSK mode gives higher clear-sky throughput but requires 3 dB more link margin and greater amplifier back-off. Develop criteria for deciding whether it should replace a 16APSK mode.

Multiple Choice Questions with Answers

Question 1

For QPSK with coding rate 3/4, what is the ideal information rate at 30 Msymbol/s?

A. 22.5 Mbit/s

B. 30 Mbit/s

C. 45 Mbit/s

D. 60 Mbit/s

Correct Answer: C. 45 Mbit/s

Question 2

What is the approximate bandwidth of a 30 Msymbol/s waveform with roll-off factor 0.20?

A. 24 MHz

B. 30 MHz

C. 36 MHz

D. 45 MHz

Correct Answer: C. 36 MHz

Question 3

Which modulation is generally more tolerant of satellite amplifier amplitude nonlinearity?

A. Constant-envelope BPSK

B. High-order rectangular QAM

C. 256-QAM

D. Unshaped multilevel QAM

Correct Answer: A. Constant-envelope BPSK

Question 4

The principal purpose of interleaving is to:

A. Increase carrier frequency

B. Convert burst errors into more distributed errors

C. Remove all channel noise

D. Increase amplifier saturation power

Correct Answer: B. Convert burst errors into more distributed errors

Question 5

In a coded system, the code rate is defined as:

A. Transmitted coded bits divided by information bits

B. Information bits divided by transmitted coded bits

C. Symbol rate divided by bandwidth

D. Carrier power divided by noise power

Correct Answer: B. Information bits divided by transmitted coded bits

Question 6

Which DVB-S2 modulation uses multiple amplitude rings?

A. BPSK

B. QPSK

C. APSK

D. FSK

Correct Answer: C. APSK

Question 7

A narrow carrier-tracking loop generally has which characteristic?

A. Better rejection of estimation noise but slower tracking

B. Faster tracking with more admitted noise

C. No sensitivity to oscillator error

D. Automatic correction of all frame errors

Correct Answer: A. Better rejection of estimation noise but slower tracking

Question 8

If 16APSK with coding rate 3/4 is transmitted at 30 Msymbol/s, ignoring overhead, what is the information rate?

A. 45 Mbit/s

B. 60 Mbit/s

C. 90 Mbit/s

D. 120 Mbit/s

Correct Answer: C. 90 Mbit/s

Question 9

Adaptive coding and modulation primarily allows a system to:

A. Keep throughput fixed regardless of channel conditions

B. Trade throughput for robustness as channel quality changes

C. Eliminate the need for synchronization

D. Operate without a link budget

Correct Answer: B. Trade throughput for robustness as channel quality changes

Question 10

Which statement about a raised-cosine roll-off factor is correct?

A. Increasing it always reduces occupied bandwidth

B. It controls excess bandwidth beyond the Nyquist bandwidth

C. It determines the FEC code rate

D. It eliminates phase noise

Correct Answer: B. It controls excess bandwidth beyond the Nyquist bandwidth