Virtual Laboratory

Baseband Analog Signal Parameters in Satellite Communication Links

For Undergraduate Satellite Communication Engineering Students

1. Laboratory Objectives

Upon completion of this virtual laboratory, the student will be able to:

Understand companding techniques (A-law and μ-law) used in satellite baseband signal processing to improve dynamic range and signal-to-noise ratio.
Analyze frequency response of audio (telephony) and video baseband channels in satellite transponders, including bandwidth limitations and filter characteristics.
Measure and quantify crosstalk between adjacent channels in FDMA satellite systems, understanding coupling mechanisms and isolation requirements.
Evaluate noise effects on baseband analog signals, including thermal noise, intermodulation noise, and their impact on signal quality in satellite links.
Assess clipping distortion by analyzing clipped vs. unclipped sinusoidal signals, understanding the importance of operating within the linear range of satellite amplifiers (TWTA/SSPA).

2. Theoretical Background

2.1 Baseband Signals in Satellite Communications

In satellite communication systems, the baseband signal refers to the original information-bearing signal before modulation onto the RF carrier. For analog satellite systems, baseband signals typically include:

Audio/Telephony signals: 300 Hz – 3.4 kHz (standard telephone channel)
Video signals: 0 Hz – 6 MHz (standard analog TV baseband)
FM multiplexed signals: Multiple voice channels multiplexed using FDM

These baseband signals are processed through various subsystems (companders, filters, pre-emphasis networks) before being modulated (typically FM) and transmitted via the satellite transponder.

2.2 Companding

Companding (COMPression + exPANDING) is a technique used to improve the dynamic range of analog signals in the presence of quantization or channel noise. In satellite telephony, companding is essential because:

Speech signals have a large dynamic range (approximately 40 dB)
Uniform quantization would waste bits on high-amplitude signals
Companding compresses high amplitudes and expands low amplitudes

A-Law Companding (ITU-T G.711, used in Europe and most of the world):

y = sgn(x) × { A|x| / (1 + ln A) for |x| ≤ 1/A
(1 + ln(A|x|)) / (1 + ln A) for 1/A < |x| ≤ 1 }
where A = 87.6 (standard value)

μ-Law Companding (ITU-T G.711, used in North America/Japan):

y = sgn(x) × ln(1 + μ|x|) / ln(1 + μ)
where μ = 255 (standard value)

Key Insight: Companding provides an effective improvement in SNR of approximately 24-30 dB for speech signals, which is critical in bandwidth-limited satellite channels.

2.3 Frequency Response of Audio and Video Channels

The satellite transponder acts as a bandpass filter with specific frequency response characteristics. For baseband signals:

Audio Channel (Telephony):

Passband: 300 Hz – 3,400 Hz
Roll-off: Typically 0.5 dB maximum variation in passband
Group delay variation: < 3 ms across the band
Stopband attenuation: > 40 dB below 100 Hz and above 4 kHz

Video Channel:

Passband: 0 Hz – 6 MHz (for standard PAL/NTSC)
Amplitude response flatness: ±0.5 dB to ±1 dB
Group delay: Linear with < 100 ns variation
Differential gain and phase must be minimized to preserve color information

Channel Transfer Function: H(f) = |H(f)| e^jφ(f)
where |H(f)| = amplitude response and φ(f) = phase response

2.4 Crosstalk in Satellite FDMA Systems

Crosstalk refers to the unwanted coupling of a signal from one channel into an adjacent channel. In satellite FDMA (Frequency Division Multiple Access) systems:

Adjacent channel interference (ACI) occurs due to non-ideal filter roll-off
Intermodulation products from the TWTA create crosstalk components
Typical crosstalk isolation requirement: > 60 dB for telephony, > 50 dB for video

Crosstalk Ratio (XTR) = 10 log₁₀(P_interfering / P_desired) [dB]
or equivalently: XTR = 20 log₁₀(V_interfering / V_desired) [dB]

2.5 Noise in Satellite Links

Noise degrades baseband signal quality in satellite links. Primary noise sources include:

Thermal Noise (AWGN): N = kTB, where k = Boltzmann's constant, T = system noise temperature, B = bandwidth
Intermodulation Noise: Generated in non-linear amplifiers (TWTA) when multiple carriers are present
Phase Noise: From local oscillators in up/down converters
Impulse Noise: From switching transients and atmospheric effects

Signal-to-Noise Ratio: SNR = P_s / P_n = P_s / (kT_sysB)
In dB: SNR(dB) = P_s(dBW) - 10log₁₀(k) - 10log₁₀(T_sys) - 10log₁₀(B)

2.6 Unclipped Sinewave and Amplifier Linearity

Satellite traveling wave tube amplifiers (TWTAs) and solid-state power amplifiers (SSPAs) have a limited linear operating range. When input signal amplitude exceeds the linear region:

Clipping occurs, causing amplitude distortion
Harmonic distortion products are generated
Total Harmonic Distortion (THD) increases rapidly
Intermodulation products affect adjacent channels

Total Harmonic Distortion: THD = √(Σ V_n² / V₁²) × 100%
where V₁ = fundamental amplitude, V_n = n-th harmonic amplitude

Satellite Design Rule: TWTAs are typically operated 3-6 dB below saturation (input back-off) to maintain linearity and minimize intermodulation distortion in multi-carrier FDMA operation.

3. Laboratory Procedure

3.1 General Setup

Each simulation below represents a measurement setup for analyzing baseband analog signal parameters in a satellite communication link. Follow these general steps:

Read the theoretical background for the parameter being measured.
Navigate to the corresponding simulation section.
Adjust the control parameters using the sliders and observe the real-time plots.
Record the measured values in your lab notebook.
Answer the questions in the report guidelines section.

3.2 Experiment 1: Companding Characteristics

Open Simulation 1: Companding Analyzer.
Select the companding law (A-Law or μ-Law).
Vary the input signal amplitude from 0 to maximum.
Observe the compression curve and compare with linear (uncompanded) response.
Measure the compression ratio at 50% and 90% of maximum input.
Switch to the expander view and verify complementary expansion.

3.3 Experiment 2: Frequency Response Measurement

Open Simulation 2: Channel Frequency Response Analyzer.
Select channel type: Audio (300-3400 Hz) or Video (0-6 MHz).
Adjust the filter roll-off factor and observe the magnitude response.
Measure the -3 dB bandwidth and compare with theoretical values.
Observe the phase response and measure group delay variation.

3.4 Experiment 3: Crosstalk Measurement

Open Simulation 3: Crosstalk Analyzer.
Set the desired signal in Channel 1 (e.g., 1 kHz tone).
Introduce crosstalk from an adjacent Channel 2 by adjusting the coupling coefficient.
Measure the crosstalk ratio in dB for different coupling levels.
Observe the combined signal and identify the interference component.

3.5 Experiment 4: Noise Analysis

Open Simulation 4: Noise and SNR Analyzer.
Set the signal amplitude and frequency (e.g., 1 kHz sine wave).
Gradually increase the noise power (decrease SNR from 40 dB to 0 dB).
Observe the signal degradation in time domain and frequency domain.
Record the SNR threshold where the signal becomes unintelligible/ unusable.

3.6 Experiment 5: Clipping Distortion Analysis

Open Simulation 5: Clipping Distortion Analyzer.
Set a clean sinusoidal signal at 1 kHz.
Gradually increase the signal amplitude beyond the amplifier's linear range.
Observe the clipped waveform and the resulting harmonic spectrum.
Measure the THD percentage as a function of clipping level.
Determine the maximum unclipped amplitude (back-off point).

1

Companding Characteristics Analyzer

Simulate A-law and μ-law companding curves. Observe how compression improves dynamic range handling in satellite telephony channels.

Companding Law

Input Amplitude: 0.5

Dynamic Range (dB): 40

Show

Input Level

0.50

Output Level

0.76

Compression Ratio

1.52

SNR Improvement

~24 dB

2

Channel Frequency Response Analyzer

Analyze the amplitude and phase response of satellite baseband channels. Compare audio (telephony) and video channel characteristics.

Channel Type

Roll-off Factor: 0.5

Passband Ripple (dB): 0.5

Plot Type

-3 dB Bandwidth

3.10 kHz

Passband Variation

0.50 dB

Stopband Attenuation

40 dB

Group Delay Variation

2.1 ms

3

Crosstalk Analyzer

Simulate crosstalk between adjacent channels in an FDMA satellite system. Measure crosstalk ratio and observe interference effects.

Desired Signal Freq (Hz): 1000

Interfering Freq (Hz): 1200

Crosstalk Coupling (dB): -30

Desired Signal Amp: 1.0

Desired Signal Power

0.00 dB

Interference Power

-30.0 dB

Crosstalk Ratio (XTR)

30.0 dB

SIR (Signal/Interference)

30.0 dB

4

Noise and SNR Analyzer

Analyze the effect of additive white Gaussian noise (AWGN) on baseband analog signals in satellite links. Observe time-domain and frequency-domain degradation.

Signal Frequency (Hz): 1000

Signal Amplitude: 1.0

SNR (dB): 20

View Mode

Signal Power

-3.01 dB

Noise Power

-23.0 dB

Measured SNR

20.0 dB

Signal Quality

Good

5

Clipping Distortion and Unclipped Sinewave Analyzer

Analyze the effect of amplifier clipping on sinusoidal signals. Understand the importance of input back-off in satellite TWTAs and SSPAs to maintain signal fidelity.

Signal Frequency (Hz): 1000

Input Amplitude: 1.0

Clip Level (Max Linear): 1.0

Plot Mode

Input Back-off

0.0 dB

THD (%)

0.00%

Clipping (%)

0.0%

Status

Linear

6. Laboratory Report Guidelines

6.1 Report Structure

Your laboratory report should contain the following sections:

Title Page: Course name, experiment title, student name, ID, date
Abstract: Brief summary (150-200 words) of objectives, methods, and key findings
Introduction: Background on baseband analog signals in satellite communications
Theory: Summarize the theoretical principles for each parameter measured
Procedure: Step-by-step description of experiments performed
Results and Discussion: Tabulated data, plots, analysis, and interpretation
Conclusion: Key findings and their significance in satellite system design
References: Cite textbooks, standards (ITU-T, IEEE), and technical papers

6.2 Required Data Tables

Include the following tables in your report:

Table 1: Companding Characteristics

Input Level (normalized)	A-Law Output	μ-Law Output	Linear Output	Compression Ratio
0.1
0.3
0.5
0.7
0.9
1.0

Table 2: Frequency Response Measurements

Channel Type	-3 dB Lower Cutoff	-3 dB Upper Cutoff	Bandwidth	Passband Ripple
Audio (300-3400 Hz)
Video (0-6 MHz)

Table 3: Crosstalk Measurements

Coupling (dB)	Desired Signal (dB)	Interference (dB)	XTR (dB)	SIR (dB)
-10
-20
-30
-40
-50
-60

Table 4: Noise Analysis

SNR (dB)	Signal Power (dB)	Noise Power (dB)	Visual Quality	Usability Threshold
40
30
20
10
0

Table 5: Clipping Distortion

Input Amplitude	Clip Level	Back-off (dB)	THD (%)	Clipping %	Status
0.5	1.0
1.0	1.0
1.5	1.0
2.0	1.0

6.3 Discussion Questions

Explain why A-law companding uses A = 87.6 specifically. What would happen if A were much smaller or much larger?
Compare the frequency response requirements for audio vs. video satellite channels. Why is group delay more critical for video?
What is the minimum acceptable crosstalk isolation for INTELSAT standard FDMA carriers? How is this achieved in practice?
Calculate the thermal noise power in a 36 MHz satellite transponder at room temperature (290 K). Express your answer in dBm.
A TWTA is operated with 4 dB input back-off. If the saturation output is 50 dBW, what is the actual output power? What is the trade-off?
How does clipping distortion generate intermodulation products that affect adjacent channels in FDMA systems?
Design a pre-emphasis/de-emphasis network for a satellite FM system. What is the typical time constant used?

6.4 Grading Rubric

Criteria	Weight	Excellent (A)	Good (B)	Satisfactory (C)	Poor (D/F)
Data Collection	20%	All tables complete, accurate	Most tables complete	Some data missing	Major data gaps
Analysis	25%	Deep insight, correct formulas	Good interpretation	Basic analysis	Incorrect or missing
Plots & Figures	15%	Clear, labeled, referenced	Mostly clear	Some labels missing	Unusable plots
Discussion	25%	Critical thinking, real-world links	Good discussion	Superficial	Missing or irrelevant
Presentation	15%	Professional formatting	Organized	Adequate	Disorganized

7. References

Roddy, D. (2006). Satellite Communications (4th ed.). McGraw-Hill. Chapters 5-7.
ITU-T Recommendation G.711 (1988). Pulse Code Modulation (PCM) of Voice Frequencies.
Pratt, T., & Bostian, C. W. (1986). Satellite Communications. John Wiley & Sons. Chapter 9: Baseband Signal Processing.
Ha, T. T. (1990). Digital Satellite Communications (2nd ed.). McGraw-Hill. Sections 3.1-3.4.
Maral, G., & Bousquet, M. (2009). Satellite Communications Systems (5th ed.). Wiley. Chapter 4: Link Budget and Baseband.
Sklar, B. (2001). Digital Communications: Fundamentals and Applications (2nd ed.). Prentice Hall. Chapter 9: SNR and Companding.
INTELSSAT IESS-308 (1992). Performance Characteristics of Intermediate Data Rate Carriers.