Satellite Traffic Intensity Study Guide

Module 1: Fundamentals of Satellite Traffic

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1.1 Unique Characteristics of Satellite Traffic

Propagation delay: Geostationary satellites (GEO) have ~250ms round-trip delay affecting traffic patterns
Coverage area: Large footprints create diverse traffic mixes from different regions
Orbital constraints: Limited satellite lifetime (15-20 years) and power limitations
Rain attenuation: Weather effects at higher frequencies require traffic margin

1.2 Key Traffic Parameters

Parameter	Symbol	Description	Typical Units
Offered Traffic	A	Total traffic attempting to use the satellite system	Erlangs (E)
Carried Traffic	Y	Traffic successfully handled by the system	Erlangs (E)
Blocking Probability	B	Probability that a call is blocked due to congestion	Dimensionless (%)
Channel Utilization	ρ	Fraction of time a channel is busy	Dimensionless (%)
Traffic Density	A_d	Traffic per unit area (e.g., per beam)	E/km²

A = λ × h (Offered Traffic = Arrival Rate × Average Holding Time)

Example Calculation:

A satellite beam covers a region with 500 users, each making an average of 2 calls per hour with average duration of 3 minutes. Calculate the offered traffic in Erlangs.

Solution:
Total arrival rate λ = 500 users × 2 calls/hour = 1000 calls/hour
Holding time h = 3 minutes = 0.05 hours
Offered traffic A = λ × h = 1000 × 0.05 = 50 Erlangs

Module 2: Satellite-Specific Constraints

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2.1 Link Budget and Traffic Capacity

The satellite's traffic capacity is fundamentally limited by the link budget equation:

C/N₀ = EIRP - Lₚ + G/T - k - Lₘ

Where:

C/N₀: Carrier-to-noise density ratio (dB-Hz)
EIRP: Equivalent isotropic radiated power (dBW)
Lₚ: Free-space path loss (dB)
G/T: Figure of merit of receiving system (dB/K)
k: Boltzmann's constant (-228.6 dBW/Hz-K)
Lₘ: Miscellaneous losses (rain, pointing, etc.) (dB)

2.2 Bandwidth-Traffic Relationship

N_max = B_total / (B_ch × (1 + α))

Where:

N_max: Maximum number of simultaneous channels
B_total: Total available bandwidth
B_ch: Bandwidth per channel
α: Guard band factor (typically 0.1-0.2)

Practice Problem:

A Ku-band satellite transponder has 36 MHz bandwidth. Each voice channel requires 64 kbps using QPSK modulation with 1.2 bps/Hz spectral efficiency. Considering 15% guard bands, calculate the maximum number of simultaneous voice channels.

Hint: First calculate bandwidth per channel, then apply the formula above.

Module 3: Multiple Access Techniques & Traffic

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3.1 FDMA (Frequency Division Multiple Access)

Traffic characteristic: Fixed allocation, inefficient for bursty traffic
Capacity calculation: N = B_total / B_channel
Advantage: Simple, no coordination needed
Disadvantage: Poor bandwidth utilization with variable traffic

3.2 TDMA (Time Division Multiple Access)

Traffic characteristic: Users transmit in assigned time slots
Frame structure: Includes preamble, traffic slots, guard times
Capacity: N = T_frame / (T_slot + T_guard)
Efficiency: Typically 90-95% due to overhead

3.3 CDMA (Code Division Multiple Access)

Traffic characteristic: All users share same frequency and time
Soft capacity: Capacity limited by interference, not hard limits
Key parameter: Processing gain (G_p = B_ss/R_b)
Capacity approximation: N ≈ G_p / (E_b/N₀ required)

Access Method	Best For	Traffic Efficiency	Complexity
FDMA	Constant rate traffic (voice, video)	Low-Medium (60-75%)	Low
TDMA	Mixed constant/bursty traffic	Medium-High (70-90%)	Medium
CDMA	Highly variable, bursty traffic	High (80-95%)	High
DAMA (Demand Assigned)	Intermittent, low-duty cycle traffic	Very High (90-98%)	Very High

Module 4: Traffic Models for Satellite Systems

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4.1 Modified Erlang Models for Satellite Systems

Satellite systems often use modified Erlang formulas accounting for:

Propagation delay effects on holding time
Multiple spot beams with handover traffic
Non-Poisson arrival patterns in some applications

B(N, A, d) = (A^N/N!) / (Σ_k=0^N A^k/k!) × f(d)

Where f(d) is a delay correction factor (typically 1.05-1.15 for GEO systems).

4.2 Multi-Beam Traffic Analysis

Modern satellites use multiple spot beams for frequency reuse:

A_system = Σ_i=1^M A_i × F_reuse

Where:

A_i: Traffic in beam i
M: Number of beams
F_reuse: Frequency reuse factor (typically 3-7)

Example: Multi-Beam Satellite Capacity

A satellite has 48 spot beams arranged in a 4-color reuse pattern. Each beam can handle 20 Erlangs at 2% blocking probability. Calculate the total system capacity.

Solution:
Capacity per beam = 20 E
Number of beams using same frequency = 48 / 4 = 12
System capacity = 12 × 20 = 240 Erlangs
Note: This assumes uniform traffic distribution across beams.

Module 5: Design Examples & Case Studies

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5.1 VSAT Network Design

Very Small Aperture Terminal networks typically use star topology with TDMA/DAMA access:

N_VSAT = (B_inbound × η) / (R_VSAT × ρ)

Where:

B_inbound: Inbound bandwidth allocated
η: TDMA efficiency factor (typically 0.8-0.9)
R_VSAT: Data rate per VSAT
ρ: VSAT duty cycle (typically 0.05-0.2 for intermittent traffic)

5.2 Satellite Internet Traffic Engineering

Modern broadband satellites (e.g., Starlink, OneWeb) use:

LEO constellations to reduce latency
Dynamic beam forming for traffic hotspots
Adaptive coding and modulation based on link conditions
Statistical multiplexing gains of 3:1 to 5:1 for Internet traffic

Design Problem: Regional Satellite System

Design a satellite system to serve a region with the following requirements:

Coverage area: 500 km diameter circle
Number of users: 10,000
Average traffic per user: 0.02 Erlangs during busy hour
Required blocking probability: 1%
Available spectrum: 50 MHz in Ku-band
Modulation: 16-QAM with 3 bps/Hz efficiency

Calculate: (a) Total offered traffic, (b) Required channels, (c) Bandwidth requirements, (d) Number of beams needed if each beam covers 100 km diameter.

Module 6: Advanced Topics & Future Trends

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6.1 High Throughput Satellites (HTS)

Frequency reuse: Up to 20x through multiple spot beams
Flexible payloads: Dynamic channel allocation based on traffic demand
Capacity: Modern HTS provide 100+ Gbps total throughput

6.2 Non-Geostationary Orbit Constellations

Orbit Type	Altitude	Latency	Traffic Handover	Examples
LEO	300-2,000 km	20-40 ms	Frequent (every few minutes)	Starlink, OneWeb
MEO	8,000-20,000 km	100-150 ms	Less frequent	O3b, GPS satellites
GEO	35,786 km	250-280 ms	None (stationary)	Traditional comm satellites

6.3 Traffic Engineering for 5G Satellite Integration

Network slicing: Virtual networks with different traffic characteristics
Edge computing: Processing traffic closer to users in satellite networks
Dynamic spectrum sharing: Between terrestrial and satellite networks

Key Takeaways

Satellite traffic engineering must account for unique constraints: delay, coverage, and propagation effects.

Multiple access technique choice significantly impacts traffic capacity and efficiency.

Modern systems use frequency reuse through multiple spot beams to dramatically increase capacity.

Non-GEO constellations are changing traditional satellite traffic patterns with lower latency but more complex handovers.

Future trends include flexible payloads, software-defined satellites, and integrated terrestrial-satellite networks.