Access Techniques

  • Introduction & Overview
  • FDMA - Frequency Division Multiple Access
  • TDMA - Time Division Multiple Access
  • CDMA - Code Division Multiple Access
  • SDMA - Space Division Multiple Access
  • Comparison & Applications

Quick Quiz

Introduction to Multiple Access Techniques

Multiple access techniques are fundamental to satellite communication systems, allowing multiple users to share the limited satellite bandwidth and transponder resources simultaneously. These techniques determine how the available resources are allocated among users and how interference is minimized.

Satellite Communication System Overview

Illustration of multiple ground stations communicating through a satellite

Ground Station 1 ──┐
Ground Station 2 ──┼──> [SATELLITE] ───> Ground Station N
Ground Station 3 ──┘

Why Multiple Access is Needed

Satellite communication systems have limited resources:

  • Limited bandwidth - The radio spectrum is a scarce resource
  • Limited transponder power - Satellite power is constrained
  • Cost efficiency - Sharing resources reduces cost per user
  • Scalability - Systems must accommodate growing numbers of users

Basic Classification

Multiple access techniques can be classified based on the resource being shared:

Technique Resource Shared Separation Method
FDMA Frequency Band Different frequency slots
TDMA Time Different time slots
CDMA Code Space Different orthogonal codes
SDMA Space/Beams Different spatial locations

Modern satellite systems often use hybrid approaches combining two or more of these techniques to optimize performance and capacity.

FDMA - Frequency Division Multiple Access

FDMA is the oldest and simplest multiple access technique where the available bandwidth is divided into multiple non-overlapping frequency bands. Each user is allocated a specific frequency band for the entire duration of their communication.

FDMA Channel Allocation

Frequency spectrum divided into multiple channels

┌───────┐ ┌───────┐ ┌───────┐ ┌───────┐
│User 1 │ │User 2 │ │User 3 │ │User 4 │
│f₁ band│ │f₂ band│ │f₃ band│ │f₄ band│
└───────┘ └───────┘ └───────┘ └───────┘
←────────────────────────────────────→
Frequency

How FDMA Works

In FDMA, each earth station transmits its signal at a different carrier frequency. The satellite transponder receives all these signals, amplifies them, and retransmits them back to Earth. Receivers use bandpass filters to select only their assigned frequency band.

Key Characteristics:

  • Continuous transmission - Users transmit continuously on their assigned frequency
  • Guard bands - Small frequency gaps between channels to prevent interference
  • Simple implementation - Requires only filters and frequency synthesizers
  • Best suited for analog signals and constant traffic

Mathematical Representation

The total bandwidth B is divided into N channels:

Bchannel = B / N

Where guard bands (Δf) are included between channels to prevent interference:

Btotal = N × Bchannel + (N-1) × Δf

Advantages & Disadvantages

Advantages Disadvantages
Simple to implement and understand Inefficient for bursty traffic
No synchronization required between stations Guard bands waste bandwidth
Well-suited for analog systems Intermodulation distortion in nonlinear transponders
Continuous transmission suitable for some applications Fixed allocation lacks flexibility

Applications

FDMA is used in:

  • Traditional analog satellite systems
  • VSAT (Very Small Aperture Terminal) networks
  • Satellite radio (e.g., SiriusXM)
  • Some legacy satellite telephone systems

TDMA - Time Division Multiple Access

TDMA divides the available bandwidth into time slots rather than frequency bands. All users transmit on the same frequency but at different times. Each user is allocated a specific time slot in a repeating frame structure.

TDMA Frame Structure

Time divided into frames, each with multiple slots

┌─────────────────────────────────────┐
│ TDMA Frame │
├─────┬─────┬─────┬─────┬─────┬─────┤
│User1│User2│User3│User4│Guard│Sync │
│Slot │Slot │Slot │Slot │Time │Bits │
└─────┴─────┴─────┴─────┴─────┴─────┘
←────────────────────────────────────→
Time

How TDMA Works

In TDMA systems, all earth stations must be synchronized to a common time reference. Each station transmits in bursts during its allocated time slot. The satellite receives these bursts, and the transponder retransmits them in the same time sequence. A reference station typically provides timing synchronization.

Key Characteristics:

  • Burst transmission - Users transmit in short, high-speed bursts
  • Requires precise synchronization between all stations
  • Guard times between slots to prevent overlap
  • More efficient for digital signals and bursty traffic

Mathematical Representation

The frame duration Tframe is divided into N slots:

Tslot = Tframe / N

Accounting for guard time (Tg) and synchronization overhead:

Tuseful = Tslot - Tg - Tsync

Advantages & Disadvantages

Advantages Disadvantages
Efficient for bursty/digital traffic Requires precise synchronization
No intermodulation distortion issues More complex than FDMA
Flexible allocation (slots can be reassigned) Higher peak power requirements
Can operate with single carrier per transponder Guard times reduce efficiency

Applications

TDMA is used in:

  • Digital satellite TV broadcasting (DVB-S2)
  • Military satellite communications
  • INMARSAT broadband services
  • Some mobile satellite systems

CDMA - Code Division Multiple Access

CDMA allows all users to transmit simultaneously on the same frequency band by using unique, orthogonal spreading codes. Each user's signal is spread across the entire bandwidth, and receivers use the corresponding code to extract their specific signal.

CDMA Spreading Principle

All users transmit simultaneously on same frequency with different codes

User 1: Data₁ × Code₁ = Spread Signal₁
User 2: Data₂ × Code₂ = Spread Signal₂
User 3: Data₃ × Code₃ = Spread Signal₃
──────────────────────────────────────
Combined: Σ(Spread Signals) → Satellite
Receiver: Combined × Code₁ → Data₁

How CDMA Works

In CDMA, each user's narrowband signal is multiplied by a unique wideband spreading code, resulting in a signal that occupies the entire bandwidth. All these spread signals are transmitted simultaneously. The receiver uses the same spreading code to despread the signal, recovering the original data while other users' signals appear as low-level noise.

Key Characteristics:

  • Spread spectrum technology - Signals spread over wide bandwidth
  • Soft capacity limit - Performance degrades gradually as users increase
  • Code orthogonality - Different codes should have low cross-correlation
  • Resistant to interference and jamming

Mathematical Representation

Each user's signal is multiplied by a spreading code ci(t):

si(t) = di(t) × ci(t)

The received signal is the sum of all transmitted signals plus noise:

r(t) = Σ si(t) + n(t)

The receiver correlates with the desired user's code:

d̂j(t) = ∫ r(t) × cj(t) dt

Advantages & Disadvantages

Advantages Disadvantages
Resistant to interference and jamming Complex signal processing required
Soft capacity limits (graceful degradation) Near-far problem (power control critical)
Enhanced privacy (coded signals) Requires precise power control
No frequency or time slot planning needed Lower spectral efficiency than TDMA

Applications

CDMA is used in:

  • GPS (Global Positioning System)
  • Military satellite communications
  • Iridium satellite phone system
  • Some satellite-based internet services

SDMA - Space Division Multiple Access

SDMA separates users based on their spatial location using directional antennas or beamforming. The satellite uses multiple spot beams to cover different geographical areas, with each beam serving a subset of users.

SDMA Spot Beam Concept

Satellite creates multiple beams pointing to different Earth regions

       ▲
       │
       └─ Satellite
       ／ │ ＼
  Beam1 Beam2 Beam3
   ↓      ↓      ↓
Region1 Region2 Region3

How SDMA Works

SDMA uses antenna arrays with beamforming capabilities to create multiple spot beams directed at different geographic areas. Each beam can reuse the same frequency band if the beams are sufficiently separated (spatial isolation). This frequency reuse dramatically increases system capacity.

Key Characteristics:

  • Frequency reuse - Same frequency used in different beams
  • Beamforming - Electronically steerable antennas create beams
  • Geographical separation - Users in different locations can use same resources
  • Often combined with other techniques (FDMA/TDMA within each beam)

Mathematical Representation

The capacity increase factor for SDMA with frequency reuse:

CapacitySDMA = N × Capacitysingle-beam

Where N is the number of beams with frequency reuse. The beam pattern for an array antenna:

G(θ,φ) = Σ wn × exp(jk·rn)

where wn are complex weights for beamforming.

Advantages & Disadvantages

Advantages Disadvantages
High capacity through frequency reuse Complex antenna systems required
Reduced interference between beams Beam steering and tracking needed
Can focus power where needed Inter-beam interference if not properly isolated
Flexible coverage patterns Higher satellite complexity and cost

Applications

SDMA is used in:

  • Modern high-throughput satellites (HTS)
  • Ka-band satellite broadband (e.g., ViaSat, HughesNet)
  • Mobile satellite systems with spot beams
  • Multi-beam satellite payloads

Comparison & Applications

Each multiple access technique has its strengths and weaknesses, making them suitable for different applications. Modern satellite systems often use hybrid approaches to leverage the advantages of multiple techniques.

Comparative Analysis

Technique Efficiency Complexity Flexibility Best For
FDMA Low-Medium Low Low Analog, continuous traffic
TDMA Medium-High Medium High Digital, bursty traffic
CDMA Medium High Medium Secure, interference-prone environments
SDMA Very High Very High High High-capacity systems with frequency reuse

Hybrid Techniques

Most modern satellite systems use hybrid approaches:

  • FDMA/TDMA: Frequency band divided into channels, each using TDMA (common in VSAT networks)
  • SDMA/FDMA: Different beams use different frequencies, with FDMA within each beam
  • CDMA/FDMA: CDMA used within each frequency channel
  • MF-TDMA (Multi-Frequency TDMA): Combination of FDMA and TDMA for flexible bandwidth allocation

Future Trends

Emerging technologies in satellite multiple access:

  • NOMA (Non-Orthogonal Multiple Access): Allows users to share same resources with different power levels
  • Massive MIMO: Advanced SDMA with hundreds of antenna elements
  • Cognitive Radio: Dynamic spectrum access based on real-time conditions
  • Laser Inter-satellite Links: Optical communication between satellites

Study Tips

  • Understand the fundamental trade-offs: bandwidth vs. time vs. code vs. space
  • Focus on how each technique handles interference between users
  • Learn to calculate capacity for simple scenarios
  • Study real-world examples of each technique in operation
  • Practice comparing techniques for different application requirements