Demand Assigned Multiple Access (DAMA)

Introduction to Multiple Access in Satellite Communication

Satellite communication systems enable long-distance communication by using satellites as relay stations. A critical challenge in these systems is how to allow multiple users to share the limited satellite resources (bandwidth and power) efficiently. This is where multiple access techniques come into play.

Traditional multiple access methods include:

• FDMA (Frequency Division Multiple Access): Different users are assigned different frequency bands
• TDMA (Time Division Multiple Access): Different users are assigned different time slots
• CDMA (Code Division Multiple Access): Different users are assigned different codes

While these fixed assignment methods work well when traffic is predictable and continuous, they are inefficient for bursty traffic patterns where users need resources only intermittently. This inefficiency led to the development of demand-based assignment methods like DAMA.

What is Demand Assigned Multiple Access (DAMA)?

Demand Assigned Multiple Access (DAMA) is a satellite communication resource allocation method where channels are assigned to users on-demand rather than being permanently allocated. In DAMA systems, resources (frequency bands, time slots, or codes) are dynamically assigned based on actual user needs.

Core Principles of DAMA:

1. Dynamic Allocation: Channels are allocated only when needed
2. Resource Pooling: All available channels form a common pool
3. Request-Grant Cycle: Users request channels, system grants access
4. Return After Use: Channels are returned to the pool after communication ends

How DAMA Works

A typical DAMA system operates through a centralized control mechanism, often managed by a master earth station or the satellite itself. The process involves several key steps:

DAMA Operation Sequence:

1. Request Phase: An earth station sends a channel request to the control station when it needs to communicate
2. Allocation Phase: The control station checks for available channels in the pool and assigns one to the requesting station
3. Communication Phase: The earth station uses the assigned channel for its communication
4. Release Phase: When communication is complete, the earth station informs the control station and returns the channel to the pool

Control Channel:

DAMA systems typically use a separate control channel (often called a signaling channel) for request and release messages. This control channel can be implemented using:

• A dedicated frequency band
• A specific time slot in a TDMA frame
• A code sequence in a CDMA system

DAMA Types & Protocols

Types of DAMA Systems:

1. Centralized DAMA: A master station controls all channel assignments
2. Distributed DAMA: Control is distributed among all earth stations
3. Hybrid DAMA: Combines elements of both centralized and distributed control

DAMA Protocols:

Various protocols govern how earth stations request and release channels:

• ALOHA-based Protocols: Random access methods where stations transmit requests randomly
• Reservation Protocols: Stations reserve slots in advance for data transmission
• Polling Protocols: The master station polls each earth station in sequence
• TDMA-based DAMA: Uses TDMA frames with dedicated request slots

DAMA with FDMA (DA-FDMA):

In DA-FDMA, the available frequency band is divided into channels, and these channels are assigned on demand. This is efficient for voice communications where channels are needed for the duration of a call.

DAMA with TDMA (DA-TDMA):

In DA-TDMA, time slots in a frame are assigned dynamically. This is particularly suitable for data communications with bursty traffic patterns.

Advantages & Limitations of DAMA

Advantages:

• High Efficiency: Better utilization of satellite resources compared to fixed assignment
• Scalability: Can accommodate a large number of users with relatively few channels
• Flexibility: Adapts to varying traffic patterns and user demands
• Cost-Effective: Reduces the required number of channels for a given user community
• Supports Bursty Traffic: Ideal for applications with intermittent communication needs

Limitations:

• Control Overhead: Requires additional signaling for request and release operations
• Setup Delay: Channel assignment takes time, causing initial setup delay
• Complexity: More complex control mechanisms than fixed assignment systems
• Single Point of Failure: In centralized DAMA, failure of the control station disrupts the entire system
• Contention: Multiple simultaneous requests may cause contention and require resolution protocols

Applications of DAMA

Military Communications:

DAMA is widely used in military satellite communications (MILSATCOM) where secure, efficient, and flexible communication is essential. Examples include the U.S. Navy's UFO (UHF Follow-On) satellite system.

Maritime and Aeronautical Communications:

INMARSAT (International Maritime Satellite Organization) uses DAMA for ship-to-shore and aircraft communications where traffic is intermittent and unpredictable.

Rural and Remote Area Communications:

VSAT (Very Small Aperture Terminal) networks in remote areas often use DAMA to efficiently share satellite bandwidth among many users with sporadic communication needs.

Disaster Recovery and Emergency Communications:

DAMA systems provide flexible communication resources that can be quickly allocated to emergency responders when needed.

SCADA (Supervisory Control and Data Acquisition):

Remote monitoring and control systems for pipelines, utilities, and other infrastructure often use DAMA for efficient data transmission from multiple remote sites.

Comparison with Other Multiple Access Techniques

Hybrid Systems:

Modern satellite systems often use hybrid approaches combining multiple techniques:

• MF-TDMA (Multi-Frequency TDMA): Combines FDMA and TDMA with DAMA control
• CDMA/DAMA: Uses CDMA for channel separation with DAMA for code assignment
• FDMA/DAMA: The most common implementation, used in many commercial VSAT networks