edVirtus Reference

What Is Fixed Assigned Multiple Access?

What Is FAMA?

Fixed Assigned Multiple Access (FAMA) is a multiple-access technique in which each user or communication service is permanently assigned a dedicated communication resource, such as a frequency channel, time slot, or code. Unlike dynamic multiple-access techniques, the assignment remains fixed regardless of whether the user is actively transmitting. FAMA is simple to implement and provides predictable performance, but it generally uses communication resources less efficiently than dynamic allocation methods.

The basic principle is straightforward. Before the system begins operation, each user or service is allocated a specific communication resource. Whenever communication is required, the user transmits using that assigned resource without requesting permission from a central controller. Since no other user is permitted to use the same resource, collisions do not occur under normal operating conditions.

A useful analogy is assigning each employee a permanently reserved parking space. The space is always available when needed, but it remains empty whenever the employee is absent. Similarly, in a fixed-assignment communication system, resources remain reserved even when they are temporarily unused.

One of the principal advantages of FAMA is its simplicity. Because communication resources are predetermined, there is no need for complex scheduling algorithms, signalling overhead, or dynamic resource management. Communication can begin immediately, making the technique attractive for systems requiring low latency, predictable performance, or continuous availability.

The principal disadvantage is reduced spectrum efficiency. During periods when a user has no information to transmit, the assigned frequency, time slot, or code cannot normally be used by other users. As the number of inactive users increases, a growing proportion of the available communication resources remains idle, reducing overall network capacity.

Fixed assignment has been widely used in conventional radio systems, satellite communications, leased communication circuits, and early analog cellular networks. Many traditional Frequency Division Multiple Access (FDMA) systems employ fixed channel assignments, with each user or service allocated a permanent frequency channel.

Modern communication systems increasingly employ Demand Assigned Multiple Access (DAMA), in which communication resources are allocated only when needed and released when the transmission ends. This dynamic approach significantly improves spectrum utilisation, particularly in networks where user activity is intermittent or unpredictable.

It is important to distinguish Fixed Assigned Multiple Access (FAMA) from Random Access techniques such as ALOHA and Carrier Sense Multiple Access (CSMA). In FAMA, communication resources are permanently allocated in advance, ensuring predictable access. In random-access systems, users compete dynamically for shared communication resources, accepting the possibility of collisions in exchange for improved spectrum efficiency.

Today, fixed assigned multiple access remains useful wherever guaranteed resource availability, predictable performance, or operational simplicity is more important than maximum spectrum efficiency. Although many modern communication systems have adopted dynamic resource allocation techniques, the principles of fixed assignment continue to underpin numerous satellite, industrial, military, and specialist communication networks where deterministic access remains essential.