8.11 ACCESS CONTROL MODES

The multiple-access techniques discussed in this chapter describe how physical channel resources are partitioned in frequency, time, code, or space. Equally important, however, is the question of how users obtain permission to use those resources. Access control determines when and under what conditions a transmitter may occupy its assigned portion of the channel.

It is therefore useful to distinguish between resource partitioning and access control. Resource partitioning defines the physical dimension in which users are separated. Access control defines the policy by which those partitions are assigned or shared.

Fixed assignment multiple access (FAMA). In FAMA, channel resources are permanently allocated to users. A given user may be assigned a fixed frequency band in FDMA, a fixed time slot in TDMA, or a fixed spreading code in CDMA. Once configured, the allocation remains stable regardless of instantaneous traffic demand. FAMA provides predictable performance, guaranteed capacity per user, and minimal real-time control overhead. However, if traffic is intermittent, fixed allocations may remain idle while still consuming spectrum or time resources. FAMA is therefore most appropriate in systems with relatively stable and continuous traffic patterns.

Demand-assigned multiple access (DAMA), In DAMA resources are allocated dynamically according to traffic requirements. A central controller or distributed coordination mechanism assigns frequency channels, time slots, or codes when users request access and releases them when no longer needed. DAMA improves spectrum utilization under variable traffic conditions by allowing idle resources to be reassigned. However, it requires additional signaling, scheduling logic, and synchronization control. Allocation delay may also increase latency relative to fixed assignment. DAMA may be applied to any physical partitioning method. An FDMA system may dynamically allocate carriers from a shared pool. A TDMA system may assign time slots on demand. A CDMA system may allocate spreading codes dynamically or adjust power levels according to load.

Contention-based access. Random or contention-based access represents a different control philosophy. Rather than explicitly assigning resources in advance, users transmit opportunistically and resolve collisions through retransmission and backoff mechanisms. Techniques such as ALOHA and CSMA fall within this category. Contention-based access is particularly effective when traffic is bursty and unpredictable. However, it provides probabilistic rather than deterministic performance guarantees and may exhibit instability under heavy load.

Access control and physical partitioning. Access control modes are orthogonal to physical partitioning techniques. The same underlying resource dimension may be combined with different control policies:

FDMA may operate under fixed assignment or dynamic carrier allocation.
TDMA may be fully scheduled, demand-assigned, or partially contention-based.
CDMA may rely on centralized power control or distributed interference management.
SDMA may use static sectorization or adaptive beam scheduling.

Recognizing this distinction clarifies the architecture of modern systems. Physical-layer separation determines how interference is avoided or managed, while access control determines how efficiently resources are utilized under varying traffic conditions.