8.7.3 Controlled ALOHA

Pure and slotted ALOHA rely entirely on random retransmission following collisions. As offered load increases, the probability of repeated collisions grows, leading to instability and reduced throughput. When multiple users retransmit simultaneously after a collision, the system can enter a congestion state in which most transmission attempts fail. Controlled ALOHA introduces additional coordination to improve stability and reduce unnecessary retransmissions.

In controlled ALOHA systems, retransmissions are not left entirely to chance. Instead, the system regulates when and how users may attempt to retransmit. This regulation may be achieved through centralized feedback, probabilistic transmission control, or limited reservation mechanisms. By moderating retransmission behavior, the system seeks to keep the offered load near the value that maximizes throughput.

Under appropriate control, the effective throughput of a slotted ALOHA system can be increased beyond the 36.8% limit achieved under purely random retransmission. By regulating retransmission probability and preventing excessive simultaneous retries after collisions, controlled-backoff techniques can raise throughput toward approximately 50% under moderate load conditions. The improvement arises because the control mechanism stabilizes the offered load near the optimal operating point instead of allowing it to grow uncontrollably during congestion.

One common approach adjusts the retransmission probability based on observed channel conditions. If the channel is heavily loaded, users reduce their probability of retransmitting in a given slot. If the channel is lightly loaded, retransmission probability may be increased. Another approach separates contention from data transmission by requiring users to compete only for short request intervals; once access is granted, data transmission proceeds in a reserved slot. In both cases, the goal is to reduce repeated collisions while retaining the flexibility of random access.

Adaptive random-access strategies based on similar principles are widely used in modern wireless and machine-type communication systems. These systems employ dynamic or exponential backoff algorithms to balance throughput and delay as traffic conditions vary. Although implementation details differ, the underlying concept remains the same: probabilistic access is moderated by feedback to improve stability and efficiency.

Controlled ALOHA therefore represents an intermediate stage between purely random access and fully deterministic scheduling. It preserves the simplicity and responsiveness of contention-based transmission while introducing mechanisms that significantly improve throughput under moderate load.