Aloha

Aloha is a multiple-access technique used in satellite communications in which multiple user terminals share a common uplink RF channel to a satellite. ALOHA is a form of asynchronous TDMA in which transmissions are not aligned to a global time reference.

In its basic form, an Earth station transmits a data frame whenever it has data to send and then waits for an acknowledgement after a round-trip propagation delay. If an acknowledgement is received, the next frame is transmitted; if not, the station waits for a random interval before retransmitting. The receiver checks each frame for errors and acknowledges only correctly received frames. Because transmissions may overlap in time, frame collisions are possible, resulting in discarded frames and retransmissions. Owing to this contention overhead, the maximum theoretical throughput of pure ALOHA is approximately 18% of the channel capacity.

An enhanced variant, known as slotted ALOHA, divides time into discrete slots equal to one frame duration, with transmissions permitted only at slot boundaries. Although collisions may still occur, they are confined to a single timeslot, improving the maximum theoretical throughput to approximately 37%. Slotted ALOHA is used, for example, on the Inmarsat-C common signaling channel.

Further refinements, collectively referred to as controlled ALOHA techniques, introduce feedback-based control mechanisms—such as adaptive retransmission delays—to reduce congestion following collisions. Under ideal conditions, these approaches can achieve throughput efficiencies approaching 50%, while remaining contention-based multiple-access schemes.