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Closed Loop Stability Controls for S-ALOHA Satellite Communications


S-ALOHA channels are intrinsically unstable and must be equipped with proper controls. The function of the controls is to dynamically adjust the ALOHA channel transmission gates in accordance with the dynamic load fluctuations. The purpose of the controls is to protect the channel from unstable behavior while optimizing channel efficiency and performance during normal operating conditions. Two control algorithms are proposed: the Closed Loop Control-Collision Detect (CLC-CD) algorithm, which assumes the capability of distinguishing collision slots from empty slots at the receiving station; and the Closed Loop Control-Collision Non-Detect (CLC-CND) algorithm, which does not require such capability. The control implementation is distributed among all stations. Channel stability and efficiency is achieved by driving the total transmission and retransmission rate to unity, using a feedback, closed loop control approach. A family of simulation runs was made to evaluate and compare the performance of the CLC schemes with that of other schemes in a variety of traffic conditions. Simulation results show that the controlled systems converge to near optimality at steady state. Futhermore, the performance of the CLC-CND algorithms is about equivalent to that of the CLC-CD algorithm, thus indicating that the requirement of distinguishing collisions from empty slots is not critical for the performance of closed loop controls. The stability properties of the CLC algorithms and their superiority over other schemes for varying load patterns are demonstrated in a series of experiments involving cyclic traffic patterns and pulse patterns. The CLC scheme displays better performance than the uncontrolled schemes as well as the previously proposed control schemes (namely, the Control Limit scheme and the Retransmission Control scheme) even when the latter are specifically tuned to handle the traffic pattern under consideration (the CLC scheme does not require any prior setting of the parameters).

Paper: PDF file of paper

Information & Date

In Data Communications Symposium, pp. 2.10-2.19, Snowbird, Utah, September. 1977


Mario Gerla
Leonard Kleinrock