SPREAD SLOTTED ALOHA TECHNIQUES FOR MOBILE AND PERSONAL SATELLITE COMMUNICATION-SYSTEMS

Mobile and personal satellite communication systems are usually characterized by low power transmission, simple receiver structures, severe propagation losses, large population of users, relatively lower duty cycle traffic, timing synchronization (e.g., time of packet arrival), and constraints on emission requirements. Developing networking techniques, particularly hybrid concepts to suit these characteristics, are essential. The objective of this paper is to evolve and study spread slotted ALOHA (SSA) techniques which combine certain characteristics of both spread spectrum systems and slotted ALOHA (SA) systems to achieve better performance than the constituent systems. The proposed system will achieve lower power transmission, serve a larger user population with simple receiver structures, and offer very high immunity to the perturbations in the time of packet arrivals. Our contributions include: 1) modeling of a spread slotted ALOHA (SSA) system; 2) derivation of computationally efficient closed form expressions for the SSA system throughput and delay taking into account receiver complexity (number of codes, number of receivers, number of interferers, etc), timing perturbations and countermeasures; and 3) present numerical results to validate the derivation as well as to substantiate the superior performance of the proposed scheme. Our results demonstrate that SSA offers significant improvements in terms of throughput, delay, and network stability against excessive loading at very acceptable levels of receiver implementation complexities. Also, SSA is found to be highly robust to errors in the time of arrivals (a serious and frequently encountered problem in satellite networks) and eliminates the need to allocate guard time. Some of the candidate SSA schemes studied here (and also elsewhere [15]-[16]) show great promise for practical implementation both from the points of views of performance and complexity.