A performance evaluation of slotted Aloha multiple access algorithms with fixed and variable frames for radiomobile networks

Radiomobile networks, among them GSM, DECT, TETRA, and PCN, are increasingly providing voice and data services to users over vast territories. In radiomobile systems, access to radio channels is controlled by a radio base station which sends and receives digital signals from mobile units in order to implement connections. All the digital messages are time-division multiplexed over one or more control channels, while the information flow is transmitted over shared radio channels. This paper evaluates the performance of the slotted Aloha access method with a superimposed frame structure. Its advantages, especially when channel occupancy is not detectable by all users and/or under particular traffic conditions, are pointed out. The proposed method, which has been incorporated in European trunking system standards such as MPT 1327, is suitable for both private (PMR) and public networks. The method is analyzed in terms of the average number of collisions per unit of time (or frame), with finite and infinite numbers of users, and with fixed and variable frame lengths. The analysis also includes hybrid access, i.e., simultaneous random and dedicated, which mobile fleets, for example, use to send data at a preset rate. In addition, the criteria for sizing frame lengths and investigating the stability of the methods are described. New ways of optimizing network performance are also presented. The results of the simulations are in good agreement with the theoretical predictions