The University of British Columbia

The protocol design for a mobile radio service supporting half-duplex, push-to-talk, voice communications over mobile satellite systems currently being developed around the world is presented. The service enables a close group of mobile users to communicate among themselves, such that a transmission from any user is received by all. A key issue in the design is the efficient utilization of satellite channels. Demand assignment multiple access (DAMA) is used to allocate channels only to those user-groups in active sessions of conversations. The unique characteristics of the half-duplex, point-to-multipoint group communications necessitate the DAMA protocol proposed for this service. Within a channel assigned to a user-group, access contentions can occur among its members. The resulting loss of capacity due to collisions is exacerbated by the very long round trip delay. A novel access control signal protocol is proposed to detect an resolve across contentions, thus improving channel utilization. Analysis and numerical results are presented to illustrate the efficacy of this protocol