Supporting Hierarchical Users in a Random Multiple Access System

Classical random access protocols support prioritized user groups by allowing a high priority user to transmit with a relatively high probability. However, when a high priority user competes for the channel with a large number of low priority users, transmission success probability of the high priority user can still diminish to zero. In this paper, a distributed medium access control (MAC) framework is proposed to support hierarchical user groups in a random multiple access system. A hierarchical user structure, such as a primary-secondary user structure, differs from a priority user structure in the following senses. First, when the number of primary users is small, the MAC framework guarantees that channel availability should stay above a pre-determined threshold no matter how many secondary users are competing for the channel. Second, when the number of primary users is large, the MAC framework drives transmission probabilities of the secondary users to zero but does not reject channel access to the primary users. These properties are achieved in a distributed environment without direct message exchange between users, without knowledge on the number of users, and without knowing whether each transmission should belong to a primary or a secondary user.