Cross-layer collision-tolerant MAC with message passing detection

A cross-layer collision-tolerant (CT) media access control (MAC) scheme is proposed in this paper. In the MAC layer, each user transmits multiple weighted replicas of a packet at randomly selected data slots in a frame, and the indices of the selected slots are transmitted in a special collision-free position slot at the beginning of each frame. Collisions of the data slots in the MAC layer are resolved by using multiuser detection (MUD) in the physical (PHY) layer. The MUD is performed by employing a modified message passing (MP) algorithm, which treats the MAC structure as a bipartite graph, with each unique packet denoted as a message node (MN), and each slot denoted as a slot node (SN). The graph is simplified by removing the nodes with 0 or 1 connection to reduce the complexity of the MP algorithm. Simulation results demonstrate that the proposed CT-MAC achieves significant performance gains over existing cross-layer MAC schemes. It can support as many as N = 2.4M simultaneous users for a system with M slots per frame, yet most existing schemes can only operate with N ≤ M.