We consider a frame-asynchronous coded slotted ALOHA (FA-CSA) system whereusers become active according to a Poisson random process. In contrast tostandard frame-synchronous CSA (FS-CSA), users transmit a first replica oftheir message in the slot following their activation and other replicasuniformly at random in a number of subsequent slots. We derive the(approximate) density evolution that characterizes the asymptotic performanceof FA-CSA when the frame length goes to infinity. We show that, if users canmonitor the system before they start transmitting, a boundary-effect similar tothat of spatially-coupled codes occurs, which greatly improves the decodingthreshold as compared to FS-CSA. We also derive analytical approximations ofthe error floor (EF) in the finite frame length regime. We show that FA-CSAyields in general lower EF, better performance in the waterfall region, andlower average delay, as compared to FS-CSA.
展开▼
