A Novel SCMA System for Coexistence of Active Users and Inactive Users

This letter proposes a new sparse code multiple access (SCMA) system for handling the coexistence of active user equipments (UEs) and inactive UEs, which share the same frequency/time resources for an uplink grant-free scenario. SCMA systems use a message passing algorithm, which considers all UEs to be active UEs. Therefore, inactive UEs cause significant decoding problem in SCMA systems. In order to solve the coexistence problem, we propose code words and a novel SCMA receiver which can distinguish the signal of active/inactive UEs. The numerical results show that the proposed SCMA receiver achieves a remarkable enhancement in false alarm probability, miss detection probability, and bit-error rate.