Pilot-Free Slotted ALOHA for Massive Access

This correspondence proposes a pilot-free slotted ALOHA scheme for a massive random access system. The user equipment (UE) utilizes coded differential modulation and transmits with the repetition-based slotted ALOHA (RSA) protocol. The base station employs non-coherent detection and cancellation for recovering all UEs' messages. We develop a belief propagation (BP) based non-coherent detection and estimation, which offers improved load-throughput performance. It is demonstrated that the pilot-free slotted ALOHA scheme can achieve almost the identical throughput of the conventional pilot-based RSA while relaxing the need for pilot sequences, further avoiding the pilot collision. We also show that the pilot-free slotted ALOHA is applicable to massive access with asynchrony and large Doppler shift.