Grant-free Random Access with Self-conjugating Metasurfaces

Recently, grant-free random access schemes have received signiﬁcant attention in the scientiﬁc community as a solution for extremely low-latency massive communications in new industrial Internet-of-things (IIoT) and digital twins applications. Unfortunately, the adoption of such schemes in the mmWave and THz bands is challenging because massive antenna arrays are needed to counteract the high path loss and provide massive access with consequent signiﬁcant signaling overhead for channel estimation and slow beam alignment procedures between the base station (BS) and user equipments (UEs), which are in contrast to the ultra-low-latency requirement, as well as to the need for low hardware complexity and energy consumption. In this paper, we propose the adoption of a self-conjugating metasurface (SCM) at the UE side, where the signal sent by the BS is reﬂected after being conjugated and phase-modulated according to the UE data. Then, a novel grant-free random access protocol is presented capable to detect new UEs and establish parallel multiple-input multiple-output (MIMO) uplink communications with almost zero latency and jitter. This is done in a blind way without the need for RF/ADC chains at the UE side as well as without explicit channel estimation and time-consuming beam alignment schemes.