  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.
