Grant-Free Random Access With Backscattering Self-Conjugating Metasurfaces

Recently, grant-free random access (GFRA) schemes have received significant attention by the research community as a solution for extremely low-latency and short packet transmissions in new industrial Internet-of-Things and digital twins applications. However, implementing such schemes in the mmWave and THz frequency bands is challenging due to the need for multiple-input multiple-output (MIMO) links to counteract the high path loss and provide sufficient spatial filtering. This results in unacceptable signaling overhead for channel estimation, slow beam alignment procedures between the access point (AP) and the sensors, as well as high sensor complexity and energy consumption. In this paper, we propose the adoption of a self-conjugating metasurface (SCM) at the sensor side, where the signal sent by the AP is backscattered after being conjugated and phase-modulated according to the data to be transmitted by the sensor. We introduce a novel SCM-based GFRA protocol enabling the detection of new sensors and the establishment of parallel MIMO uplink communications with extremely low latency. This is achieved in a blind manner, eliminating the need for radiofrequency chains and digital processing at the sensor side, as well as explicit channel estimation and time-consuming beam alignment schemes.