Establishing Multi-User MIMO Communications Automatically Using Retrodirective Arrays

Communications in the mmWave and THz bands will be a key technological pillar for nextgeneration wireless networks. However, the increase in frequency results in an increase in path loss, whichmust be compensated for by using large antenna arrays. This introduces challenging issues due to powerconsumption, signalling overhead for channel estimation, hardware complexity, and slow beamformingand beam alignment schemes, which are in contrast with the requirements of next-generation wirelessnetworks. In this paper, we propose the adoption of a retro-directive antenna array (RAA) at the userequipment (UE) side, where the signal sent by the base station (BS) is reflected towards the source afterbeing conjugated and phase-modulated according to the UE data. By making use of modified PowerMethods for the computation of the eigenvectors of the resulting round-trip channel, it is shown that,in single and multi-user multiple-input multiple-output (MIMO) scenarios, ultra-low complexity UEs canestablish parallel communication links automatically with the BS in a very short time. This is done ina blind way, also by tracking fast channel variations while communicating, without the need for ADCchains at the UE as well as without explicit channel estimation and time-consuming beamforming andbeam alignment schemes.