Stabilizing Terahertz MIMO Channel Capacity with Controlled Diffuse Reflections

Communication at the terahertz band is increasingly seen as vital for future short-range very high datarate channels. However, these channels suffer from significant environmental impairments and, as a result, providing coverage in indoor settings requires the use of directional line of sight (LoS) paths to visible users and reflected paths, using smooth metal reflectors, for users in the shadow of an obstruction. Previous work has shown that these types of reflected paths display similar characteristics to LoS paths and we call them R-LoS (reflected LoS). MIMO for LoS and R-LoS channels is feasible at terahertz frequencies and delivers very high capacity at some distances. Unfortunately, channel capacity varies greatly with small changes in distance (the channel matrix fluctuates between full rank and rank 1) which is undesirable for communication systems. In this paper, we utilize diffusive reflectors to create multiple reflections such that the MIMO channel capacity for R-LoS is better behaved. We conduct experiments at 410 GHz for reflections from different artificially created diffuse surfaces. We use measurements to estimate channel capacity for 2x2 MIMO when the only path is the diffuse reflected one. We show that by creating multiple controlled reflections, it is possible to achieve relatively stable capacity up to 13 - 16 bits/sec/Hz at varying distances. We also analyze the phase of the received signals and the beam profile in detail. Overall, our results indicate that by utilizing artificially created reflections, we can maintain a stable MIMO channel at high capacity.