Creating Spatial Degrees of Freedom for Long-Range LoS MIMO using Reflect-arrays

A fundamental bottleneck for long-range Line-of-Sight (LoS) MIMO is transceiver form factor: for a given carrier frequency, the product of the transmit and receiver apertures required to sustain a given number of spatial degrees of freedom scales quadratically with the link distance. In this paper, we propose and investigate the feasibility of a novel approach for sidestepping this bottleneck by creating large effective apertures using multiple reflect-arrays (RAs) placed near the transceivers. The introduction of an RA between a transmitter and receiver results in an end-to-end gain scaling as $1/d_1^2d_2^2$, where d1 is the distance between the transmitter and RA and d2 is the distance between the RA and the receiver. While this leads to poor scaling when d1 and d2 are both comparable to the link range, this problem is alleviated in our proposed setting, in which RAs are deployed near the transmitter and receiver. By benchmarking the link budget of our system model against a SISO link using transceivers with comparable form factors, we provide analytical guidelines for choice of system parameters such as the required RA sizes and the distance of the RAs from the associated transceivers. Simulation results based on detailed modeling of the channel matrices validate our analytical framework. Two key findings are as follows: (a) in order to avoid excessive degradation in link budget relative to the SISO benchmark, we must deploy "large enough" RAs at "small enough" distances making near-field beam focusing between RAs and transceivers essential, (b) coarse (2-bit) quantization of RA phases suffices to implement the required combination of near-field focusing and long-range beamforming with relatively small performance loss. We illustrate our ideas for a 6.4 Gbps link at 1.5 km using 4-fold spatial multiplexing and 800 MHz of bandwidth operating at 28 GHz.