ScatterMIMO: Enabling Virtual MIMO with Smart Surfaces

Mobicom 2020

Authors: Manideep Dunna, Chi Zhang, Daniel Sievenpiper, Dinesh Bharadia

[Paper] [Slides] [Video]

Motivation

In the last decade, the bandwidth expansion and MIMO spatial multiplexing have promised to increase data throughput by ordersof magnitude. However, we are yet to enjoy such improvementin real-world environments, as they lack rich scattering and pre-clude effective MIMO spatial multiplexing.

Overview

We present ScatterMIMO, which uses smart surface to increase the scattering in the environment, to provide MIMO spatial multiplexing gain. Specifically, smart surface pairs up with a wireless transmitter device say an active AP and re-radiates the same amount of power as any active access point (AP), thereby creating virtual passive APs. ScatterMIMO avoids the synchronization, interference, and power requirements of conventional distributed MIMO systems by leveraging virtual passive APs, allowing its smart surface to provide spatial multiplexing gain, which can be deployed at a very low cost. We show that with optimal placement, these virtual APs can provide signals to their clients with power comparable to real active APs, and can increase the coverage of an AP. Furthermore, we design algorithms to optimize ScatterMIMO’s smart surface for each client with minimal measurement overhead and to overcome random per-packet phase offsets during the measurement. Our evaluations show that with commercial off-the-shelf MIMO WiFi (11ac) AP and unmodified clients, ScatterMIMO provides a median throughput improvement of 2x over the active AP alone.

ScatterMIMO prototype

We hierarchically implement the ScatterMIMO smart surface to simplify the design and ensure scalability. Each smart surface can contain multiple tiles, which are coordinated by a central controller through a UART bus. The central controller communicated with the ScatterMIMO AP to obtain the appropriate phase information, and deliver them to each tile. Each of the tiles hosts a matrix of phase-shifted reflecting antennas, or namely, reflector units. For our implementation, we put 4×4 = 16 reflector units on each tile. The 2-dimensional design allows us to steer the reflected beam both horizontally and vertically. The reflector unit uses a patch antenna for maximum reflection. The antenna is then connected to a HMC7992 5-GHz RF switch, which further connects to 4 open-ended transmission lines. The transmission lines provide 0, π/2, π, and 3π/2 round-trip phase shifts.