Beyond Massive MIMO: The Potential of Data Transmission With Large Intelligent Surfaces

In this paper, we consider the potential of data transmission in a system with a massive number of radiating and sensing elements, thought of as a contiguous surface of electromagnetically active material. We refer to this as a large intelligent surface (LIS), which is a newly proposed concept and conceptually goes beyond contemporary massive MIMO technology. First, we consider capacities of single-antenna autonomous terminals communicating to the LIS where the entire surface is used as a receiving antenna array in a perfect line-of-sight propagation environment. Under the condition that the surface area is sufficiently large, the received signal after a matched-filtering operation can be closely approximated by a sinc-function-like intersymbol interference channel. Second, we analyze a normalized capacity measured per unit surface, for a fixed transmit power per volume unit with different terminal deployments. As terminal density increases, the limit of the normalized capacity [nats/s/Hz/volume-unit] achieved when wavelength λ approaches zero is equal to half of the transmit power per volume unit divided by the noise spatial power spectral density. Third, we show that the number of independent signal dimensions that can be harvested per meter deployed surface is 2/λ for one-dimensional terminal deployment, and π/λ² per square meter for two- and three-dimensional terminal deployments. Finally, we consider implementations of the LIS in the form of a grid of conventional antenna elements, and show that the sampling lattice that minimizes the surface area and simultaneously obtains one independent signal dimension for every spent antenna is the hexagonal lattice.

I. Introduction

A large Intelligent Surface (LIS) is an entirely new concept in wireless communication [1], [2], where we envision a future where man-made structures become more and more electronically active, with integrated electronics and wireless communication making the entire environment intelligent. The LIS concept can be seen as an extension of traditional massive multi-input multi-output (MIMO) [3]– [7], scaled up beyond the traditional large antenna-array concept. As an extension of traditional massive MIMO systems, LIS retains all the advantages such as allowing for an unprecedented focusing of energy in three-dimensional space which enables wireless charging, remote sensing with extreme precision and unprecedented data-transmissions. This makes it possible to fulfill the most grand visions for the next generation of communication systems and the concept of Internet of Things (IoT) [8], [9], where billions of devices are expected to be connected to the Internet. In Fig. 1, we show an example of three terminals communicating to a LIS in indoor and outdoor scenarios, respectively.