  With the potential to provide a clean break from massive multiple-input
multiple-output, large intelligent surfaces (LISs) have recently received a
thrust of research interest. Various proposals have been made in the literature
to define the exact functionality of LISs, ranging from fully active to largely
passive solutions. Nevertheless, almost all studies in the literature
investigate the fundamental spectral efficiency performance of these
architectures. In stark contrast, this paper investigates the implementation
aspects of LISs. Using the fully active LIS as the basis of our exposition, we
first present a rigorous discussion on the relative merits and disadvantages of
possible implementation architectures from a radio-frequency circuits and
real-time processing viewpoints. We then show that a distributed architecture
based on a common module interfacing a smaller number of antennas can be
scalable. To avoid severe losses with analog signal distribution, multiple
common modules can be interconnected via a digital nearest-neighbor network.
Furthermore, we show that with such a design, the maximum backplane throughput
scales with the number of served user terminals, instead of the number of
antennas across the surface. The discussions in the paper can serve as a
guideline toward the real-time design and development of LISs.
