In this work, we consider a transmit architecture where few active antennas
(sources), each equipped with a dedicated radio frequency chain, illuminate a
reconfigurable intelligent surface (RIS) that control the beam-steering
capability of the whole system. In this framework, we tackle the beampattern
design problem, where the waveform emitted by the sources and the phase shifts
introduced by the RIS are designed so that the realized beampattern matches, in
a least-square sense, the desired one. The design of this architecture can be
useful in many areas, such as radar detection and tracking, millimeter wave,
sub-THz, and THz communications, and integrated sensing and communications. We
provide a sub-optimum solution to the beampattern design problem, and we report
an example to show that this RIS-based transmit architecture can be competitive
with respect to fully-digital MIMO systems, especially if constant-modulus
waveforms are required.Comment: Submitted for possible publication to IEEE Transactions on Signal
Processin