  Next-generation wireless networks are expected to utilize the limited radio
frequency (RF) resources more efficiently with the aid of intelligent
transceivers. To this end, we propose a promising transceiver architecture
relying on stacked intelligent metasurfaces (SIM). An SIM is constructed by
stacking an array of programmable metasurface layers, where each layer consists
of a massive number of low-cost passive meta-atoms that individually manipulate
the electromagnetic (EM) waves. By appropriately configuring the passive
meta-atoms, an SIM is capable of accomplishing advanced computation and signal
processing tasks, such as multiple-input multiple-output (MIMO)
precoding/combining, multi-user interference mitigation, and radar sensing, as
the EM wave propagates through the multiple layers of the metasurface, which
effectively reduces both the RF-related energy consumption and processing
delay. Inspired by this, we provide an overview of the SIM-aided MIMO
transceiver design, which encompasses its hardware architecture and its
potential benefits over state-of-the-art solutions. Furthermore, we discuss
promising application scenarios and identify the open research challenges
associated with the design of advanced SIM architectures for next-generation
wireless networks. Finally, numerical results are provided for quantifying the
benefits of wave-based signal processing in wireless systems.
