  Multiple-input multiple-output (MIMO) has been a key technology of wireless
communications for decades. A typical MIMO system employs antenna arrays with
the inter-antenna spacing being half of the signal wavelength, which we term as
compact MIMO. Looking forward towards the future sixth-generation (6G) mobile
communication networks, MIMO system will achieve even finer spatial resolution
to not only enhance the spectral efficiency of wireless communications, but
also enable more accurate wireless sensing. To this end, by removing the
restriction of half-wavelength antenna spacing, sparse MIMO has been proposed
as a new architecture that is able to significantly enlarge the array aperture
as compared to conventional compact MIMO with the same number of array
elements. In addition, sparse MIMO leads to a new form of virtual MIMO systems
for sensing with their virtual apertures considerably larger than physical
apertures. As sparse MIMO is expected to be a viable technology for 6G, we
provide in this article a comprehensive overview of it, especially focusing on
its appealing advantages for integrated sensing and communication (ISAC)
towards 6G. Specifically, assorted sparse MIMO architectures are first
introduced, followed by their new benefits as well as challenges. We then
discuss the main design issues of sparse MIMO, including beam pattern
synthesis, signal processing, grating lobe suppression, beam codebook design,
and array geometry optimization. Last, we provide numerical results to evaluate
the performance of sparse MIMO for ISAC and point out promising directions for
future research.
