This chapter has overviewed the principles of LSAS systems, as one of the dominant candidates for 5G communication networks and beyond. It is the remarkable advantages of LSAS, promised by communications theory that form the main driver for the adoption of LSAS systems in future networks. The implementation of LSAS is however subject to severe challenges, which pose the question of whether the promised benefits can be materialised in practice, and to what extent. This has been the central stimulus for the pursuit of practical approaches for LSAS deployment. While academic and industrial research on LSAS has been gaining momentum globally, a number of research problems remain open to this day, as identified throughout this chapter, promising a fruitful research field in the years to come.