  Intelligent reflecting surface (IRS) is envisioned to become a key technology
for the upcoming six-generation (6G) wireless system due to its potential of
reaping high performance in a power-efficient and cost-efficient way. With its
disruptive capability and hardware constraint, the integration of IRS imposes
some fundamental particularities on the coordination of multi-user signal
transmission. Consequently, the conventional orthogonal and non-orthogonal
multiple-access schemes are hard to directly apply because of the joint
optimization of active beamforming at the base station and passive reflection
at the IRS. Relying on an alternating optimization method, we develop novel
schemes for efficient multiple access in IRS-aided multi-user multi-antenna
systems in this paper. Achievable performance in terms of the sum spectral
efficiency is theoretically analyzed. A comprehensive comparison of different
schemes and configurations is conducted through Monte-Carlo simulations to
clarify which scheme is favorable for this emerging 6G paradigm.
