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An emerging technology to enhance the spectral efficiency (SE) and energyefficiency (EE) of wireless communication systems is reconfigurable intelligentsurface (RIS), which is shown to be very powerful in single-carrier systems.However, in multi-user orthogonal frequency division multiplexing (OFDM)systems, RIS may not be as promising as in single-carrier systems since anindependent optimization of RIS elements at each sub-carrier is impossible inmulti-carrier systems. Thus, this paper investigates the performance of variousRIS technologies like regular (reflective and passive), simultaneously transmitand reflect (STAR), and multi-sector beyond diagonal (BD) RIS in multi-usermultiple-input multiple-output (MIMO) OFDM broadcast channels (BC). Thisrequires to formulate and solve a joint MIMO precoding and RIS optimizationproblem. The obtained solution reveals that RIS can significantly improve thesystem performance even when the number of RIS elements is relatively low.Moreover, we develop resource allocation schemes for STAR-RIS and multi-sectorBD-RIS in MIMO OFDM BCs, and show that these RIS technologies can outperform aregular RIS, especially when the regular RIS cannot assist the communicationsfor all the users.

Maximizing Spectral and Energy Efficiency in Multi-user MIMO OFDM Systems with RIS and Hardware Impairment