Study of Iterative Detection and Decoding for Multiuser Systems and MMSE Refinements with Active or Passive RIS

An iterative detection and decoding (IDD) scheme is proposed for multiuser multiple-antenna systems assisted by an active or a passive Reconfigurable Intelligent Surface (RIS). The proposed approach features an IDD strategy that incorporates Low-Density Parity-Check (LDPC) codes, RIS processing with refinements of soft information in the form of log likelihood ratios (LLRs) and truncation. Specifically, a minimum mean square error (MMSE) receive filter is used for refinement of LLRs and truncation at the RIS, and for soft interference cancellation at the receiver. An analysis of the proposed MMSE refinement is also devised along with a study of the computational complexity of the proposed and existing schemes. Simulation results demonstrate significant improvements in system capacity and bit error rate in the presence of block-fading channels