Performance Evaluation of Short-Packet Communication Systems Assisted by Passive RIS and Active Relays

To address the limitations of passive reconfigurable intelligent surfaces （RIS） and active relays in enhancing short-packet communication （SPC） performance， this paper proposes a hybrid SPC system assisted by both a passive RIS and an active decode-and-forward （DF） relay. In the proposed system， the source node transmits short-packet data to the destination node with the cooperative assistance of the RIS and the DF relay. Following the DF protocol， the transmission slot is divided into two equal sub-slots. In the first sub-slot， the source node simultaneously transmits the signal to both the RIS and the relay； the RIS reflects the signal to the destination. If the relay successfully decodes the data， it forwards the decoded data to the destination in the second sub-slot， again assisted by the RIS. The destination node then applies maximum ratio combining （MRC） to coherently combine the received signals from both sub-slots. To evaluate the performance of the system， a closed-form expression for the overall block error rate （BLER） is derived. Monte Carlo simulation results confirm the accuracy of the analytical expression and demonstrate that the proposed system significantly improves BLER performance compared with existing methods.