On the Performance of Coexisting Passive RIS and Active Relay-Assisted Short-Packet Communications

Although coexisting passive reconfigurable intelligent surface (RIS) and active relay have been considered for enhancing long-packet communications, their potential in supporting short-packet communications (SPC) where the direct link is blocked has not been sufficiently studied. Motivated by this, this work investigates the performance of the coexisting passive RIS and active relay-assisted SPC network, where a source (S) transmits its short-packet data to the destination (D) with the help of a passive RIS and an active relay (R). More specifically, in the first time slot of the two-hop transmission, S transmits its short-packet signal to both the RIS and R. The RIS then reflects the incident signal towards D. If D is unable to decode the short-packet information while R successfully decodes it, R forwards the decoded signal to D with the assistance of the RIS in the second time slot. Following a decode-and-forward (DF) relaying protocol, we analytically derive the closed-form expression of the block error rate (BLER) for the short-packet transmission. Simulation results verify the correctness of the derived expression and show that the combination of the passive RIS and active relay can enhance the BLER performance as compared with the benchmarks that employ only a passive RIS or an active relay.