Abstract
This work investigates the communication reliability of the URLLC-oriented decode-and-forward two-way relaying (DF-TWR) within FBL regime, where the network and superposition coding (NSC) is employed at the relay. The block error probabilities (BLEPs) are analysed for all links. It is found that the uplink BLEP is convex w.r.t. the uplink blocklength (UBL); and each downlink BLEP is convex w.r.t. the downlink blocklength (DBL) and the relay power allocation (RPA) ratio used for the superposition coding. To improve the overall reliability under the latency guarantees for the URLLC-oriented DF-TWR, we propose a framework of minimizing the overall BLEP under the constraints of maximum allowed overall blocklengths by jointly optimizing the UBL, DBL, and RPA. Both the instantaneous CSI (ICSI)-assisted and statistical CSI (SCSI)-assisted optimization scenarios are considered, where the optimizations are conducted online and offline respectively. Based on the convexity of the BLEPs of all links, it is further found that the overall BLEP is jointly convex w.r.t. the UBL and DBL; and convex w.r.t. the RPA ratio. By utilizing the convexity of the overall BLEP, the dynamic joint blocklengths and RPA optimization (BLRPAO) as well as static BLRPAO algorithms are designed for the considered two scenarios respectively. Numerical results validate the theoretical analyses as well as the effectiveness of the proposed algorithms and NSC. It is also shown that both algorithms converge rapidly.