Variable-Length XP-HARQ for URLLC

A variable-length cross-packet hybrid automatic repeat request (VL-XP-HARQ) is proposed to accommodate ultrareliable low-latency communications (URLLC). The spectral efficiency (SE) is firstly derived in terms of the outage probabilities, with which the SE is proved to be upper bounded by the ergodic capacity (EC). Moreover, to facilitate the maximization of the SE, the asymptotic outage probability is obtained at high signal-to-noise ratio (SNR), with which the SE is maximized by properly choosing the number of new information bits while guaranteeing outage requirement. By applying Dinkelbach's transform, the fractional objective function is transformed into a subtraction form, which can be decomposed into multiple sub-problems through alternating optimization. By noticing that the asymptotic outage probability is a convex function, each sub-problem can be easily relaxed to a convex problem by adopting successive convex approximation (SCA). Finally, numerical results are presented for verification.