Variable-Rate Incremental-Redundancy HARQ for Finite Blocklengths

Incremental redundancy (IR) hybrid automatic repeat request (HARQ) is a staple component of modern wireless systems, instrumental for efficient and reliable low-latency communication. To further improve the performance, the blocklengths—and therefore the incremental rates—of the various transmissions can be released from being fixed. This paper optimizes these variable blocklengths in truncated IR-HARQ, with the goal of maximizing the overall throughput at any desired operating point (meaning any combination of signal-to-noise ratio and target error rate). The optimization relies on a finite-blocklength information-theoretical analysis, whereby the block error rate emerges as a function of the channel capacity and the channel dispersion. Numerical results confirm that the optimized variable-rate IR-HARQ significantly outperforms its fixed-rate counterpart, both with ideal codes and with a 5G commercial code. Additionally, a heuristic scheme that mimics the behavior of the optimized solution, but is simpler to implement, is set forth and shown to essentially attain the same performance.