Frequency Diversity Enhanced Repetition for URLLC over Flat Fading Channels: a Practical Improvement Design

The frequency diversity enhanced repetition (FDER) scheme, which allows users (UEs) to select K resource blocks from a shared resource pool and transmit K replicas of the same packet, has been considered as a promising approach to achieve ultra-reliable and low latency communications (URLLC) with grant-free access. However, practical designs of such an FDER scheme tailored for URLLC remain underexplored. Moreover, flat fading within the coherence bandwidth can significantly diminish the frequency diversity gains offered by the FDER scheme. By considering such fading effects, this work analyzes the performance of both random access FDER scheme and the deterministic FDER scheme, and derives the corresponding packet loss probabilities. For the deterministic FDER scheme, a practical FDER design tailored to URLLC is proposed, along with an efficient algorithm that maximizes the number of users (UEs) supported by the proposed design. In addition, tractable methods are developed to characterize the decoding error probability of the proposed design under both selection combining (SC) and maximum ratio combining (MRC) techniques. Simulation results show that the proposed design supports a larger number of UEs than existing approaches, and that the analytical results closely match the corresponding simulation results.