5G New Radio for Non-Terrestrial Networks: Analysis and Comparison of HARQ and RLC ARQ Performance Over Satellite Links

In this paper, we study the extension of 5G New Radio (NR) to Non-Terrestrial Networks (NTN). For terrestrial ones, Hybrid Automatic Repeat reQuest (HARQ) is the main retransmission solution used by 5G NR at the physical and MAC layers, enhancing decoding performance through diversity and coding gain. However, for NTN, its implementation faces challenges due to the significant delays caused by the long distances of satellites. In the first part, we begin by investigating the minimum number of HARQ processes required for various LEO scenarios, as well as the relationship between the number of processes and the coherence time of the satellite link. Next, while the performance of 5G retransmission schemes over AWGN and terrestrial channels is well explored, this is not the case for realistic satellite channel models. To address this, we have developed an open-source simulator that accurately implements all the blocks of the data channel transmission and reception chain, including the retransmission schemes and the Land Mobile Satellite (LMS) channel. We consider the 5G NR Physical Downlink Shared Channel (PDSCH) and we present and discuss a number of results over the LMS channel, which are important to understand the HARQ performance for the NTN satellite scenario. In the second part, we consider the 5G NR alternative retransmission solution, RLC ARQ, which is available at the Radio Link Control (RLC) layer. This method might be interesting for satellite links, because it adds minimal complexity to the receiver side, but it provides less enhancement to signal reception capabilities and more latency. We first present an analytic model to compute its performance over the LMS channel, then we analyze its behavior. Finally, we provide a detailed comparison and discussion of HARQ and RLC ARQ performance in terms of block error rate, spectral efficiency, and latency. This extensive analysis provides valuable insights for researchers and space agencies interested in applying 5G NR to satellite-based Non-Terrestrial Networks.