Beacon Signalling for Expedited Cell Search Procedures in NTN NB-IoT

Three cellular standards have been considered for Non-Terrestrial Networks (NTN): NB-IoT, eMTC and NR, each having had features introduced to accommodate the challenges of the NTN case. In Terrestrial Networks (TNs), it is reasonable to expect continuous coverage when a UE is stationary within reach of a base-station (eNB) with rare exceptions of downtime due to failures or catastrophic events. The same continuity cannot be assumed in NTN for sparse eNB constellations or during the rollout of dense eNB constellations. Therefore, a feature of the NTN IoT protocols - NTN NB-IoT & NTN eMTC - is the support of discontinuous RAN coverage. Cell search is a core task of NTN UEs serviced by non-geostationary (NGSO) constellations. Initially, when UEs are booted up, unless a recent ephemeris has been provisioned to it, the UE must first discover a valid eNB by employing repeated cell searching. UEs will have to keep doing cell search each time they wish to access a cell again after losing or dropping connectivity. Intermittent coverage gaps, which occur in dense constellations due to system failures, during rollout or inherently in sparse constellations, exaggerate the number of cell search attempts required by a UE before finding an appropriate cell to camp on. These latter cases of intermittent coverage can be mitigated by the coverage prediction features for discontinuous coverage. In this paper, a beacon signal, which can be transmitted within the white-spaces of stand-alone NB-IoT, is introduced. The beacon signal is designed to expedite the cell search procedure in NTN NB-IoT in NGSO constellations by: (1) Allowing for easy and early detection of the presence of a cell, (2) encoding preliminary information for the UE to assess whether to continue cell search at that early point and (3) providing helpful information to the synchronisation procedure. The performance of the beacon signal is simulated and evaluations show a fair improvement over utilizing legacy synchronization signals for cell detection both in terms of speed and SNR