Figure 1 -GEO transparent mode system overview
Service and feeder link constraints
Several service and feeder link challenges exist, including doppler, delay, signal changes and path loss, all of which depend on the satellite, its elevation angle and position in the sky.
High-orbit GEO satellites operating 36,000 km above the ground will experience significantly higher path loss than lower orbit LEO (NGSO) satellites at e.g., 600 km. Signal delay and NGSO-affected doppler will also vary as a satellite moves closer or further away.
“The device’s physical layer requires adaptation enabled by GNSS positioning information to compensate for these challenges. Satellites will broadcast their position and timing information in this compensation”, explains Henrik.
Another area of focus is how satellites handle millions of constantly moving IoT devices, cover multiple tracking areas simultaneously in a short period and stay connected to NGSO networks:
“Devices are given a list of tracking area identifiers similar to terrestrial networks. The NodeB will broadcast its current tracking identifier and a list of tracking areas in service, which will change over time as satellites move. This list will guide device paging, and only devices with an intersection between the lists can be paged. Devices may send their locations to the network, providing for optimisation”.
Higher frequency equals greater signalling challenges
Release 17’s NB-IoT NTN specifications will begin operating on S-band frequencies which range between 2-4 gigahertz (GHz), crossing the conventional ultra-high frequency (UHF) and super high frequency (SHF) band boundaries at 3 GHz. Aviation, shipping and space industries utilise S-band for their optimised two-way communication and content delivery capabilities for mobile networks and handheld devices.
However, Svend states that higher frequencies “involve major technical challenges”, affecting satellite-connected 5G service quality, efficiency and reliability worldwide. This challenge primarily affects high-orbit GEOs operating at frequencies of above 6 GHz. “Discussions are ongoing between 3GPP and the International Telecommunication Industry (ITU) to address frequency spectrum challenges and how they affect NB-IoT connectivity. Unfortunately, these discussions haven’t been resolved, but we believe they will in the future”.