The use cases and goals of the 5G initiative are well-known. What might not be as well-known is the role and potential use of satellites in the 5G ecosystem. Afterall, there has not been significant discussion on how satellites fit in. For Release 16, the 3GPP initiated a study item focused on the role of satellites in the 5G ecosystem.
For 5G, the 3GPP community has expanded their scope to address application requests from the traditional contributor body but also a diverse set of new players. As 5G will unlock the use of wireless technologies in new applications, expanding the scope is not only prudent but essential to spur wider adoption.
You may be wondering, “Why satellites?” Satellites or non-terrestrial networks (NTNs) have some advantages that terrestrial links cannot offer, such as much greater geographic coverage including offering broad band access to under-served regions of the world and areas where deploying terrestrial equipment is either infeasible or impossible (i.e., jungles, deserts, mountains, oceans, seas and lakes). In addition, satellites remain active in the face of natural disasters or physical attacks.
Aligning with the 3GPP makes sense for NTN technology companies because new, orthogonal standards will not leverage the economies of scale built inherently with the vast deployments of commercial mobile networks. Not to mention, NTNs when combined with 5G mobile access will expand the possibilities, applications and use cases creating more economic value.
Including NTNs in a mobile standard comes with some challenges. The first is that satellites are located very far away from the users and in fact much farther than typical cell phones relative to their serving base stations. The distance introduces higher delays which impact timing and specifically the timing advance for the uplink, random access and HARQ timing (retrying when data is corrupted), just to name a few.
The distance also pressures the link budget, but assuming the satellites can deliver high power PAs with high gain antennas, this problem may be addressed with further study. However, the UE will have to most certainly deal with lower or sub-optimal SNR conditions implying the need for higher coding and judicious choice of modulation, which may impede the overall data rate.
Finally, the effects of motion must be considered in an NTN system whether Geostationary, Medium Earth Orbit, Low Earth Orbit, and/or even High Altitude Platform Systems such as drones and balloons. The distance from the Earth impacts the delay but also the Doppler Shift has many of these types of satellites move with respect to a user or ground based relay node. For these satellite networks, the Doppler effect causes carrier frequency offset that must be corrected within the 5G NR limits or the standard will have to be modified.
To facilitate the diverse use cases, the 3GPP study group is considering several different configurations taking advantage of the 5G NR’s new distributed architecture – essentially distributing the functions of a traditional base station among geographically diverse nodes. Utilizing this new flexible topology, operators can use satellites for additional access. The satellite can operate in a “relay” mode to provide backhaul connections to challenging access locations, and gNB capability could then be added to the satellite to reduce latency and simplify the connection to NGC (next generation core network).
In summary, NTN could be potentially augment 5G mobile access networks to increase coverage and provide redundancy in network service beneficial for the next generation of commercial wireless communication systems, but the technical challenges remain unanswered for now, and it will be interesting to see the results of the 5G NR NTN study item in the next few months. The 3GPP will invariably have to make tradeoffs in terms of cost/benefit to realize of the use NTN in the 5G ecosystem.