 mobility has to be accounted for. The higher Doppler shifts in the case of LEO based NTN pose another issue to be addressed in the context of timing and frequency acquisition and tracking by a UE. HARQ timers and buffer sizes may need to be extended to support NTN propagation delays; alternatively, NR may need to be modified to support limited HARQ function- ality or disabling HARQ altogether for NTN access. For terrestrial links, the timing advance (TA) typically does not require very fast updates, which may not be the case with NTN. While TA variations are bigger with NTN, they are also more predictable, which opens up possibilities for optimizations of TA adjustments. 5.3.2 Market Drivers For the cellular ecosystem, NTN can be a complementary solution addressing new market segments that are currently hard to serve using terrestrial networks. Thanks to the wide ser- vice coverage capabilities and reduced vulnerability of space/airborne vehicles to physical attacks and natural disasters, NTNs are expected to: Foster the rollout of 5G services in unserved areas that cannot be covered by terrestrial 5G networks (isolated/remote areas, on board aircraft, or ships) and underserved areas (e.g. suburban/rural areas) to improve the performance of limited terrestrial networks in a cost effective manner; Reinforce the 5G service reliability by providing service continuity for M2M/IOT devices or for passengers on board moving platforms (e.g. passengers on air vehicles, ships, high-speed trains, and buses) or by ensuring service availability anywhere, especially for critical communications and future railway/maritime/aeronautical communications; 5.3 Non-terrestrial Networks Enable 5G network scalability by providing efficient multicast/broadcast resources for data delivery toward the network edges or even the user terminal. These benefits relate to either NTNs operating alone or to an integrated solution encom- passing terrestrial and NTNs. The design choice will have an impact on coverage, user bandwidth, system capacity