NetSim NTN Q&A

Q7. Which orbits and altitudes are supported?

LEO from 160 to 2000 km with presets at 600 and 1200 km, MEO from 2000 km up to geostationary altitude with presets at 10000 and 20000 km, and GEO at 35786 km. Altitude is a user input within those ranges rather than a fixed set of choices.

Q8. What is the difference between a single-satellite and a multi-satellite study?

Single-satellite serves the whole UE region from one platform, which suits link-budget checks, beam coverage studies and traffic runs where every UE is served through the same satellite.

Multi-satellite places two or more satellites over the same region, which is what satellite diversity, coverage overlap, beam association, inter-satellite interference and service continuity require.

Q9. Can I simulate a real constellation such as Starlink?

Yes. Satellites are loaded from live TLE data by constellation category, with a maximum satellite count to keep the scenario tractable, and each satellite follows its own SGP4-propagated trajectory.

Position is taken from the propagated orbit at every mobility update, so passes, elevation changes and link delay follow the real two-line elements rather than a fixed analytical track.

Q10. How do I define a synthetic constellation instead?

Walker Delta and Walker Star patterns, generated from altitude, inclination, plane count, satellites per plane, RAAN spacing and phase offset. NetSim reports the resulting satellite count before beam setup.

A Cesium 3D globe renders node placement, satellite tracking and link geometry against real Earth geometry, so tracks, beam footprints and coverage movement can be inspected before the network is created. UEs, gateways and core devices can be placed directly on the globe, taking their coordinates from the drop point.

Q11. What is the difference between earth-fixed and earth-moving beams?

Earth-fixed beams steer to hold a ground point, so they need target locations, usually supplied as a CSV. This is the established mode, and it is what a single-satellite study uses.

Earth-moving beams point at the satellite nadir and sweep with the satellite, needing no target file, with the beam centre and ground footprint recomputed at every mobility update. This mode belongs to the multi-satellite workflow; confirm its status in the build you evaluate.

Earth-fixed beams maximise the time a UE stays under one satellite, which for LEO is the time the satellite remains above the horizon, typically 7 to 10 minutes.

Q12. How does a UE choose its satellite and beam?

Only satellites above the configured elevation mask are considered; anything below it is ignored for that UE at that time. For each eligible satellite, the full link budget is computed across its beams, covering carrier frequency, slant range, elevation, path loss, shadow fading, clutter, antenna pattern gain, receiver G/T, thermal noise, SNR and SINR.

The UE attaches to the satellite and beam giving the highest SNR, re-evaluated as satellites and UEs move. If no satellite passes the coverage check, the UE is treated as outside NTN coverage for that evaluation.

Q19. How is uplink scheduling done, given the propagation delay?

Terrestrial dynamic grant assumes sub-millisecond round trips: scheduling request, grant, buffer status report, then PUSCH. In NTN, one-way delay runs from about 4 ms at LEO 600 km to 240 ms at GEO, so round trips reach 8 ms to over 500 ms. A four-step handshake would add hundreds of milliseconds before any data moves.

NetSim therefore implements Configured Grant Type 1 as a cyclic round-robin scheduler in time-division mode:

• One dedicated uplink slot per UE per cycle, with the cycle length equal to the number of associated UEs.
• All available PRBs to the scheduled UE, after overhead deduction.
• An independent schedule per beam.
• MCS selected per UE from the uplink budget: SNR to CQI to MCS.

Q20. Which downlink schedulers are available?

Round Robin today, with full PRB-level resource allocation on FDD carriers. Proportional Fair and Max C/I are planned, so scheduling policy can be compared across the same constellation.

Q21. Is the feeder link modelled as a real radio link?

Not today. Following the simplification permitted by TR 38.821, feeder-link SNR impairments are treated as negligible and only propagation latency is modelled, which is why link-budget computation happens on the service link.

Gateway selection itself does work today: for each satellite, the visible gateway with the highest elevation above its mask is chosen, and the association is recalculated as the satellite moves.

What is not there yet is a feeder-link budget with its own antenna, EIRP, G/T and loss terms, Q and V band feeder carriers alongside Ka, feeder-link switchover, and gateway selection that accounts for feeder-link quality rather than elevation alone. Those matter at Ka, Q and V band, where the feeder link carries a real budget.

Q22. Is handover modelled?

Serving satellite, beam and SNR are tracked per slot, and the association is re-evaluated as described in Q12. The switch itself is instantaneous, with no control-plane exchange and no measurement reporting.

Handover as a measured procedure is planned: measurement reporting, decision, execution and completion, with configurable margin, time to trigger and interruption time; conditional handover triggered on predicted satellite visibility, which is what the short LEO visibility window demands; and KPIs for attempts, successes, failures, ping-pong, interruption time and packets lost or buffered per handover.

Q23. Is Doppler modelled?

Today Doppler compensation is assumed ideal. Every UE is taken to have GNSS and to pre-compensate Doppler and delay using satellite ephemeris and its own position, with perfect timing advance.

Explicit Doppler shift and rate computed per link from satellite and UE velocity along the slant path at every mobility update, common network-side pre-compensation with the residual frequency offset carried into the link abstraction so its effect on demodulation is visible rather than assumed away, and ephemeris-assisted acquisition with a configurable K-offset per Release 17, are planned.

Q24. How many satellites can it simulate?

Today a multi-satellite scenario runs the full stack across the satellites in the scenario, and the practical ceiling depends on beam count, UE count and run length rather than on a fixed number. Tell us the study and we will size it.

The reason there is a ceiling at all is worth stating, because it is not specific to one product. The cost of a system-level wireless simulation is dominated by the channel computation between transmitter and receiver pairs, and that cost grows quadratically with the number of nodes sharing a channel. Tools reporting results at thousands of satellites generally do so by abstracting the radio layer down to geometry, routing and latency, while tools that keep a full protocol stack have typically been demonstrated on one or a few satellites.

To raise that ceiling without giving up the stack, two-tier fidelity in a single simulation is planned:

The idea: satellites are promoted into the full-stack tier as they come into service and demoted as they pass out of view, so the tier a satellite occupies follows the orbital geometry. A UE experiences a full constellation, while the simulator pays for a full stack only on the links that carry its traffic.

Q30. What is not modelled today?

• Inter-satellite links.
• Regenerative payloads. The whole gNB sits on the ground, with no CU/DU split.
• HARQ, which is disabled, with RLC in UM mode only.
• Native rain attenuation. Rain and gaseous absorption enter today as a user-entered atmospheric-loss value in dB, applied in the link budget.
• Terrestrial to NTN coexistence and handover.
• Broadcast and multicast transmissions.
• SIB19, with SIB and MIB behaviour following terrestrial 5G.
• TDD carriers, since NTN carriers are FDD today.
• Feeder-link switchover, since a single terrestrial gNB is modelled.
• Antenna polarisation.

Four things are idealised rather than absent, and are worth knowing before interpreting a result: the RACH procedure is assumed ideal, channel state information is perfect at both transmitter and receiver, UE, satellite and gNB are perfectly synchronised in frequency and time, and Doppler compensation is assumed at the UE as described in Q23.

Several of these are planned developments rather than permanent limits. The gNB CU/DU split with configurable DU placement, on the ground beside the gateway or on board with F1 carried over the Satellite Radio Interface, is planned; because the DU placement is the MAC scheduler placement, it determines the scheduling round-trip time and therefore the achievable link adaptation behaviour. A full regenerative payload terminating NR-Uu on board, inter-satellite links with routing across the constellation, HARQ sized for the NTN round-trip time with RLC AM alongside UM, TDD with NTN-sized guard periods, and native ITU-R P.618 rain attenuation with P.676 gaseous absorption are also planned.

Q31. Can I study multi-orbit systems, for example steering traffic between GEO and LEO?

Not in a single scenario today. A GEO layer and one or more LEO shells in one run, each with its own band plan, beam layout and gateway set, is planned.

The intent is that the serving orbit is chosen per bearer from QoS class and traffic priority rather than SNR alone, so latency-sensitive traffic steers to LEO and capacity-oriented or delay-tolerant traffic to GEO, with the delay, packet loss and throughput transient measured on each switch as for a handover.