LEO constellations · GEO · Direct-to-device · VSAT · Link budget
NetSim's NTN library is a standards-based, end-to-end, full-stack, packet-level simulator
for 5G non-terrestrial networks. Model single-satellite and multi-satellite scenarios with
uplink and downlink budgets, and measure what UEs and VSATs receive as satellites rise,
serve and set.
Walker and live-TLE constellationsSatellite and gateway switchingDirect-to-device handheldTransparent payload (A1)TR 38.821 link budget
A LEO satellite relays traffic from a ground gateway to UEs spread across a
wide area. Link lines and per-UE throughput update as the run plays.
NetSim · NTN simulation playback
What you can do with it
Standards-based simulation of non-terrestrial networks, from the link budget
up to end-to-end application performance.
Full-stack 5G NTN
End-to-end, full-stack, packet-level simulation of 5G NTN satellite networks, with the 5G
core and application traffic in the loop.
Constellations at scale
Run one satellite or several hundred. NetSim propagates Walker patterns, live TLE catalogues and GEO slots with SGP4 and animates them on a 3D globe.
Sweep key parameters
Evaluate performance across orbit height, elevation angle, constellation size, number of
spot beams, and frequency reuse factor.
File-based mobility
Drive UEs and VSATs along time-stamped geodetic waypoints, and satellites along their own
trajectories, from CSV files.
Multi-satellite operation
As the constellation moves, NetSim decides which satellite serves each UE and
which gateway serves each satellite, and logs every change.
Serving satellite selection
Each UE attaches to the satellite and beam with the highest downlink SSB SNR among satellites
above the elevation mask. A UE with no candidate is out of coverage, and its traffic pauses
until a satellite rises.
Elevation mask default: 10°
Switching with hysteresis
The UE switches to another satellite only when that satellite's SNR exceeds the serving SNR by the handover margin. This stops ping-pong between satellites of similar strength. Beam changes on the same satellite are immediate.
Handover margin default: 3 dB
Gateway and feeder-link switch
Each satellite uses the gateway with the highest elevation above that gateway's own mask. NetSim checks the choice again at every satellite position update. One gateway can serve many satellites at once.
Gateway mask default: 10°
Constellation refresh
NetSim applies the satellite positions from the SGP4 trajectory once per time step and evaluates all UEs together. A step therefore never mixes old and new geometry.
Trajectory step: user set, 30 s typical
Standards and architecture
The RAN follows NTN A1 mode (transparent payload) per TR 38.821, with the
feeder link carrying F1 over the Satellite Radio Interface.
Standards
3GPP alignment
TR 38.821 – NTN architecture, scenarios and link budget
TR 38.811 – channel model, LOS probability, shadowing and clutter tables
TS 38.321 – MAC procedures (incl. Configured Grant Type 1)
Architecture
Transparent payload (A1)
RAN architecture per NTN A1 mode (transparent payload)
Satellite Radio Interface (SRI) on the feeder link transports the F1 protocol
NR-Uu radio interface on the service link between satellite and UE
Orbits
LEO, MEO, GEO
Single-satellite and multi-satellite scenarios in LEO, MEO and GEO, with UE elevation
masks
Feeder-link SNR impairments are negligible per TR 38.821 Table 6.1.1.1-5. NetSim models only the feeder-link propagation latency.
Traffic paths
NetSim simulates every hop. The satellite is a transparent relay. Each radio leg adds its own propagation delay, and the feeder link adds delay only.
Direct-to-device
A handheld UE talks to the satellite directly, with no ground base station in between. This is the same architecture as the direct-to-cell services launched with LEO constellations in 2025 and 2026. Downlink and uplink share the bent-pipe path above.
UE-to-UE through the core
Two UEs under different satellites and different gateways exchange traffic through the UPF. The satellites do not switch traffic on board, and there is no sidelink. The path and its delay are therefore the same as in a transparent-payload network today.
Specifications
Gateways, satellites, terminals, 5G core and remote servers, configurable
from the constellation down to the radio.
Constellations
Constellation options
Choose a source in the multi-satellite wizard. NetSim propagates the positions with SGP4, writes them to a trajectory file, and replays them in the simulation and on the 3D globe.
Synthetic Walker
Walker Delta or Walker Star from your own orbital parameters.
Orbital planes and satellites per plane
Altitude and inclination
RAAN spacing and phase offset
Live TLE constellations
Satellites loaded from public TLE catalogues, capped at a chosen count.
CelesTrak or N2YO as the source
Starlink, OneWeb and Iridium groups
GNSS and GEO operator groups
GEO and IGSO
Geostationary satellites placed by longitude slot.
Any number of slots
Inclination up to 15° for IGSO
Altitude fixed at 35786 km
Single satellite
One satellite over the scenario area, for link-level and beam studies.
Presets: LEO 600, LEO 1200, MEO 10000, GEO 35786
Custom altitude from 1 km to 35786 km
Fixed, or moved along a trajectory file
Components
Network components
One or more satellite gateways, satellites, terminals, 5G core, remote servers
gNB located outside the service beams, reached over the feeder link
Wired nodes, routers and switches from the rest of NetSim
Terminals
Terminal types
Handheld UE: 23 dBm, 0 dBi, 7 dB noise figure, FR1 (the TR 38.821 handheld)
VSAT: directive antenna up to 35 dBi, FR1 or FR2
Either type static or on a file-based mobility path
Mixed populations of UEs and VSATs in one run
Beams
Spot beam configuration
Any number of Earth-fixed beams, one-to-one with cells, with built-in presets of 1, 7
and 19
Earth-moving beams that travel with the satellite footprint
Per-beam minimum elevation: a fixed beam is inactive until its satellite clears it
Timed reconfiguration: change a beam's pointing or channel at a set simulation time
Frequency reuse factor FRF 1, 2, 3 or 4, with the hexagonal inter-site distance from the beam diameter
Setup
Scenario setup modes
Multi-satellite wizard: source, region and time, constellation, beam setup
Standard setup: predefined single-satellite parameters per 3GPP
The NTN radio measurement log records the full link budget per TTI.
Constellation studies
Multi-satellite scenarios run in NetSim, with the configuration and the
measured results.
Study 1
Multiple satellites over a city: SNR and SINR with adjacent-satellite interference
One handheld UE near Tokyo under a 300-satellite LEO constellation for five hours. Every
co-channel beam in view, on the serving satellite and on its neighbours, contributes
interference through the exact geometric model.
Constellation
300 LEO satellites, 10 planes, 30 per plane
Altitude
550 to 568 km, SGP4 trajectories
Band and bandwidth
S-band n256 (FR1), 5 MHz
EIRP density
45 dBW/MHz
Beam and antenna
One Earth-moving beam, Gaussian pattern
UE
Handheld, 0 dBi, elevation mask 10°
Interference
Exact geometric model
Simulation time
18000 s (5 h)
SNR at 2:00−11.63 dB
SINR at 2:00−13.96 dB
Loss to adjacent satellites2.33 dB
SNR and SINR over the run. Each peak is a satellite that passes near zenith. Where a co-channel neighbour is also in view, the SINR is a few dB lower than the SNR.
Study 2
Mixed terminals across cities: throughput and SINR by terminal type
Handheld UEs and VSATs spread over four cities, each city with its own satellite gateway
and core, under a 225-satellite LEO constellation. All terminals share one 5 MHz S-band
channel.
The 35 dBi VSAT antenna lifts the average SINR by about 36 dB over the
handheld UE. Both terminal types share the same 5 MHz channel and the same satellites.
Featured examples
Single-satellite link-budget studies, ready to load and extend.
LEO altitude vs SNR and path loss
Across S-band (2.185 GHz, handheld) and Ka-band (18.75 GHz, VSAT), path loss rises with
altitude. Ka-band has higher path loss yet higher SNR, thanks to the 30 dBi VSAT antenna
gain.
SNR vs transmit power, rural and urban
SNR rises with EIRP and Tx power in both environments. The rural SNR is always higher. Because of clutter and NLOS, dense urban needs more power or beamforming to match it.
SNR vs elevation angle
In an S-band downlink at 600 km, a lower elevation angle increases slant distance, which
raises path loss and reduces SNR.
Worked application
3GPP TR 38.821 reference scenario
Build and run the System Level Simulator reference scenario from TR 38.821 in NetSim. The orbit, beams, link budget and end-to-end performance follow the 3GPP baseline.
Fly a Bangalore to Delhi route over a Ka-band LEO constellation and compare 50 against
100 satellites: handover timeline, throughput, SINR, delay, and availability.
The NTN library simulates the network, and NetSim Astra plans it, with constellation design, link budgets and coverage studies over real geography. The NTN library flies the constellation that you design in Astra. A coverage plan therefore becomes an end-to-end performance study, and you do not enter any orbit again.
No inter-satellite links and no regenerative payload (Rel-19 items, on the roadmap)
HARQ is disabled at the gNB and the UE. Block errors follow a target BLER.
RLC UM mode only
O-RAN CU-DU-RU split is not modelled
SIB19 is not modelled. System information follows terrestrial 5G.
Terrestrial–NTN coexistence and handovers not currently available
Perfect Doppler compensation and timing advance assumed in the devices
Frequently asked questions
Short answers, with links to the detailed NTN Q&A.
What can I simulate with NetSim's NTN library?
You can simulate 5G non-terrestrial networks per 3GPP TR 38.821. The library covers a single LEO, MEO or GEO satellite or a constellation of hundreds. It models a transparent payload, spot beams, uplink and downlink link budgets, and the full 5G protocol stack. It measures application traffic end to end, packet by packet.
Does the NTN library compute satellite link budgets?
Yes. NetSim computes uplink and downlink budgets per 3GPP methodology. You can study SNR, path loss and throughput against orbit height and elevation angle. For satellite RF planning,
coverage maps and constellation design over real geography, use
NetSim Astra.
How does NetSim compare with Ansys STK and the MATLAB Satellite Communications
Toolbox?
STK models orbital mechanics and coverage. The MATLAB toolbox models link-level waveforms.
NetSim simulates the complete network: protocol stack, scheduling, transport, and application
traffic, end to end. Many teams use the tools together. See the
detailed comparison.
Can I simulate satellite handover in a LEO constellation?
Yes. The satellite and beam with the highest SNR above the elevation mask serve each UE. The UE switches to another satellite only when that satellite's SNR exceeds the serving SNR by a handover margin (3 dB by default). Each satellite also selects the gateway with the highest elevation,
so the feeder link switches as the constellation moves. The
in-flight
connectivity study walks through a handover timeline over a full flight.
Terrestrial-to-NTN handover is not modelled.
Can two UEs under different satellites talk to each other?
Yes, through the 5G core. Traffic from UE1 goes up to its serving satellite and down the feeder link to its gateway. It passes through the gNB and the UPF, then goes out through the second gateway and satellite to UE2. The satellites are transparent relays, so there is no on-board switching and
no sidelink.
Useful links
Documentation, a reference scenario, and support to take an NTN project
further.
