Model Features

Under the Physical Layer, two types of links are modeled in the Satellite communication system.

  • Abstract Link

  • Satellite Link

Modulation and coding (MODCOD) schemes selection

The Modulation and Coding Scheme (MCS) determines the combination of modulation order and coding rate used in the physical layer for both the Forward and Return links.

NetSim supports two MCS configuration modes: Fixed MCS and Adaptive MCS.

Fixed MCS Mode

  • In Fixed MCS mode, the modulation and coding rate remain constant throughout the simulation.

  • The user selects the desired modulation type (e.g., QPSK, 8PSK, 16APSK, etc.) and coding rate from the GUI.

  • The selected MCS values are applied to all devices, irrespective of variations in the channel or SNR conditions.

Modulation and coding schemes supported:

  1. QPSK with coding rates 1/3, 1/2, 1/4, 2/5, 3/5, 2/3, 3/4, 4/5, 5/6, 8/9, 9/10

  2. 8PSK with coding rates 3/5, 2/3, 3/4, 5/6, 8/9, 9/10

  3. 16APSK with coding rates 2/3, 3/4, 4/5, 5/6, 8/9, 9/10

  4. 16QAM with coding rates 3/4, 5/6

  5. 32APSK with coding rates 3/4, 4/5, 5/6, 8/9, 9/10

  6. These modulation and coding rates are specified in Table 12 on page 32 of the ETSI EN 302 307 – European Standard.

Adaptive MCS Mode

In Adaptive MCS mode, the physical layer dynamically adjusts the modulation and coding rate based on the Signal-to-Noise Ratio (SNR) calculated from the link-budget model.

MCS Configuration files

  • NetSim uses external configuration files to define the mapping between SNR and the corresponding Modulation and Coding Scheme (MCS).

  • Separate CSV files are used for the Forward and Return links:

  • ForwardLink_Modulation.csv

  • ReturnLink_Modulation.csv

  • The format of the CSV files is as shown below:

  • Index, Modulation, Coding Rate and Spectral Efficiency

Modulation CodingRate SpectralEfficiency
QPSK 0.333333 0.56
QPSK 0.5 0.87
QPSK 0.666667 1.26
QPSK 0.75 1.42
QPSK 0.833333 1.6
8PSK 0.666667 1.7
8PSK 0.75 1.93
8PSK 0.833333 2.13
16QAM 0.75 2.59
16QAM 0.833333 2.87
Return link Spectral efficiency values
QPSK 0.25 0.490243
QPSK 0.333333 0.656448
QPSK 0.4 0.789412
QPSK 0.5 0.988858
QPSK 0.6 1.188304
QPSK 0.666667 1.322253
QPSK 0.75 1.487473
QPSK 0.8 1.587196
QPSK 0.833333 1.654663
QPSK 0.888889 1.766451
8PSK 0.6 1.779991
QPSK 0.9 1.788612
8PSK 0.666667 1.980636
8PSK 0.75 2.228124
8PSK 0.833333 2.478562
16APSK 0.666667 2.637201
8PSK 0.888889 2.646012
8PSK 0.9 2.679207
16APSK 0.75 2.966728
16APSK 0.8 3.165623
16APSK 0.833333 3.300184
16APSK 0.888889 3.523143
16APSK 0.9 3.567342
32APSK 0.75 3.703295
32APSK 0.8 3.951571
32APSK 0.833333 4.11954
32APSK 0.888889 4.397854
32APSK 0.9 4.453027

Adaptive MCS Selection Logic

  • Obtain the Signal-to-Interference-Noise Ratio (SINR).

  • Convert SNR from dB to Linear Scale.

\[\begin{equation} SINR_{Linear}=10^{\left(\frac{SINR_{dB}}{10}\right)} \end{equation}\]

  • Compute Spectral Efficiency (SE)

\[\begin{equation} \text{Spectral Efficiency}=\log_{2}(1+SINR_{Linear}) \end{equation}\]

  • Select the Appropriate MCS files based on the link direction.

  • The selected CSV file defines how spectral efficiency maps to the corresponding modulation and coding rate.

  • The entry with the largest spectral-efficiency threshold that is less than or equal to the computed SE is chosen.

  • This Adaptive MCS mechanism enables dynamic adjustment of modulation and coding according to real-time channel conditions.

  • As the SNR increases, higher-order modulation and coding rates are selected to enhance spectral efficiency and throughput.

The modulation, coding rate and spectral efficiency values used in NetSim’s Adaptive MCS configuration are derived from the OpenSAND MODCOD database (https://github.com/CNES/opensand/wiki/acm).

Satellite PHY

Given below is the data rate calculation methodology for both forward and return links. The parameter values used are the default values in NetSim GUI.

\[\begin{equation} \text{Symbol Rate}=\frac{BW}{(1+(\text{Roll off factor}))} \end{equation}\]

\[\begin{equation} \text{Bit Rate}=\text{Symbol rate}\times \text{Modulation order}\times \text{CodeRate} \end{equation}\]

\[\text{Bandwidth }(Hz)=\text{Frame\_Bandwidth }(Hz)=10^{6}\text{ Hz}\]

\[\text{Central Frequency }(Hz)=\text{Base Frequency }(Hz)+\frac{\text{Bandwidth }(Hz)}{2.0}\]

\[\text{Central Frequency }(Hz)=26\times 10^{9}+ \frac{10^{6}}{2}=2.60005\times 10^{10}\text{ Hz}\]

\[\begin{equation} \text{Effective Bandwidth }(Hz)=\frac{ \text{Carrier Bandwidth }(Hz)}{(\text{RollOffFactor}+1.0)\times (\text{SpacingFactor}+1.0)} \end{equation}\]

\[\text{Effective Bandwidth }(Hz)=\frac{10^{6}}{(1.0+1.0)\times (1.0+1.0)}=25\times 10^{4}\text{ Hz}\]

\[\text{Symbol Rate}=\text{Effective Bandwidth }(Hz)=25\times 10^{4}\text{ Hz}\]

\[\text{Modulation Bits}=2\]

The number of Modulation Bits depends on the modulation scheme per the table below:

Modulation bits for different modulations.
Modulation Modulation bits
QPSK 2
8PSK 3
16APSK/16QAM 4
32APSK 5

Satellite PHY: Land Satellite Channel Model

Propagation

The distance between the ground nodes and the satellite determines the propagation delay and path loss of the radio signal. The distance is computed based on the cartesian distance between the ground nodes and the satellite. NetSim computes the propagation delay of the radio signal traveling from the source node to the destination node at the speed of light. The propagation model calculates the weakening of the radio signal as it propagates from the source node per the pathloss and fading model.

Earth fixed spot beams and cells

NetSim provides three methods for configuring satellite beams:

Standard setup:

The Standard Setup option allows users to quickly configure satellite beams using predefined parameters without manually entering beam coordinates or importing external files. NetSim automatically calculates the beam geometry, radius, and coverage area based on the selected inputs.

  • Number of spot beams.

  • NetSim can presently support configurations of 1, 7, or 19 spot beams.

  • The 7-cell setup consists of a central hexagonal cell surrounded by 6 adjacent cells.

  • The 19-cell configuration has two layers of surrounding cells around a central hexagonal cell.

  • NetSim will automatically compute the tessellated beams (cells) based on number of spot beams.

User-defined beam configuration:

This option allows users to upload an Excel/CSV file containing beam parameters such as beam count, Frequency reuse, Satellite altitude (km), HPBW (theta), Aperture radius (m), Band, Beam centers, Antenna model and Beam radius (km) details instead of relying on automatic or standard setup.

User-defined beam configuration.

Manual Placement:

The Manual Placement option allows complete user control over beam locations, coverage regions, and satellite altitude. Unlike the Standard Setup or CSV Upload options, no beam parameters are auto-generated. This mode is intended for advanced users who want full flexibility in defining exact beam geometry.

Pathloss Model – Friis Free Space Propagation

The free space propagation model is used to predict received signal strength when the transmitter and receiver have a clear, unobstructed line-of-sight path between them. Satellite communication systems and microwave line-of-sight radio links typically undergo free space propagation. The mathematical expression for free-space path loss is given by the Friis Free-Space Equation:

\[\begin{equation} PL_{FS}= 20\log_{10}\left(\frac{4\pi d}{\lambda }\right) \label{eq:fspl} \end{equation}\]

Where \(\lambda =\frac{c}{f},\)

\(d\) is the slant height in meters,

\(\lambda\) is the wavelength in meters.

Fading model

NetSim uses a 3-state (state 1, state 2 and state 3) Markov model to simulate fading.

The conditional probabilities of state \(s_{n+1}\) given the state \(s_{n}\) are described by state transition probabilities \(p_{ij}\)

Where \(S_{1}\), \(S_{2}\), \(S_{3}\) denote respective channel states, \(P_{ij}\) is the probability the Markov process goes from state \(i\) to state \(j\).

Switching of three-state Markov process.

The switching among each state is described by a transition matrix P, which is

\[\begin{equation} P= \begin{pmatrix} p_{11} & p_{12} & p_{13} \\ p_{21} & p_{22} & p_{23} \\ p_{31} & p_{32} & p_{33} \end{pmatrix} \end{equation}\]

Each state of the three states of the Markov model obeys the Loo distribution with different parameters, while the state transition is modeled as a first-order Markov random process.

The Loo distribution considers the received signal as a sum of two signal components. A log-normally distributed direct signal expresses the slow fading component corresponding to varying shadowing conditions of the direct signal. A Rice distribution characterizes the fast-fading component due to multipath effects.

The Loo parameter triplet consists of the mean, the standard deviation for the log-normally distributed direct signal, and the average multipath power.

\[\begin{equation} N(\mu , \sigma^{2})+R \end{equation}\]

Depending on the current state interval and on the environment of the terminal, a new random Loo parameter triplet is generated. The output of the channel model is a time-series of the received signal in the form of a complex envelope.

And finally, the model computes the Loo distributed time-series including Doppler shaping for every new state interval, which is the output of the proposed LMS channel model.

The Satellite LMS channel Model.

Antenna Models

TR 38.811 Antenna model

The normalized antenna gain pattern, corresponding to a typical reflector antenna with a circular aperture, is considered

\[\begin{equation} G(\theta) = \begin{cases} 1 & \text{for } \theta = 0 \\[6pt] 4\left|\dfrac{J_{1}(ka\sin\theta)}{ka\sin\theta}\right|^{2} & \text{for } 0 < |\theta| \leq 90^{\circ} \end{cases} \label{eq:tr38811} \end{equation}\]

where \(J_1(x)\) is the Bessel function of the first kind and first order with argument \(x\), \(a\) is the radius of the antenna’s circular aperture, \(k =\frac{2\pi f}{c}\) is the wave number, \(f\) is the frequency of operation, \(c\) is the speed of light in a vacuum and \(\theta\) is the angle measured from the boresight of the antenna’s main beam. Note that \(ka\) equals the number of wavelengths on the circumference of the aperture and is independent of the operating frequency. The above expression provides the gain in linear scale and it needs to be converted to dB scale. The normalized gain pattern for \(a = 10\frac{c}{f}\) (aperture radius of 10 wavelengths) is shown below

Antenna gain pattern for aperture radius 10 wavelengths, \(a= 10 \frac{c}{f}\).

ITU-R S.672 Antenna model

To model the off-axis behavior of satellite antennas, the ITU-R S.672 recommendation specifies that, for single-feed circular or elliptical beam spacecraft antennas operating in the fixed-satellite service (FSS), the following radiation pattern should be used as the design objective outside the coverage area.

\[\begin{align} G(\psi) &= G_m - 3 \left(\frac{\psi}{\psi_b}\right)^{\alpha} \;\text{[dBi]} &\text{for } \psi_{b} \leq \psi \leq a\psi_b \label{eq:itu1}\\ G(\psi) &= G_m + LN + 20\log(z) \;\text{[dBi]} &\text{for } a\psi_b < \psi \leq 0.5b\psi_{b} \label{eq:itu2a}\\ G(\psi) &= G_m + LN \;\text{[dBi]} &\text{for } 0.5b\psi_{b} < \psi \leq b\psi_{b} \label{eq:itu2b}\\ G(\psi) &= X - 25 \log \psi \;\text{[dBi]} &\text{for } b\psi_{b} < \psi \leq Y \label{eq:itu3}\\ G(\psi) &= LF \;\text{[dBi]} &\text{for } Y < \psi \leq 90^{\circ} \label{eq:itu4a}\\ G(\psi) &= LB \;\text{[dBi]} &\text{for } 90^{\circ} < \psi \leq 180^{\circ} \label{eq:itu4b} \end{align}\]

Where: \[\begin{equation} X = G_m + LN + 25 \log(b\psi_{b}) \quad\text{and}\quad Y = b\psi_{b} \cdot 10^{0.04(G_m + LN - LF)} \end{equation}\]

  • \(G(\psi):\) gain at the angle \(\psi\) from the main beam direction (dBi)

  • \(G_m:\) maximum gain in the main lobe (dBi)

  • \(\psi_{b}\): one-half the 3 dB beamwidth in the plane of interest (3 dB below \(G_m\)) (degrees)

  • \(LN\): near-in-side-lobe level in dB relative to the peak gain required by the system design

  • \(LF = 0\) dBi: far side-lobe level (dBi)

  • \(z\): (major axis/minor axis) for the radiated beam

  • \(LB\): \(15 + LN + 0.25 G_m + 5 \log z\) dBi or \(0\) dBi whichever is higher

Figure 3-8: LN selection to get \(a\), \(b\) and \(\alpha\).
\(LN\,(dB)\) \(a\) \(b\) \(\alpha\)
\(-20\) \(2.58 \cdot \sqrt{(1 - \log(z))}\) 6.32 2
\(-25\) \(2.58 \cdot \sqrt{(1 - 0.8 \cdot \log(z))}\) 6.32 2

The numeric values of \(a\), \(b\), and \(\alpha\) for \(LN = -20\) dB and \(-25\) dB side-lobe levels are given in the above table.

Off Boresight Angle Calculation

Conceptually, the off-boresight angle, \(\theta\), in the antenna gain formula is defined as the angle between two vectors: one along the boresight of the beam (i.e., from the satellite to the beam centre, which corresponds to the sub-satellite nadir point), and the other from the satellite to the UE. In NetSim, \(\theta\) is computed as the arccosine of the normalized dot product of these two vectors.

Standard ENU-Based Look Angle Computation (Satellite to Earth)

This section describes the standard ENU (East–North–Up) method for computing satellite look angles (azimuth and elevation) as seen from an earth station (user terminal).

Notation

\(\varphi_{t}\) : Latitude of earth station (degrees)

\(\lambda_{t}\) : Longitude of earth station (degrees)

\(H_{t}\) : Altitude of earth station above sea level (km)

\(\varphi_{s}\) : Latitude of satellite (degrees)

\(\lambda_{s}\) : Longitude of satellite (degrees)

\(H_{s}\) : Altitude of satellite above sea level (km)

Step 1: Convert Geodetic Coordinates to ECEF

Convert both earth station and satellite geodetic coordinates to Earth-Centered Earth-Fixed (ECEF) Cartesian coordinates.

  • WGS-84 ellipsoid constants

  • WGS-84 defines the Earth as an oblate ellipsoid with:

  • Semi-major axis (equatorial radius):

\[a = 6378137.0 \text{ m}\]

  • Flattening:

\[f = \frac{1}{298.257223563}\]

  • First eccentricity squared:

\[e^{2}=2f-f^{2}\]

  • Numerically:

\[e^{2} \approx 0.00669437999014\]

  • Prime vertical radius of curvature

\[N(\phi) = \frac{a}{\sqrt{1 - e^{2}\sin^{2}(\phi)}}\]

  • This is to compensate for Earth’s flattening.

  • ECEF Conversion formulas (WGS-84)

\[X = (N + h)\cos(\phi)\cos(\lambda)\]

\[Y = (N + h)\cos(\phi)\sin(\lambda)\]

\[Z = \left(N(1 - e^{2}) + h\right)\sin(\phi)\]

This gives Earth-Centered Earth-Fixed (ECEF) coordinates:

  • \(X\) axis: intersection of equator and Greenwich meridian

  • \(Y\) axis: \(90^{\circ}\) east on equator

  • \(Z\) axis: North pole

Step 2: Line-of-Sight Vector in ECEF

Compute the line-of-sight (LOS) vector from the earth station to the satellite:

\[\Delta r = r_{s} - r_{t} = (\Delta X, \Delta Y, \Delta Z)\]

Step 3: Rotate LOS Vector into Local ENU Frame

Define the standard ECEF-to-ENU rotation matrix at the earth station latitude \(\varphi_{t}\) and longitude \(\lambda_{t}\):

\[\begin{equation} \begin{bmatrix} E \\ N \\ U \end{bmatrix} = \begin{bmatrix} -\sin(\lambda_{t}) & \cos(\lambda_{t}) & 0 \\ -\sin(\phi_{t}) \cos(\lambda_{t}) & -\sin(\phi_{t}) \sin(\lambda_{t}) & \cos(\phi_{t}) \\ \cos(\phi_{t}) \cos(\lambda_{t}) & \cos(\phi_{t}) \sin(\lambda_{t}) & \sin(\phi_{t}) \end{bmatrix}\begin{bmatrix} \Delta X \\ \Delta Y \\ \Delta Z \end{bmatrix} \end{equation}\]

This yields the local ENU components (\(E\), \(N\), \(U\)) of the satellite relative to the earth station.

Step 4: Slant Range

The straight-line distance between the earth station and the satellite is:

\[\begin{equation} D_{ts}= \sqrt{E^{2}+ N^{2}+ U^{2}} \end{equation}\]

Step 5: Elevation Angle

The elevation angle (above the local horizontal plane) is computed as:

\[\begin{equation} el = \text{atan2}\left(U, \sqrt{E^{2}+ N^{2}}\right) \end{equation}\]

Elevation is in the range \(-90^{\circ}\) to \(+90^{\circ}\).

Step 6: Azimuth Angle

The azimuth angle, measured clockwise from true North, is:

\[\begin{equation} az = \text{atan2}(E, N) \end{equation}\]

The result should be wrapped to the range \(0^{\circ}\)–\(360^{\circ}\).

Step 7: Visibility Check

Given a minimum elevation mask angle (maskAngle):

\[\text{visible} = 1 \quad\text{if } el \geq \text{maskAngle}\]

\[\text{visible} = 0 \quad\text{otherwise}\]

If \(\sqrt{E^{2}+ N^{2}} \approx 0\) (satellite directly overhead), the azimuth is indeterminate.

Frequency Reuse

NetSim supports FR1 (\(N_{reuse}=1\)), FR2 (\(N_{reuse}=2\)), FR3 (\(N_{reuse}=3\)) and FR4 (\(N_{reuse}=4\)) configurations.

With NetSim GUI, users can configure the number of beams (1, 7, and 19) and select the frequency reuse factor (FRF) accordingly. When \(N_{reuse}>1\), NetSim assigns virtual “channel IDs” to the beams. The available resources and bandwidth are then equally divided among the channels. The bandwidth per channel is calculated per the following expression

\[\begin{equation} BW_{channel}= \frac{BW_{Total}}{N_{channels}} \end{equation}\]

Interference models

Exact Geometric Interference

Geometric interference arises when multiple beams sharing the same channel ID overlap, leading to co-channel interference at user terminals. The level of interference is influenced by the number of beams and the configured frequency reuse factor. In FR1, interference occurs from all available beams, whereas in FR2, FR3 and FR4 it is limited to beams using the same channel ID.

CIR-based Interference

Per TR 38.821 section 6.1.3.1 the Carrier-to-noise-and-interference ratio (CNIR) of the transmission link between satellite and UE can be derived by carrier-to-noise ratio (CNR) and carrier-to-interference ratio (CIR) as follows

\[\begin{equation} CNIR\;[\text{dB}]=-10\log_{10}\left(10^{-0.1\cdot CNR\;[\text{dB}]}+10^{-0.1\cdot CIR\;[\text{dB}]}\right) \label{eq:cnir} \end{equation}\]

Carrier-to-Interference Ratio (CIR) is the user input and

CNR (dB) is computed from the link budget, based on EIRP, G/T, path loss and the antenna gain.

Interference power calculations for CIR based Interference

\[\begin{equation} \text{Interference}\;[\text{mW}]=\text{Noise}\;[\text{mW}] \times \left(\frac{CNR\;[\text{mW}]}{CNIR\;[\text{mW}]}-1\right) \end{equation}\]

\[\begin{equation} \text{Interference}\;[\text{dB}]=10\times \log_{10}(\text{Interference}\;[\text{mW}]) \end{equation}\]

Results

Please refer to the NetSim User Manual, section 8, for Results and Analysis.

Satellite Log

NetSim Satellite Log file records UT Satellite association, calculated superframe, frame, slot, bandwidth, etc. This log can be enabled/disabled by going to Plots option and checking/unchecking the Satellite Log option under the Network Logs section as shown below:

Enabling Satellite Log file.

A log file specific to satellite communication, is generated post-simulation as shown in the screenshot below.

Result Window.

On opening, the satellite log file would look like the image below.

NetSim Satellite communication log file.

This file logs details such as

  • UT – Satellite Gateway association

  • Calculated Super frame, frame, slot, bandwidth, carrier count etc. for each satellite.

  • Frame by frame transmissions with time stamps

Satellite Radio Measurements Log

NetSim Satellite Radio Measurements Log file records Time (ms), Transmitter name, Receiver name, Slant height (km), EIRP (dBW), Elevation Angle (\(^{\circ}\)), RXG_T, Pathloss (dB), Fading loss (dB), Additional loss (dB), Total loss (dB), Angular gain (dB), Rx power (dBm), SNR (dB), Thermal noise (dBm), Channel Id, Beam Id, MCS Index and Coding rate. This log can be enabled/disabled by going to Logs option and checking/unchecking the Satellite Radio Measurements Log option under the Network Logs section as shown below:

Enabling Satellite Radio Measurements Log file.

The Satellite Radio Measurements.csv file will contain the following information:

  • Time in Milliseconds

  • Transmitter Name

  • Receiver Name

  • Slant height (km)

  • EIRP (dBW)

  • Elevation Angle (\(^{\circ}\))

  • RXG_T

  • Pathloss (dB)

  • Fading loss (dB)

  • Additional loss (dB)

  • Total loss (dB)

  • Angular gain (dB)

  • Rx power (dBm)

  • SNR (dB)

  • Interference (dBm)

  • Thermal noise (dBm)

  • Channel Id

  • Beam Id

  • MCS Index and

  • Coding rate

Satellite Radio Measurements log files will be available under the Logs in the results window as shown below:

Result Window.

Users can see Tx Power, Rx power, pathloss, fading-loss, Total loss, Thermal noise, and SNR values in the Log files for each forward and return link.

Satellite Radio Measurements log file.

Satellite Beam Association Log

The NetSim Satellite Beam Association Log file is useful for observing the values recorded for each beam, including Time (ms), TX Name, RX Name, EIRP (dBW), Beam Id, Channel Id, Theta (\(^{\circ}\)), Angular Gain, RXG_T (dB), FSPL (dB), Total loss (dB), SNR (dB), Interference (dBm), Interfering Beam Count, MCS Index, and Coding rate. This log can be enabled/disabled by going to Logs option and checking/unchecking the Satellite Beam Association Log option under the Network Logs section as shown below:

Results window.

Users can see Angular Gain, Interference and SNR values in the Log files for each forward and return link. These parameters are logged separately for each beam.

Satellite Beam Association log file.

Omitted Features

  • Regenerative transponder where the signal is demodulated, decoded, re-encoded and modulated aboard the satellite.

  • Impact of Rain/Weather on signal propagation

  • Forward Error Coding in Layer 2

  • IPv6 Addressing

  • No support for LEO, MEO