Model Features

Satellite PHY: Data Rate

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.

\[Symbol\ Rate = \frac{BW}{\left( 1 + (Roll\ of\ factor) \right)}\]
\[Bit\ Rate = Symbol\ rate \times Modulation\ order \times CodeRate\]
\[Bandwidth\ (Hz) = Frame\_ Bandwidth\ (Hz) = 10^{6}\ Hz\]
\[Central\ Frequency\ (Hz) = Base\ Frequency\ (Hz) + \frac{Bandwidth\ (Hz)}{2.0}\]
\[Central\ Frequency\ (Hz) = 26 \times 10^{9} + \ \frac{10^{6}}{2} = 2.60005\mathbf{\times}10^{10}Hz\]
\[Effective\ Bandwidth\ (Hz) = \frac{\ Carrier\ Bandwidth\ (Hz)}{(RollOfFactor + 1.0) \times (SpacingFactor + 1.0)}\]
\[Effective\ Bandwidth\ (Hz) = \frac{10^{6}}{(1.0 + 1.0) \times (1.0 + 1.0)} = 25 \times 10^{4}\ Hz\]
\[Symbol\ Rate = Effective\ Bandwidth\ (Hz) = 25 \times 10^{4}Hz\]
\[Modulation\ Bits = 2\]

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

Modulation

Modulation bits

QPSK

2

8PSK

3

16APSK/16QAM

4

32APSK

5

Table-2: Modulation bits for different modulation

\[Slots = Slot\ Count\ in\ Frame + Pilot\ Header\ (slots) = 360 + 1 = 361\]
\[Data\ Symbols = Slots \times Symbol\ per\ Slot = 361 \times 90 = 32490\]
\[Pilot\ Slot = \frac{Slots}{Pilot\ Block\ Interval} = \frac{361}{16} = 22\]
\[Pilot\ Symbol = Pilot\ Slot \times Pilot\ block\ Size\ (symbols) = 22 \times 36 = 792\ Symbols\]
\[Total\ Symbol = Pilot\ Symbol + Data\ Symbols = 792 + 32490 = 33282\]
\[Frame\ length = \frac{Total\ Symbol}{Symbol\ Rate} \times 1000000 = \frac{33282}{250000} \times 1000000 = 133128\ \mu s\]
\[Pilot\ Block\ Length = \frac{Pilot\ block\ Size\ }{Symbol\ Rate} \times 1000000 = \frac{36}{250000} \times 1000000 = 144\ \mu s\]
\[Slot\ Length = \frac{Symbol\ per\ Slot}{Symbol\ Rate} \times 1000000 = \frac{90}{250000} \times 1000000 = 360\ \mu s\]
\[SuperFrame\ Duration = Frame\ length \times Frames\ per\ SuperFrame = 133128 \times 10 = 1331280\ \mu s\]
\[Bits\ per\ Slot = Symbol\ per\ slot\ \times Modulation\ Bits \times Coding\ Rate = 90 \times 2 \times \frac{1}{2} = 90\]
\[Bits\ per\ Frame = Bits\ per\ Slot \times \ Slot\ Count\ in\ Frame = 90 \times 360 = 32400\]
\[Data\ Rate = \frac{Bits\ per\ Slot}{Slot\ Length} = \frac{90\ bits}{360\ \mu s} = 0.25 \times 10^{6}\ bits/sec = 0.25\ Mbps\]

Analytical throughput estimation

Let us an example in which the Packet Size (App layer) is 1460B which translates to 1488B at the PHY layer after addition of overheads, with QPSK modulation and \(\frac{1}{2}\) coding rate. For this modulation and coding rate the raw PhyRate of the channel is 162249 bps using the formulas given in 3.4. The analytical throughput estimate for such a scenario would be:

\[PacketTransmissionTime = \frac{PacketSize(at\ PHY) \times 8}{PhyRate(bps)} = \frac{1488 \times 8}{162249} = 0.0733687s = 73368.7\mu s\]
\[PacketsPerFrame = \lfloor\frac{FrameTime}{PacketTransmissionTime}\rfloor = \lfloor\frac{133128}{73368.7}\rfloor = \lfloor 1.81\rfloor = 1\]

\(PacketsPerFrame\) is the number of packets that can be packed in a frame, and hence the greatest integer or floor function is used.

\[BytesPerFrame = PacketsPerFrame \times PacketSize(B) = 1488 \times 1 = 1488\]
\[NumberOfFramesPerSecond = \frac{1}{Frame\ Duration(s)} = \frac{1}{0.133128} = 7.51\]
\[PhyThroughput = NumberOfFramesPerSecond \times (BytesPerFrame \times 8) = 7.51 \times (1488 \times 8) = 89399.04\ bps = 0.089\ Mbps\]
\[ApplicationThroughput = \frac{1460}{1488} \times PhyThroughput = 0.087\ Mbps\]

PHY rate for various modulations and coding rates

Modulation

Modulation bits

Slot Count in a frame

Coding Rate

PHY Rate (Mbps)

QPSK

2

360

1/3

0.167

1/2

0.250

1/4

0.125

2/5

0.200

3/5

0.300

2/3

0.333

3/4

0.375

4/5

0.400

5/6

0.417

8/9

0.444

9/10

0.450

8PSK

3

240

3/5

0.450

2/3

0.500

3/4

0.561

5/6

0.625

8/9

0.667

9/10

0.675

16APSK

4

180

2/3

0.667

3/4

0.750

4/5

0.800

5/6

0.833

8/9

0.889

9/10

0.900

16QAM

4

180

3/4

0.750

5/6

0.833

32APSK

5

144

3/4

0.936

4/5

1.000

5/6

1.042

8/9

1.111

9/10

1.125

Table-3: List of support modulation schemes and coding rates, and their respective PHY Rates

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.

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:

\[\ P_{r}\ = \ P_{t} + \ G_{t} + \ G_{r} + \ 20\log_{10}{\left( \frac{\lambda}{(4*\pi*do)} \right)\ }\ + \ \left( 10 \times 2 \times \log_{10}\left( \frac{do}{d} \right) \right)\]

where \(P_{t}\)is the transmitted power.

\(P_{r}\) is the received power.

\(G_{t}\) is the transmitter antenna gain.

\(G_{r}\) is the receiver antenna gain.

d is the T-R separation distance in meters.

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

_images/Figure-41.png

Figure-4: Switching of three-state Markov process

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

\[\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ p_{11}\ \ \ \ p_{12}\ \ \ \ p_{13}\]
\[\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ P = \ \ \ \ \ \ \ \ \ {\ p}_{21}\ \ \ \ \ p_{22}\ \ \ \ p_{23}\]
\[\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ p_{31}\ \ \ \ p_{32}\ \ \ \ p_{33}\]

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.

\[N\left( \mu,\ \sigma^{2} \right) + R\]

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 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.

_images/Figure-51.png

Figure-5: The Satellite LMS channel Model

SNR - BER Calculation

\[{SNR\ (dBm) = \ log}_{10}\left( \frac{Received\ power\ (in\ mW)}{\ Thermal\ Noise\ (in\ mW)} \right)\]

The SNR is calculated separately for each ‘hop’ of each link. This means the calculation is done from Gateway to Satellite and then separately again from Satellite to UT, and vice versa.

\(Noise = k_{B}\ T\ B\) where \(k_{B}\)is the Boltzman’s constant, B is the carrier bandwidth and \(T\) is the temperature calculated per user input of \(\frac{G}{T}(dBK)\ \)in NetSim UI.

NetSim provides three options for BER.

  • Model Based: The BER is then calculated for each link based on the SNR. Please see Propagation-Models.pdf document for detailed information on BER calculation.

  • Fixed: the BER value can be input in the GUI. If this option is chosen, the SNR (derived from propagation model) is not used.

  • File Based: SNR – BER table should be provided in a file per the format given below. This table should be in increasing order of SNR. The SNR is calculated by NetSim from the RF propagation model. For this SNR, the appropriate BER is selected from this table. BER is 1.0 for any SNR value below SNR1, and BER is 0.0 for any SNR greater than SNRn.

SNR1, BER1

SNR2, BER2

…

SNRn, BERn

Note: Users can enable the Satellite Propagation Log to see the SNR calculated from RF propagation model and then choose appropriate entries of SNR, BER values into the BER-File to see the impact on throughput.

Results

Please refer 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:

_images/Figure-61.png

Figure-6: Enabling Satellite Log file.

A log file specific to satellite communication, is generated post simulation as shown in screen shot below,

_images/Figure-71.png

Figure-7: Result Window

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

_images/Figure-81.png

Figure-8: 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:

_images/Figure-91.png

Figure-9: Enabling Satellite Radio Measurements Log file.

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

  • Time in Milliseconds

  • Transmitter Name

  • Receiver Name

  • Transmitter Power in dBm

  • Pathloss in dB

  • Shadowing Loss in dB

  • Fading Loss in dB

  • Total Loss in dB

  • Received Power in dBm

  • Noise in dBm

  • SNR in dB

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

_images/Figure-101.png

Figure-10: 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.

_images/Figure-111.png

Figure-11: Satellite Radio Measurements 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