Introduction#
NetSim satellite library models end-to-end, full stack, packet level communication between terrestrial nodes and Geostationary satellites. Geo satellites have the unique property of remaining permanently fixed in exactly the same position in the sky as viewed from any fixed location on Earth. This means ground-based antennas do not need to track them but can remain fixed in one direction. These satellites have orbital period that is the same as Earth’s rotation period and are the most common type of communications satellites.
The Satellite MAC layer protocol supported in NetSim is TDMA for forward link and MF-TDMA for return link (based on the DVB S2 standards). The forward link is in the Ku band (12 – 18GHz) while the return link is in the Ka band (24 – 40 GHz)
The satellite can be thought of as a relay station. It operates on the bent-pipe (transparent star) principle, sending back to Earth what comes in, with only amplification and a shift from uplink to downlink frequency.
In NetSim, the satellite communication network library interfaces with Internetworks library. This means users can connect Satellite gateway and User Terminals to devices such as Routers, Switches Wired nodes, Access point and Wireless nodes etc
The PHY layer models include:
a. Channel model: Friis free space path loss with Loo Markov fading model.
b. Modulation: QPSK, 8PSK, 16APSK, 16QAM, 32APSK with appropriate coding rates.
c. Tx, Rx Antenna gains.
d. Antenna gain to noise temperature.
All the choices of transport protocols, and all types of applications in unicast mode can be run. NetSim’s protocol source C code shipped along with (standard / pro versions) is modular and customizable to help researchers to design and test their own sat-com protocols.
Simulation GUI#
Open NetSim, Go to New Simulation → Satellite Comm. Networks
Create Scenario#
Satellite Communication Networks palette features various devices like Wired Nodes, L2 Switch, Access Point, Wireless node, UT Router (User Terminal Router), Router, UT Node (User Terminal Node), Satellite Gateway, and Satellite.
Devices specific to NetSim Satellite Comm. Library#
- UT- User Terminal. The user terminals are part of the same communication network as the Satellite Gateway. The User Terminals in NetSim are UT_Node and UT_Router
- UT Router - User Terminal Router. A UT_Router is used when a separate communication network is required. The typical use case is where there are multiple devices downstream who seek to utilize the sat-com link. The UT Router cannot be asource of any traffic.
- Satellite Gateway: Each gateway has two interfaces, a satellite interface and multiple wired interfaces. The satellite interface connects via the forward link to the satellite. The wired interface allows for connection to routers via the wired interface. When connected to a satellite, the user terminals mapped to the gateway are part of the same network. Multiple gateways can be configured per satellite, and round-robin scheduling is run (at the Network control center (NCC) which is not displayed in NetSim GUI)
- Satellite: Since the satellite model is a bent pipe the satellite does not have an IP. Each satellite can be connected to multiple gateways and to multiple User_Terminals. The satellite node cannot be the source of any traffic. The default altitude of the Satellite is 35,768,000 meters, which represents the circular geosynchronous orbit. Multiple satellites can be configured per scenario. However, no interference is modeled when multiple satellite communication occurs simultaneously.
- Coordinate System: NetSim uses a Geodetic co-ordinate system. The altitude is from Mean Seal level. The geocentric co-ordinate system uses distance from the centre of the earth.
Placement of devices on the grid environment#
- Add a User Terminal (UT) – Click the User_Terminal > UT_Node icon on the toolbar and place the device in the grid. UT_Node must be connected to Satellite.
- Add a UT Router – Click the User_Terminal > UT_Router icon on the toolbar and place the device in the grid. UT_Router must be connected to a Node or to a L2_Switch or to a Router or to an Access_Point or Satellite.
- Add a Satellite – Click the Satellite icon on the toolbar and place the Satellite in the grid. Satellite must be connected to a Satellite_Gateway or to a UT_Node or to a UT_Router.
- Add a Satellite_Gateway – Click the Satellite_Gateway icon on the toolbar and place the Satellite_Gateway in the grid. Satellite_Gateway must be connected to a Satellite or to a Router.
- Add a Router – Click the Router icon on the toolbar and place the Router in the grid.
- Add a Wired Node – Click the Wired_Node icon on the toolbar and place the device in the grid.
- Add a L2_Switch – Click the L2_Switch icon on the toolbar and place the device in the grid.
- Add an Access_Point – Click the Access_Point icon on the toolbar and place the Access_Point in the grid.
- Add a Wireless Node – Click the Wireless Node icon on the toolbar and place the device in the grid.
Note: It is recommended not to connect multiple satellite gateways to a single satellite since this can leads to IP address and static route complications
GUI Configuration Parameters#
The SATELLITE parameters can be accessed by right clicking on a Satellite, Satellite Gateway, UT Router or UT and selecting Interface (SATELLITE) Properties → Datalink and Physical Layers.
| Satellite Properties | |||
|---|---|---|---|
| Interface (Satellite) – Physical Layer | |||
| Parameter | Type | Range | Description |
| G/T (dBk) | Local | 0-100000dBk | Antenna gain-to-noise-temperature is (G/T) where G is the antenna gain in decibels at the receive frequency, and T is the equivalent noise temperature of the receiving system in kelvins. |
| Tx Power | Local | 0-10000dBW | It is the signal intensity of the transmitter. The higher the power radiated by the transmitter's antenna the greater the reliability of the communications system. |
| Access Protocol | Fixed | TDMA | TDMA allows a number of clients to access a single radio-frequency channel without interference by allocating unique time slots to each user within each channel, reducing the loss of packets and improving the data rate thereby delivering QoS to the clients. |
| Fixed | MF-TDMA | Multi-frequency time-division multiple access is a technology for dynamically sharing bandwidth resources in an over-the-air two-way communications network. | |
| Base Frequency (GHz) | Local | Ku-band: 1218GHz,Ka-band: 2640GHz | The “band” in use refers to the radio frequencies used to and from the satellite:Ku-band services uses the 12 - 18GHz, andKa-band services uses the 26- 40GHz segment of the electromagnetic spectrum |
| Band | Fixed | KU | Microwave frequency band used for satellite communication and broadcasting, using frequencies in the range of 12 -18 GHz |
| Fixed | KA | Microwave frequency band used for satellite communication and broadcasting, using frequencies in the range of 26 - 40 GHz | |
| Rolloff Factor | Local | 0-1 | In NetSim,Symbol Rate = BW / (1+Roll of factor) and Bit Rate =Symbolrate* Modulation order * CodeRate |
| Spacing Factor | Local | 0-1 | In NetSim EffectiveBandwidth(Hz) = AllocatedBandwidth (Hz) /((RollOffFactor +1.0) *(SpacingFactor + 1.0)); |
| A spacing factor should be in the range of [0,1] | |||
| Carrier Bandwidth (Hz) | Local | 0-1000000 Hz | The bandwidth of the carrier in Hz |
| Frame count in Superframe | Local | 0-1000000 | A number of frames are present in a superframe. |
| Frame Bandwidth (Hz) | Local | 0-1000000 Hz | The bandwidth of the frame in Hz. |
| Frame Usage Mode | Local | NORMAL SHORT | Baseband frame usage modes. |
| Modulation | Local | QPSK,8PSK, 16APSK, 16QAM ,32APSK | Modulation is the process of varying one waveform in relation to another waveform. It is used to transfer data over an analog channel. |
| Coding Rate | Local | 1/3,1/2, 3/5, 2/3,3/4, 4/5, 5/6, 8/9,9/10 | It states what portion of the total amount of information is useful(non-redundant). This code rate is typically a fractional number. |
| Slot Count in Frame | Local | Short Frame:QPSK-90, 8PSK-60,16APSK/16QAM-45,32APSK-36 Normal Frame:QPSK-360,8PSK-240,16APSK/16QAM-180,32APSK-144 | The number of slots per frame. The number of slots per frame is based on modulation and frame type chosen. |
| Symbol Rate | Local | 0-1000000 | It is the ratio of total bandwidth and (1+ Roll of Factor) |
| Symbol per Slot | Local | 0-1000000 | The number of TDMA symbols within a slot, the default symbol value per slot is 90. |
| Pilot Block Size (Symbols) | Local | 0-1000000 symbols | Size of pilot block in symbols |
| Pilot Block Interval (Slots) | Local | 0-1000000 slots | Interval (in symbols) between Pilot blocks |
| Pilot Header (Slots) | Local | 0-1000000 slots | The pilot block header size in slots. |
| Frame Header Length (Bytes) | Local | 0-1000000 bytes | Baseband frame header length in bytes |
| BER Model | Local | Fixed | BER value is based on the user input. |
| FILE-BASED | File-Based is a feature in NetSim with which users can define the BER. Users will have to provide a BER_FILE.txt file as input to NetSim by clicking on the Open file link the Physical LayerProperties of the device. | ||
| MODEL_BASED | The BER model, calculates the BER via the pathloss model for the particular scenario. | ||
| BER | Local | 0.00000001-1 | This is the rate at which errors occur in the transmission of digital data. |
| UT Properties | |||
|---|---|---|---|
| Interface (Satellite) – Physical Layer | |||
| Parameter | Type | Range | Description |
| Tx Antenna Gain (dB) | Local | 0-1000000dB | A relative measure of an antenna’s ability to direct or concentrate radio frequency energy in a particular direction or pattern at the transmitter side. |
| Rx Antenna Gain (dB) | Local | 0-1000000dB | A relative measure of an antenna’s ability to receive radio frequency energy in a particular direction or pattern at the receiver side. |
Table 2-1: Satellite, Satellite Gateway, UT Router or UT and selecting Interface (SATELLITE) Properties → Datalink and Physical Layers Description
| Propagation Model | |||
|---|---|---|---|
| Link Properties | |||
| Parameter | Type | Range | Description |
| Propagation Medium | Link | Air | Medium of propagation in NetSim would be Air for RF waves. |
| Channel Characteristics | Fixed | Pathloss and Fading and Shadowing | Path loss and fading and shadowing: In pathloss models, for a fixed distance between source and destination, path loss is same. We get varied path loss for some distance between source and destination in shadowing and fading is variation of the attenuation of a signal with various variables. These variables include time, geographical position, and radio frequency. |
| Shadowing model | Fixed | NONE | |
| Pathloss Model | Link | Friis Free Space | It Used to model the LOS path loss incurred in the channel. the Friis Free space model is restricted to unobstructed clear path between the transmitter and the receiver. |
| Pathloss Exponent ($\eta$) | Fixed | 2 | Path loss exponent indicates the rate at which the path loss increases with distance. The value depends on the specific propagation environment. |
| Fading Model | Fixed | Markov Loo | Each state of the three-state Markov channel models obeys the Loo distribution with different parameters; while the state transition is modeled as a firstorder Markov random process. |
| Direct Signal Mean (dB) | Link | $-\infin$ to $\infin$ | Mean value of the direct signal, value can be differentiated according to the state. |
| Direct Signal Standard Dev (dB) | Link | 0 to $\infin$ | Standard Deviation of the direct signal value can be differentiated according to the state. |
| RMS Multipath Power (dB) | Link | $-\infin$ to $\infin$ | RMS squared multipath power in dB |
| Number of Direct Signal Oscillators | Link | 0 to $\infin$ | Number of direct signal oscillator is used for frequency conversion process in superheterodyne receiver. |
| Number of Multipath Oscillators | Link | 0 to $\infin$ | Number of multipath oscillators is used to generate higher oscillation frequencies. |
| Direct Signal Doppler (Hz) | Link | 0 to $\infin$ | |
| Multipath Doppler (Hz) | Link | 0 to $\infin$ | The normalized PSD (its integral in the whole frequency range equals to one) constitutes the PDF for the Doppler frequencies, arising from the different angles of arrival the multipath components have with respect to the receiver’s motion. |
| Initial Probability | Link | 0 to 1 | An initial probability distribution, defined on S, specifies the starting state. Usually this is done by specifying a particular state as the starting state. |
Table 2-2: Propagation Model/Wireless Link Properties Description
Mapping of User_Terminal (UT_Note / UT_Router) to Satellite_Gateway#
Each satellite can be connected to multiple Satellite_Gateways and to Multiple User_Terminals. The following screen shot shows how to map the User_Terminal to Satellite_Gateway as shown in below Figure
In order to Map User_Terminal (UT_Node / UT_Router) to Satellite_Gateway right click go to the properties of UT_Node/UT_Router → INTERFACE1_(SATELLITE) → DATALINK_LAYER → Gateway user can map the Satellite_Gateway with UT_Node / UT_Router accordingly.
Additionally, in the UT_Router/UT_Node -> Interface_Satellite the default gateway IP should be set as the IP of the connected Satellite_Gateway. Incorrect mapping of the Satellite_Gateway and/or the default_Gateway IP address, in the properties of the UT_Node / UT_Router could lead application crash or NIL application throughputs.
Configuring Static Routes#
After mapping the UT_Router/UT_Node to a Satellite_Gateway, static routes need to be configured in the devices to forward traffic. Let us consider the following network scenario as shown in below Figure
In this network scenario, for UDP traffic to be sent from UT_Node_2 to UT_Node_3, static routes need to be set in UT_Node_2 and in the Satellite_Gateway_4.
If TCP traffic needs to be sent from UT_Node_2 to UT_Node_3, then static routes need to be set in UT_Node_3 as well. This is essential for connection establishment and sending acknowledgements.
Refer the featured example on Configuring applications from UT Node to UT Node for detailed information on static route configuration.
Multiple gateways connected to a single satellite#
An example where Multiple gateways are connected to the single Satellite is shown in below screenshot in below Figure
In order to Map User_Terminal (UT_Node / UT_Router) to Satellite_Gateway, right click /properties of UT_Node/UT_Router → INTERFACE1_(SATELLITE) → DATALINK_LAYER → Gateway. Here the user must map the Satellite_Gateway1 for the UT_Node / UT_Router. For UT_Node_6, the satellite gateway is 3, and for UT_Node_5 the Satellite gateway is 2.
In this network scenario, for UDP traffic is sent from UT_Node_5 to UT_Node_6. The traffic flow is UT_Node_5 > Satellite_1 > Sat_Gateway_2 > Router_4 > Sat_Gateway_3 > Satellite_1 > UT_Node_6. Appropriate static routes need to be configured in UT_Node_5, Sat_Gateway_2, Router_4, and Sat_Gateway_3.
Refer the featured example on Configuring applications from UT Node to UT Node for detailed information on static route configuration.
Model Features#
TDMA Forward Link and MF TDMA Return Link#
In NetSim, a Forward link is defined as the direction from Satellite Gateway to Satellite to UT_Node / UT_Router. A Return link is defined as the direction from the UT_Node / UT_Router to Satellite to the Satellite Gateway.
The protocol operating in the Forward link is Time Division Multiple Access (TDMA). The protocol operating in the Return link is Multi Frequency Time Devision Multiple Access (MF-TDMA).
Both the Forward link and Return link transmissions in NetSim are modeled as Layer-2 transmissions. The framing is as explained in the subsequent paragraph. Each Super Frame is composed of a number of Frames. This is taken as a user input, given by the attribute Framecount_in_SuperFrame available in Satellite -> Interface_Satellite -> Physical_Layer properties. The frames in turn are composed of carriers (in frequency) and slots (in symbols). The number of carriers would be $$Number \ of \ carriers = \frac{frame \ Bandwidth(Hz)}{Carrier \ Bandwidth(Hz)}$$ The number of slots per frame is determined by the modulation scheme chosen by the user.
Modulation and coding schemes supported#
- QPSK with coding rates 1/3, 1/2, 3/5, 2/3, 3/4, 4/5, 5/6, 8/9, 9/10
- 8PSK with coding rates 3/5, 2/3, 3/4, 5/6, 8/9, 9/10
- 16APSK with coding rates 2/3, 3/4, 4/5, 5/6, 8/9, 9/10
- 16QAM with coding rates 3/4, 5/6
- 32APSK with coding rates 3/4, 4/5, 5/6, 8/9
Physical layer framing for forward and return links#
| $\eta_{𝒍𝒅𝒑𝒄} = 𝟔𝟒𝟖𝟎𝟎 (𝒏𝒐𝒓𝒎𝒂𝒍 𝒇𝒓𝒂𝒎𝒆)$ | $𝒍 = 𝟏𝟔𝟐𝟎𝟎 (𝒔𝒉𝒐𝒓𝒕 𝒇𝒓𝒂𝒎𝒆)$ | ||
|---|---|---|---|
| $\eta_{𝑴𝑶𝑫}$% (𝒃𝒊𝒕𝒔 /𝑯𝒛) | $S$ | $\eta% 𝑛𝑜 − 𝑝𝑖𝑙𝑜𝑡$ | $𝑆$ |
| 2 | 360 | 99.72 | 90 |
| 3 | 240 | 99.59 | 60 |
| 4 | 180 | 99.45 | 45 |
| 5 | 144 | 99.31 | 36 |
The normal frame and short frame setting can be done using the Frame_Usage_Mode parameter in the GUI as shown in below Figure.
Changing the Modulation scheme in UI would change the value of S (Slot_count_in_frame)
Default NetSim GUI settings
- Symbols per slot: 90
- Pilot Block size (symbols): 36
- Pilot block interval (slots): 16
- PL header size (slots): 1
- Frame header size (In bytes): 10 (per ETSI EN 302 307 V1.3.1)
- Frame Type: Normal (Options are normal or short)
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. $$Symbol \ rate = \frac{BW}{(1+(Roll \ of \ factor))}$$ $$Bit \ rate = Symbol \ rate \times Modulation \ order \times Code \ Rate$$ $$Bandwidth(Hz) = Frame \ Bandwidth(Hz) = 10^6 \ Hz$$ $$Central \ frequency (Hz) = Base \ frequency(Hz) + \frac{Bandwidth(Hz)}{2.0}$$ $$Central \ frequency (Hz) = 26 \times10^9+ \frac{10^6}{2} = 26000500000 Hz$$ $$Effective \ Bandwidth(Hz) = \frac{Carrier \ Bandwidth(Hz)}{(RollOfFactor+1)\times(SpacingFactor+1)}$$ $$Effective \ Bandwidth (Hz) = \frac{10^6}{(1.0+1.0)\times(1.0+1.0)} = 25 \times 10^4 Hz$$ $$Symbol \ rate = Effective \ Bandwidth(Hz) = 25 \times 10^4 Hz$$ $$Modulation \ Bits = 2$$
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 |
$$𝑆𝑙𝑜𝑡𝑠 = 𝑆𝑙𝑜𝑡 \ 𝐶𝑜𝑢𝑛𝑡 \ 𝑖𝑛 \ 𝐹𝑟𝑎𝑚𝑒 + 𝑃𝑖𝑙𝑜𝑡 \ 𝐻𝑒𝑎𝑑𝑒𝑟 \ (𝑠𝑙𝑜𝑡𝑠) = 360 + 1 = 361$$ $$𝐷𝑎𝑡𝑎 \ 𝑆𝑦𝑚𝑏𝑜𝑙𝑠 = 𝑆𝑙𝑜𝑡𝑠 \times 𝑆𝑦𝑚𝑏𝑜𝑙 \ 𝑝𝑒𝑟 \ 𝑆𝑙𝑜𝑡 = 361 \times 90 = 32490$$ $$𝑃𝑖𝑙𝑜𝑡 \ 𝑆𝑙𝑜𝑡 =\frac{𝑆𝑙𝑜𝑡𝑠}{𝑃𝑖𝑙𝑜𝑡 \ 𝐵𝑙𝑜𝑐𝑘 \ 𝐼𝑛𝑡𝑒𝑟𝑣𝑎𝑙}=\frac{361}{16} = 22$$ $$𝑃𝑖𝑙𝑜𝑡 \ 𝑆𝑦𝑚𝑏𝑜𝑙 = 𝑃𝑖𝑙𝑜𝑡 \ 𝑆𝑙𝑜𝑡 × 𝑃𝑖𝑙𝑜𝑡 \ 𝑏𝑙𝑜𝑐𝑘 \ 𝑆𝑖𝑧𝑒 (𝑠𝑦𝑚𝑏𝑜𝑙𝑠) = 22 × 36 = 792 𝑆𝑦𝑚𝑏𝑜𝑙𝑠$$ $$𝑇𝑜𝑡𝑎𝑙 \ 𝑆𝑦𝑚𝑏𝑜𝑙 = 𝑃𝑖𝑙𝑜𝑡 \ 𝑆𝑦𝑚𝑏𝑜𝑙 + 𝐷𝑎𝑡𝑎 \ 𝑆𝑦𝑚𝑏𝑜𝑙𝑠 = 792 + 32490 = 33282$$ $$𝐹𝑟𝑎𝑚𝑒 \ 𝑙𝑒𝑛𝑔𝑡ℎ = \frac{𝑇𝑜𝑡𝑎𝑙 \ 𝑆𝑦𝑚𝑏𝑜𝑙}{𝑆𝑦𝑚𝑏𝑜𝑙 \ 𝑅𝑎𝑡𝑒}\times 1000000 =\frac{33282}{250000}\times 1000000 = 133128 \mu 𝑠$$ $$𝑃𝑖𝑙𝑜𝑡 \ 𝐵𝑙𝑜𝑐𝑘 \ 𝐿𝑒𝑛𝑔𝑡ℎ = \frac{𝑃𝑖𝑙𝑜𝑡 \ 𝑏𝑙𝑜𝑐𝑘 \ 𝑆𝑖𝑧𝑒}{𝑆𝑦𝑚𝑏𝑜𝑙 𝑅𝑎𝑡𝑒}\times 1000000 =\frac{36}{250000} \times 1000000 = 144 \mu s $$ $$𝑆𝑙𝑜𝑡\ 𝐿𝑒𝑛𝑔𝑡ℎ = \frac{𝑆𝑦𝑚𝑏𝑜𝑙 \ 𝑝𝑒𝑟 \ 𝑆𝑙𝑜𝑡}{𝑆𝑦𝑚𝑏𝑜𝑙\ 𝑅𝑎𝑡𝑒}\times 1000000 =\frac{90}{250000} \times 1000000 = 360 \mu s$$ $$𝑆𝑢𝑝𝑒𝑟 \ 𝐹𝑟𝑎𝑚𝑒 \ 𝐷𝑢𝑟𝑎𝑡𝑖𝑜𝑛 = 𝐹𝑟𝑎𝑚𝑒 \ 𝑙𝑒𝑛𝑔𝑡ℎ \times 𝐹𝑟𝑎𝑚𝑒𝑠 \ 𝑝𝑒𝑟 \ 𝑆𝑢𝑝𝑒𝑟𝐹𝑟𝑎𝑚𝑒 = 133128 × 10 = 1331280 \mu s$$ $$𝐵𝑖𝑡𝑠 \ 𝑝𝑒𝑟 \ 𝑆𝑙𝑜𝑡 = 𝑆𝑦𝑚𝑏𝑜𝑙 \ 𝑝𝑒𝑟\ 𝑠𝑙𝑜𝑡 \times 𝑀𝑜𝑑𝑢𝑙𝑎𝑡𝑖𝑜𝑛 \ 𝐵𝑖𝑡𝑠 \times 𝐶𝑜𝑑𝑖𝑛𝑔 \ 𝑅𝑎𝑡𝑒 = 90 \times 2 \times \frac{1}{2}= 90$$ $$𝐵𝑖𝑡𝑠 \ 𝑝𝑒𝑟 \ 𝐹𝑟𝑎𝑚𝑒 = 𝐵𝑖𝑡𝑠 \ 𝑝𝑒𝑟 \ 𝑆𝑙𝑜𝑡 × 𝑆𝑙𝑜𝑡 \ 𝐶𝑜𝑢𝑛𝑡 \ 𝑖𝑛 \ 𝐹𝑟𝑎𝑚𝑒 = 90 \times 360 = 32400$$ $$𝐷𝑎𝑡𝑎 \ 𝑅𝑎𝑡𝑒 =\frac{𝐵𝑖𝑡𝑠 \ 𝑝𝑒𝑟 \ 𝑆𝑙𝑜𝑡}{𝑆𝑙𝑜𝑡 \ 𝐿𝑒𝑛𝑔𝑡ℎ}=\frac{90 𝑏𝑖𝑡𝑠}{360 \mu 𝑠}= 0.25 \times 10^6 𝑏𝑖𝑡𝑠/𝑠𝑒𝑐 = 0.25 𝑀𝑏𝑝s$$
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 $\frac{1}{3}$ 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:
$$𝑃𝑎𝑐𝑘𝑒𝑡𝑇𝑟𝑎𝑛𝑠𝑚𝑖𝑠𝑠𝑖𝑜𝑛𝑇𝑖𝑚𝑒 = \frac{𝑃𝑎𝑐𝑘𝑒𝑡𝑆𝑖𝑧𝑒(𝑎𝑡 𝑃𝐻𝑌) × 8}{𝑃ℎ𝑦𝑅𝑎𝑡𝑒(𝑏𝑝𝑠)} = \frac{1488 × 8}{162249} = = 0.0733687𝑠 = 73368.7 \mu s$$ $$𝑃𝑎𝑐𝑘𝑒𝑡𝑠𝑃𝑒𝑟𝐹𝑟𝑎𝑚𝑒 =\lfloor \frac{𝐹𝑟𝑎𝑚𝑒𝑇𝑖𝑚𝑒}{𝑃𝑎𝑐𝑘𝑒𝑡𝑇𝑟𝑎𝑛𝑠𝑚𝑖𝑠𝑠𝑖𝑜𝑛𝑇𝑖𝑚𝑒} \rfloor = \lfloor\frac{133128}{73368.7}\rfloor = \lfloor1.81\rfloor = 1$$
$𝑃𝑎𝑐𝑘𝑒𝑡𝑠𝑃𝑒𝑟𝐹𝑟𝑎𝑚𝑒$ is the number of packets that can be packed in a frame, and hence the greatest integer or floor function is used. $$𝐵𝑦𝑡𝑒𝑠𝑃𝑒𝑟𝐹𝑟𝑎𝑚𝑒 = 𝑃𝑎𝑐𝑘𝑒𝑡𝑠𝑃𝑒𝑟𝐹𝑟𝑎𝑚𝑒 \times 𝑃𝑎𝑐𝑘𝑒𝑡𝑆𝑖𝑧𝑒(𝐵) = 1488 \times 1 = 1488$$ $$𝑁𝑢𝑚𝑏𝑒𝑟𝑂𝑓𝐹𝑟𝑎𝑚𝑒𝑠𝑃𝑒𝑟𝑆𝑒𝑐𝑜𝑛𝑑 = \frac{1}{𝐹𝑟𝑎𝑚𝑒 𝐷𝑢𝑟𝑎𝑡𝑖𝑜𝑛(𝑠)} = \frac{1}{0.133128} = 7.51$$ $$𝑃ℎ𝑦𝑇ℎ𝑟𝑜𝑢𝑔ℎ𝑝𝑢𝑡 = N𝑢𝑚𝑏𝑒𝑟𝑂𝑓𝐹𝑟𝑎𝑚𝑒𝑠𝑃𝑒𝑟𝑆𝑒𝑐𝑜𝑛𝑑 \times(𝐵𝑦𝑡𝑒𝑠𝑃𝑒𝑟𝐹𝑟𝑎𝑚𝑒 \times 8)$$ $$= 7.51 \times (1488 \times 8) = 89399.04 𝑏𝑝𝑠 = 0.089 𝑀𝑏𝑝s$$ $$ApplicationThroughput = \frac{1460}{1488} \times PhyThroughput = 0.087 Mbps$$
| Modulation | Modulation bits | Slot Count in a frame | Coding Rate | PHY Rate (Mbps) |
|---|---|---|---|---|
| QPSK | 2 | 360 | 1/3 | 0.167 |
| 1/2 | 0.250 | |||
| 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 |
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 free space power received by a receiver antenna which is separated from a radiating transmitter antenna by distance d, is given by the Friis free space equation
$$P_r = P_t+G_t+G_r+20log_{10}\bigg(\frac{\lambda}{(4*\pi\ast d0)}\bigg)+\bigg(10 \times 2 \times log_{10} (\frac{d0}{d})\bigg)$$
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 𝑆1, 𝑆2, 𝑆3 denotes respective channel state, $𝑃_{𝑖𝑗}$ is the probability the Markov process goes from state i to state j
The switching among each state is described by a transition metrix P, which is
$$\begin{pmatrix} p_{11} & p_{12} & p_{13}\\ p_{21} & p_{22} & p_{23} \\ p_{31} & p_{32} & p_{33} \end{pmatrix}$$
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. $$𝑁(\mu, \sigma ^2) + R$$
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.
SNR - BER Calculation#
$$𝑆𝑁𝑅 (𝑑𝐵𝑚) = 𝑙𝑜𝑔_{10} \bigg(\frac{𝑅𝑒𝑐𝑒𝑖𝑣𝑒𝑑 𝑝𝑜𝑤𝑒𝑟 (𝑖𝑛 𝑚𝑊)}{𝑇ℎ𝑒𝑟𝑚𝑎𝑙 𝑁𝑜𝑖𝑠𝑒 (𝑖𝑛 𝑚𝑊)}\bigg)$$
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. $𝑁𝑜𝑖𝑠𝑒 = 𝑘_𝐵\ast 𝑇\ast 𝐵$ 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. $$SNR1, BER1$$ $$SNR2, BER2$$ $$…$$ $$SNRn, BERn$$
Results#
Please see NetSim User manual, Results and Analysis section.
Satellite Log File#
A log file specific to satellite communication, is generated post simulation as shown in screen shot below,
On opening it would look like the image below
This file logs details such as
- UE – Satellite Gateway association
- Calculated Super frame, frame, slot, bandwidth, carrier count etc. for each satellite.
- Frame by frame transmissions with time stamps
Enable Propagation log#
A Sat comm. propagation log file can be enabled by the user, in the file Satellite.h, by uncommenting the line #define SATELLITE_PROPAGATION_LOG
Then Rebuild the source codes of Satellite Project and run the simulation.
Additional Satellite Propagation log files will be available under the Log Files menu in the left panel of the Results Window as shown below:
Users can see pathloss, fading-loss, noise, and SNR values in the Log files for each forward and return link.
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
Featured Examples#
Reference Documents#
- ETSI EN 301 545-2 V1.2.1 (2014-04). Digital Video Broadcasting (DVB); Second Generation DVB. Interactive Satellite System (DVB-RCS2); Part 2: Lower Layers for Satellite standard
- ETSI EN 302 307 V1.2.1 (2009-08). Digital Video Broadcasting (DVB); Second generation framing structure, channel coding and modulation systems for Broadcasting, Interactive Services, News Gathering and other broadband satellite applications (DVB-S2)
- Lu Lu, Daoxing Guo, Aijun Liu and Maoqiang Yang (2012). Analysis of Channel Model for GEO Satellite Mobile Communication System. In National Conference on Information Technology and Computer Science (CITCS 2012)
- Chun Loo (1985). A Statistical Model for a Land Mobile Satellite Link. IEEE Transactions on Vehicular Technology, 1985, 34(8): 122-127
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Default Gateway IP address is an IP layer address configuration, and is different from Satellite Gateway which is a device ↩