Model Features
The following are the 802.22 features implemented in NetSim:
Primary user: A user who has higher priority or legacy rights to use a specific part of the spectrum. Also known as Incumbent user. In NetSim, the primary user is part of the base station.
Secondary user: A user who has a lower priority or legacy rights to use a specific part of the spectrum, and therefore uses the spectrum in such a way that it does not cause interference to the primary users.
Keep-out distance: The minimum distance between the primary user and the secondary user to ensure that there is no interference.
Spectrum sensing: A process of monitoring and sensing the spectrum to capture information such as holes in the spectrum and the interference, and the primary users in the geographical area.
For more information about spectrum sensing in NetSim, see
You can find the definition of the Spectrum sensing function the SpectrumManager.c file. If you want to modify this function at run-time, you must write a new sensing algorithm that checks if the incumbent user’s signal is present. This changes the keep-out distance.
The following is the Spectrum sensing function:
struct stru_802_22_SSFOutput* fn_NetSim_CR_CPE_SSF(struct
stru_802_22_SSFInput* input,NETSIM_ID nDevId,NETSIM_ID nInterfaceId)
UCS notification: UCS or Urgent Coexistence Situation notification is a notification that the secondary user sends when it senses that the primary user is back to use the channel. In NetSim, this happens when the distance between the primary user and the secondary user is less than the keep-out distance. UCS notifications are generated at the end of the quiet period (network-wide quiet periods when all network traffic is suspended and the base stations and CPEs perform in-band sensing.
Channel switching: A process where the secondary user switches the channel when the primary user of the channel comes back to use it.
In NetSim, the secondary user does not switch the channel to an adjacent one. The secondary user switches the channel to a channel that is next to an adjacent channel.
For example, if the primary user has returned to channel 1, the secondary user switches to channel 3 and not channel 2.
Operating frequency: The frequency band at which the incumbent operates. This band can range from 54 MHz to 862 MHz. The bandwidth of every channel is 6 MHz.
For example, if the operating frequency is set from 54 MHz to 72 MHz, then
Channel 1 will be 54 to 60 MHz.
Channel 2 will be 60 to 66 MHz.
Channel 3 will be 66 to 72 MHz.
ON duration: The duration of time for which the primary or incumbent user operates.
In NetSim, you can specify a duration between 1 second and 100,000 seconds.
OFF duration: The time interval between two successive ON durations of an incumbent.
For example, if you specify an ON duration on 5 seconds, the incumbent operates once every 5 seconds. If you specify an ON duration of 0, the incumbent remains always active.
PHY rate: The PHY rate in the IEEE 802.22 standard depends on the following parameters:
Number of bits per symbol
Coding rate
Cyclic prefix
Symbol duration
PHY rate in Cognitive Radio is calculated as follows:
\[PHY\ rate = \frac{Bit\ count\ in\ One\ symbol}{Symbol\ duration}\]\[Bit\ count\ in\ One\ symbol = Subcarrier\ count \times Number\ of\ bits\ \times Coding\ Rate\]\[Bit\ count\ in\ One\ symbol = 1440 \times 2 \times 2\left( \frac{1}{2} \right),\ where\ Subcarrier\ count = 1440\]
For Subcarrier count, refer the table 201 in the document available here - https://ieeexplore.ieee.org/document/7098301.
The following table lists the different modulation techniques and the number of bits per symbol see Table-1, the modulation technique uses.
Modulation technique |
# of bits per symbol |
|---|---|
QPSK |
2 |
16-QAM |
4 |
64-QAM |
6 |
Table-1: Different Modulation techniques vs. number of bits per symbol
The following displays the coding rate.
Coding rate |
Data bits |
Redundant bits (n-k) |
|---|---|---|
1/2 (default) |
1 |
1 |
2/3 |
2 |
1 |
3/4 |
3 |
1 |
4/5 |
4 |
1 |
Table-2: Different Coding rates vs. Data bits vs. Redundant bits
The following table displays the cyclic prefix.
Cyclic prefix |
|---|
1/4 |
1/8 |
1/16 |
1/64 |
Table-3: Cyclic prefix
In NetSim, the base station allocates a maximum of One symbol per CPE. If the generation rate is more than the data filled in one symbol, the allocation fails and this results in Zero throughput. The first symbol is reserved for CR control frames or any broadcast PDU.
Coding rate: A fractional number used for error correction. Coding rate specifies what part of the redundant message is meaningful. If the code rate is k/n, for every ‘k’ bits of useful information, the coder generates a total of ‘n’ bits of data, of which n-k are redundant.
How to avoid low Application Layer Throughput
When you simulate a Cognitive Radio network in NetSim, the throughput in the Application layer is lesser than the throughput in the Datalink layer throughput because of the following factors:
TCP connection establishment.
ARP set up.
Service flow created for the CPE to the BS and the BS to the CPE.
Bandwidth request.
To avoid the above effects:
Set the application traffic model to Custom.
Set the Downlink/Uplink ratio to 1:1 so, the BS transmits whatever it receives.
Run UDP in the Transport layer.
Use static ARP.
Run the simulation for more than 100 seconds.
Segmentation
CR segments packets of 100B. In the application settings if the packet size is greater than 100B, then those packets will be segmented. The segment IDs can be viewed in the packet trace.
How to Modify Device Parameters at Run-time
For information about how to change the device parameters at run-time, see