Table Of Contents
Table Of Contents

Crosstalk between adjacent IEEE 802.11 channels

Goals

By default, 802.11 hosts and access points in INET are configured to use the same Wifi channel (Channel 1.) In reality, however, it is rarely the case. There are many Wifi networks at the same location, typically spread out on all channels (especially in the case of 2.4 GHz Wifi where only a few channels are available, compared to 5 GHz.) Transmissions on many adjacent channels also overlap in frequency and can cause interference.

INET has support for simulating communication on the different Wifi channels, both in the 2.4 GHz and 5 GHz frequency range. This showcase demonstrates using both overlapping and non-overlapping Wifi channels in simulations, and how transmissions on different channels interfere with each other.

This showcase divides the topic of the simulation of different Wifi channels into three cases:

  • Completely overlapping frequency bands: all nodes communicate on the same Wifi channel

  • Independent frequency bands: nodes communicate on different channels that don’t affect each other

  • Partially overlapping frequency bands: nodes communicate on adjacent channels, which interfere with each other

Each case is demonstrated with an example simulation.

INET version: 4.2
Source files location: inet/showcases/wireless/crosstalk

The model

The 2.4 GHz frequency range in 802.11g, for example, can use a limited number of channels (13 in the EU.) The bandwidth of transmissions in 802.11g is 20MHz, and channels are spaced 5MHz apart. Thus adjacent channels overlap, and transmissions on these channels can interfere. There can be a few independent channels, where there is no cross-channel interference, e.g. Channels 1, 6, and 11, as illustrated below.

../../../../_images/channels1.png

In INET, the scalar analog model represents signals with a scalar signal power, and a constant center frequency and bandwidth. The scalar model can only handle situations when the spectra of two concurrent signals are identical or don’t overlap at all. When using the dimensional analog model, signal power can change in both time and frequency; more realistic signal shapes can be specified. This model is also able to calculate the interference of signals whose spectra partially overlap.

There are example simulations for the three cases outlined in the Goals section. In the cases of completely overlapping channels and independent channels, the simulations use the scalar analog model; in the case of the partially overlapping channels, the dimensional analog model is used.

All simulations use variations of the same network, which is illustrated by the image below:

../../../../_images/basenetwork.png

The networks contain four AdhocHost’s, named host1 to host4. The networks also contain an Ipv4NetworkConfigurator module, an IntegratedVisualizer module, and one or two radio medium modules. The number and type of the radio medium modules vary in the networks for the different simulations. All hosts are within communication range of each other. The hosts are arranged in a rectangle; host1 is configured to send UDP packets to host2, and similarly, host3 sends UDP packets to host4. The configuration keys common to all simulations, specifying e.g. traffic generation and visualization, are defined in the General configuration in omnetpp.ini. The following sections detail the three configurations.

Completely Overlapping Frequency Bands

The simulation for this case demonstrates the hosts communicating on the same Wifi channel, the default Channel 1. The simulation can be run by selecting the CompletelyOverlappingFrequencyBands configuration from the ini file. The configuration doesn’t specify anything beyond the keys of the General configuration, so it’s empty:

[Config CompletelyOverlappingFrequencyBands]
network = CrosstalkShowcaseOneScalarRadioMediumModule

Since the frequency and bandwidth of transmissions for all hosts is exactly the same, inferring which transmissions interfere is obvious (all of them). In this case, a scalar analog model is sufficient. The following video shows the node-pairs communicating, the number of sent/received packets is displayed above the nodes, as well as the state of the contention modules of the transmitting hosts.

At first the two source nodes, host1 and host3, start transmitting at the same time. The transmissions collide, and neither destination host is able to receive any of them correctly. The collision avoidance mechanism takes effect, and host3 wins channel access. Both nodes can transmit their data successfully after one another.

Independent Frequency Bands

In this case, we are modeling host-pairs that are communicating on different, non-overlapping Wifi channels (e.g. Channels 1 and 6.) Since the channels are independent, it is obvious that there won’t be any interference. The scalar analog model is sufficient for this case.

In the first configuration for this case, the hosts use the same radio medium module. The simulation can be run by choosing the IndependentFrequencyBandsOneScalarRadioMediumModule configuration from the ini file. The radios of the two host pairs are configured to use the non-overlapping Channels 1 and 6:

[Config IndependentFrequencyBandsOneScalarRadioMediumModule]
network = CrosstalkShowcaseOneScalarRadioMediumModule

*.host{1..2}.wlan[*].radio.channelNumber = 0
*.host{3..4}.wlan[*].radio.channelNumber = 5

[Config IndependentFrequencyBandsTwoScalarRadioMediumModules]
network = CrosstalkShowcaseTwoScalarRadioMediumModules

*.host{1..2}.wlan[*].radio.radioMediumModule = "radioMedium1"
*.host{3..4}.wlan[*].radio.radioMediumModule = "radioMedium2"

# Wifi channel settings - not required
*.host{1..2}.wlan[*].radio.channelNumber = 1
*.host{3..4}.wlan[*].radio.channelNumber = 6

Note

The channel numbers are set to 0 and 5 because in INET’s 802.11 model, the channels are numbered from 0, so that this setting corresponds to Wifi Channels 1 and 6.

The video below shows the hosts communicating:

Since host-pairs communicate on independent channels, there is no interference. host1 and host3 can transmit simultaneously, and their transmissions are correctly receivable by both destination hosts. Note that all transmissions are sent to all hosts by the radio medium module.

Since they transmit/receive on different, non-interfering channels, it is obvious that host4 cannot receive host1’s transmissions, just as host2 cannot receive host3’s transmissions. Yet the radio medium module sent all transmissions to all hosts, where the radio module decided that some of the transmissions cannot be received because the host’s receiver is set to a different channel.

The simulation can be optimized by omitting these unnecessary message sends by the radio medium, by using two radio medium modules and configuring the non-interfering host-pairs to use different radio mediums. By using two radio medium modules, the simulation scales better as the number of nodes increases.

The second example simulation demonstrates the use of two radio medium modules to optimize the simulation. The simulation can be run by choosing the IndependentFrequencyBandsTwoScalarRadioMediumModules configuration from the ini file:

[Config IndependentFrequencyBandsTwoScalarRadioMediumModules]
network = CrosstalkShowcaseTwoScalarRadioMediumModules

*.host{1..2}.wlan[*].radio.radioMediumModule = "radioMedium1"
*.host{3..4}.wlan[*].radio.radioMediumModule = "radioMedium2"

# Wifi channel settings - not required
*.host{1..2}.wlan[*].radio.channelNumber = 1
*.host{3..4}.wlan[*].radio.channelNumber = 6

Here, the radios of each host pair are set to use one of the two radio medium modules (by default, radios use the one named radioMedium). Also, the non-overlapping channels are configured, but from the perspective of interference, it doesn’t make any difference, as the use of two radio modules prevents interference anyway.

The following video shows the host-pairs communicating:

The host pairs communicate without interference. Notice that there are only message sends between hosts using the same radio medium module.

Partially Overlapping Frequency Bands

In this case, the host pairs communicate on different Wifi channels, which partially overlap in frequency. The scalar analog model is insufficient to simulate partially overlapping channels, thus we use the dimensional analog model. The example simulation for this case uses the CrosstalkShowcasePartiallyOverlappingFrequencyBands network, which contains a Ieee80211DimensionalRadioMedium module. The simulation is specified in the PartiallyOverlappingFrequencyBands configuration. The hosts are configured to have Ieee80211DimensionalRadio modules. The host pairs are set to adjacent Wifi Channels 1 and 2. Also, a more realistic signal spectrum is configured, based on the spectral mask of OFDM transmissions, as in the 802.11 standard:

../../../../_images/spectralmask_wifi2.png

Here is the configuration in omnetpp.ini:

[Config PartiallyOverlappingFrequencyBands]
network = CrosstalkShowcasePartiallyOverlappingFrequencyBands

*.host*.wlan[*].radio.typename = "Ieee80211DimensionalRadio"
*.radioMedium.backgroundNoise.powerSpectralDensity = -110dBmWpMHz
*.radioMedium.backgroundNoise.power = nan

*.host{1..2}.wlan[*].radio.channelNumber = 0
*.host{3..4}.wlan[*].radio.channelNumber = 1

*.host*.wlan[*].radio.transmitter.frequencyGains = "left c-b*1.5 -40dB linear c-b -28dB linear c-b*0.5-1MHz -20dB linear c-b*0.5+1MHz 0dB linear c+b*0.5-1MHz 0dB linear c+b*0.5+1MHz -20dB linear c+b -28dB linear c+b*1.5 -40dB right"

The following video shows the host pairs communicating:

Even though they are on different channels, the transmissions interfere. In the beginning, host1 and host3 transmit simultaneously, and neither transmission can be successfully received. Due to the collision avoidance mechanism, one of the transmitting hosts defer from transmitting, and the subsequent transmissions are successful.

Sources: omnetpp.ini, Crosstalk.ned

Discussion

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