Spectrum Module¶
The Spectrum module aims at providing support for modeling the frequency-dependent aspects of communications in ns-3. The model was first introduced in [Baldo2009Spectrum], and has been enhanced and refined over the years.

Spectrogram produced by a spectrum analyzer in a scenario
involving wifi signals interfered by a microwave oven, as simulated
by the example adhoc-aloha-ideal-phy-with-microwave-oven
.
Model Description¶
The module provides:
- a set of classes for modeling signals and
- a Channel/PHY interface based on a power spectral density signal representation that is technology-independent
- two technology-independent Channel implementations based on the Channel/PHY interface
- a set of basic PHY model implementations based on the Channel/PHY interface
The source code for the spectrum module is located at src/spectrum
.
Design¶
Signal model¶
The signal model is implemented by the
SpectrumSignalParameters
class. This class provides the following
information for a signal being transmitted/received by PHY devices:
- a reference to the transmitting PHY device
- a reference to the antenna model used by the transmitting PHY device to transmit this signal
- the duration of the signal
- its Power Spectral Density (PSD) of the signal, which is assumed to be constant for the duration of the signal.
The PSD is represented as a set of discrete scalar values each
corresponding to a certain subband in frequency. The set of frequency subbands
to which the PSD refers to is defined by an instance of the
SpectrumModel
class. The PSD itself is implemented as an instance
of the SpectrumValue
class which contains a reference to the
associated SpectrumModel
class instance. The SpectrumValue
class provides several arithmetic operators to allow to perform calculations
with PSD instances. Additionally, the SpectrumConverter
class
provides means for the conversion of SpectrumValue
instances from
one SpectrumModel
to another.
For a more formal mathematical description of the signal model just described, the reader is referred to [Baldo2009Spectrum].
The SpectrumSignalParameters
class is meant to include only
information that is valid for all signals; as such, it is not meant to
be modified to add technology-specific information (such as type of
modulation and coding schemes used, info on preambles and reference
signals, etc). Instead, such information shall be put in a new class
that inherits from SpectrumSignalParameters
and extends it with
any technology-specific information that is needed. This design
is intended to model the fact that in the real world we have signals
of different technologies being simultaneously transmitted and
received over the air.
Channel/PHY interface¶
The spectrum Channel/PHY interface is defined by the base classes SpectrumChannel
and SpectrumPhy
. Their interaction simulates the transmission and
reception of signals over the medium. The way this interaction works is depicted in Sequence diagram showing the interaction between SpectrumPhy and SpectrumChannel:

Sequence diagram showing the interaction between SpectrumPhy and SpectrumChannel
Spectrum Channel implementations¶
The module provides two SpectrumChannel
implementations:
SingleModelSpectrumChannel
and MultiModelSpectrumChannel
. They
both provide this functionality:
- Propagation loss modeling, in two forms:
- you can plug models based on
PropagationLossModel
on these channels. Only linear models (where the loss value does not depend on the transmission power) can be used. These models are single-frequency in the sense that the loss value is applied equally to all components of the power spectral density.- you can plug models based on
SpectrumPropagationLossModel
on these channels. These models can have frequency-dependent loss, i.e., a separate loss value is calculated and applied to each component of the power spectral density.- Propagation delay modeling, by plugging a model based on
PropagationDelayModel
. The delay is independent of frequency and applied to the signal as a whole. Delay modeling is implemented by scheduling theStartRx
event with a delay respect to theStartTx
event.
SingleModelSpectrumChannel
and MultiModelSpectrumChannel
are
quite similar, the main difference is that
MultiModelSpectrumChannel
allows to use different
SpectrumModel
instances with the same channel instance, by
automatically taking care of the conversion of PSDs among the
different models.
Example model implementations¶
The spectrum module provides some basic implementation of several components that are mainly intended as a proof-of-concept and as an example for building custom models with the spectrum module. Here is a brief list of the available implementations:
SpectrumModel300Khz300GhzLog
andSpectrumModelIsm2400MhzRes1Mhz
are two exampleSpectrumModel
implementationsHalfDuplexIdealPhy
: a basic PHY model using a gaussian interference model (implemented inSpectrumInterference
) together with an error model based on Shannon capacity (described in [Baldo2009Spectrum] and implemented inSpectrumErrorModel
. This PHY uses theGenericPhy
interface. Its addditional custom signal parameters are defined inHalfDuplexIdealPhySignalParameters
.WifiSpectrumValueHelper
is an helper object that makes it easy to createSpectrumValues
representing PSDs and RF filters for the wifi technology.AlohaNoackNetDevice
: a minimal NetDevice that allows to send packets overHalfDuplexIdealPhy
(or other PHY model based on theGenericPhy
interface).SpectrumAnalyzer
,WaveformGenerator
andMicrowaveOven
are examples of PHY models other than communication devices - the names should be self-explaining.
Usage¶
The main use case of the spectrum model is for developers who want to develop a new model for the PHY layer of some wireless technology to be used within ns-3. Here are some notes on how the spectrum module is expected to be used.
SpectrumPhy
andSpectrumChannel
are abstract base classes. Real code will use classes that inherit from these classes.If you are implementing a new model for some wireless technology of your interest, and want to use the spectrum module, you’ll typically create your own module and make it depend on the spectrum module. Then you typically have to implement:
- a child class of
SpectrumModel
which defines the (sets of) frequency subbands used by the considered wireless technology. Note: instances ofSpectrumModel
are typically statically allocated, in order to allow severalSpectrumValue
instances to reference the sameSpectrumModel
instance.- a child class of
SpectrumPhy
which will handle transmission and reception of signals (including, if appropriate, interference and error modeling).- a child class of
SpectrumSignalParameters
which will contain all the information needed to model the signals for the wireless technology being considered that is not already provided by the baseSpectrumSignalParameters
class. Examples of such information are the type of modulation and coding schemes used, the PHY preamble format, info on the pilot/reference signals, etc.The available
SpectrumChannel
implementations (SingleModelSpectrumChannel
andMultiModelSpectrumChannel
, are quite generic. Chances are you can use them as-is. Whether you prefer one or the other it is just a matter of whether you will have a single SpectrumModel or multiple ones in your simulations.Typically, there will be a single SpectrumChannel instance to which several SpectrumPhy instances are plugged. The rule of thumb is that all PHYs that are interfering with each other shall be plugged on the same channel. Multiple SpectrumChannel instances are expected to be used mainly when simulating completely orthogonal channels; for example, when simulating the uplink and downlink of a Frequency Division Duplex system, it is a good choice to use two SpectrumChannel instances in order to reduce computational complexity.
Different types of SpectrumPhy (i.e., instances of different child classes) can be plugged on the same SpectrumChannel instance. This is one of the main features of the spectrum module, to support inter-technology interference. For example, if you implement a WifiSpectrumPhy and a BluetoohSpectrumPhy, and plug both on a SpectrumChannel, then you’ll be able to simulate interference between wifi and bluetooth and vice versa.
Different child classes of
SpectrumSignalParameters
can coexist in the same simulation, and be transmitted over the same channel object. Again, this is part of the support for inter-technology interference. A PHY device model is expected to use theDynamicCast<>
operator to determine if a signal is of a certain type it can attempt to receive. If not, the signal is normally expected to be considered as interference.
Helpers¶
The helpers provided in src/spectrum/helpers
are mainly intended
for the example implementations described in Example model implementations.
If you are developing your custom model based on the
spectrum framework, you will probably prefer to define your own
helpers.
Attributes¶
- Both
SingleModelSpectrumChannel
andMultiModelSpectrumChannel
have an attributeMaxLossDb
which can use to avoid propagating signals affected by very high propagation loss. You can use this to reduce the complexity of interference calculations. Just be careful to choose a value that does not make the interference calculations inaccurate.- The example implementations described in Example model implementations also have several attributes.
Output¶
Both
SingleModelSpectrumChannel
andMultiModelSpectrumChannel
provide a trace source calledPathLoss
which is fired whenever a new path loss value is calclulated. Note: only single-frequency path loss is accounted for, see the attribute description.The example implementations described in Example model implementations also provide some trace sources.
The helper class
SpectrumAnalyzerHelper
can be conveniently used to generate an output text file containing the spectrogram produced by a SpectrumAnalyzer instance. The format is designed to be easily plotted withgnuplot
. For example, if your run the exampleadhoc-aloha-ideal-phy-with-microwave-oven
you will get an output file calledspectrum-analyzer-output-3-0.tr
. From this output file, you can generate a figure similar to Spectrogram produced by a spectrum analyzer in a scenario involving wifi signals interfered by a microwave oven, as simulated by the example adhoc-aloha-ideal-phy-with-microwave-oven. by executing the following gnuplot commands:unset surface set pm3d at s set palette set key off set view 50,50 set xlabel "time (ms)" set ylabel "freq (MHz)" set zlabel "PSD (dBW/Hz)" offset 15,0,0 splot "./spectrum-analyzer-output-3-0.tr" using ($1*1000.0):($2/1e6):(10*log10($3))
Examples¶
The example programs in src/spectrum/examples/
allow to see the
example implementations described in Example model implementations in action.
Troubleshooting¶
- Disclaimer on inter-technology interference: the spectrum model makes it very easy to implement an inter-technology interference model, but this does not guarantee that the resulting model is accurate. For example, the gaussian interference model implemented in the
SpectrumInterference
class can be used to calculate inter-technology interference, however the results might not be valid in some scenarios, depending on the actual waveforms involved, the number of interferers, etc. Moreover, it is very important to use error models that are consistent with the interference model. The responsibility of ensuring that the models being used are correct is left to the user.
Testing¶
In this section we describe the test suites that are provided within the spectrum module.
SpectrumValue test¶
The test suite spectrum-value
verifies the correct functionality of the arithmetic
operators implemented by the SpectrumValue
class. Each test case
corresponds to a different operator. The test passes if the result
provided by the operator implementation is equal to the reference
values which were calculated offline by hand. Equality is verified
within a tolerance of 10^{-6} which is to account for
numerical errors.
SpectrumConverter test¶
The test suite spectrum-converter
verifies the correct
functionality of the SpectrumConverter
class. Different test cases
correspond to the conversion of different SpectrumValue
instances
to different SpectrumModel
instances. Each test passes if the
SpectrumValue
instance resulting from the conversion is equal to the reference
values which were calculated offline by hand. Equality is verified
within a tolerance of 10^{-6} which is to account for
numerical errors.
Describe how the model has been tested/validated. What tests run in the test suite? How much API and code is covered by the tests? Again, references to outside published work may help here.
Interference test¶
The test suite spectrum-interference
verifies the correct
functionality of the SpectrumInterference
and
ShannonSpectrumErrorModel
in a scenario involving four
signals (an intended signal plus three interferers). Different test
cases are created corresponding to different PSDs of the intended
signal and different amount of transmitted bytes. The test passes if
the output of the error model (successful or failed) coincides with
the expected one which was determine offline by manually calculating
the achievable rate using Shannon’s formula.
IdealPhy test¶
The test verifies that AlohaNoackNetDevice
and
HalfDuplexIdealPhy
work properly when installed in a node. The
test recreates a scenario with two nodes (a TX and a RX) affected by a path loss such
that a certain SNR is obtained. The TX node transmits with a
pre-determined PHY rate and with an application layer rate which is
larger than the PHY rate, so as to saturate the
channel. PacketSocket
is used in order to avoid protocol
overhead. Different
test cases correspond to different PHY rate and SNR values. For each
test case, we calculated offline (using Shannon’s formula) whether
the PHY rate is achievable or not. Each test case passes if the
following conditions are satisfied:
- if the PHY rate is achievable, the application throughput shall be within 1\% of the PHY rate;
- if the PHY rate is not achievable, the application throughput shall be zero.