.. _sec-spectrum-module:
Spectrum Module
---------------
.. include:: replace.txt
.. highlight:: cpp
.. heading hierarchy:
------------- Chapter
************* Section (#.#)
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The Spectrum module aims at providing support for modeling the frequency-dependent
aspects of communications in |ns3|.
The model was first introduced in
[Baldo2009Spectrum]_, and has been enhanced and refined over the years.
.. _fig-spectrum-analyzer-example:
.. figure:: figures/spectrum-analyzer-example.*
:align: center
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 :ref:`fig-spectrum-channel-phy-interface`:
.. _fig-spectrum-channel-phy-interface:
.. figure:: figures/spectrum-channel-phy-interface.*
:align: center
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 the ``StartRx`` event with a delay respect to the
``StartTx`` 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.
.. _sec-example-model-implementations:
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`` and
``SpectrumModelIsm2400MhzRes1Mhz`` are two example ``SpectrumModel`` implementations
* ``HalfDuplexIdealPhy``: a basic PHY model using a gaussian
interference model (implemented in ``SpectrumInterference``)
together with an error model based on Shannon capacity (described
in [Baldo2009Spectrum]_ and implemented in ``SpectrumErrorModel``. This PHY
uses the ``GenericPhy`` interface. Its addditional custom signal
parameters are defined in ``HalfDuplexIdealPhySignalParameters``.
* ``WifiSpectrumValueHelper`` is an helper object that makes it easy
to create ``SpectrumValues`` representing PSDs and RF filters for
the wifi technology.
* ``AlohaNoackNetDevice``: a minimal NetDevice that allows to send
packets over ``HalfDuplexIdealPhy`` (or other PHY model based on
the ``GenericPhy`` interface).
* ``SpectrumAnalyzer``, ``WaveformGenerator`` and ``MicrowaveOven`` are examples of PHY
models other than communication devices - the names should be
self-explaining.
References
==========
.. [Baldo2009Spectrum] N. Baldo and M. Miozzo, "Spectrum-aware Channel and PHY layer modeling for ns3",
Proceedings of ICST NSTools 2009, Pisa, Italy
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`` and ``SpectrumChannel`` 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 of ``SpectrumModel`` are typically statically allocated,
in order to allow several ``SpectrumValue`` instances to reference
the same ``SpectrumModel`` 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
base ``SpectrumSignalParameters`` 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`` and ``MultiModelSpectrumChannel``,
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 the
``DynamicCast<>`` 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 :ref:`sec-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`` and
``MultiModelSpectrumChannel`` have an attribute ``MaxLossDb`` 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 :ref:`sec-example-model-implementations` also have several attributes.
Output
======
* Both ``SingleModelSpectrumChannel`` and
``MultiModelSpectrumChannel`` provide a trace source called
``PathLoss`` 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 :ref:`sec-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 with ``gnuplot``. For example, if your run the
example ``adhoc-aloha-ideal-phy-with-microwave-oven`` you will get
an output file called ``spectrum-analyzer-output-3-0.tr``. From
this output file, you can generate a figure similar to
:ref:`fig-spectrum-analyzer-example` 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 :ref:`sec-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 :math:`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 :math:`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
:math:`1\%` of the PHY rate;
* if the PHY rate is not achievable, the application throughput shall
be zero.
Additional Models
*****************
TV Transmitter Model
====================
A TV Transmitter model is implemented by the ``TvSpectrumTransmitter`` class.
This model enables transmission of realistic TV signals to be simulated and can
be used for interference modeling. It provides a customizable power spectral
density (PSD) model, with configurable attributes including the type of
modulation (with models for analog, 8-VSB, and COFDM), signal bandwidth,
power spectral density level, frequency, and transmission duration. A helper
class, ``TvSpectrumTransmitterHelper``, is also provided to assist users in
setting up simulations.
Main Model Class
################
The main TV Transmitter model class, ``TvSpectrumTransmitter``, provides a
user-configurable PSD model that can be transmitted on the ``SpectrumChannel``.
It inherits from ``SpectrumPhy`` and is comprised of attributes and methods to
create and transmit the signal on the channel.
.. _spectrum-tv-cofdm:
.. figure:: figures/spectrum-tv-cofdm.*
:align: center
8K COFDM signal spectrum generated from ``TvSpectrumTransmitter`` (Left) and
theoretical COFDM signal spectrum [KoppCOFDM] (Right)
One of the user-configurable attributes is the type of modulation for the TV
transmitter to use. The options are 8-VSB (Eight-Level Vestigial Sideband
Modulation) which is notably used in the North America ATSC digital television
standard, COFDM (Coded Orthogonal Frequency Division Multiplexing) which is
notably used in the DVB-T and ISDB-T digital television standards adopted by
various countries around the world, and analog modulation which is a legacy
technology but is still being used by some countries today. To accomplish
realistic PSD models for these modulation types, the signals’ PSDs were
approximated from real standards and developed into models that are scalable by
frequency and power. The COFDM PSD is approximated from Figure 12 (8k mode) of
[KoppCOFDM], the 8-VSB PSD is approximated from Figure 3 of [Baron8VSB], and the
analog PSD is approximated from Figure 4 of [QualcommAnalog]. Note that the
analog model is approximated from the NTSC standard, but other analog modulation
standards such as PAL have similar signals. The approximated COFDM PSD model is
in 8K mode. The other configurable attributes are the start frequency,
signal/channel bandwidth, base PSD, antenna type, starting time,
and transmit duration.
``TvSpectrumTransmitter`` uses ``IsotropicAntennaModel`` as its antenna model by
default, but any model that inherits from ``AntennaModel`` is selectable, so
directional antenna models can also be used. The propagation loss models used
in simulation are configured in the ``SpectrumChannel`` that the user chooses to
use. Terrain and spherical Earth/horizon effects may be supported in future ns-3
propagation loss models.
After the attributes are set, along with the ``SpectrumChannel``,
``MobilityModel``, and node locations, the PSD of the TV transmitter signal can
be created and transmitted on the channel.
.. _sec-tv-helper-class:
Helper Class
############
The helper class, ``TvSpectrumTransmitterHelper``, consists of features to
assist users in setting up TV transmitters for their simulations. Functionality
is also provided to easily simulate real-world scenarios.
.. _spectrum-tv-8vsb:
.. figure:: figures/spectrum-tv-8vsb.*
:align: center
North America ATSC channel 19 & 20 signals generated using
``TvSpectrumTransmitterHelper`` (Left) and theoretical 8-VSB signal
[Baron8VSB] (Right). Note that the theoretical signal is not shown in dB
while the ns-3 generated signals are.
Using this helper class, users can easily set up TV transmitters right after
configuring attributes. Multiple transmitters can be created at a time. Also
included are real characteristics of specific geographic regions that can be
used to run realistic simulations. The regions currently included are
North America, Europe, and Japan. The frequencies and bandwidth of each TV
channel for each these regions are provided.
.. _spectrum-tv-rand-geo-points:
.. figure:: figures/spectrum-tv-rand-geo-points.*
:align: center
Plot from MATLAB implementation of CreateRegionalTvTransmitters method in
``TvSpectrumTransmitterHelper``. Shows 100 random points on Earth’s surface
(with altitude 0) corresponding to TV transmitter locations within a 2000 km
radius of 35° latitude and -100° longitude.
A method (CreateRegionalTvTransmitters) is provided that enables users to
randomly generate multiple TV transmitters from a specified region with a given
density within a chosen radius around a point on Earth’s surface. The region,
which determines the channel frequencies of the generated TV transmitters, can
be specified to be one of the three provided, while the density determines the
amount of transmitters generated. The TV transmitters' antenna heights
(altitude) above Earth's surface can also be randomly generated to be within a
given maximum altitude. This method models Earth as a perfect sphere, and
generated location points are referenced accordingly in Earth-Centered
Earth-Fixed Cartesian coordinates. Note that bodies of water on Earth are not
considered in location point generation--TV transmitters can be generated
anywhere on Earth around the origin point within the chosen maximum radius.
Examples
########
Two example simulations are provided that demonstrate the functionality of the
TV transmitter model. ``tv-trans-example`` simulates two 8-VSB TV transmitters
with adjacent channel frequencies. ``tv-trans-regional-example`` simulates
randomly generated COFDM TV transmitters (modeling the DVB-T standard)
located around the Paris, France area with channel frequencies and bandwidths
corresponding to the European television channel allocations.
Testing
#######
The ``tv-spectrum-transmitter`` test suite verifies the accuracy of the
spectrum/PSD model in ``TvSpectrumTransmitter`` by testing if the maximum power
spectral density, start frequency, and end frequency comply with expected values
for various test cases.
The ``tv-helper-distribution`` test suite verifies the functionality of the
method in ``TvSpectrumTransmitterHelper`` that generates a random number of TV
transmitters based on the given density (low, medium, or high) and maximum
number of TV channels. It verifies that the number of TV transmitters generated
does not exceed the expected bounds.
The CreateRegionalTvTransmitters method in ``TvSpectrumTransmitterHelper``
described in :ref:`sec-tv-helper-class` uses two methods from the
``GeographicPositions`` class in the Mobility module to generate the random
Cartesian points on or above earth's surface around an origin point which
correspond to TV transmitter positions. The first method converts Earth
geographic coordinates to Earth-Centered Earth-Fixed (ECEF) Cartesian
coordinates, and is tested in the ``geo-to-cartesian`` test suite by comparing
(with 10 meter tolerance) its output with the output of the geographic to ECEF
conversion function [MatlabGeo] of the MATLAB Mapping Toolbox for numerous
test cases. The other used method generates random ECEF Cartesian points around
the given geographic origin point, and is tested in the ``rand-cart-around-geo``
test suite by verifying that the generated points do not exceed the given
maximum distance radius from the origin point.
References
##########
.. [Baron8VSB] Baron, Stanley. "First-Hand:Digital Television: The Digital
Terrestrial Television Broadcasting (DTTB) Standard." IEEE Global History
Network. <http://www.ieeeghn.org/wiki/index.php/First-Hand:Digital_Television:_The_Digital_Terrestrial_Television_Broadcasting_(DTTB)_Standard>.
.. [KoppCOFDM] Kopp, Carlo. "High Definition Television." High Definition
Television. Air Power Australia. <http://www.ausairpower.net/AC-1100.html>.
.. [MatlabGeo] "Geodetic2ecef." Convert Geodetic to Geocentric (ECEF)
Coordinates. The MathWorks, Inc.
<http://www.mathworks.com/help/map/ref/geodetic2ecef.html>.
.. [QualcommAnalog] Stephen Shellhammer, Ahmed Sadek, and Wenyi Zhang.
"Technical Challenges for Cognitive Radio in the TV White Space Spectrum."
Qualcomm Incorporated.