VISSIM-A NOVEL SIMULATION
APPROACH FOR MODELLING OF COMMUNICATION SYSTEMS
(M.L.S.N. Swarajya Lakshmi1, Niranjan
Prasad2)
K. L. University,
Vijayawada; DLRL, Hyderabad 500005
Abstract:
A
typical communication link includes, ata minimum, three key elements: a
transmitter, a communication medium (or channel), and areceiver. The ability to
simulate all three of these elements is required in order to successfullymodel
any end-to-end communication system. In order to achieve this target we have
used simulation software “VisSim” or Visual Simulator that allows us to use a
graphical approach tosimulation and modeling..The visual simulatorallows us to
model end-to-end communication systems at the signal or physical level.. In
this paper we explored the advantages of VisSimand simulation of variety of models includinganalog, digital and
mixed mode designs, with including their simulating behavior using the
VisSim/Comm software and graphical programming.
Keywords:VisSim,
communication systems, modeling, simulation, real-time.
Introduction:
A communication system is a collection of
individual communication networks, transmission
Systems,relay
stations, tributary stations, and data terminal equipment (DTE) usually capable
ofinterconnection and interoperation to form an integrated whole.
The
components of acommunication system serve a common purpose, are technically
compatible, use commonprocedures, respond to controls, and operate in unison.As
such any communication systemconsists of subsystems which work together to
achieve a common link,through achieving itsown functionality.
Thetransmitter
and receiver elements can in turn be further subdivided into
sub-systems.Theseinclude a data source (analog or digital), an optional data
encoder, a modulator, a demodulator,an optional data decoder, and a signal
sink. To understand the process of such a communicationwe need to visualize or
simulate such a link,so as to have a better understanding of the
processinvolved.With graphical programming, the diagram isthe source code, depicted as an arrangement of nodes connected
by wires. Each piece of data flows through the wires, to be consumed by nodes that
transform the data mathematically or perform some action such as I/O(1).
The
concept of a dataflow diagram (which, unlike a flowchart, shows the motion of
data rather than the motion of logic) is nothing new. In fact, even the idea of
letting a dataflow diagram be the sole input to a compiler or interpreter has
been put into practice for years. A number of graphical programming tools are
available today, each tailored to a particular industry.
this
might be the case formultiple subsystems simultaneously.
VISSIM:
VisSim
is one of the fastest, most intuitive simulation software package around and
anindispensable tool for anyone who needs to model dynamicprocessesand systems. VisSim can shorten the design
stage of any project, reduce costs, provide dependable outcomes for virtual
prototypes and provide moreresults for less effort.
VisSim
is an excellent software program for the modeling and simulation of
complexdynamic systems. It combines an intuitive drag & drop block diagram
interface with apowerful simulation engine. The visual block diagram interface
offers a simple method forconstructing, modifying and maintaining complex
system models. The simulation engineprovides fast and accurate solutions for
linear, nonlinear, continuous time, discrete time, timevarying and hybrid
system designs. With VisSim, users can quickly develop software
or"virtual" prototypes of systems or processes to demonstrate their
behavior prior to buildingphysical prototypes.
In
viewing the purpose of ease of modeling VisSim has following advantages:
VisSim is a block diagram
language for creating complex nonlinear dynamic systems. To create a model,
simply drag blocks in the workspace and connect them with wires. Then click the
Go button to initiate your simulation. The response is instantaneous. You can
choose to display your response in 2D or 3D plots, gauges, bar charts, meters,
digital readouts, and even 3D animated scenes. All are driven in real time
using the VisSim engine.
VisSim
is one of the fastest, most intuitive simulation software package around and
anindispensable tool for anyone who needs to model dynamicprocessesand systems. VisSim can shorten the design
stage of any project, reduce costs, provide dependable outcomes for virtual
prototypes and provide moreresults for less effort.
VisSim
is an excellent software program for the modeling and simulation of
complexdynamic systems. It combines an intuitive drag & drop block diagram
interface with apowerful simulation engine. The visual block diagram interface
offers a simple method forconstructing, modifying and maintaining complex
system models. The simulation engineprovides fast and accurate solutions for
linear, nonlinear, continuous time, discrete time, timevarying and hybrid
system designs. With VisSim, users can quickly develop software
or"virtual" prototypes of systems or processes to demonstrate their
behavior prior to buildingphysical prototypes.
In
viewing the purpose of ease of modeling VisSim has following advantages:
VisSim is a block diagram
language for creating complex nonlinear dynamic systems. To create a model,
simply drag blocks in the workspace and connect them with wires. Then click the
Go button to initiate your simulation. The response is instantaneous. You can
choose to display your response in 2D or 3D plots, gauges, bar charts, meters,
digital readouts, and even 3D animated scenes. All are driven in real time
using the VisSim engine.
VisSim's
highly tuned math engine executes your diagram directly with no compilation
delay. In addition to accelerating development with rapid turnaround for
changes, VisSim's fast execution speed is perfect for model based operator
training, off-line controller tuning, and hardware-in-the-loop testing. Its efficient C code generator makes it an ideal platform
for model-based embedded system
development.
By combining the simplicity and clarity of a block diagram
interface with a high-performance mathematical engine, VisSim provides fast and
accurate solutions for linear, nonlinear, continuous time, discrete time, SISO,
MIMO, multi-rate, and hybrid systems. With VisSim's wide selection of block
operations and expression handling, complex systems can be quickly entered into
VisSim.
VisSim's tightly integrated development platform makes it easy to
pass freely among the stages of model construction, simulation, optimization,
and validation. This means you can create virtual prototypes on your desktop
and make sure they're working properly before committing to the design. And
because VisSim eliminates traditional programming, your learning time is
minimal.
VisSim has libraries of preconstructed components giving you access to high level models of subsystems like HVAC units, AC and DC electric motors, hydraulic components, gas turbines, human respiration and blood flow, 6-DOF airframe dynamics, counters, timers, logic, and much more. Just right click to get a dialog box, configure to your system parameters, and connect the wires and go.
For specialized engineering problems, VisSim offers a
comprehensive set of companion products for frequency domain analysis, C code
generation, communications system modeling, DSP and embedded system design,
neural networks, OPC, CAN, and real-time analog and digital I/O. These addon
modules are listed in the Products sidebar.
With
over 100 thousand users across a broad range of industries and disciplines,
VisSim is a excellent software for
dynamic simulation and model-based
development.
Table 1: Various VisSim Software Tools and their Functions
Vissim/Comm:
Based on the core
VisSim engine, VisSim/Comm) is used for the modeling and simulating end-to-end
communication systems at the signal or physical level. With a full complement
of communication blocks and a powerful, time-domain simulation engine, VisSim/Comm
provides fast and accurate solutions for analog, digital, and mixed-mode
communication system designs. The communication block set includes RF, UWB,
Bluetooth, 802.x, Turbo Codes, Costas loop, PLL, VCO, BPSK, QPSK, DQPSK, QAM,
BER, Eye Diagram, Viterbi, Reed-Solomon and much more. VisSim/Comm was rebadged
as "Commsim"
byElectronic
Workbench and has been used by them for extensive applications.
VisSim/Comm
supports a wide range of customizable filters, including FIR, IIR, gaussian,
raisedcosine and root raised cosine filters. Additional blocks, such as the
complex FFT block, make it easy to view gain and phase responses of any filter.
Furthermore,
for designs that requireadaptive filters, fractionally-spaced LMS equalizer
blocks are included. VisSim/Comm highly interactive interface makes it easy to
perform ‘what if’ simulations and carry out performance trade-offs. For
example, in analog modulation we can keep amplitude modulation and frequency
modulation side by side and evaluate their envelope shapes, simultaneously.
Using
VisSim/Comm, you can freely move among the stages of model construction,
simulation, optimization, and validation. This tightly integrated development
platform allows you to simulate and view signal waveforms at any stage of the
communication system chain. And all modeling and simulation tasks can be
completed without writing a line of code. In short, VisSim/Comm makes it easy
to build, modify and maintain even the most complex system models. Many
manufacturers of cellular and PCS consumer electronics, has been using
VisSim/Comm for years in their research and development efforts(3).
Models in VisSim/comm:
Few
examples:
Transmitter
and Receiver models:
Communication
system design can be divided into two categories: transmitter design
andreceiver design. VisSim/ Comm lets us build build both transmitter and
receiver models, from afirst principles perspective, by simply selecting and
connecting predefined blocks. We simulate avariety of models including analog,
digital and mixed mode designs, and quickly simulatetheirbehavior. The
VisSim/Comm block set provides a variety of modulators and
demodulators,including standard analog, PSK, QAM and differential formats.
Channel
Models:
VisSim/Comm
includes a variety of predefined channel models supporting both fixed and
mobileservice scenarios. Included are fading, multipath, bandlimited, and Gaussian
noise models.Further all VisSim/Comm blocks, can modify model parameters to
suit their specific needs in designing of Filters and Equalizers(4).
Application areas of VisSim:
·
Aerospace
and defense
·
Communications
·
Electronics
·
HVAC
·
Control
engineering
·
Medical
devices
·
Power
and gas
·
Transportation
Some
of the simulations usingVisSim:
Combination of AM and FM:
The
major advantage that we can have with VisSim is that we can plot the Input
Signal, The AMModulated Signal and the FM Modulated signal simultaneously which
helps us in comparing thetwo outputs with a single source
Here
the basic components that are involved:
·
Input
Signal(which is a combination of many sine waves )
·
AM
Modulator (which modulates the input signal w.r.t. Amplitude)
·
FM
Modulator(which modulates the input signal w.r.t. Frequency)
·
Complex
to Real (converts the complex quantity into real & imaginary part)
The outputs that we achieve here
are:
1. AM Modulated Signal and
Envelope
2. FM Modulated Signal
Fig
1: AM VS FM.
MULTIPLEXER:
In
electronics, a multiplexer or mux (occasionally the term muldex is also found,
for acombination multiplexer-demultiplexer) is a device that performs
multiplexing; it selects one ofmany analog or digital input signals and outputs
that into a single line. An electronic multiplexermakes it possible for several
signals to share one expensive device or other resource, for exampleone A/D
converter or one communication line, instead of having one device per input
signal.
In
electronics, a demultiplexer (or demux) is a device taking a single input
signal and selectingone of many data-output-lines, which is connected to the
single input. A multiplexer is oftenused with a complementary demultiplexer on
the receiving end. An electronic multiplexer can beconsidered as a
multiple-input, single-output switch, and a demultiplexer as a
single-input,multiple-output switch.
Fig 2: Multiplexer
An
equalization (EQ) filter, or an equalizer is a filter, usually adjustable,
&mainly meant toCompensate for the unequal frequency response of some other
signal processing circuit orSystem. An EQ filter typically allows the user to
adjust one or more parameters that determine theoverall shape of the filter's
transfer function. It is generally used to improve the fidelity of sound,to
emphasize certain instruments, to remove undesired noises(2).
Equalizers may be designed withPeaking
filters, shelving filters, band pass filters, plop filters or high-pass and
low-pass filters. Fig.3Shown below is the block diagram of a 5 tap adaptive
filter that takes in input as well as error toadaptively equalize the channel.
Furtherchannel equalization of a QAM link simulation using VisSim has shown in
Fig4..
Fig
3:Block Diagram of a 5 Tap Adaptive
Filter
Fig 4: Channel equalization of a QAM link:
A
mobile channel is often characterized by multi-path propagation. What really
happens is that
the
presence of reflecting objects and scaterers in the channel creates a
constantly changingenvironment that dissipates the signal energy in amplitude
phase and time. These effects result inmultiple versions of the same
transmitted signal that arrive at the receiving antenna ,displacedwith respect
to one another in time and spatial orientation. The random phases and
amplitudes of
different
multipath components cause fluctuation in signal strength there by inducing
small scalefading ,signal distortion and or both.
Below
we simulate a multi path channel ,showing the congruence between a single
multipathchannel and several single channels with different propagation delays.
The simulation is donewith the help of communication module in Vissim. The
essential block here is the Multipathchannel block.
This
block implements a multipath channel, in which multiple time and phaseshifted
versions of a signal are modeled as arriving simultaneously at a receiver.
Multipathchannels are commonly used to model the interaction between a direct
signal and multiplereflected path signals. The reflected signals affect both
the amplitude and phase of the received
signal.
Block parameters include the number of total paths, and the individual path’s
delay,relative gain, and phase rotation. This block takes a complex signal as
its input, and outputs acomplex signal.
x
= Complex input signal [Re, Im]
y
= Complex output signal [Re, Im]
Fig 5: Multipath propagation:
A
simulation of a mobile fading channel has been shown below. Here we derive the
signal fadingas it passes through a mobile channel and also generate the Doppler
fading spectrum of thechannel.The essential blocks used here were:
Mobile
Fading:This block implements a mobile Raleigh fading channel suitable
formodeling mobile communications systems. This block is similar to the Jakes
Mobile block,but uses a different approach for shaping the spectrum of the
fading process. While the JakesMobile block approximates a Rayleigh fading
process via the summation of multiplecomplex sinusoids, the Mobile Fading block
does so by passing a uniform fading spectrumthrough an appropriate FIR shaping
filter. Block parameters include the Doppler shiftfrequency and the desired
number of taps for the FIR fading filter(2).
Spectrum:
This block outputs the complex power spectrum of the input signal. Thespectrum
can be continuously updated (once started by the external trigger) or produced
atuser-defined intervals (again, using the external trigger). Results are
viewed using a plotblock configured in XY mode with an external trigger. An
output trigger line and x-axisoutput are provided for driving the plot block.
Block parameters are FFT window, trigger mode, spectral output and power
spectrum units
Conclusion:
However
like any other approach to coding, graphical programming is not a suitable
criteria to meet all software needs. Besides the obviously more expensive
hardware required to create and viewdataflow diagrams, there are far fewer
cheap or free software tools available. . Despite their ability to be compiled,
graphical programs still rely on hefty runtime libraries that may
slowperformance. Additionally, the dataflow model proves unsettling and
unproductive for somecoders and inappropriate for some jobs.
Its efficient C code generator makes it an ideal platform for
model-based embedded system development.
The
visual presentation of this software is direct and reliable. The ability to
prototype rapidly and call on a wide range of industry-specific libraries leads
to productivity increase for certain tasks.
REFERENCES:
1.
Principles of Communication System – Taub Schilling
2.
Communication Systems – Simon Haykin
3.
Visual Simulator User Guide
4.DigitalTransmission:A
Simulation Aided Introduction To Vissim/Comm-Guimaraes
Thus
the graphical programming approach used in Vissim eases the simulation by
creating aplatform for visual implementation of such communication systems. VisSim's fast execution speed is perfect for model based
operator training, off-line controller tuning, and hardware-in-the-loop
testing.
