VISSIM-A NOVEL SIMULATION APPROACH FOR
MODELLING OF COMMUNICATION SYSTEMS
M.L.S.N. SWARAJYA LAKSHMI AND NIRANJAN PRASAD
K. L. University, Vijayawada; DLRL, Hyderabad .
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-toend 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
VisSim, communication systems, modeling, simulation, real-time.
INTRODUCTION:
Acommunication 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 subsystems.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.
Thetool in use, ”VisSim” ,has a special communication module that allows us to create accurate
simulation environment of the communication system involved. It is a software program for modeling endto-end communication systems at the signal or physical level.Execution is determined by the structure of a
graphical block diagram on which the programmer connects different function-nodes by drawing wires.
These wires propagate signals and any subsystem execute as soon as all its input data become available.
Since 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'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.
VISSIM/COMM:
Based on the core VisSim engine, VisSim/Comm) is used for the modeling and simulating end-toend 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 Receivermodels:
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 OFTHE 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
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.
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..
Amobile 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]
Asimulation 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
FFTwindow, 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.
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.
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:ASimulation Aided Introduction To Vissim/Comm-Guimaraes
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