Simulation Of Communication Systems Using Vissim Computer Science Essay

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The objective of this report is to determine type of modulation or type of Communication system can be associated with the experiments individually. Determining the output variations with the change in circuit parameters.

VisSim/Comm software is an application in which the model communication system block diagrams are formed with the components available in the VisSim/Com tool kit. It is also termed as the block diagram language for the simulation of the dynamic systems and embedded systems and this software is developed initially by Visual Solutions of Westford, Massachussetts.

WHY VisSim/Com?

In the modern world communication was undergoing drastic change day by day. Whatever be the type of communication. The principle of operation of the inbuilt communication system is almost the same to know the proper working condition of the communication links we useVisSim/Comm software simulation which provides a wide range of communication components to get connected within workspace and to observe the changes in the outputs with the changes in the parameters mainly (Frequency, Amplitude, Phase ,Gain etc)and this is the simplest way of estimating the communication system.

APPLICATION-VISSIM?

Main application of this software is in system design like

1) Control System Design

2) Multi Domain Design and Simulation in Digital Signal Processing

3) Model Based Embedded System development

ROLE OF AD633

AD633 is a functionally complete, four-quadrant, analog multiplier. It includes high impedance summing input(Z), differential X and Y inputs and a high impedance. The low impedance output voltage is a nominal 10V full scale provided by the buried Zener. The AD633 is the first product to offer these features at low priced 8-lead DIP and SOIC packages.

APPLICATION 0F AD633

1) Used for Multiplication, Division and Squaring of signals

2) Applicable in Phase detection, modulation and Demodulation of signals

3) Used in electronic component construction like VC(voltage controlled) Amplifiers, Filters and Attenuators

COMMUNICATION SYSTEM BLOCK

DATA SOURCE

Data source generates data or information signal that is to be transmitted. Signals can be either analog such as speech, or digital such as binary data sequence. This signal is typically a baseband signal represented by a voltage level. A microphone is a simple example of a data source.

ENCODE R AND DECODER

Encoder performs encoding by which the actual messages are converted into symbols for transmission. Here the sequence of characters is put into a special format for efficient transmission. The decoder decodes the encoded signal at the receiving end to retrieve the original information which was transmitted. This process of encoding and decoding ensures the security of the transmitted data. If

the data is transmitted without encoding then there are chances that the data may be trapped by knowing the frequency of transmission. Even if encoded data is tapped it cannot be decoded unless and until the way in which it is encoded is known. Thus encoding increases the reliability of information transfer over unreliable communication channels.

The encoder can also be used to implement Forward Error Correction (FEC). It is the process of adding some redundancy to the digital data stream in the form of additional data bits in a way that provides an error correction capability at the receiving end. This whole process is termed as Forward Error Correction. The most popular FEC schemes are,

Convolutional coding.

Block Coding.

Trellis Coding.

MODULATOR AND DEMODULATOR

Modulation is the process of converting the information so that it can be successfully sent through a medium. In a communication system modulator modulates the information to be transmitted over the channel at the transmitting end and demodulator, at the receiving end demodulates the received signal. Modulation is the process of mapping the original information signal into a carrier signal and then transmitting the modulated carrier signal leaving the original signal behind. This process is indispensable as it allows different signals to coexist simultaneously without interfering with each other. This is achieved by allocating each modulated signal to a slightly different region of the available frequency spectrum. At a high level modulation techniques can be subdivided into two basic groups, Analog Modulation and Digital Modulation.

Analog Modulation

In analog modulation, the transmitted signal can be varied continuously over a specified range as opposed to assuming a fixed no of predetermined states. The aim of analog modulation is to transmit an analog baseband signal (low pass signal) like a TV signal or audio signal over an analog bandpass channel, for example a cable TV network channel. Analog modulation includes three types.

Amplitude Modulation(AM)

Frequency Modulation(FM)

Phase Modulation(PM)

Amplitude Modulation

In amplitude modulation the amplitude of the carrier signal is varied with respect to the amplitude of modulating signal or information signal. The frequency of the carrier signal remains unchanged throughout. The resultant AM signal consist of carrier frequency plus upper and lower sidebands. This is the basic AM scheme or Double Sideband - Amplitude Modulation (DSB-AM). Sometimes the carrier frequency may be suppressed or transmitted at a low level, which requires the carrier frequency to be generated at the receiver for demodulation. This type of transmission is known as Double Sideband - Suppressed Carrier (DSB - SC).

Fig 2: Amplitude Modulation

Frequency Modulation

In frequency modulation the centre frequency of the carrier wave is varied in accordance to the amplitude of the information signal. FM is more immune to noise than AM since amplitude of the carrier remains unchanged and hence improves the signal-to-noise ratio of the communication system. But the bandwidth requirement of FM signal is greater than AM signal. One of the main applications of FM is in radio broadcasting.

Fig 3: Frequency Modulation

Phase Modulation

In phase modulation the phase of the carrier signal is varied with respect to the amplitude of the input signal. Pm is easily adaptable to data modulation applications. PM is not commonly used in radio transmission since it will require very complex receivers. PM finds application in digital music synthesizers.

Fig 4: Phase Modulation

Digital Modulation

In digital modulation the transmitted signal can assume only a fixed no of predetermined states, usually referred to as the alphabet size or constellation size of the modulated signal. These include discrete amplitude levels, discrete phases, discrete frequencies or their combination. Digital modulation has inherent benefits over analog modulation as its distinct transmission states can be easily detected at a receiver even in the presence of noise than an analog signal which can assume infinite values. In analog modulation tradeoff occurs always at encoding stage since some information is lost in the quantization process. Types of digital modulation are,

Phase Shift Keying(PSK)

Quadrature Amplitude modulation(QAM)

Frequency Shift Keying(FSK)

Phase Shift Keying Modulation

In PSK modulation the digital information is transmitted by varying the carrier phase between known phase states, keeping amplitude constant. As all constellation points have equal power, the envelope characteristics remain constant. The bandwidth is directly proportional to symbol rate. This eliminates the need of increased bandwidth with constellation size. Even though higher power is required to maintain a given BER performance level, as the constellation points move closer together.

Quadrature Amplitude Modulation

Here modulation is carried out by varying both the carrier phase and amplitude between known constellation points in the (I, Q) plane. QAM makes efficient use of available bandwidth.

Frequency Shift Keying Modulation

In FSK the digital information is transmitted by assigning discrete output frequencies to each of the possible input symbols keeping the carrier amplitude constant. Bandwidth occupied by an FSK signal is directly proportional to the signalling rate.

Pulse Position Modulation

In pulse position modulation information is transmitted by varying the occurrence of a rectangular or shaped pulse within a predefined symbol frame. The location of the pulse is proportional to the input signal level.

Fig 5: FSK and QPSK Modulation Formats

Fig 5: Pulse Position Modulation

Fig 6: Quadrature Amplitude Modulation

EXPERIMENT 1 - AMPLITUDE MODULATION

CIRCUIT DIAGRAM

Fig 7: Amplitude Modulation

The circuit diagram for simulating an amplitude modulation system is shown above. To build this system on the VisSim, slide the blocks off the block menu into the work area and wire them together with the help of mouse. The output of the simulation will appear as a two dimension plot for viewing and analysing. Most blocks have user settable parameters associated with them that allow us to set the simulation invariant properties of the block functions. To run the simulation click the Go button on the toolbar or choose Go command from simulate menu. We can enter a block by right clicking on it.

Input Signal Block

Fig 8: Inside of Input Signal Block

To enter the input signal block just right click on it. As we can see several sinusoidal waveform generators are connected to a summer. The wave forms generated by each generator are added to form the output signal. The source parameters of each waveform generator can be varied so as to produce output signal of desired frequency and amplitude. The output of input signal block is fed to an Amplitude modulator.

Fig 12: Sinusoidal Source Parameters

Bias Block

Fig 9: Inside of Bias Block

It provides biasing to the input signal. The constant block produces a constant signal which is added to the input signal.

AM Modulator Block

Performs modulation process. The input signal is used to modulate a carrier wave which is generated by the block to commence transmission. The carrier can be set to desired frequency and amplitude by opening the AM modulator properties window. The window provides provision for changing the modulation factor and carrier wave phase.

Fig 13: AM Modulator Properties Window

HOW DOES IT WORK?

The input sinusoidal signal is fed to an AM modulator block where modulation is carried out. The amplitude of the carrier signal is varied according to the amplitude of the input signal. Finally the modulated signal is passed through a complex to Re/Im converter block which converts complex vector input into its real and imaginary parts. Initially the carrier frequency, Fc and amplitude are set to 10 Hz and 3 volt, keeping the modulation factor as one. When we run the simulation we get the output as shown below.

Fig 10: Output of AM simulation

OBSERVATION

The blue wave form is the modulated output signal which is to be transmitted over the channel. It is observed that the amplitude of the carrier signal is varied in accordance with the amplitude of input sinusoidal signal. The red graph indicates the envelop characteristics of the modulated signal. As we can see the envelop characteristics of amplitude modulated signal is not a constant. Simulation is carried out by changing the frequency and amplitude of both carrier and input signal and corresponding waveforms are observed.

Fig 11: Variation in Modulation with varying parameters

EXPERIMENT 2 - DELAY ESTIMATOR

CIRCUIT DIAGRAM

Fig 12: Delay Estimator Circuit Diagram

The circuit diagram of a delay estimator is shown above. It mainly consists of a sinusoidal wave form generator, a constant block, a time delay block and a delay estimator block. The input signal is fed to a time delay block. The delay block imparts a delay on the input fed to it depending on the output of constant signal block.

Constant Signal Block:

This block generates a constant signal output which influences the delay imparted on the input signal. The constant signal generated is fed to time delay block. By changing the constant value the delay can be varied.

Fig 13: Constant properties Window

Time Delay Block:

Time delay block delays the signal for an absolute time. This block is intended to model a continuous delay in a continuous simulation.

Fig 14: Time Delay Block

Required delay is implemented by the equation

Fig 15: Time Delay Block properties

Initial Condition: Sets an initial condition for the delay. Default value is zero.

Max Buffer Size: Controls the granularity of the resulting time delay signal. The default value is 128.

Delay Estimator Block

This block estimates the propagation time delay from input to output in a simulation. The delay is estimated by performing a sliding correlation between the desired output signal and an undelayed version of the input signal (or, reference signal). The output signal can be a distorted version of the input signal. The size of the correlation window is specified as a parameter. An output flag is provided that indicates when the entire delay range was successfully searched. The result flag is 0 during computation and toggles to 1 upon completion. The delay estimate output is 0 at simulation start. The total simulation time should be greater than the sum of the correlation start time, the correlation window size (expressed in seconds), and the maximum delay.

Fig 16: Delay Estimator

Fig 17: Delay Estimator Properties

Window Size: Specifies the size of correlation window used in simulation steps.

Max Delay: Specifies the upper end of delay search range. Search range starts with zero delay.

Start Time: Specifies starting time of correlation process in seconds.

Signal Consumer Block

This block displays the current value of input in any number of significant digits.

Fig 18: Display properties

Value: Controls the current value in the display. Default value is one.

Display Digits: Indicates the no of displayed significant digits. Default value is 6.

HOW DOES IT WORK?

The input signal is supplied to a time delay block. The delay block imposes a certain amount of delay depending upon the value of constant block. The output of delay block can be expressed as,

y=x (t-Td)

This delay estimator estimates the propagation time delay from input to output in a simulation. The delay is estimated by performing a sliding correlation between the desired output signal and an undelayed version of the input signal. The output of the simulation is as shown.

Fig 19: Delay Estimator Output

OBSERVATION

The red wave is the actual input signal and the blue wave is the delayed input. As we can see the input is delayed by certain amount. This delay is imposed by the time delay block depending upon the value of constant signal block.The simulation is carried out by varying the value of constant block and the corresponding variation in delay is observed.

Fig 20: Variation in delay with delay value

CONCLUSION

EXPERIMENT 1- AM MODULATOR:

Modulation is an essential part of any communication system. Modulation ensures proper transmission of weak information signals over long distance with minimum data loss, thus making the communication system more reliable and effective. Through this simulation the amplitude modulation of a carrier wave by an input signal was carried out. The variation in carrier amplitude with respect to input signal amplitude was observed. Also studied how variation in carrier frequency and carrier amplitude affects the modulation.

EXPERIMENT 2- DELAY ESTIMATOR

Through this simulation the working of delay estimator as mean to impose an overall transmission delay to the communication system was observed. Also, how the input signal is delayed to any desired time period by varying the delay value was experimented.

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