am generation in matlab

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CE00038-2-Communications Individual Assignment Page 1of 29

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Level 2 Asia Pacific Institute of Information Technology 2014

Table of content

S.no Topic Page no.

1. Acknowledgement 2

2. Abstract 3

3. Question 1 4-10

4. Question 2 10-28

5. References 29


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Acknowledgement

I have taken efforts in this assignment. However, it would not have been possible without the

kind support and help of many individuals. I would like to extend my sincere thanks to all of

them.

I am highly indebted to my module teacher Mrs. Monika Gambhir for her guidance and constant

supervision as well as for providing necessary information regarding the assignment & also for

her support in completing the assignment.

My thanks and appreciations also go to my colleague in developing the assignment and people

who have willingly helped me out with their abilities.


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Abstract

The assignment basically deals with communication system. The first part of assignment consists

of a formal report on Modulation and Demodulation techniques. Modulation and demodulation

are briefly described. By going through the assignment one can know how modulation

techniques are useful in communication system. Different schemes of analog and digital

modulation are described.

The second part of the assignment covers the simulation performed in MATLAB. DSBAM

(Double Side Band Amplitude Modulation) modulation and demodulation models are build on

MATLAB and then simulated at different modulation depths. It consists of diagrams of

waveforms at different modulation depth. Analysis is done on the basis of waveforms. Model of

DSBSC (Double Side Band Suppressed Carrier) is also build and simulated. Parameters of

DSBSC are theoretically calculated.


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a) MODULATION

Modulation is an important step of communication system. Modulation is the process of placing

the message signal over some carrier to make it suitable for transmission over a long distance.

The carrier signal is basically of higher frequency than that of message. Modulation is also

defined as the process whereby some characteristic (amplitude, frequency, phase of a high

frequency signal wave(carrier wave) is varied in accordance with instantaneous value of low

frequency signal wave (modulating wave.) Either of three characteristics can be varied in

accordance with the modulating signal.

DEMODULATION

Demodulation is the process of separating message signal from the modulated carrier signal. The

process is used in the receivers to recover the original signal coming from the sender end in

modulating form. When the signals reach the destination i.e. at the receiver end, then the signal

strength will be very less. This weak signal is amplified with the help of other signals. After

amplification this signal is filtered from the other signals which were used earlier to modify it.

When the signal becomes ready for demodulation process, then the below steps are performed

for demodulation. These steps are basically the functions of the receiver.

Demodulating and amplifying the received signal

Filtering of the original received signal from the non necessary signals

Power display of the received signal after the completion of demodulation process

Fig. 1: Block diagram of demodulator

Modulation makes possible to transmit several modulating signal over a common channel and

the technique is known as multiplexing. It is simultaneous transmission of multiple messages

(more than one message) over a common channel. The channel may be pair of wires (called

transmission lines) or free space. If transmitted without modulating, the different message signals


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over a single channel will interfere with one another. This is because their baseband (spectrum)

is identical or overlapping. However, different message signals can be transmitted over a same

channel without interference using multiplexing techniques. There are two types of multiplexing

techniques- frequency division multiplexing and time division multiplexing. The frequency

division multiplexing uses analog modulation systems, whereas the time division multiplexing

uses pulse modulating systems. Multiplexing reduces the cost of installation and maintenance of

more channels.

b) Types of analog pulse modulation are

PAM (Pulse Amplitude Modulation)

In pulse amplitude modulation, the amplitude of periodic sequential pulses are varied in

accordance with sample values of the baseband signal. It can be generated by using an AND

gate. PAM can be demodulated by passing through low pass filter with cut-off frequency as the

highest signal frequency.

PDM (Pulse Duration Modulation)

In pulse duration modulation, the duration of periodic sequential pulses are varied in accordance

with sample values of the baseband signal. It is also known as pulse width modulation. It can be

generated using a monostable multivibrator. It can be demodulated by feeding the PWM signal

to an integrating circuit.

PPM (Pulse Position Modulation)In PPM the position of pulse or the time of occurrence of the pulses is changed in accordance

with the instantaneous magnitude of the modulating signal. It can be generated in the similar way

as PWM but the pulse is kept constant from the starting point of occurrence of pulse. It can be

demodulated by converting into PWM using a flip-flop.


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Table 1: Difference between PAM, PDM, PPM

Sr.No.Pulse Amplitude

Modulation

Pulse Width/

Duration

Modulation

Pulse Position

Modulation

1.

Amplitude of the pulse is proportional to amplitude of

modulating signal.

Width of the pulse is proportional to

amplitude ofmodulating signal

The relative positionof the pulse is

proportional to theamplitude of

modulating signal.

2.

The bandwidth of the

transmission channel

depends or width of the pulse.

Bandwidth of


depends on rise time ofthe pulse.

Bandwidth of


depends on risingtime of the pulse.

3.

The instantaneous power ofthe transmitter varies.

The instantaneous power of the

transmitter varies.

The instantaneous power of the

transmitter remainsconstant.

4.

Noise interference is high. Noise interference is

minimum.

Noise interference is

minimum.

5.

System is complex. Simple to implement. Simple toimplement.

6.

Similar to amplitude. Similar to frequency. Similar to phase

modulation.

c) The move to digital modulation provides more information capacity, compatibility, with digital

data services, higher data security, better quality communications, and quicker system

availability. Digital modulation is very useful in communication system due to following

reasons-

Compatibility and flexibility- A complex and costly transmission is far more useful if it can

sustain a variety of information types and patterns of usage. Conversion of all data sources to a

common format, bits, means that can be handled by the same equipment.

Privacy- Privacy has become increasingly difficult to guarantee, so that many users demand

encryption of their data, a much easier process with digital signal.


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Fidelity of reproduction and error control- digital transmission may be favored by the nature of

channel. Moreover digital communication makes it easier to guarantee a given data error rate or

fidelity of reproduction.

Types of digital modulation techniques-

Amplitude Shift Keying (ASK)

In ASK, the baseband signal is used to modulate the amplitude of an analog carrier. One ways to

perform ASK would be to transmit the carrier signal with constant amplitude for one level of

digitally encoded baseband signal (e.g. NRZ) voltage and sending nothing for the other level.

ASK is a very simple mechanism and is a frequent choice in optical communications where a

large amount of bandwidth is available.

Frequency shift keying (FSK)

In FSK, two signals with different carrier frequencies are used to represent data. Since these

carriers have different frequencies, they can easily be distinguished at the receiver. It has the

added benefit over the ASK that if a bit is lost during transmission, it is known that the bit is lost,

since there should always be a carrier signal for zero or one. As compared to ASK signal

synchronization is easier to maintain in FSK. In FSK, the difference between the carrier

frequencies is kept large enough so that energy is not trapped in their byproducts.

Fig. 2: Block diagram of ASK modulation


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Phase shift keying (PSK)

In this type of digital modulation, the data information is embedded in the phase of the carrier.

The same carrier frequency is used for both types of bits (0 and 1) but the phase is inverted for

one or other. In simple binary PSK, two carrier signals are defined with same frequency and

same amplitude but opposite phases. In differential BPSK, instead of defining two carrier signals

with opposite phases, phase inversion is used to modulate one of the two types of binary signals.

d) Pulse code modulation (PCM) is a digital scheme for transmitting analog data. It converts an

analog signal into digital form. Using PCM, it is possible to digitalize all forms of analog data,

including full-motion video, voice, music, telemetry, etc.

To obtain a PCM signal from an analog signal at the source (transmitter) of a communications

circuit, the analog signal is sampled at regular intervals. The sampling rate is several times the

maximum frequency of the analog signal. The instantaneous amplitude of the analog signal at

each sample is rounded off to the nearest of several specific, predetermined levels (quantization)

the number of levels is always power of 2. The output of a pulse code modulator is a series of

binary numbers, each represented by some power of 2 bits. At the destination of the

communications circuit, the pulse code modulator converts the binary numbers back into the

Fig. 3: Block diagram of FSK modulation

Fig. 4: Block diagram of PSK modulation


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pulses having the same quantum levels as those in the modulator. These pulses are further

processed to restore the original analog waveforms.

Fig. 5: Block diagram of PCM technique

Assume that the modulating signal be a sinuosoidal voltage, having peak amplitude A m. Let this

signal cover the complete excursion of representation levels.

The power of this signal will be,

=

Here V= rms value

= [ √ 2]

When R=1, the power is normalized P i.e.,

Normalised power:

= 2 Therefore, signal to noise ratio is given by equation

=

3 × 2

Here

= 2 =

Putting these values in the above equation, = 1 . 5 × 2

Expressing signal to noise power ratio in dB,

() =101 . 5 × 2


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= 1 . 8 + 6 e) Application of Analog Modulation

Amplitude modulation is used in computer modems, VHF aircraft radio and portable two way

radio.

Frequency modulation is commonly used for broadcasting music and speech, magnetic tape

recording systems, two way radio systems and video transmission systems.

Generally phase modulation is used for transmitting waves is an essential part of many digital

transmission coding schemes that underlie a wide range of technologies like GSM, WiFi, and

satellite television.

Applications of Digital Modulation

ASK is used to transmit digital data over optical fiber.

FSK is used over voice lines, in high frequency radio transmission.

PSK are used in DTH satellite broadcasting system.

2.

a)

DSBAM model is build and simulated in MATLAB. The diagram below shows the model of

DSBAM modulator.

Fig. 6: Model of DSBAM Modulator (Time domain)


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Parameters of components used in Simulink model are given below

= 1 = 100

= 7

= As mentioned in the question we have to vary the value of VDC to get desired modulation depth

Table 2: Value of amplitude of message signal and constant

S.no. =

1.

1 4 0.25

2. 1 2 0.5

3. 1 1 1

4. 1 2/3 1.5

5.

1 1/5 2

6. 1 1/100 100


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Fig. 7: Waveform at modulation index 0.25 (Time domain)



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Fig. 9: Waveform at modulation index 1 (Time domain)



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Fig.11: Waveform at modulation index 2 (Time domain)

Fig. 12: Waveform at modulation index 100 (Time domain)


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Fig.13: Model DSBAM (Frequency domain)

Fig.14: Waveform at modulation index 0.25 (Frequency domain)


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Fig.16: Waveform at modulation index 1 (Frequency domain)


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Conclusion

When modulation index is less than 1, there is no distortion in waveforms. The ideal condition is

when modulation index is 1.

As modulation index goes beyond 1, modulated waveform becomes distorted. It results in a

condition called over modulation. As we can see the waveform is flattened at zero line. Moreover

data is also lost in over modulation. The negative peak of the modulating waveform is clipped and

[Ac+m(t)] goes negative, which mathematically appears as phase reversal of 1800 rather than a

clamped level. These phase reversals give additional sidebands resulting from the phase reversals

(phase modulation) than extend out, in theory to infinity. This can cause serious inference to other

users if not filtered.



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b)

Model for DSBAM demodulator is build and simulated at different value of modulation depth by

changing the value of constant.

Fig.20: Model of DSBAM Demodulator (Time domain)


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Fig. 21: waveform at modulation depth 0.25 (Time domain)

Fig.22: Waveform at modulation depth 0.5 (Time domain)


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Fig.23: Waveform at modulation depth 1 (Time domain)

Fig.24: Waveform at modulation depth 1.5 (Time domain)


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Fig.27: DSBAM demodulation (Frequency domain)



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Conclusion

When modulation index is below 1 or 1 the bandpass or modulating signal can be recovered with

negligible change in original data. It can be observed from waveforms when modulation index is leesthan 1 or equal to 1.

When modulation depth is more than 1, the baseband signal recovered from the modulated signal is

not preserved. It means that the baseband signal recovered is distorted. As phase reversal takes place

in modulated waveforms, the demodulated signal is inverted and becomes – m(t).



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c)

Output of PM1: The output consists of two sidebands as follows:

LSB = 10kHz -0.3kHz to 10kHz -3kHz

= 9.7kHz to 7kHz

USB = 10kHz +0.3kHz to 10kHz +3kHz

- 10.3kHz to 13kHz

Output of BPF1- Assume that this BPF passes only the USB

∴ = 10.3 13 Output of PM2 : Output of PM2 consists of the following two sidebands

→ 1 + 10.3 1 + 13 → 1.0103 1.013

→ 1 − 10.3 1 − 13 → 0.9897 0.987

Output of BPF2: let the BPF2 select the upper sideband from the PM2 output.

∴ 2 → 1.0103 1.013 Passbands of the two BPF:

1. Passband of BPF1:10.3kHz to 13kHz

2. Passband of BPF2: 1.0103MHz to 1.013MHz

Guardband of BPF1:The guardband of BPF1 extends from the lowest frequency of the USB to the higher frequency

of LSB.

∴ 1 = 9.7 10.3 Guardband of BPF2:

Similarly the guardband of BPF2 extends from 0.9897 1.0103 DSBSC model is build and simulated in MATLAB. The diagram given below illustrates the

model of DSBSC.

Conclusion

The detection process for DSB-SC requires a local oscillator at the receiver end. The frequency

and phase of the locally generated carrier signal and the carrier signal at the transmitter carrier

must be identical. This means that the local oscillator signal must be exactly coherent or

synchronized with the carrier signal at the transmitter, both in frequency and phase, otherwise the


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detected signal would get distorted. Thus the demerits of the synchronous detection is that it

requires an additional system at the receiver to ensure that the locally generated carrier is

synchronized with the transmitter carrier making the receiver complex and costly.

Fig.34: Model of DSBSC in MATLAB

Fig.35: Waveform of DSBSC


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References

Anderson,J.B,(1986), Digital Phase Modulation,Berlin,Springer Science & Business

Media,pp.76-77.

Borda,M,(2011), Fundamentals in Information Theory and Coding ,Berlin,Springer Science &

Business Media,pp.33-34.

Borth,D.E,(2013),Telecommunication. Available:http://www.britannica.com/EBchecked/topic/58

5799/telecommunication,Last accessed 18th Oct 2014.

Chitode,J.S,(2009), Principles Of Communication,Pune,Technical Publications,pp.4-7.

Godse,A.P,(2009), Communication Engineering ,Pune, Technical Publications,pp.16-18.

Rajput,R.K,(2011), Basic Electrical and Electronics Engineering ,New Delhi,Laxmi

Publications,pp.525-526.

Singh,R.P,(2008),Communication Systems,New Delhi,Tata McGraw-Hill Education,pp.112-113.

Sturley,K.R,(2012), Modulators and Demodulators .Available:

http://www.britannica.com/EBchecked/topic/1262240/radiotechnology/25124/Modulator

s-and-demodulators. Last accessed 17th Oct 2014.

Waggener,B,(1994). Pulse Code Modulation Techniques. London,Springer Science & Business

Media. p221-223.

Xiong,F,(2006), Digital Modulation Techniques, Pune, Artech House,pp.110-111.
http://www.britannica.com/EBchecked/topic/1262240/radiotechnology/25124/Modulatorhttp://www.britannica.com/EBchecked/topic/1262240/radiotechnology/25124/Modulator

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