Experiment No. 1 -...

Analog & Digital Communications Lab Manual – Part II

Refer Text-book: `Analog & Digital Communications’, T. L. Singal, Tata McGraw-Hill, 2012

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Experiment No. 1

Aim: To obtain modulated and demodulated waveforms of amplitude shift keying

technique.

Apparatus/Test Equipments:

1. TDM Pulse Code Modulation Transmitter Trainer, Scientech ST2103 or equivalent.

2. TDM Pulse Code Modulation Receiver Trainer, Scientech ST2104 or equivalent.

3. Data Formatting and Carrier Modulation Transmitter Trainer, Scientech ST2106 or

equivalent

4. Carrier Demodulation and Data Reformatting Receiver Trainer, Scientech ST2107 or

equivalent

5. Digital Storage Oscilloscopes, Tektronix TDS1002 or equivalent.

6. Connecting Wires.

Theory:

The simplest method of modulating a carrier with a data stream is to change the

amplitude of the carrier wave every time the data changes. This modulation technique is

known as amplitude shift keying. The simplest way of achieving amplitude shift keying is

by switching ‗On‘ the carrier whenever the data bit is '1' & switching off. Whenever the

data bit is '0' i.e. the transmitter outputs the carrier for a' 1 ' & totally suppresses the carrier

for a '0'. This technique is known as ‗On-Off’ keying Figure 1 illustrates the amplitude

shift keying for the given data stream. Thus, Data = 1 carrier transmitted Data = 0 carrier

suppressed.

ASK Modulation Wave

Figure 1: ASK Modulation wave

The ASK waveform is generated by a balanced modulator circuit, also known as

a linear multiplier. As the name suggests, the device multiplies the instantaneous signal at



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its two inputs. The output voltage being product of the two input voltages at any instance

of time one of the input is AC coupled 'carrier' wave of high frequency. Generally, the

carrier wave is a sine wave since any other waveform would increase the bandwidth,

without providing any advantages. The other input which the information is signal to be

transmitted, is DC coupled.

Figure 2: ASK MODULATOR

It is known as modulating signal. In order to generate ASK waveform it is necessary

to apply a sine wave at carrier input & the digital data stream at modulation input. The

double - balanced modulator is shown in Figure 2.

The data stream applied is unipolar i.e. 0 volts at logic '0' & + 5 Volts at logic '1'.

The output of balanced modulator is a sine wave, unchanged in phase when a data bit ‗l' is

applied to it. In this case the carrier is multiplied with a positive constant voltage when the

data bit '0' is applied, the carrier ismultiplied by 0 volts, giving rise to 0 volt signal at

modulator's output.

Figure 3: ASK Demodulator

Step A: The ASK waveform is rectified by a diode rectifier, giving a positive going

signal. This signal is too rounded to be used as digital data.

Step B: After rectification, the signal is passed through the low pass filter to remove the

carrier component. This result in slightly rounded pulses of unreliable amplitude



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Step C: These rounded pulsed are then 'Squared Up' (i.e. shaped in a square wave

Fashion) by passing it through voltage comparator set at a threshold level. If the

input voltage exceeds the threshold level, the comparator output is a +5V signal.

EXPERIMENT SET-UP DIAGRAMS

Figure 4: Set up Diagrams

Procedure:



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1. Ensure that all trainers are switched off, until the complete connections are made.

2. Check ST2104 Trainer's clock regeneration circuit Set up for correct operation.

3. Set up the following conditions on ST2103 trainer

a. Mode switch set in FAST position.

b. Pseudo -random sync code generator switched on.

c. Error check code selector switches A & B in A=0 & B=0 positions.

d. All switched faults ‗Off‘

4. Set ST2106 trainer's mode switch in position 1

5. Set up following conditions on ST2104 trainers:

a. Mode switch set in FAST position

b. Pseudo -random sync code detector in ‗On‘ position.

c. Error check code selector switch A & B in A = 0 & B = 0 position.

d. All switched faults to be kept ‗Off‘

6. Make the following connections between ST2103, ST2106 trainers.

a. TX clock output (TP3) to TX clock input

b. PCM output (TP44) to TX data input

7. Connect the TX to output (TP4) on ST2103 trainer to external trigger input of the

oscilloscope. Set to negative edge triggered mode in oscilloscope. It may be necessary

to adjust the trigger level manually to obtain a stable waveform.

8. ST2103 trainer make following connections.

a. DC 1to CH 0 input.

b. CH 0 input to CH 1 input.

This is done to supply the same voltage level to each of the two time division

multiplexed channels. Thus we are able to get the same data stream for any timeframe.

9. Make the rest of the connections as shown in configuration Figure 4.

10. Switch ‘on’ the power

11. On ST2103 trainer adjust the DC1 potentiometer until the 7 bit code displayed at A/D

converter

12.Observe the data clock output at TP4 on ST2106 trainer's data format block with

Oscilloscope adjust the oscilloscopes time base & position control until each rising

edge of data clock coincides with one of scope's vertical line.

13. Examine the NRZ (L) TP5 in data format block of ST2106 trainer on other channel of

the oscilloscope. The NRZ (L) waveform must be identical.



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14. Switch off the power. Make the connections as given in Experimental Set-up Diagram

Figure 4.

a. On ST2106 trainer :

i) NRZ (L) output (TP5) to carrier modulation circuit‘s modulation input (TP27).

ii) 1.44 MHz carrier output (TP16) to carrier input socket of modulator circuit

(TP26).

b. Between ST2106 and ST2107 :

i) Modulator 1 output (TP28) to ASK demodulator input (TP21).

c. On ST2106 and ST2107 :

i) ASK demodulator output (TP22) to Low Pass Filter 1 input (TP23).

ii.) Low Pass Filter 1 output (TP24) to comparator 1 input (TP46)

d On ST2104 trainer :

i) PCM. Data input (TP1) to clock regent circuit input (TP3)

ii) Clock regent circuit output TP8 to RX clock input (TP46)

e. Connect:

i.) Comparator 1 output (TP47) on ST2107 to PCM data input (TP1) on ST2104.

15. Turn ‘On’ the trainers: Monitor NRZ (L) output (TP5) from ST2106 trainer on one

channel of the oscilloscope Use the other channel to monitor the output of modulator

1 (TP28) in ST2106 trainer

16. Three variables have been provided in the modulators block. Their use may be

necessary to obtain a required ASK waveform.

These variables are:

a. Gain: This pot adjusts the amplification of the modulator's output. Adjust this pot

till the output is not a 2Vpp signal in ‗On‘ state.

b. Modulation Offset: This control is used to adjust the amplitude of the ‗Off‘ signal.

Adjust this control till the amplitude of the ‗Off‘ signal is an close to zero as possible.

C. Carrier Offset: This control adjusts the ‗Off‘ bias level of the ASK waveform.

Adjust this control till the ‗Off‘ level occurs midway between the ‗On‘ signal peaks.

17. To see the demodulation process, observe the output at the ASK demodulator (TP22)

&low pass filter (TP24) on ST2107 trainer.

18. The last stage of demodulation is 'squaring up' of filter output. In order to achieve this

it is necessary to adjust the bias level for comparator 1 so that the output has the

correct pulse width.



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19. Adjust it till the output signal pulse width is not similar to the NRZ (L) data pulse

width. You can observe these two simultaneously on the dual trace oscilloscope. The

two will be identical once the bias level is adjusted except for short delay between

them.

20. Turn ‗On‘ the pseudo-random sync code generator on ST2103 trainer. This pulls the

transmitter& receiver in 'Frame - Synchronization'. Observe the A/Converter LEDs on

ST2103 trainer &D/A Converter LEDs on ST2104 trainer. Now they will be carrying

the same data. Change the position of the DC l potentiometer Observe that the same

change is reflected at the receiver.

21. Turn ‘Off’ the power: Disconnect CH0 & CH1. Instead connect them to 1 KHz &2

KHz signal respectively. Turn ‗On‘ the trainers. & check the reconstructed analog

output on ST2104 CH 0 & CH 1 (TP33 & 36).

22. You can use any data format available on ST2106 trainer to modulate the carrier.

Remember to demodulate the carrier as described above & convert the date format back

to NRZ (L) format by means of reformatting techniques described in the earlier

sections.

23. The same experiment can be performed by using 960 KHz (1) signal instead of1.44

MHz

Note *Remember*

Amplitude shift keying is fairly simple to implement in practice, but it is less

efficient, because the noise inherent in the transmission channel can deteriorate the

signal so much that the amplitude changes in the modulated carrier wave due to noise

addition, may lead to the incorrect decoding at the receiver. This is particularly true

when the noise added is comparable to the comparator threshold level. Hence, this

technique is not widely used is practice. Application wise, it is however used in

diverse areas and old as emergency radio transmissions and fiber-optic

communications.



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Results /Observed Waveforms:

Figure 5 (a): ASK Modulated Waveforms

Figure 5 (b): ASK Demodulated Waveforms

Result/Observed Waveforms:

The candidates should paste the observed waveforms here.

Inference: Hence we conclude generation of modulated and demodulated waveforms of

amplitude shift keying.



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MCQs and Viva-voce Questions:

Q1 State that bandwidth requirement of ASK system.

(a) fb (b) 5 fb (c) 4 fb, (d) ≥2fb

Q2 How many balance modulator are used in ASK modulation?

(a) 2 (b) 1 (c) 3 (d) 4

Q3 ASK balance modulator act as _________.

(a) Low pass filter (b) Subtractor (c) Bandpass filter (d) Multiplier

Q4 Which type of Technique is used in ASK demodulation?

(a) Envelop detector (b) Phase lock loop (c) Ratio detector (d) Balanced

slope detector

Q5 What is level of noise immunity in ASK?

(a) Low (b) High (c) Very high (d) Medium

Q6 _____Parameters of an analog sine-wave carrier signal can be modulated by

Digital data information, leading to the most common basic types of digital

modulation technique.

(a) Amplitude (b) frequency (c) Phase (d) any one of the above

Q7 _____ gives maximum probability of error.

(a) ASK (b) BFSK (c) BPSK (d) DBPSK

Q8 What is multi-level phase and amplitude modulation technique?

Q9 What is meant by figure of merit in digital radio systems?

Q10 Differentiate between bit rate and baud rate.



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Experiment No. 2

Aim: To obtain modulated and demodulated waveforms of Frequency shift keying

technique.





equivalent


equivalent



Theory:

Frequency shift keying, the carrier frequency is shifted in steps (i.e. from one

frequency to another) corresponding to the digital modulation signal. If the higher

frequency is used to represent a data '1' & lower frequency a data '0',

Figure 1: FSK Generator

On a closer look at the FSK waveform, it can be seen that it can be represented as the sum

of two ASK Let us assume that we apply the above data stream to an ASK modulator

using the higher frequency carrier. Let us now invert the original data stream.

Original Data Steam 0 1 1 0 0 0 1 0 1 1

Inverted Data Steam 1 0 0 1 1 1 0 1 0 0



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We now apply the inverted data stream to the ASK modulator using a lower

stream frequency carrier. The result is the original data '0' filled with the lower frequency

carrier shown in Figure 3.

Lastly, we have to sum the two ASK waveforms, to get a FSK wave. The functional

blocks required in order to generate the FSK signal Figure 1 .The two carriers have

different frequencies & the digital data is inverted in one case.

FSK Demodulator

The demodulation of FSK waveform can be carried out by a phase locked loop.

As known, the phase locked loop tries to 'lock' to the input frequency. It achieves this by

generating corresponding output voltage to be fed to the voltage controlled oscillator, if

any frequency deviation at its input is encountered. Thus the PLL detector follows the

frequency changes & generates proportional output voltage. The output voltage from PLL

contains the carrier components. Therefore the signal is passed through the low pass filter

to remove them. The resulting wave is too rounded to be used for digital data processing.

Also, the amplitude level may be very low due to channel attenuation. The signal is

'Squared Up' by feeding it to the voltage comparator. Figure 2 shows the functional blocks

involved in FSK demodulation.

Figure 2: FSK Demodulator

Since the amplitude change in FSK waveform does not matter, this modulation

technique is very reliable even in noisy & fading channels. But there is always a price to

be paid to gain that advantage. The price in this case is widening of the required

bandwidth. The bandwidth increase depends upon the two carrier frequencies used & the

digital data rate. Also, for a given data, the higher the frequencies & the more they differ

from each other, the wider the required bandwidth. The bandwidth required is at least



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doubled than that in the ASK modulation. This means that lesser number of

communication channels for given band of frequencies



Procedure:



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oscilloscope. Set to negative edge triggered mode in oscilloscope. It may be necessary

to adjust the trigger level manually to obtain a stable waveform.





multiplexed Channels. Thus we are able to get the same data stream for any time

frame.




converter

12. Observe the data clock output at TP4 on ST2106 trainer's data format block with





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the oscilloscope. The NRZ (L) waveform must be identical.

14 Switch off the power. Make the connections as given in Experimental Set-up Diagram

Figure 3

a. On ST2106 Trainer :

i) NRZ (L) output (TP5) to modulation input of unipolar-bipolar converter (TP27)

ii) Modulation input (TP27) to data inverter input (TP32)

iii) Modulator output (TP28) to summing amplifier input A (TP34)

iv) Data inverter output (TP33) to modulation input of modulator 2 (TP30)

v) 1.44 MHz carrier (TP16) to Modulator 1 carrier input (TP26)

vi) 960 KHz (1) carrier (TP17) to modulator 2 carrier input (TP29)

Vii) Modulator 2 output (TP31) to summing amplifier input (TP35)

b. Between ST2106 & ST2107 trainer :

i) Summing amplifier output (TP36) to PLL detector input (TP16)

c. On ST2107 trainer :

i) PLL detector output (TP17) to low pass filter 1 input (TP23)

ii) Low pass filter 1 Output (TP24) comparator 1 input (TP46)

d. Between ST2107 & ST2104 trainer :

i) Comparator 1 output (TP24) to PCM. data input (TP1)

e. On ST2104 trainer :

i) PCM data input (TP1) to clock regeneration circuit input (TP3)

ii) Clock regeneration circuit output (TP8) to RX clock input (TP46)

15. Monitor the output of modulator 1 (TP28) on ST2106 trainer. Make the adjustments of

the given controls in the modulator block as follows.

a. Gain: Adjust this pot until the amplitude of the ‗On‘ signal is 2Vpp.

b. Modulation off set: It is used to control the amplitude of ‗Off‘ signal. Adjust it till

the ‗Off ‗Signal level doesn't approach as close to zero as possible.

c. Carrier Offset: This control adjusts the ‗Off‘ bias level of the ASK waveform.

Adjust this control till the ‗Off‘ level occurs midway between the ‗On‘ signal peaks.

16. Observe the output of the summing amplifier on the ST2106 trainer at (TP36) Note

that it is the FSK waveform for the given data.

17. Adjust the ‗Gain‘ control of modulator 2, if necessary to make the amplitude the two

frequencies Common



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18. Display the FSK waveform simultaneously with NRZ (L) output. Observe that for data

bit '0'the FSK signal is at lower frequency (960KHz) & for data bit '1‘ the FSK signal

is at higher frequency (1.44 MHz).

19. Now, to study about demodulation, examine the input (TP16) and the output (TP17) of

ST2107FSK demodulator. The PLL detector has been used as the FSK demodulator

on this trainer.

20. Observe that the output voltage of the PLL detector is greater for higher incoming

frequency. Also, observe that for both incoming carrier frequencies, the demodulator's

output also contains component at that frequency

21. The unwanted frequency component is removed by passing it through the low pass

filter. On a dual trace oscilloscope examine the input (TP23) & (TP24) of ST2107

low pass filter simultaneously. Observe that the output contains no carrier frequency

components.

22. The rounded output of the low pass filter is removed by passing it through the data

squaring circuit. But prior to it, the BIAS level of the comparator I is to be adjusted to

a value until the output pulse width (TP47) is same as the NRZ (L) input (TP5) on

ST2106. For this purpose, display them simultaneously. Observe that the comparator

output is slightly delayed then NRZ L) input.

23. Turn ‗On‘ the pseudo-random sync generator. This locks the transmitter & receiver in

'frame synchronization. Therefore the data on A/D converter LEDs on ST2103 trainer

is same as that present on D/A converter LEDs on ST2104 trainer. You can examine

this fact by varying the data on transmitter trainer ST2103 by the DC1 pot variation.

24. To examine FSK modulation & demodulation for a time variant data/wave, connect

CH0 and CH1 input to ~1 KHz & ~2 KHz function generator outputs instead of

DC1/DC2 input.

25. Remember to connect/disconnect the links only with the trainers in switched off

position. Switch the oscilloscope for internal triggering. Check the reconstructed

waveform CH0 & CH1 outputs on ST2104 trainer. They should be identical to the

input waveforms. Remember, the two outputs are independent of each other & thus

interference free. Any interference if present can be removed by adjusting phase

generator delay adjusts control.

26. You can try experimenting FSK modulation / demodulation by using any other data

format. Equally, any of the other binary outputs from the Data Formatting Circuits



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can be used. But remember to reformat it to NRZ (L) waveform on ST2107 board,

after demodulation, before feeding it to the ST2104 trainer.

Observations

Figure 4 (a): FSK Waveform

Figure 4 (b): Generation of FSK Waveform from ASK WaveformFigure 4 (c):

Figure 4 (c): ASK Waveform

Figure 4 (d): ASK Wave form using Lower Frequency Carrier with Inverted Data

Stream



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Inference:

Hence we conclude generation of modulated and demodulated waveforms of Frequency

shift keying.


Q1. As the bit rate of an FSK signal increases, the bandwidth _______

a) Decreases b) Increases c) Remains the same d) Doubles

Q2. For FSK, as the difference between the two carrier frequencies increases, the

bandwidth ____________

a) Decreases b) Increases c) Remains the same d) Halves

Q3. Modulation of an analog signal can be accomplished through changing

the_______ of the carrier signal.

(a) Amplitude b) Frequency c) Phase d) Any of the above

Q4. A modulated signal is formed by

a) changing the modulating signal by the carrier wave

b) changing the carrier wave by the modulating signal

c) quantization of the source data

d) sampling at the Nyquist frequency used

Q5. ____________ gives maximum probability of error.

a) ASK b) BFSK c) BPSK d) DBPSK

Q6. Why is FSK suitable for HF radio applications?

Q7. Specify typical application of binary FSK

Q8. GMSK is better than conventional FSK. Justify

Q9. How are analog carrier modulation (AM, FM) connected conceptually with

digital modulation (ASK, FSK, PSK)?

Q10. What is the limiting factor for FSK modems for use over telephone lines?



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Experiment No. 3

Aim: To obtain modulated and demodulated waveform of phase shift keying

technique.





equivalent


equivalent



Theory:

Phase shift keying involves the phase change of the carrier sine wave

between ‗ and in accordance with the data stream to be transmitted. Phase shift

keying is also known as phase reversal keying (PRK). The PSK waveform for a given data

is as shown in Figure 1.

Figure 1: PSK Modulator

Functionally, the PSK modulator is very similar to the ASK modulator. Both

uses balanced modulator to multiply the carrier with the modulating signal. But in contrast

to ASK technique, the digital signal applied to the modulation input for PSK generation is

bipolar i.e. have equal positive and negative voltage levels. When the modulating input is



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positive the output of modulator is a sine wave in phase with the carrier input. Whereas for

the negative voltage levels, the output of modulator is a sine wave which is shifted out of

phase by 180 from the carrier input.

This happens because the carrier input is now multiplied by the negative constant level.

Thus the output in phase when a change in polarity of the modulating signal results shows

in Figure 1.

The unipolar-Bipolar converter converts the unipolar data stream to bipolar data. At

receiver, the square loop detector circuit is used to demodulate the transmitted PSK signal.

Functionally, the demodulator is as shown in Figure 2.

Figure 2: PSK Demodulator

The incoming PSK signal with 0 & 180 phase changes is first fed to the

signal squarer, which multiplies the input signal by itself. The output of this block is a

signal of twice the frequency with the frequency of the output doubled; the 0 &

180 phase changes are reflect as 0 & 360 phase changes. Since phase change of 360 is

same as 0 phase change, it can be said that the signal squarer simply removes the phase

transitions from the original PSK waveform. The PLL block locks to the frequency of the

signal square output & produces a clean square wave output of same frequency. To derive

the square wave of same frequency as the incoming PSK signal, the PLL's output is

divided by two in frequency domain is the divided by 2 circuit. The following phase adjust

circuit allows the phase of the digital signal to be adjusted with respect to the input PSK

signal. Also its output controls the closing of an analog switch. When the output is high

the switch closes & the original PSK signal is switched through the detector. When the

phases adjust block's output is low, the switch opens & the detector's output falls to 0

Volts. The demodulator output contains positive half cycles when the PSK input has one

phase & only negative half cycles when the PSK input has another phase. The phase



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adjust potentiometer is adjusted properly. The average level information of the

demodulator output which contains the digital data information is extracted by the

following low pass filter. The low pass filter output is too rounded to be used for digital

processing. Therefore it is 'Squared Up' by a voltage comparator.

Since the sine wave is symmetrical, the receiver has no way of detecting whether

the incoming phase of the signal is 0 This phase ambiguity create two different

possibilities for the receiver output i.e. the final data stream can be either the original data

stream or its inverse.

This phase ambiguity can be corrected by applying some data conditioning to

the incoming stream to convert it to a form which recognizes the logic levels by changes

that occur & not by the absolute value. One such code is NRZ (M) where a change or the

absence of change conveys the information. A change in level represents data '1' & no

change represents data '0'. This NRZ (M) waveform is used to change the phase at the

modulator. The comparator output at receiver can again be of two forms, one being the

logical inverse of the other. But now it is not the absolute value in which we are interested.

Now the receiver simply locks for changes in levels, a level change representing a '1' and

no level changes representing a '0' thus the phase ambiguity problem does not makes

difference any more. This is known as differential phase shift keying.From the differential

bit decoder output is a data '1' when it encounters a level change & a '0' when no change

occurs. Thus the output from the differential bit decoder is a NRZ (L) waveform. Figure 3

shows the functional block diagram of the PSK receiver.

Figure 3: PSK Receiver System



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Procedure:


















oscilloscope.

Set to negative edge triggered mode in oscilloscope. It may be necessary to adjust the

trigger level manually to obtain a stable waveform.





multiplexed channels. Thus we are able to get the same data stream for any time frame.




converter

12. Observe the data clock output at TP4 on ST2106 trainer's data format block with





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the oscilloscope. The NRZ (L) waveform must be identical to ASK.

14. Switch off the power. Make the connections as given in Experimental Set-up Diagram

Figure 4.

a. On ST2106 trainer :

i) Carrier input of modulator 1 (TP26) to 960KHz (1) carrier (TP17)

ii) NRZ (M) output (TP6) to unipolar-bipolar converter input (TP20)

iii) Unipolar-bipolar converter output (TP21) to modulator 1 input (TP27)

b. Between ST2106 & ST2107 trainers :

i.) Modulator 1 output (TP28) to PSK demodulator input (TP10)

c. On ST2107 trainer :

i.) PSK demodulator output (TP15 to low pass filter 1 input (TP23)

ii.) Low pass filter 1 output (TP24) to comparator 1 input (TP46)

iii.) Comparator 1 output (TP47) bit decoder input (TP39)

d. Between ST2107 and ST2104 trainers :

i.) Bit decoder output (TP40) to PCM data input (TP1)

ii.) Bit decoder input (TP39) to clock regeneration circuit input (TP3)

iii.) Bit decoder clock input (TP41) to clock regeneration circuit output (TP8)

e. On ST2104 trainers :

i.) Clock regeneration circuit output (TP8) to clock input (TP46)

15. Switch on the trainers.

16. Monitor the modulator 1 output (TP28) in ST2106 trainer with reference to its input

(TP27) by using a dual trace oscilloscope. The three controls in modulator block may

require some setting.

a. Gain: This controls the amplitude of the modulator output signal. Vary it until the

amplitude of the output is 2Vpp

b. Modulation offset: This controls the peak to peak amplitudes of 0 phases

relative to each other. Vary it till the amplitudes for both faces become equal.

c. Carrier offset: This control the DC offsets of two phases namely 0 &1 phases,

relative to each other. Vary the control till the DC off set for them is reduced to as

close as zero volts.

displaying the NRZ (M) input with the PSK modulated waveform helps to

understand the PSKmodulation concept. Notice that every time the NRZ (M)



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waveform level changes, PSK modulated waveform undergoes an 18 phase

change.

17. To see the PSK demodulation process, examine the input of PSK demodulator (TP10)

on ST2107 trainer with the demodulator's output (TP15). Adjust the phase adjust

control & see its effect on the demodulator's output. Check the various test points

provided at the output of the Demodulation technique.

18. The output of the demodulator goes to the low pass filter 1's input. Monitor the filter's

output (TP24) with reference to its input (TP28). Notice that the filter has extracted the

average information from the demodulator output. Adjust the PSK demodulator's phase

adjust control until the amplitude of filter's output is maximum.

19. The low pass filter's output is rounded & cannot be used for digital processing. In

order to square up the waveform, comparators are used (data squaring circuit). The

bias control is adjusted so that the comparator‘s output pulse width at TP47 is same as

the NRZ (M) pulse width.

20. Switch on the pseudo-random sync code generator. Notice that the data on A/D

converter LEDs is same as that of the D/A converter LEDs. You can verify this by

varying the DC l pot on ST2103trainer. The data on ST2104 trainer always copy the

ST2103 trainer's data output. This proves that the transmitter & receiver are now in

frame synchronization.

21. Switch off the trainers. Select internal triggering mode for the oscilloscope disconnect

the CH 0& CH 1 inputs on ST2103 trainer instead connect CH 0 to1 KHz signal &

CH l to KHZ 2 signal.

22. Switch on the power. Notice the outputs CH0 & CH1 on ST2104 trainers. They are

replica of the input signals from ST2103 trainers. Also notice that they are

independent of each other (i.e. free interference) & variation in one does not affect the

other. If some interference is present, it can be removed adjust on ST2104 trainer.

23. Perform the same experiment with NRZ (L) data and observe the phase ambiguity of

the detector.

24. The same experiment can be done by using biphase (Mark) code. Just remember to

reformat the signal as described in biphase (Mark) code.

25. The carrier frequency used in the above experiment was 960KHz. 1.44 MHz carrier

can well be used. The only change to be made is to put the PSK demodulator switch in

1.44 MHz position.



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Observations



Inference:

Hence we conclude generation of modulated and demodulated waveforms of Phase

shift keying.


Q1. In a dibit system, the symbol rate or baud rate is _________ the bit rate.

a) Half b) same as c) double d) four times

Q2. ________ type of digital modulation technique is used for high-speed telephone

modems.

a) QAM b) GMSK c) QPSK d) 8-PSK

Q3. __________________ parameters of an analog sine-wave carrier signal can be

modulated by digital data information, leading to the most common basic types

of digital modulation technique.

a) Amplitude b) frequency c) phase d) any one of the above

DPSK Waveform

PSK Waveform

Bipolar Data Stream

Bipolar Data Stream



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Q4. What is the upper limit for the data rate using an 8-PSK modulator over a

conventional telephone line having bandwidth specified as 300 Hz – 3000 Hz?

a) 2700 bps b) 5400 bps c) 8100 bps d) 21600 bps

Q5. What is the limitation imposed on PSK modems?

a) Phase shifts of -10° b) Phase shifts of -22.5° c) Phase shifts of -45°

d) Phase shifts of

Q6. How is PSK different from conventional phase modulation?

Q7. What is the principle of coherent detection of digital signal?

Q8. How does a DBPSK receiver decode the incoming symbols into binary bits?

Q9. State the reason as why the error performance of M-ary PSK demodulator is poor?

Q10. What is meant by differential binary PSK?



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Experiment No. 4

Aim: To study constellation diagram of QPSK technique and power spectrum

density (PSD) waveforms


1. Advanced Digital Communication, Falcon DCS-02 or equivalent

2. Multiple Power Supply, Falcon make or equivalent.

3. Digital Storage Oscilloscope Tektronix TDS1002 or equivalent.

4. Connecting wires.

THEORY:

QPSK sometimes known as quaternary or quadriphase PSK, 4-PSK, or 4-QAM,

QPSK uses four points on the constellation diagram, equi-spaced around a circle. With

four phases, QPSK can encode two bits per symbol, shown in the diagram with Gray

coding to minimize the BER — twice the rate of BPSK. Analysis shows that this may be

used either to double the data rate compared to a BPSK system while maintaining the

bandwidth of the signal or to maintain the data-rate of BPSK but halve the bandwidth

needed. As with BPSK, there are phase ambiguity problems at the receiver and

differentially encoded QPSK is used more often in practice Implementation The

implementation of QPSK is more general than that of BPSK and also indicates the

implementation of higher-order PSK. Writing the symbols in the constellation diagram in

terms of the sine and cosine waves used to transmit them:

Si (t) = 4122cosE2 s Tf

T c 4,3,2,1

This yields the four phase‘s π/4, 3π/4, 5π/4 and 7π/4 as needed.

This results in a two-dimensional signal space with unit basis functions

tfT

t cs

2cos21

cts

ft

t 2sin22

The first basis function is used as the in-phase component of the signal and the second as

the quadrature component of the signal. Hence, the signal constellation consists of the

signal-space points



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2,

2ss EE

The factors of 1/2 indicate that the total power is split equally between the two

carriers. Comparing this basis function with that for BPSK shows clearly how QPSK can

be viewed as two independent BPSK signals. oE QPSK systems can be implemented in

a number of ways. An illustration of the major components of the transmitter and receiver

structure is shown below Figure 1.

Figure 1: QPSK Generator

Conceptual transmitter structure for QPSK The binary data stream is split into

the in-phase and quadrature-phase components. These are then separately modulated onto

two orthogonal basis functions. In this implementation, two sinusoids are used.

Afterwards, the two signals are superimposed, and the resulting signal is the QPSK signal.

Note the use of polar non-return-to-zero encoding. These encoders can be placed before

for binary data source, but have been placed after to illustrate the conceptual difference

between digital and analog signals involved with digital modulation.

Figure 2: QPSK Demodulation



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Receiver structure for QPSK the matched filters can be replaced with correlators. Each

detection device uses a reference threshold value to determine whether a 1 or 0 is detected.

Bit error rate Although QPSK can be viewed as a quaternary modulation, it is easier to

see it as two independently modulated quadrature carriers. With this interpretation, the

even (or odd) bits are used to modulate the in-phase component of the carrier, while the

odd (or even) bits are used to modulate the quadrature-phase component of the carrier.

BPSK is used on both carriers and they can be independently demodulated. As a result, the

probability of bit-error for QPSK is the same as for BPSK.

EXPERIMENT SET-UP DIAGRAMS:


PROCEDURE:

1. Do the Connections as per block diagram shown Figure 3

2. Connect the power supply to the kit and switch it ON.



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3. Select the QPSK Experiment using SW1. Observe the corresponding LED indication.

4. Select 8 bit data pattern for modulation using SW6. Observe the corresponding LED

indication.

5. Set the data pattern as shown in block diagram using SW3.Odserve the 8 bit serial data

a SRIAL DATA post.

6. Observe the carrier used for modulation at SIN 1, SIN 2, SIN3 and SIN4 posts.

7. Connect SERIAL DATA to DATA IN 2 of DIBIT ENCODER section. Observe the

Clock post. Compare the dibit encoded data at EVEN and ODD post with EVEN CLK

and ODD CLK respectively.

8. Connect EVEN and ODD post to C1 and C2 respectively in CARRIER

MODULATOR post

9. Observe QPSK modulated signal at MOD OUT post of CARRIER MODULATOR.

Compare modulated signal with EVEN and ODD of DIBIT ENCODER.

10. To Demodulate the QPSK signal Connect MOD OUT to IN23 in QPSK

DEMODULATOR

Observe the demodulated EVEN and ODD signal at test point provided.

11. Observe the decoded signal at OUT 22 of QPSK DEMODULATOR and Compare it

with original signal i.e. with signal at SERIAL DATA.

12. To observe the constellation diagram connect X and Y test points from

CONSTELLATION OUTPUT section to X and Y channel OF CRO. Keep CRO in

XY mode.

13. NOTE: observe the intermediate signals during QPSK demodulation at TP1. Change

the jumper position to serve other signal.

OBSERVATION:

1. Input Data at SERIAL DATA.

2. Carrier frequency SIN 1 to SIN 4

3 Dibit Clock EVEN CLK and ODD CLK

4 Dibit generated data I bit & Q bit at DIBIT ENCODER.

5. QPSK modulated signal at MOD OUT.

6 Intermediate signal during demodulation at TP1.

7. Recovered data bits (Even & Odd bits) at EVEN and ODD test points.

8 Recovered data from Even & Odd bits at OUT 22, the output of DATA DECODER.



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Observations

Figure 4(a): QPSK MODULATION



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QPSK DEMODULATION

Figure 4(b): QPSK DEMODULATION



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Inference:

In BPSK we deal individually with each bit of duration Tb. In QPSK we lump two

bits together to form a SYMBOL. The symbol can have any one of four possible

values corresponding to two-bit sequence 00, 01, 10, and 11. We therefore arrange to

make available for transmission four distinct signals. At the receiver each signal

represents one symbol and, correspondingly, two bits. When bits are transmitted, as

in BPSK, the signal changes occur at the bit rate. When symbols are transmitted the

changes occur at the symbol rate which is one-half the bit rate. Thus the symbol time


The candidates should paste the observed waveforms here..


Q1._____ gives minimum probability of error.

(a) ASK (b) BFSK (c) BPSK (d) DBPSK

Q2._____type of digital modulation technique allows more bits per symbol and, therefore,

greater speed in a given bandwidth than other digital modulation techniques.

(a) QAM (b) OQPSK (c) π/4-QPSK (d) 16-PSK

Q3. How many bits are grouped to form a QPSK symbol?

(a) 2 bits per symbol (b) 3 bits per symbol (c) 4 bits per symbol (d) 6 bits per symbol

Q4. The phase angle difference between symbols for a QPSK modulator is

(a) 0° (b) 45° (c) 90° (d) 180°

Q5. The symbol rate for a 2100-bps data stream is _______ using a QPSK modulator.

(a) 4200 sps (b) 2100 sps (c) 1050 sps (d) 525 sps

Q6.Why coherent QPSK or OQPSK systems are not employed in mobile communications

application?

Q7. How can the coherent detection problem be overcome in QPSK?

Q8. How is OQPSK derived from conventional QPSK?

Q9. What is the reason for higher probability of errors in QPSK?

Q10. What causes OQPSK to exhibit minimum probability of errors?



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Experiment No. 5

Aim: To study the constellation diagram of MSK technique and compute

transmission power and bandwidth from its PSD wave forms.


5. Advanced Digital Communication, Falcon DCS-02 or equivalent


7. Digital Storage Oscilloscope Tektronix TDS1002 or equivalent.


Theory:

In digital modulation, minimum-shift keying (MSK) is a type of continuous-

phase frequency –shift keying that was developed in the late 1960s. Similar to OQPSK,

MSK is encoded with bits alternating between quaternary components, with the Q

component delayed by half the symbol period However, instead of square pulses as

OQPSK uses, MSK encodes each bit as a half sinusoidal. This results in a constant

modulated signal, which reduces problems caused by non-linear distortion. In addition to

being viewed as related to OQPSK, MSK can also be viewed as a continuous phase

frequency shift keyed (CPFSK) signal with a frequency separation of one-half the bit rate.

Minimum Shift Keying, MSK is a form of FSK (frequency shift keying) or phase

shift keying (PSK), MSK uses changes in phase to represent 0's and 1's, with the phase

shift used depending on the previous phase value. MSK acts like FSK with minimum

difference between the frequencies of the two FSK signals, resulting in a power spectral

density that falls off much faster than QPSK (Quadrature phase shift keying) so MSK can

operate in a smaller radio bandwidth than QPSK. GSM uses a variant of MSK and QPSK,

GMSK (Gaussian MSK)

Minimum frequency-shift keying or minimum-shift keying (MSK) is a

particularly spectrally efficient form of coherent FSK. In MSK the difference between the

higher and lower frequency is identical to half the bit rate. Consequently, the waveforms

used to represent a 0 and 1 bit differ by exactly half a carrier period. This is the smallest

FSK modulation index that can be chosen such that the waveforms for 0 and 1 are

orthogonal. A variant of MSK called GMSK is used in the GSM mobile phone standard.



Page | 34



POCEDURE:

1. Do the Connections as per block diagram shown Figure 1.

2. Connect the power supply to the kit and switch it ON.

3. Select the MSK Experiment using SW1. Observe the corresponding LED indication.

4. Select 8 bit data pattern for modulation using SW6. Observe the corresponding LED

indication.

5. Set the data pattern as shown in block diagram using SW3.Odserve the 8 bit serial data

at SERIAL DATA post.

6. Observe the carrier used for modulation at SIN 1 and SIN2 posts.

7. Connect SERIAL DATA to C1 in CARRIER MODULATOR post.

8. Observe MSK modulated signal at MOD OUT post of CARRIER MODULATOR.

9. To Demodulate the MSK signal Connect MOD OUT to IN22 in MSK

DEMODULATOR section.



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10. Observe the demodulated signal at OUT 21 of MSK DECODER and Compare it with

original signal i.e. with signal at SERIAL DATA.

OBSERVATIONS:


2. Carrier frequency SIN 1 to SIN 2

3. MSK modulated signal at MOD OUT.

4. Recovered data at OUT 21 post of MSK demodulator.

Observation Waveforms: -- The candidates should paste the observed waveforms here

1. CH-1: CLK2 CH-2: SERIAL

DATA

Figure 2(a):

2. CH-1: SERIAL DATA CH-2: EVEN

Figure 2(b):

3. CH-1: SERIAL DATA CH-2: ODD

Figure 2(c)

4. CH-1: ODD CH-2: MOD OUT

Figure2(d):



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5. CH-1: SERIAL DATA CH-2: OUT 21

Figure 2(e):



Inference:

Hence we conclude generation of modulated and demodulated waveforms of minimum-

shift keying.


Q1. Size of MSK side lobes is always ______ as compared to side lobe is of QPSK.

a) small b) large c) equal d) medium

Q2. Main lobe for MSK is_________ as compared to side lobe is of QPSK.

a) small b) large c) equal d) medium

Q3. What is the band width of MSK?

a) 1.5fb b) 2fb c) 5fb d) 3fb

Q4. The main lobe of MSK PSD is smaller than ___________ .

a) MSK b) PSK c) QPSK d) OQPSK

Q5. What is the operating principle of MSK?

Q6. Why MSK is called ―shaped QPSK‖?

Q7. What is disadvantage of MSK?

Q8. What is the difference between FSK and MSK?

Q9. What is difference between OQPSK and MSK?



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Experiment No. 6

Aim: GMSK modulation and demodulation.

Apparatus/Test Equipment:

1. Advanced Digital Communication, Falcon ADCL03 or equivalent


3. Digital Storage Oscilloscope Tektronix TDS1002.


Theory:

Prior to discussing GMSK in detail we need to review MSK, from which

GMSK is derived. MSK is a continuous phase modulation scheme where the modulated

carrier contains no phase discontinuities and frequency changes occur at the carrier zero

crossings. MSK is unique due to the relationship between the frequency of a logical zero

and one: the difference between the frequency of a logical zero and a logical one is always

equal to half the data rate. In other words, the modulation index is 0.5 for MSK, and is

defined as

m = f x T

Where, f = |f logic 1 – f logic 0|

T = 1/bit rate

For example, a 1200 bit per second baseband MSK data signal could be composed of 1200

Hz and 1800 Hz frequencies for a logical one and zero respectively (see Figure 1).

Figure 1: 1200 baud MSK data signal; a) NRZ data, b) MSK signal.

Baseband MSK, as shown in Figure 1, is a robust means of transmitting data in

wireless systems where the data rate is relatively low compared to the channel BW. MX-

COM devices such as the MX429 and MX469 are single chip solutions for baseband MSK

systems, incorporating modulation and demodulation circuitry on a single chip. An

alternative method for generating MSK modulation can be realized by directly injecting

NRZ data into a frequency modulator with its modulation index set for 0.5 (see Figure 2).



Page | 38

This approach is essentially equivalent to baseband MSK. However, in the direct approach

the VCO is part of the RF/IF section, whereas in baseband MSK the voltage to frequency

conversion takes place at baseband.

Figure 2: Direct MSK modulation

The fundamental problem with MSK is that the spectrum is not compact enough to

realize data rates approaching the RF channel BW. A plot of the spectrum for MSK

reveals side lobes extending well above the data rate (see Figure 4). For wireless data

transmission systems which require more efficient use of the RF channel BW, it is

necessary to reduce the energy of the MSK upper side lobes. Earlier we stated that a

straightforward means of reducing this energy is low pass filtering the data stream prior to

presenting it to the modulator (pre-modulation filtering). The pre-modulation low pass

filter must have a narrow BW with a sharp cutoff frequency and very little overshoot in its

impulse response. This is where the Gaussian filter characteristic comes in. It has an

impulse response characterized by a classical Gaussian distribution (bell shaped curve), as

shown in Figure 3. Notice the absence of overshoot or ringing.

Time-Domain Response

Figure 3: Gausssian filter impulse response for BT = 0.3 and BT = 0.5



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Figure 3 depicts the impulse response of a Gaussian filter for BT = 0.3 and 0.5. BT is

related to the filters - 3dB BW and data rate by

Hence, for a data rate of 9.6 kbps and a BT of 0.3, the filter‘s -3dB cutoff

frequency is 2880Hz. Notice that a bit is spread over approximately 3 bit periods for

BT=0.3 and two bit periods for BT=0.5. This gives rise to a phenomenon called inter-

symbol interference (ISI). For BT=0.3 adjacent symbols or bits will interfere with each

other more than for BT=0.5. GMSK with BT=_ is equivalent to MSK. In other words,

MSK does not intentionally introduce ISI. Greater ISI allows the spectrum to be more

compact, making demodulation more difficult. Hence, spectral compactness is the primary

trade-off in going from MSK to Gaussian pre-modulation filtered MSK. Figure 4 displays

the normalized spectral densities for MSK and GMSK. Notice the reduced side lobe

energy for GMSK. Ultimately, this means channel spacing can be tighter for GMSK when

compared to MSK for the same adjacent channel interference.

Frequency Response

Figure 4: Spectral density for MSK and GMSK

Performance Measurements

The performance of a GMSK modem is generally quantified by measurement of the

signal-to-noise ratio (SNR) versus BER. SNR is related to Eb/N0 by



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GMSK MODULATOR

In ADCL-03for GMSK, Gaussian low pass filter has a BT product of 0.3 accordingly, for

data rate of 256 kbits/sec the filters -3.0dB Bandwidth is 76.8 kHz.

Filters -3dB cutoff = BT product× Bit rate

= 0.3×256e3

= 76.8e3

= 76.8 kHz

Modulation index is 0.5. The modulation index (µ) is the frequency deviation divided by

bit rate therefore:

Frequency deviation = µ ×bit rate

= 0.5×256 e3

= 0.128 e6

= 128 kHz

With a frequency deviation of 128 kHz, the RF carrier for a binary 0 will be offset - 64

kHz and a binary 1 will produce a + 64 kHz offset

GMSK MODULATOR

Figure 5: GMSK MODULATOR

Input NRZ sequence x(t) is fed to the integrator.



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The integrator integrates the NRZ sequence to give the phase plot.

(t) = x (t) d

To smooth out the phase transition, output of integrator is passed through the Gaussian

Filter which gives the continuous phase change.

Y (t) = h (t) * (t)

Th e I and Q components are obtained from cos LUT and sin LUT, using the Gaussian

Filter output as an address. This I and Q are the quadrature components of the baseband

GMSK equivalent signals.

I(t) = cos [y(t)]

Q (t) = sin [y (t)]

By multiplying the I and Q components with the corresponding cos(Fc) and –sin(Fc)

carriers the modulate I and Q signals are obtained.

By adding these two modulated signals GMSK modulated signal is obtained.

M (t) =I (t) cos (Fc) –Q (t) sin (Fc)

GMSK

DEMODULATOR

Figure 6: GMSK Demodulator Block Diagram

Modulated GMSK signal m (t) is fed to the GMSK Demodulator. Multipliers at

demodulator remove the phase from the modulated signal. Low pass filter bypass the

high frequency component and follow the shape of its input. The demodulated I and Q

are obtained at the output of low pass Filters.

Tan-1 block is used to recover the phase plot. I.e. Tan-1 [Tan y (t)] = y(t)

Where tan[y (t)] = dmod Qdmod I =

siny(t)cos[y(t)]

Derivator is used to recover the input NRZ signal.



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X (t) = d y(t)

dt

PROCEDURE

1. Refer the Kit Figure 5 and carry out the following connections.

2. Connect power supply in proper polarity to the kit ADCL-03 and switch it on.

3. Select a data pattern ―00011011‖ using switch SW1 of DATA GENRATOR block.

4. Connect SERIAL DATA generated on board to DATA IN of INTEGRATOR of

GMSK MODULATOR section.

5. Select a DAC selection jumper (j8) position as shown in kit to observe the respective

output of the GMSK modulator section. By the setting of jumper (J8) the respective

LED glow. When LED 8 and LED 9 glow then observe the output at Ɵ (t) and y1(t) test

points. When LED 10 and LED 11 glow then observe the output at I and Q test points.

6. Observe the GMSK modulated signal at GMSK MOD test point GMSK

MODULATOR section

7. Output of GMSK MODULATOR is connected to GMSK DEMODULATOR

internally. Observe the signal of various block of GMSK Demodulator at their

respective test points.

8. Observe various waveforms at respective test points as mentioned below.

OBSERVATION

Observe the following signal on the oscilloscope for the selected data pattern using the

steps given in the procedure of this experiment.

1. SERIAL DATA with respect to DATA CLK.

2. Output of INTEGRATOR at post Ɵ (t) with respect to SERIAL DATA.

3. Output of GAUSSIAN FILTER1 at post y1 (t) with respect to SERIAL DATA.

4. Y1 (t) with respect to Ɵ (t).

NOTE: - Integrator is implemented using up-down counter and it is working on 4

MHz clock. When SERIAL DATA is logic ‗1‘ it is incremented and for logic ‗0‘ it is

decremented. For the Data pattern ―00011011‖ integrator works smoothly For Data

pattern other than there will be possibility of over flow i.e. integrator output gives

sudden transition due to which glitches will be observed. (Therefore do the whole

experiment for Data pattern ―00011011‖ by observing each signal at its respective test

point.)

5. Output of COS block at post I and output of SIN block at post Q

6. Output of ADDER at post GMSK MOD with present to SERIAL DATA.



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7. Output of LPF block of GMSK DEMODULATOR section at DMOD I with respect to

output of COS block at post I.

8. Output of LPF block of GMSK MODULATOR section at post DMOD Q with respect

to output of SIN block at post Q.

9. Output of tan-1 at its test point with respect to output of Gaussian filter1 at post Y1 (t).

10. Output of DERIVATOR at post GMSK DMOD with respect to SERIAL DATA.

OBSERVED WAVE FORMS:

The candidates should paste the observed waveforms here



Page | 44

Inference:

In this experiment we have studied GMSK Modulation and Demodulation technique using

I and Q modulation whose modulation index is 0.5 and observe various waveforms.


Q1. ___ requires a high signal-to-noise ratio.

(a) MSK (b) GMSK (c) QAM (d) GFSK

Q2. They ____________________ are good at detecting and correcting random errors

such as errors occurring at random positions of the code word.

(a) CRC codes (b) Hamming codes (c) Parity codes (d) Block codes

Q3.The optimum coding of a digital communication channel is determined by

(a) probability of error, Pe (b) bandwidth and probability of error

(c) SNR and bandwidth (d) source entropy equal to channel capacity

Q4. Differentiate between bit rate and baud rate

Q5. What is GMSK modulation?

Q6. What is the essential features of GMSK digital modulation technique?

Q7. What is the significance of (BTb) product in GMSK?

Q8. Differentiate between GMSK and GFSK?

Q9. What is Gaussian filter?

Q10. What are applications of GMSK?

Q11. What is disadvantage in GMSK?



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Experiment No. 7

Aim: To study constellation diagram of QAM technique and its PSD waveform.


1. Advanced Digital Communication, Falcon DCS-02 or equivalent.




THEORY:

Quadrature amplitude modulation (QAM) is both an analog and a digital

modulation scheme. It conveys two analog message signals, or two digital bit streams, by

changing (modulating) the amplitudes of two carrier waves, using the amplitude-shift

keying (ASK) digital modulation scheme or amplitude modulation (AM) analog

modulation scheme. These two waves, usually sinusoids, are out of phase with each other

by 90° and are thus called quadrature carriers or quadrature components hence the name

of the scheme. The modulated waves are summed, and the resulting waveform is a

combination of both phase-shift keying (PSK) and amplitude shift keying (ASK), or in the

analog case of phase modulation (PM) and amplitude modulation. In the digital QAM

case, a finite number of at least two phases and at least two amplitudes are used. PSK

modulators are often designed using the QAM principle, but are not considered as QAM

since the amplitude of the modulated carrier signal is constant.

Ideal Structure Transmitter:

The following block diagram shows Figure 1 the ideal structure of a QAM

transmitter, with a carrier frequency f0 and the frequency response of the transmitter's

filter Ht:

Figure 1: QPSK Transmitter

First the flow of bits to be transmitted is split into two equal parts: this process generates

two independent signals to be transmitted. They are encoded separately just like they were



Page | 46

in an amplitude- shift keying (ASK) modulator. Then one channel

(the one "in phase") is multiplied by a cosine, while the other channel (in "quadrature") is

multiplied by a sine. This way there is a phase of 90° between them. They are simply

added one to the other and sent through the real channel. Thesent signal can be expressed

in the form:

Where Vc[n] and Vs[n] are the voltages applied in response to the nth symbol to the

cosine and sine waves respectively. The receiver simply performs the inverse process of

the transmitter. It‘s ideal structure is shown Figure 2 in the picture below with Hr the

receive filter's frequency response:

QPSK RECEIVER (Figure 2):

Figure 2: QPSK RECEIVER

Multiplying by a cosine (or a sine) and by a low-pass filter it is possible to extract the

component in phase (or in quadrature). Then there is only an ASK demodulator and the

two flows of data are merged back.

In practice, there is an unknown phase delay between the transmitter and receiver

that must be compensated by synchronization of the receiver‘s local oscillator, i.e. the sine

and cosine functions in the above figure. In mobile applications, there will often be an

offset in the relative frequency as well, due to the possible presence of a Doppler shift

proportional to the relative velocity of the transmitter and receiver. Both the phase and

frequency variations introduced by the channel must be compensated by properly tuning

the sine and cosine components, which requires a phase reference, and is typically

accomplished using a Phase-Locked Loop (PLL). In any application, the low-pass filter

will be within hr (t): here it was shown just to be clearer.



Page | 47



PROCEDURE:

1) Do the Connections as per block diagram shown.

2) Connect the power supply to the kit and switch it ON.

3) Select the QAM Experiment using SW1. Observe the corresponding LED indication.

4) Select 24 bit data pattern for modulation using SW6. Observe the corresponding LED

indication.

5) Set the data pattern as shown in block diagram using SW3, SW4 and SW5.Odserve the

24 bit serial data at SERIAL DATA post.

6) Observe the carrier used for modulation at SIN 1, SIN 2, SIN3 and SIN4 posts.

7) Connect SERIAL DATA to DATA IN 1 of TRIBIT ENCODER section. Observe the

Clock at CLK1 post. Compare the tri-bit encoded data at EVEN, ODD and C post

with EVEN CLK, ODD CLK and C CLK respectively.

8) Connect EVEN, ODD and C post to C1, C2 and C3 respectively in CARRIER

MODULATOR post.

9) Observe QAM modulated signal at MOD OUT post of CARRIER MODULATOR.

Compare modulated signal with EVEN, ODD and C of TRIBIT ENCODER.



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10) To Demodulate the QAM signal Connect MOD OUT to IN26 of QAM

DEMODULATOR section.

11) Observe the demodulated EVEN, ODD and C signal at test point provided.

12) Observe the decoded signal at OUT 25 of QAM DEMODULATOR and Compare it

with original signal i.e. with signal at SERIAL DATA.

13) To observe the constellation diagram connect X and Y test points from

CONSTELLATION OUTPUT section to X and Y channel OF CRO. Keep CRO in

XY mode.

NOTE: observe the intermediate signals during QPSK demodulation at TP1. Change

the jumper position to observe other signal.

OBSERVATION:


2. Tribit encoder data EVEN, ODD & C BIT at the output of Tribit encoder.

3. Tribit encoder clock at EVEN CLK, ODD CLK and C CLK.

4. Carrier signal SIN 1 to SIN 4.

5. QAM modulated signal at MOD OUT of carrier modulator.

6. Intermediate signal during demodulation at TP1.

7. Recovered data bits (Even, Odd and C bits) at EVEN, ODD and C test points.

8. Recovered data from Even, Odd and C bits at OUT 25, the output of DATA

DECODER.

Inference:

In this experiment we have studied QAM Modulation and Demodulation

technique using I and Q modulation whose modulation index is 0.5 and observe various

waveforms.



Page | 49

Observed Waveforms:

The candidates should paste the observed wave form



Page | 50


Q1.________ type of digital modulation technique is used for high-speed telephone

modems.

(a) QAM b) GMSK c) QPSK d) 8-PSK.

Q2.How many different symbols are possible at the output of a 16-QAM modulator?

a) 8b) 16 c) 64 d) 256

Q3.An ideal communication channel is defined for a system in which

Q4.Which speech signal formatting is used along with digital modulation?

Q5.What is multi-level phase and amplitude modulation technique?

Q6.Distinguish between coherent, non-coherent and differential-coherent detection.?

Q7.What do you understand by non-cohrent detection

Q8.What is significant advantages of channel coding?

Q9. What is QAM?

Q10. Why is amplitude change along with phase?



Page | 51

Experiment No. 8

Aim: to obtain DSSS modulated and demodulated digital pattern for given data

sequence.

Apparatus/Test Equipment’s:

1. CDMA Mobile Communication, Falcon CDMA-02 or equivalent.


3. Digital Storage Oscilloscopes Tektronix TDS1002 or equivalent,

4. Spectrum analyzer 9 kHz to 3 GHz, GWINSTEK GSP 830 or equivalent.


Theory:

Experimental Contents

a. Observe the waveform before and after baseband signal spread spectrum (spectrum).

b. Observe the waveform before and after PSK modulation (spectrum).

Experimental Principles

Spread spectrum communication system is to use a certain spread spectrum

function to expand the information which need to be transmitted and make it to be

wideband signal then send it to communication channel to transmit, utilize corresponding

method to decompress signals at receiver and gain transmission information. That is to say

RF bandwidth during transmitting same signals is much wider than all kinds of required

modulation method which we are very familiar with. Spread spectrum wideband is at least

tens of or tens of thousands of information wideband. Information will not be an important

factor to decide modulation signal‘s wideband. The wideband of modulation signal is

mainly decided by spread spectrum function.

This definition includes the following meanings in three aspects:

Signal spectrum is expanded. In normal communication, we always adopt

the signals which generally have the same wideband to transmit signals in order to

improve the utilization rate of frequency. That is to say, wideband of RF signal and the

wideband of submitted information are belong to a same magnitude in radio

communication, but the ratio of signal wideband in spread Spectrum communication to

communication wideband is high up to 100 ～1000, belongs to broadband

communication, the reason is to upgrade the anti-jamming ability, this is the basic thought

and theoretical basis of spread spectrum communication. The wider expended spectrum of

spread spectrum communication system is, the stronger ability it has to deal with the plus



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and anti-jamming. Spread signal spectrum with the method of spread spectrum code

sequence modulation. We know from signal theory that the more narrow pulse signal is,

the wider CDMA-02: CDMA Mobile Communication Trainer Kit - 30 - Wireless

Communication the spectrum is, the wideband of signal and the width of pulse is inverse

ratio approximately, so transmitted information modulated by the more narrow pulse, the

wider signal wideband it can generate. Spread spectrum code sequence is a kind of very

narrow pulse sequence. In the receiver, use the spread spectrum code sequence to de-

spread the same with the sending terminal. Spread spectrum technologies

Theory basis qualitative discuss is in the following points:

Firstly, the theory basis of spread spectrum technology can be described by

Shannon channel capacity formulary: log (1 /) 2 C =W + S N .In the formulary: C is

channel capacity, W is system transmission wideband S/N is Signal-to-Noise in system

transmission.

The formulary indicate that when the Signal-to-Noise in transmission system

reduces in Gaussian channel, can enlarge the wideband W of system transmission to keep

channel capacity C stable. For the given arbitrarily Signal-to-Noise, can enlarge

transmission wideband to gain lower error rate of information. Spread spectrum

technology is using this principle, use high speed spread spectrum code to achieve the goal

that spreader the digital signal information which is being transmitted. So under the same

Signal-to-Noise condition, it has stronger ability of anti-noise interference. Shannon

indicates that: Under gauss noise interference, the signal realizing effective and reliable

channel in the channel which limits the average power is the signal which has white noise

statistical characteristics. Presently, people find some pseudo-random sequence‘s

statistical characteristics approximate to gauss white noise statistical characteristics. Used

in spread spectrum system, it can make the signal to be transmitted approximate to the

best signal format which gauss channel requires. In the early 1950‘s, Russian scientist

proved in theory: conquer multipath fading interference, the best signal form to be

transmitted in the channel should also be with white noise statistical characteristics. As

spread spectrum function approximate to white noise statistical characteristics, the spread

spectrum communications have the ability to conquer multipath fading interference.

The ways we usually use to spread spectrum are in below:



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1.Direct Series Spread Spectrum CDMA（DS-CDMA）: use information which is to be

transmitted multiply high speed pseudo-random code sequence to control the parameter of

one certain radio frequency signal to realize spreading spectrum.

2. Frequency hopping spread spectrum CDMAFH-CDMA after adding Module Two of

digital signal binary pseudo-random code sequence, subtract discretely control radio

frequency carrier oscillator‘s output frequency to make the emission signal‘s frequency

jump with the change of pseudo-random code.

3. Time-hopping spread-spectrum CDMA（TH-CDMA）: Time-hopping is to start and

close the sending time and duration of signals with pseudo-random code sequence. Yes or

No of the sending signal is pseudo-random the same with pseudo-random CDMA-02:

CDMA Mobile Communication Trainer Kit - 31 - Wireless Communication code

sequence.

4. Hybrid way: Combined with two or three of the above basic spread spectrum methods

to form some mixed spread spectrum system, such as FH/DS, DS/TH, FH/TH, etc. In this

experiment, we are using Direct Series spread spectrum.

Figure 1 and Figure 2 are the spectrums of PSK signal before and after spreading

spectrum.

PSK signal’s spectrum after spreading spectrum (Figure 1):

Figure 1: PSK signal’s spectrum before spreading spectrum.



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Figure 2: PSK signal’s spectrum after spreading spectrum.

Via comparison, we can find that PSK signal‘s spectrum has been widely broadened.

CDMA-02: Figure 3 is the diagram of direct series spread spectrum:

Figure 3: Direct Sequence Spread Spectrum

The process of direct series spread spectrum communication is multiply information code

element which to be transmitted and pseudo-random sequence, convoluting the spectrum

of the two in frequency domain, spreading signal‘s spectrum, spreader spectrum showed

narrowband gauss characteristic, then send it out after carrier wave modulation, in this

way we get carrier wave signal which is modulated by information code element, then do

carrier wave synchronization, after demodulation we can get information code element.



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We adopt ―Spread Spectrum Plus‖ GP ‗s conception to describe the good or bad of anti-

jamming ability of spread spectrum system, its definition is the ratio for output Signal-to-

Noise of dispreading receiver and input Signal-to-Noise, that is It shows after spread

spectrum, in the meantime of enhance the signal; restrain the ability of interference signal

which is inputted into receiver. The bigger the ratio is, the stronger the anti-jamming

ability is. In direct series spread spectrum system, spread spectrum plus GP:

From the above formula, we can see that increase the speed of spread spectrum or reduce

the speed of information code can both improve spread spectrum plus.

Procedures

1. Equipped transmitting antenna and receiving antenna.

2. Plug in; turn on the AC switch in the right side of the main case. Press switch

POWER100, POWER101, POWER200 and POWER201, corresponding LED

LED100, LED101, LED200 and LED201 emit light, the transmitter and receiver of

CDMA system start to work.

3. Dial all the dial-bit switches ―signal code speed‖, ―spread-spectrum code rate‖, ―spread

spectrum‖, ―coding‖ of the transmitter, dial all the dial-bit switches of receiver ―signal

code speed‖, ―spread-spectrum code speed rate‖, ―track‖, ―decode‖. Now the signal

code speed rate of the system is 1Kbit/s, spread-spectrum code speed is 100Kbit/s.

4. Experiment of observing the change of waveform (spectrum) before and after baseband

signal‘s

a) spectrum. Setting ―SIGN1 Position‖ as not all 0 or all 1 code word, setting ―GOLD1

Position‖

b) Observing the waveforms of ―SIGN1‖ and ―S1-KP‖ with oscilloscope.

c) (Select) Use oscilloscopes which have FFT function to observe spectrums of

―SIGN1‖ and―S1-KP‖, and do the comparison.

d) Change signal code speed and spread-spectrum speed of transmitter, repeat the

above steps.

5. (Select) Experiment of observing PSK modulation spectrum before and after spread-

spectrum.

a) Do not change code word setting, dial ―spread spectrum‖ switch, and observe the

spectrum of ―PSK1‖ with spectrum analyzer.

b) Set ―spread spectrum‖ switch up, observe the spectrum of ―PSK1‖, and compare

with the result in



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c) Change digital code speed and spread spectrum speed of transmitter separately,

repeat the above steps.

Explanation: after the change of dial-bit switch, press reset key.

6. De-spread experiment

a) Reset dial-bit switch to the position as experiment step 3 required, presses

―Transmitter Reset‖

b) Joint ―First Channel‖ of dial-bit switch, disconnection the ―Second Channel‖ of dial-

bit switch, now transmitter output GOLD1 is the first channel spread-spectrum signal of

spread-spectrum code.

c) Turn dial-bit switch ―GOLD3 Position‖ to the same position with ―GOLD1

Position‖, press ―Transmitter Reset‖ button.

d) Follow the step 8 ～11 of experiment two, adjust ―capture‖ and ―track‖ button,

making the receiver and transmitter GOLD code complete consistency, now output in

―TX2‖ is the PSK signal after de-spread.

e) Use oscilloscope dual-trace to observe the waveforms in ―SIGN1‖ and ―TX2‖.

f) (Select) Use spectrum analyzer to observe the spectrum in ―TX2‖, comparing the

result with step 5 of experiment.

7. Thinking Questions

1. When ―SIGN1 Setting‖ dial switches be set to all 1 or all 0, compare the signal‘s

waveforms of ―S1-KP‖ and ―GOLD1‖, analyze the reasons of those phenomenon.

2. Presently what kind of spread spectrum method does commercial CDMA system adopt?

3. Change transmitter‘s signal code speed and spread spectrum code speed separately in

step 5.3, what kind of changes occurs on spectrums before and after spread spectrum,

and what kind of problems are explained?

Inference: Observe the waveform before and after baseband signal spread spectrum and

observe the waveform before and after PSK modulation (spectrum).

Observed Waveforms:




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Q1. The ________ technique expands the bandwidth of a signal by replacing each data

bit with n bits.

a) FDM b) DSSS c) FHSS d) TDM

Q2. In _______, we combine signals from different sources to fit into a larger bandwidth.

Q3. ____ is designed to be used in wireless applications in which stations must be able to

share the medium without interception by an eavesdropper and without being subject

to jamming from a malicious intruder.

a) Spread spectrum b) Multiplexing c) Modulation d) none of the above

Q4. The _______ technique uses M different carrier frequencies that are modulated by the

source signal. At one moment, the sign modulates one carrier frequency; at the next

moment, the signal modulates another carrier frequency.

a) FDM b) DSSS c) FHSS d) TDM

Q5. In general, Spread Spectrum communications is distinguished by which of these

elements?

a. The signal occupies a bandwidth much greater than that which is necessary to send

the information

b. The bandwidth is spread by means of a code which is independent of the data

c. The receiver synchronizes to the code to recover the data

Q6.Which of this is the way to spread the bandwidth of the signal?

a. Frequency hopping

b. Time hopping

c. Direct sequence

d. All of the above

Q7.The digital data is directly coded at a much higher frequency. The code is generated

pseudo-randomly, the receiver knows how to generate the same code, and correlates

the received signal with that code to extract the data. Which type of spread spectrum

is this?


b. Time hopping

c. Direct sequence

d None of the above



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Q8.The signal is transmitted in short bursts pseudo-randomly, and the receiver knows

before hand when to expect the burst. Which type of spread spectrum is this?


b. Time hopping

c. Direct Sequence

d. None of the above

Q9.What is the data rate for Adaptive Differential Pulse Code Modulation (ADPCM)?

a 8 Kbits/sec

b.16 Kbits/sec

c. 32 Kbits/sec

d. 64 Kbits/sec

Q10.CDMA technology is inherently resistant to

a. interference

b. jamming

c. Both 1 & 2

d. None of the above



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Experiment No. 9

Aim: Study of Convolution Encoding and Hard Decision Viterbi Decoding and

Rate=1/2.


1. Convolution Encoding and Hard Decision Viterbi Decoding kit, Falcon ADCL06.

2. 9 Volt power Supply.



Theory:

A convolution code works by adding some structured redundant information

to the user's data and then correcting errors using this information. A convolution encoder

is a linear system. A binary convolution encoder can be represented as a shift register. The

outputs of the encoder are modulo 2 sums of the values in the certain register's cells. The

input to the encoder is either the uuencoded sequence (for non-recursive codes) or the

uuencoded sequence added with the values of some register's cells (for recursive codes).

As per the standard the generator polynomials for K = 7, R = 1/2 areG0(x) =

1+x2+x3+x5+x6

G1(x) = 1+x+x2+x3+x6 i.e. G0(x) = 133(octal) & G1(x) = 171(octal).

The implementation depicted below and is used in conjunction with an

R=1/2, K=7 Hard Decision Viterbi Decoder. The intent of this experiment is to help

clarify the terms used to define the convolution encoding and Viterbi decoding as well as

to explain how convolution encoding and Hard decision Viterbi decoding takes place

theoretically and to observe and verify the results practically. We can approach the



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encoder in terms of its impulse response i.e. the response of the encoder to a single ―one‖

bit that moves through it. Consider the contents of the register in Fig B.

Register

contents

Branch Word

U1 U2

1000000 1 1

0100000 0 1

0010000 1 1

0001000 1 1

0000100 0 0

0000010 1 0

0000001 1 1

Input sequence: 1 0 0 0 0 0 0

Output sequence: 11 01 11 11 00 10 11

The output sequence for the input ―one‖ is called the impulse response of the encoder.

Then for the input sequence m = 1 1 1 1 1 1 1, the output may be found by the

superposition or the linear addition of the time shifted input ―impulses‖ as

Follows:

Observe that this is the same output obtained in Fig (d), demonstrating that convolution

codes are linear. It is from this property of generating the output by the linear addition of

time shifted impulses, or the convolution of the input sequence with the impulse response

of the encoder, that we derive the name convolutional encoder.



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An example of Convolution encoding technique:

Fig (e): Convolution Encoder (rate ½, K = 3)

The Fig (e) illustrates a (2, 1) convolution encoder with constraint length K = 3.There are

n = 2 modulo-2 adder; thus the code rate k/n = ½. At each input bit time, a bit is shifted

into the leftmost stage and the bits in the register are shifted one position to the right.

Next, the output switch samples the output of each modulo-2 adder (i.e. first the upper

adder, then the lower adder), thus forming the code symbol pair making up the branch

word associated with the bit just inputted. The sampling is repeated with for each inputted

bit. The choice of connection between the adders and the stages of the register gives rise to

the characteristics of the code. Any change in the choice of connections results in a

different code. The connections are, of course, not chosen or changed arbitrarily. The

problem of choosing connections having good distance properties is complicated and has

not been solved in general; however, good codes have been found with the help of

computer search for all constraint length less than 20. We can approach the encoder in

terms of its impulse response i.e. the response of the encoder to a single ―one‖ bit that

moves through it. Consider the contents of the register in

Fig.

Input sequence: 1 0 0



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Output sequence: 11 01 11

The output sequence for the input ―one‖ is called the impulse response of the encoder.

Then for the input sequence m = 1 1 0 1 1, the output may be found by the superposition

or the linear addition of the time shifted input ―impulses‖ as follows:

Hard decision Viterbi’s decoding of above example:

Before proceeding towards the hard decision viterbi decoding of above example letus

discuss about the state representation and state diagram of Convolutional Encoder.

State Representation and State diagram:

A convolution encoder belongs to a class of devices known as finite-state machines, which

the general name is given to machines that have a memory of past signals. The adjective

finite refers to the fact that there are only a finite number of unique states that the machine

can encounter. In the most general sense, the state consists of the smallest amount of

information that, together with a current input to the machine, can predict the output of the

machine. For rate 1/n convolutional encoder, the state is represented by, the rightmost K -

1 stages. Since the rightmost K - 1 stages of the register in the example of convolution

encoder represented by Fig (e) are 2. Hence there are 22 = 4 possible states for the

encoder. These states of the encoder are represented as a = 00, b = 10, c = 01, d = 11. State

diagram can be drawn from the state transition table. Before proceeding further let us

focus on how the state transition table is prepared.

State transition Table: For simplicity let us designate the registers of the convolution

encoder described By the Fig.

Since there are 4 states, there arise 4 cases for the input states.



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Case 1:

Input state is a. Since a is represented as a = 00 then m2 = 0 and register m3 = 0.

Now the register m1 can either be 0 or 1. If m1 = 0 then

Note that the content of the register m1 and m2 decide the output state, since m1=0, m2=0

the output state is a. Similarly if the input state is a and if m1 = 1 then

Case 2:

Similarly if the input state is b then using the steps mentioned in case 1

Case 3:

Similarly if the input state is c then using the steps mentioned in case 1



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Case 4:

Similarly if the input state is d then using the steps mentioned in case 1

Now using the above cases the state transition table can be prepared as:

State Diagram

One way to represent the simple encoder is with a state diagram; such a

representation for the encoder in Fig is shown in Fig (i). The states, shown in the boxes of

the diagram, represent the possible contents of the rightmost K – 1 stages of the register,

and the paths between the states represent the output branch words resulting from such

state transitions. The states of the register are designated a = 00, b = 10, c = 01, and d =

11; the diagram shown in Fig illustrates all the state transitions that are possible for the

encoder in Fig. There are only two transitions emanating from each state, corresponding to

the two possible input bits. Next to each path between states is written the output branch

word associated with the state transition. In drawing the path, we use the convention that a

solid line denotes a path associated with an input bit, one. Notice that it is not possible in a

single transition to move from a given state to any arbitrary state. As a consequence of

shifting-in one bit at a time, there are only two possible state transitions that the register

can make at each bit time. For example, if the present encoder state is 00, the only

possibilities for the state at the next shift are 00 or 10.



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Fig (i): Encoder state diagram (rate = ½, K = 3)

Trellis Diagram:

In drawing the trellis diagram, we use the same convention that we introduced

with the state diagram Figure 2– a solid line denotes the output generated by an input bit

zero, and a dashed line denotes the output generated by an input bit one. The nodes of the

trellis characterize the encoder states; the first row nodes correspond to the state a = 00,

the second and subsequent rows correspond to the states b = 10, c = 01 and d = 11. At

each unit of time, the trellis requires 2k-1 nodes to represent

The 2k-1 possible encoder states the trellis in our example assumes a fixed periodic

structure after trellis depth 3 is reached (at time t4). In the general case,

Figure 2

The fixed structure prevails after depth K is reached. At this point and there-

after, each of the states can be entered from either of two preceding states. Also, each of

the states can transition to one of two states. Of the two outgoing branches, one

corresponds to an input bit zero and the other corresponds to an input bit one. On Fig the

output branch words corresponding to the state transitions appear as labels on the trellis

branches.



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Hard decision Viterbi Convolution Decoding:

For simplicity, a BSC is assumed; thus Hamming distance is a proper distance

measure. The encoder for this example is shown in Fig and the encoder trellis diagram is

shown in Fig. A similar trellis can be used to represent the decoder, as shown in Fig 3

Figure 3: Decoder trellis diagram (rate ½, K = 3)

We start at time t1 in the 00 state. Since in this example there are only two possible

transitions leaving any state, not all the branches need to be shown initially. The full trellis

structure evolves after time after t3. The basic idea behind the decoding procedure can

best be understood by examining the encoder trellis in concert with the Fig (k) decoder

trellis. For the decoder trellis it is convenient at each time interval, to label each branch

with the hamming distance between the received code symbols and the branch word

corresponding to the same branch from the encoder trellis.

The example in Figure 4 shows a message sequence m, the corresponding

code sequence U, and a noise corrupted received sequence Z = 11 01 01 10 01. The

branch words seen on the encoder trellis branches characterize the encoder in Fig (e) and

are known a priori to both the encoder and decoder. These encoder branch words are the

code symbols that would be expected to come from the encoder output as a result of each

of the state transitions. The labels on the decoder trellis branches are accumulated by the

decoder on the fly. That is, as the code symbols are received, each branch of the decoder

trellis is labeled with a metric of similarity (Hamming distance) between the received code

symbols and each of the branch words for that time interval. From the received sequence

Z. shown in Fig (k), we see that the code symbols received at (following) time t1 are 11.

In order to label the decoder branches at (departing) time t1 with the appropriate

Hamming distance metric, we look at the Figure 4 encoder trellis. Here we see that a state

00 00 transition yields an output branch word of 00. But we received 11. Therefore, on the



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decoder trellis we labeled the 00 00 state transition with Hamming distance between them,

namely 2. Looking at the encoder trellis again, we see that a state 00 10 transition yields

an output branch word of 11, which corresponds exactly with the code symbols we

received at time t1. Therefore, on the decoder trellis, we label the state 00 10transition

with a Hamming distance of 0. In summary, the metric entered on a decoder trellis branch

represents the difference (distance) between what was received and what ―should have

been‖ received had the branch word associated with that branch been transmitted. In

effect, these metric describe a correlation like measure between a received branch word

and each of the candidate branch words. We continue labeling the decoder trellis branches

in this way as the symbols are received at each time t1. The decoding algorithm uses these

Hamming distance metrics to find the most likely (minimum distance) path through the

trellis.

Figure 4: Path metrics for two merging path

The basis of Viterbi decoding is the following observation: If any two paths in

the trellis merge to a single state, one of them can always be eliminated in the search for

an optimum path. For example, Fig shows two paths merging at time t5 to state 00. Let us

define the cumulative hamming path metric of a given path at time ti as the sum of the

branch Hamming distance metrics along that path up to time ti. In Fig the upper path has

metric 4; the lower has metric 1. The upper path cannot be a portion of the optimum path

because the lower path, which enters the same state, has a lower metric. This observation

holds because of the Markov nature of the encoder state: The present state summarizes the

encoder history in the sense that previous states cannot affect future states or future states

or future states or future output branches.

At each time ti there are 2K -1 states in the trellis, where K is the constraint length,

and each state can be entered by means of two paths. Viterbi decoding consists of

computing the metrics for the two paths entering each state and eliminating one of them.

This computation is done for each of the 2K – 1 states or nodes at time ti; then the decoder



Page | 72

moves to time ti + 1 and repeats the process. At a given time, the winning path metric for

each state is designated as the state metric for that state at that time. The first few steps in

our decoding example are as follows (see Fig). Assume that the input data sequence m,

code word U, and the received sequence Z are as shown in Fig. (This assumption is not

necessary in practice, but simplifies the explanation).At time t1 the received code symbols

are 11. From state 00 the only possible state transitions are to state 00 or state 10 as shown

(a) of Fig. State 00 00 transitions has branch metric 2; state 00 10 transition has branch

metric 0. At time t2 there two possible branches leaving each state as shown in (b) of Fig

the cumulative metrics of these branches are labeled state metrics Γa, Γb, Γc and Γd,

corresponding to the terminating state. At time t3 in (c) of Fig (m) there are again two

branches diverging from each stateare again two branches diverging from each state

As a result, there are two paths entering each state at time t4. One path entering

each state can be eliminated, namely, the one having the larger cumulative path metric. If

the metrics of two entering path be of equal value, one path is chosen for elimination by

using an arbitrary rule. The surviving path into each state is shown in (d) of Fig. At this



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point in the decoding process, there is only a single surviving path, termed the common

stem, between times t1 and t2. Therefore, the decoder can now decide that the state

transition which occurred between t1 and t2 was 00 10. Since this transition is produced

by an input bit one, the decoder outputs a one as the first decoded bit. Here we can see

how

At each succeeding step in the decoding process, there will always be two

possible paths entering each state; one of the two will be eliminated by comparing the path

metrics. (e) of Fig (m) shows the next step in the decoding process. Again at time t5 there

are two paths entering each state, and one of each pair can be eliminated. (f) of Fig (m)

shows the survivors at time t5 . Notice that in our example we cannot yet make a decision

on the second input data bit because there still are two paths leaving the state 10 node at

time t2. At time t6 (g) of Fig (m) we again see the pattern of remerging paths, and in (h) of

Fig (m) we see the survivors at time t6. Also in (h) of Fig (m) the decoder output one as

the second bit, corresponding to the single surviving path between t2 and t3 The decoder

continues in this way to advance deeper into the trellis and to make decisions on the input

data bits by eliminating all paths but one.

Pruning the trellis (as paths remerge) guarantees that there are never more paths than there

are states. For Example, verify that after each pruning in (b), (d), (f), (h) Fig (m) there are

4 paths. Compare this to attempting a ―brute force‖ maximum likelihood sequence



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estimation without using the Viterbi algorithm. In that case, the number of possible paths

(representing possible sequences) is an exponential function of sequence length. For a

binary code word sequence that has a length of L branch word, there are 2L possible

sequences.

EXPERIMENT SET-UP DIAGRAMS (Figure 1):

PROCEDURE

1. Refer to the Fig. 1.1 and carry out the following connections and switch settings.

2. Connect power supply in proper polarity to the kit ADCL-06 and switch it on.

3. Keep the Data clk select switch SW2 towards slow position.

4. Select data pattern using select switch SW1 in the Data Generator block.

5. Connect SERIAL DATA generated on board to DATA IN of CONVOLUTION

ENCODER.

6. Observe RDY1 pin, convolution ally encoded data will be observed at OUT1 and

OUT2 post. The convolution ally encoded data are valid from the instant when RDY1

goes high.

7. Connect OUT1 and OUT2 post of Convolution Encoder block IN1 and IN2 of Hard

Decision Viterbi Decoder block.

8. Observe the decoded data at the DATA OUT1 post of Hard Decision Viterbi Decoder

block.

9. Repeat the procedure by keeping the data clk select switch towards fast position.



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Note1: The reason for the provision of data cl low frequency, encoded and decoded

data · Note2: It is advised to observe the decoded data i.e. output of the Hard decision

Viterbi Decoder in fast modebecause it takes approximately four minutes by the Viterbi

decoder to decode the data since the operating frequency is very low.

OBSERVATION

Observe the following signal on the DSO for the selected data pattern, using the steps

given in the theory of this experiment Also note that in the theory we have taken input

data of only five bits f but ADCL-06 supports 8 bit input data pattern which is

continuous in nature, so while solving for different data pattern always take two to three

samples of 8 bit input data pattern.

1. SERIAL DATA with respect to DATA CLOCK FIG. 1.2. (a)

2. OUT 1 with respect to RDY1 FIG. 1.2. (b)

3. OUT 2 with respect to RDY1 FIG. 1.2. (c)

4. OUT 1 with respect to OUT 2 FIG. 1.2. (d)

5. DATA OUT1 with respect to SERIAL DATA FIG. 1.2. (e)

Observed Waveforms:


CH 1: DATA CLK (256 KHz or 1 Hz) & CH 2: SERIAL DATA (00011011)



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Inference:

Thus we have studied convolution encoding and hard decision viterbi decoding technique

of serial data and also observed how a serial data is convolution ally encoded and when it

is passed through an error using Hard Decision Viterbi Decoder


1. The ____________________ is very popular channel code for error control purpose in

cellular phone applications.

(A) CRC codes (B) BCH codes (C) RS codes

(D) Convolutional codes



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2. They ____________________ are an important sub-class of no binary BCH codes.

(A) CRC codes (B) BCH codes (C) RS codes

(D) Convolutional codes

3. The ____________________ technique is employed when both random and burst

errors occur.

(A) Source coding (B) channel coding (C) interleaving

(D) modulation

4. They ____________________ are powerful cyclic codes that can correct any number

of errors and can be designed from the specification of the error-correcting capability.

(A) CRC codes (B BCH codes (C) RS codes (D Convolutional codes

5. The ____________________ is a special case of BCH code whose error correcting

ability is 1.

(A) CRC code (B) Hamming code (C) RS code (D) Convolutional code

6. Distinguish between quantizes and block coders.

7. How is linear block code characterized?

8. How is a convolutional code described?

9. What is the unique characteristic of convolutional codes which makes it different from

linear block codes?

Experiment No. 1 -...

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