GATE Online Coaching Classes
Digital Communications Online Class-2
• By • Dr.B.Leela Kumari • Assistant Professor, • Department of Electronics and Communications Engineering • University college of Engineering Kakinada • Jawaharlal Nehru Technological University Kakinada
6/15/2020 Dr. B. Leela Kumari UCEK JNTUK Kakinada 1 6/15/2020 Dr. B. Leela Kumari UCEK JNTUK Kakinada 1
Session -2 Baseband Transmission
• Introduction to Pulse analog Transmission • Quantization and coding • Classification of Uniform Quantization • Line Coding • Non Uniform Quantization • Objective type Questions & Illustrative Problems
6/15/2020 Dr. B. Leela Kumari UCEK JNTUK Kakinada 3 Pulse Analog Transmission
Pulse modulation: One of the parameters of the carrier is varied in accordance with the instantaneous value of the modulating signal Classification: Pulse Amplitude Modulation(PAM) Pulse Time Modulation(PTM) Pulse Width Modulation(PWM) Pulse Position Modulation(PPM)
Pulse Amplitude Modulation(PAM) Pulse Width Modulation(PWM) Pulse Position Modulation(PPM) Comparisions Quantization Pulse analog modulation systems are not completely digital Sampled signals are used to get a complete digital signal Sampled signals are first quantized and then coded to get complete digital signal
Input Signal digital signal Sampler Quantizer Encoder
Sampler samples the analog signal Quantizer: Does the Quantizing (approximation) Quantized signal is an approximation to the original signal The Quality of approximation improved by reducing the step Size Encoder translates the Quantized signal more appropriate format of signal
Operation of Quantization Process of Quantization
Consider message signal m(t) ,whose range is from VL to VH
Divide this range in to M equal intervals, each size S, Step size
S= (VL –VH )/M
In the center of each of these steps ,locate Quantization levels,m0,m1,m2….. m7.
Generate the Quantized signal mq(t).
Generation of Quantized signal mq(t) Whenever m(t) in the range Δ0 the signal mq(t) maintains the constant level m0 Whenever m(t) in the range Δ1 the signal mq(t) maintains the constant level m1 and so on. signal mq(t) will always be at one of these levelsm0,m1,m2….. m7. The transition in mq(t) from m0 to m1 is made abruptly passes the transition level L01 which is mid way between m0 & m1 and so on. Separation of extremes VL & VH from its nearest Quantization level is S/2
m (t)- mq(t) is the Quantization error , has magnitude less than or equal to S/2 t every instant The maximum Quantization Error is S/2
Classification of Uniform Quantizer
Uniform Quantizer • Mid Tread Type • Mid Rise Type
• The quantizer characteristics have a stair case shape irrespective of the type of quantizer • Mid Tread Type: The origin is in the tread portion • Mid Rise Type :The origin is in the rise portion
• The no. of levels in Mid rise Quantizer L= 2n
• The no. of levels in Mid tread Quantizer L= 2n-1 Coding Coding : It translates the discrete sequence of values to a more appropriate format of signal. process of converting binary data to digital signal
Coding
Advantages: Effect of noise implementation will be less Security
Line Coding
Line coding converts stream of binary digits into a formal or code which is more suitable for transmission over a cable or any other medium process of converting binary data to digital signal Characteristics: Signal level and data level Pulse rate and Bit rate DC component Self Synchronization Signal level : The number of values allowed in particular signal
data level : The number of values to represent data Binary data has two values 0 and 1 Pulse rate: Number of pulses per second Pulse is defined as the minimum amount of time required to transmit symbol.
Bit rate: Number of bits per second If one pulse corresponds to one bit then pulse rate equal to the bit rate. if pulse carries more than to one bit then pulse rate is lower than the bit rate. L Bit rate= Pulse rate x log2 = Pulse rate x m L= 2m m is number of bits per sample
DC component: The average voltage of the given signal
Self Synchronization : Clock frequency should be same and synchronized to match data at transmitter and receiver
Classification of Line Coding Techniques Unipolar NRZ Polar NRZ Bipolar NRZ UniPolar RZ Polar RZ Bipolar RZ Polar NRZ Polar Manchester or Split Phase Alternate Mark inversion (AMI) Note: NRZ indicates Non Return to Zero RZ indicates Return to Zero
Unipolar NRZ Signalling Polar NRZ
Bipolar NRZ
• UniPolar RZ
Polar RZ
1 0 1 1 0 0
Symbol ‘1’ and ‘0’ are represented by pulse of ( half symbol wide) of equal +ve and –ve amplitudes Bipolar RZ : 1 0 1 1 0 0 A
-A
+ ve and –ve Rectangular pulses ( half symbol wide) of equal amplitude are used alternatively for symbol ‘1’ and no pulse for ‘0’ . • Polar Manchester or Split Phase
• Alternate Mark Inversion Code
Bit rate and Baud Rate
Bit rate (R) : It is number of bits per second Baud Rate ( r): It is number of Symbols per second If ‘n’ indicates number of bits/symbol r = R/n The total number of symbols L= 2n Band width = 1/ Minimum Pulse Width Possible
Band width = 1/Tb =Rb Prob: An analog signal carries 4 bits/symbol elements. If 1000 signal elements are sent per second, Find bit rate and total number of elements. Sol: n=4 bits/symbol elements r= 1000 signal elements /second Bit Rate R= nr =4x1000 =4kbps The total number of elements L= 2n=24 =16
Prob: An analog signal has a bit rate of 8000bps and a baud rate of 1000 baud. How many data elements are carried by each signal element? How many signal elements do we need? Sol: R= 8000bps r= 1000 baud n=? L=? n= R/r = 8000/1000 =8 bits/element L= 2n =28=256
Non-Uniform Quantization • In non-uniform quantization, the step size is not fixed. It varies according to certain law or as per input signal amplitude. The following fig shows the characteristics of Nonuniform quantizer.
Companding PCM System Non-uniform quantizers are difficult to make and expensive. An alternative is to first pass the speech signal through nonlinearity before quantizing with a uniform quantizer. The nonlinearity causes the signal amplitude to be compressed. The input to the quantizer will have a more uniform distribution. At the receiver, the signal is expanded by an inverse to the nonlinearity. The process of compressing and expanding is called Companding.
Companding The word Companding is a combination of Compressing and Expanding, which means that it does both. This is a non-linear technique used in PCM which compresses the data at the transmitter and expands the same data at the receiver. The effects of noise and crosstalk are reduced by using this technique.
There are two types of Companding techniques. They are A-law Companding Technique µ-law Companding Technique
µ-law Companding Technique µ-law Companding is continuous in nature Uniform quantization is achieved at µ = 0, where the characteristic curve is linear and no compression is done. µ-law has mid-tread at the origin. Hence, it contains a zero value. µ-law companding is used for speech and music signals. µ-law is used in North America and Japan.
A-law Companding Technique Uniform quantization is achieved at A = 1, where the characteristic curve is linear and no compression is done. A-law has mid-rise at the origin. Hence, it contains a non-zero value. A-law companding is used for PCM telephone systems.
μ Law Companding:
The input and Out Put relation ship is given by
A Law Companding:
The input and Out Put relation ship is given by
µ-law Companding Technique For given value of µ ,reciprocal slope of compression curve defines Quantum steps dx/dy = log(1+µ)/µ = 1+µ|x| A-law Companding Technique: For given value of A ,reciprocal slope of compression curve defines Quantum steps dx/dy = (1+logA)/A 0≤|x|≤1/A = (1+logA)|x| 1/A≤|x|≤1
Multiplexing Schemes Multiplexing techniques are used to combine several message signals into a single composite message so that they can be transmitted over a common channel.
The multiplexing technique ensures that the different message signals in the composite signal do not interfere with each other and that they can be conveniently separated out at the receiver end.
Types of multiplexing :
1. Frequency-division multiplexing 2. Time-division multiplexing
Frequency-Division Multiplexing
Frequency-division multiplexing is used with signals that employ analog modulation techniques.
In case of frequency-division multiplexing (FDM), different message signals are separated from each other in the frequency domain.
Different message signals modulates a different carrier.
Most commonly used modulation technique is the single sideband (SSB) modulation.
On the receiving side, band-pass filters (BPF) separate out the signals, which are then coherently demodulated as shown.
FDM is used in telephony, commercial radio broadcast (both AM and FM), television broadcast, communication networks and telemetry.
In case of commercial AM broadcast, the carrier frequencies for different signals are spaced 10 kHz apart.
This separation is definitely not adequate if we consider a high- fidelity voice signal with a spectral coverage of 50 Hz to -15 kHz. Because of this reason, AM broadcast stations using adjacent carrier frequencies are usually geographically far apart to minimize interference.
In case of FM broadcast, the carrier frequencies are 200 kHz apart.
In case of long-distance telephony, 600 or more voice channels each with a spectral band of 200 Hz to 3.2 kHz can be transmitted over a coaxial or microwave link using SSB modulation and a carrier frequency separation of 4 kHz.
Time Division Multiplexing Time -division multiplexing is used with signals that employ Digital modulation techniques.
Time-division multiplexing (TDM) is used for simultaneous transmission of more than one pulsed signals over a common communication channel.
Multiple-pulsed signals are fed to a type of electronic switching circuitry called commutator.
All the message signals, which have been sampled at least at the Nyquist rate (the sampling is usually done at 1.1 times the Nyquist rate to avoid aliasing problem), are fed to the commutator.
The commutator interleaves different samples from different sampled message signals so as to form a composite interleaved signal.
This composite signal is then transmitted over the link.
In case all message signals have the same bandwidth, one commutation cycle will contain one sample from each of the messages.
But in case signals have different bandwidths, then one would need to transmit more number of samples per second of the signals having larger bandwidth.
As an illustration, if there are three message signals with respective sampling rates of 2.4, 2.4 and 4.8 kHz, then each cycle of commutation will have one sample each from the first two messages and two samples from the third message. At the receiving end, the composite signal is demultiplexed using a similar electronic switching circuitry that is synchronized with the one used at the transmitter. TDM is widely used in telephony, telemetry, radio broadcast and data processing. If T is the sampling time interval of the time-multiplexed signal of n different signals each having a sampling interval of Ts, then
Also, if time-multiplexed signal is considered as a low pass signal having a
bandwidth of fTDM and fm is the bandwidth of individual signals, then
Objective Type questions 1. In pulse amplitude modulation, a. Amplitude of the pulse train is varied b. Width of the pulse train is varied c. Frequency of the pulse train is varied d. None of the above
2. Pulse time modulation (PTM) includes a. Pulse width modulation b. Pulse position modulation c. Pulse amplitude modulation d. Both a and b
3. Drawback of using PAM method is a. Amplitude of carrier is constant b. Varying amplitude of carrier varies the peak power required for transmission c. Due to varying amplitude of carrier, it is difficult to remove noise at receiver d. Both b and C
4. The process of converting the analog sample into discrete form is called a. Modulation b. Multiplexing c. Quantization d. None of the above
5. The modulation techniques used to convert analog signal into digital signal are a. Pulse code modulation b. Delta modulation c. Adaptive delta modulation d. All of the above
6. An analog voltage in the range 0 to 8 V is divided in 16 equal intervals for conversion to 4-bit digital output. The maximum quantization error (in V) is ______a.0.25 b.0.5 c. 2 d. 1
7. The line code that has zero dc component for pulse transmission of random binary data is (a) non-return to zero (NRZ) (b)return to zero (RN) (c) Alternate Mark Inversion (AMI) (d)none of the above
8. The characteristics of compressor in μ-law companding are a. Continuous in nature b. Logarithmic in nature c. Linear in nature d. Discrete in nature
9. The bit rate required to digitize the human voice assuming human voice frequencies to be in the range of 0 to 4000 Hz and number of bits per sample to be eight is
(a) 32 kbps (b) 64 kbps (c) 8 kbps (d) 4 kbps
Given that the human voice contains frequencies from 0 to 4000 Hz. Therefore, sampling rate = 4000 × 2 = 8000 samples/s Bit rate = Sampling rate × Number of bits per sample = 8000 × 8 = 64000 bps = 64 kbps
10. In uniform quantization process ( )
a. The step size remains same b. Step size varies according to the values of the input signal c. The quantizer has linear characteristics d. Both a and c are correct
11. The format in which the positive half interval pulse is followed by a negative half interval pulse for transmission of ‘1’ is ( ) a. Polar NRZ format b. Bipolar NRZ format c. Manchester format d. None of the above
12. In Pulse time modulation (PTM), a. Amplitude of the carrier is constant b. Position or width of the carrier varies with modulating signal c. Pulse width modulation and pulse position modulation are the types of PTM d. All of the above
13. In pulse width modulation, a. Synchronization is not required between transmitter and receiver b. Amplitude of the carrier pulse is varied c. Instantaneous power at the transmitter is constant d. None of the above
14. The compressor characteristics of A Law companding are a. Piece wise mode of linear segments for low input levels b. Logarithmic segment for high input levels c. Both a and b d. None of the above
15. Non uniform quantization can be achieved by a. Companding b. Compounding c. Computing d. None of the above
16. In Pulse Position Modulation, the drawbacks are a. Synchronization is required between transmitter and receiver b.Width is not constant c. Both a and b d. None of the above 17. An analog signal is band-limited to 4 KHz, sampled at the Nyquist rate and the 4 samples levels are assumed to be independent and equally probable. If we transmit two quantized samples per second, the information rate is
(a) 1 bit/sec (b)2 bits/sec (c) 3 bits/sec (d)4 bits/sec
18. In polar RZ format for coding, symbol ‘0’ is represented by a. Zero voltage b. Negative voltage c. Pulse is transmitted for half the duration d. Both b and c are correct
19. In a uni-polar RZ format, a. The waveform has zero value for symbol ‘0’ b. The waveform has A volts for symbol ‘1’ c. The waveform has positive and negative values for ‘1’ and ‘0’ symbol respectively d. Both a and b are correct
20. Polar coding is a technique in which a. 1 is transmitted by a positive pulse and 0 is transmitted by negative pulse b. 1 is transmitted by a positive pulse and 0 is transmitted by zero volts c. Both a & b d. None of the above
21. The polarities in NRZ format use a. Complete pulse duration b. Half duration c. Both positive as well as negative value d. Each pulse is used for twice the duration
22. The format in which the positive half interval pulse is followed by a negative half interval pulse for transmission of ‘1’ is a. Polar NRZ format b. Bipolar NRZ format c. Manchester format d. None of the above
23.The maximum synchronizing capability in coding techniques is present in a. Manchester format b. Polar NRZ c. Polar RZ d. Polar quaternary NRZ
24. The advantage of using Manchester format of coding is a. Supress the DC component b. Polarity sense at the receiver c. Noise immunity d. None of the above
25. Alternate Mark Inversion (AMI) is also known as a. Pseudo ternary coding b. Manchester coding c. Polar NRZ format d. None of the above
26. Companding is used in the case of signals a. Having uniform amplitude distribution b. Having the large peak-to-peak amplitude but small verge power c. Having a large peak at high amplitude levels in their amplitude probability density function d. With large verge powers
27. Four independent messages have bandwidths of 100, 100, 200 and 400 Hz, respectively. Each is sampled at the Nyquist rate, and the samples are time-division multiplexed (TDM) and transmitted. Find the transmitted sample rate (in Hz).
28. Three analog signals having bandwidths 1200, 600 and 600 Hz are sampled at their respective Nyquist rates, encoded with 12- bit words and time-division multiplexed. The bit rate for the multiplexed signal is
(a) 115.2 kbps (b) 28.8 kbps (c) 57.6 kpbs (d) 38.4 kbps
Solution. Overall sampling frequency fs = fs1 + fs2 + fs3 = 2 × 1200 + 2 × 600 + 2 × 600 = 4.8 kHz Bit rate, Rb = nfs = 12 × 48 × 103 = 57.6 kbps
6/15/2020 69 29. Four messages band-limited to W, W, 2W and 3W, respectively, are to be multiplexed using time division multiplexing (TDM). The minimum bandwidth required for transmission of this TDM signal is
(a) W (b) 3W (c) 6W (d) 7W
6/15/2020 70 30
References 1. communication systems by B.P.Lathi 2. Principles of communication systems by Herber Taub.Donald & Schilling,Goutam Saha 3. Digital Communications ,Design for real World by Andy Bateman 4. Digital Communications by Dr.K.N.Hari Bhat ans Dr.D.Ganesh Rao 5. Communication Syatems,by R.P.Singh , S D Sapre 6. Digital Communications second edition by CH. Kranthi Rekha 7. Digital Communications theory ,techniques and applications by R.N.Mutagi 8. Digital Communications by P.Rama Krishna Rao 9. Wiley Acing the GATE Examination For Electronics and Communication Engineering 10. https://www.aceenggacademy.com/ 11. gate study.com 12. https://www.madeeasy.in/
Disclaimer: The material presented in this presentation is taken from various standard Textbooks and Internet Resources and the presenter is acknowledging all the authors.
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