Digital Communications GATE Online Coaching Classes
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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 n 4 The total number of elements L= 2 =2 =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 n 8 L= 2 =2 =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.