Digital Communication and Switching Lecture Notes PDF 2024-25

EETE 4220 Digital Communication and Switching Semester I AY 2024-25 Class Lecture -1 Outcome #1-Various elements of Digital Communication systems Course Lect...

EETE 4220 Digital Communication and Switching Semester I AY 2024-25 Class Lecture -1 Outcome #1-Various elements of Digital Communication systems Course Lecturer :Lenin Joseph (LJ) E&E Section. Dept. of Engineering. ACT Dec 24, 2024 1 OUTCOME # 1 1.Describe various elements of Digital Communication systems. EETE 4220 Digital Communication & Switching -E&E Section. Dept. of Engine Dec 24, 2 ering. ACT 2024 ELECTRONIC COMMUNICATION SYSTEMS  Collection of electronics circuit capable of transmitting and/or receiving information by electric means. EETE 4220 Digital Communication & Switching -E&E Section. Dept. of Engine Dec 24, 3 ering. ACT 2024 BLOCK DIAGRAM OF A COMMUNICATION SYSTEM message transmitted received Estimate of signal signal signal message Information Channel / signal User of Transmitter (communication Receiver Source medium) Information Noise EETE 4220 Digital Communication & Switching -E&E Section. Dept. of Engine Dec 24, 4 ering. ACT 2024 ANALOG AND DIGITAL TRANSMISSION  Analog transmission is the transfer of a continuously varying analog signal while digital transmission is the transfer of discrete messages.  In digital communication, the messages are either represented by a sequence of pulses by means of a line code ( baseband transmission), or by a limited set of continuously varying wave forms (passband transmission), using a digital modulation method. EETE 4220 Digital Communication & Switching -E&E Section. Dept. of Engine Dec 24, 5 ering. ACT 2024 BLOCK DIAGRAM OF DIGITAL COMMUNICATION SYSTEM EETE 4220 Digital Communication & Switching -E&E Section. Dept. of Engine Dec 24, 6 ering. ACT 2024 ELEMENTS OF DIGITAL COMMUNICATION SYSTEM INFORMATION SOURCES  The message generated by the information source can be an analog signal human voice, or a digital signal such as computer data. ANALOG TO DIGITAL CONVERTER  The A/D converter samples and quantizes the analog signal and represents the samples in digital form (0s and 1s).  Conversion of Analog signal to Discrete signal can be done through the following steps: (a) Sampling (b) Quantization (c) Coding EETE 4220 Digital Communication & Switching -E&E Section. Dept. of Engine Dec 24, 7 ering. ACT 2024 ANALOG TO DIGITAL CONVERTER Sampling  Process of converting the continuous time signal to a discrete time signal.  Sampling is done by taking “Samples” at specific times spaced regularly. EETE 4220 Digital Communication & Switching -E&E Section. Dept. of Engine Dec 24, 8 ering. ACT 2024 Analog to Digital Conversion  The closer the Ts (spacing of the sampling time) value, the closer the sampled signal resemble the original signal.  Note that some values of the original signal were lost, which are the parts between each successive samples. EETE 4220 Digital Communication & Switching -E&E Section. Dept. of Engine Dec 24, 9 ering. ACT 2024 Analog to Digital Conversion Nyquist Sampling Theorem  The original information signal can be reconstructed at the receiver with minimal distortion if the sampling rate is equal or greater than twice the maximum information signal frequency. fs > 2fm(max) where: fs = minimum Nyquist sampling rate/frequency fm(max) = maximum information signal frequency  For bandwidth limited signals with upper and lower limits of f2 and f1, the Nyquist sampling rate is equal or greater than twice the bandwidth, or 2 (f2 –f1). EETE 4220 Digital Communication & Switching -E&E Section. Dept. of Engine Dec 24, 10 ering. ACT 2024 Analog to Digital Conversion Aliasing  If fs is less than 2 times fm(max) an impairment called as alias or fold-over distortion occurs.  An alias is a signal that is mistakenly sampled when the sampling frequency used for digital transmission is less than twice the input frequency.  To eliminate this problem, a low pass filter called antialiasing filter is usually placed between modulating signal source and the A/D converter input. EETE 4220 Digital Communication & Switching -E&E Section. Dept. of Engine Dec 24, 11 ering. ACT 2024 Analog to Digital Conversion Sampling methods: (a) Ideal Sampling  An impulse at each sampling instant (b) Natural Sampling  Sampling of analog signal, so that the sample amplitude follows that of the original signal for the duration of the sample. EETE 4220 Digital Communication & Switching -E&E Section. Dept. of Engine Dec 24, 12 ering. ACT 2024 Analog to Digital Conversion Sampling methods: (c) Flat-top Sampling  Sampling of analog signal using sample and hold circuit, such that the sample has the same amplitude for the whole duration. EETE 4220 Digital Communication & Switching -E&E Section. Dept. of Engine Dec 24, 13 ering. ACT 2024 Analog to Digital Conversion Example 1: An information signal to be transmitted digitally is a rectangular wave with a period of 71.4us. It has been determined that the wave will be adequately passed if bandwidth includes the fourth harmonic. Calculate: (a) the signal frequency= f = 14 KHz (b) the fourth harmonic=4f = 56 KHz (c) the minimum sampling frequency (Nyquist rate)=fs T= 71.4 us f =1/T 1/(71.4 x 10-6) = 14005.60 Hz = 14 K 4f=4 x 14 K = 56K fs= 2x fm max = 2X 56 K= 112 KHz EETE 4220 Digital Communication & Switching -E&E Section. Dept. of Engine Dec 24, 14 ering. ACT 2024 Analog to Digital Conversion 2. Quantization  Process of assigning the analog signal samples to a pre- determined discrete level.  In the quantization process, the available range of a signal voltages is divided into L amplitude levels denoted by mk, where k = 1, 2, 3,.. L Quantization Level  The number of quantization levels, L depends on the number of bits per sample, n L 2 n where: L = number of quantization level n = number of bits in binary to represent the value of the samples EETE 4220 Digital Communication & Switching -E&E Section. Dept. of Engine Dec 24, 15 ering. ACT 2024 Analog to Digital Conversion Quantization Step / Zone Width S max  S min  L where: Smax = maximum signal amplitude Smin = minimum signal amplitude L = number of quantization level Δ = quantization step / zone width EETE 4220 Digital Communication & Switching -E&E Section. Dept. of Engine Dec 24, 16 ering. ACT 2024  Analog to Digital Conversion How quantization is done?  Assume we have a voltage signal with amplitudes S min=0V and Smax=+10V.  If we use L=4 quantization levels, we will divide the signal into 4 zones.  Quantization Step /Zone width  = (10 - 0)/4 = 2.5  The 4 zones are: m1  [ 0 to 2.5 ] m2  [ 2.5 to 5 ] m3  [ 5 to 7.5] m4  [ 7.5 to 10] EETE 4220 Digital Communication & Switching -E&E Section. Dept. of Engine Dec 24, 17 ering. ACT 2024 Analog to Digital Conversion How quantization is done?  For every zone, the midpoint is calculated. Each value of signal falling in a zone is then approximated to the value of the midpoint. M1 = 1.25 if the signal in m1 M2 = 3.75 if the signal in m2 Q [ v(t) ] = M3 = 6.25 if the signal in m3 M4 = 8.75 if the signal in m4 EETE 4220 Digital Communication & Switching -E&E Section. Dept. of Engine Dec 24, 18 ering. ACT 2024 Analog to Digital Conversion Quantization Error  When a signal is quantized, we introduce an error since the coded signal is just an approximation of the actual amplitude value.  The difference between actual and coded value (midpoint) is referred to as the quantization error.  The more quantization levels we take, the smaller the error between the original and quantized signal.  The smaller the Δ, the smaller the quantizing error.  However, increasing the quantization level would increase the number of bits per sample, thus increasing the data rate (D). D = fs * n (in bps) where: fs = sampling rate in Hz and n = number of bitsDecper EETE 4220 Digital Communication & Switching -E&E Section. Dept. of Engine 24, sample 19 ering. ACT 2024 Analog to Digital Conversion Example 2: Calculate the number of quantization levels if the number of bits per sample is: (a) 8 (as in telephony) (b)16 (as in compact disc audio system) Solution- (a) L=2n then, n=8 give you L= 28 =256 (b) L=2n then, n=16 give you L= 216 =65536 EETE 4220 Digital Communication & Switching -E&E Section. Dept. of Engine Dec 24, 20 ering. ACT 2024 Analog to Digital Conversion Example 3: Calculate the minimum data rate needed to transmit an audio signal with a sampling rate of 40KHz and 16 bit per sample. Solution D= fs * n Given fs= 40 KHz n=16 minimum data rate D= 40 KHz* 16 D= 640 Kbps EETE 4220 Digital Communication & Switching -E&E Section. Dept. of Engine Dec 24, 21 ering. ACT 2024  Analog to Digital Conversion 3. Coding  Assigning a binary code to each quantization level (or zone).  The number of bits required to encode the zones, or the number of bits per sample as it is commonly referred to, is obtained as follows: n = log2 L Example:  For signal divided into 4 quantization level (or zones), n = 2.  The level codes are : 00, 01, 10, 11  Assigning codes to zones: 00  m1 [ 0 to 2.5 ] 01  m2 [ 2.5 to 5 ] 10  m3 [ 5 to 7.5] 11  m4 [ 7.5 to 10] EETE 4220 Digital Communication & Switching -E&E Section. Dept. of Engine Dec 24, 22 ering. ACT 2024 ELEMENTS OF DIGITAL COMMUNICATION SYSTEM SOURCE ENCODER  The source encoder accepts the digitized source and encodes it to a shorter digital signal.  Source coding (also called entropy encoding) refers to the process of compressing data. This is typically done by replacing long binary codes (named codewords) that occur frequently by shorter ones, and those that occur less frequently by longer codes.  The aim of the source encoding is to remove the redundancy in the transmitting information, so that bandwidth required for transmission is minimized. EETE 4220 Digital Communication & Switching -E&E Section. Dept. of Engine Dec 24, 23 ering. ACT 2024 ELEMENTS OF DIGITAL COMMUNICATION SYSTEM SOURCE ENCODER  The important parameters of a source encoder are block size, code word lengths, average data rate and the efficiency of the coder. TYPES OF SOURCE ENCODING (a) Lossless Compression (encoding) - reduces bits by identifying and eliminating statistical redundancy. Data can be recovered without any missing information. Example: Huffman Coding, Run Length Encoding and Lempel-Ziv (b) Lossy Compression (encoding) - reduces bits by identifying unnecessary information and removing it. Data removed in encoding can not be recovered again. Example: MPEG and JPEG compression EETE 4220 Digital Communication & Switching -E&E Section. Dept. of Engine Dec 24, 24 ering. ACT 2024 ELEMENTS OF DIGITAL COMMUNICATION SYSTEM CHANNEL ENCODER  The channel encoder accepts the output digital signal of the source encoder and systematically adds extra bits to it. The extra bits or redundancy do not convey any information but helps the receiver to detect and / or correct some of the errors in the information bearing bits.  Error control is accomplished by the channel coding operation. EETE 4220 Digital Communication & Switching -E&E Section. Dept. of Engine Dec 24, 25 ering. ACT 2024 ELEMENTS OF DIGITAL COMMUNICATION SYSTEM METHODS OF CHANNEL CODING 1. Block Coding: The encoder takes a block of k information bits from the source encoder and adds r error control bits, where r is dependent on k and error control capabilities desired. Example: Hamming codes 2. Convolution Coding: The information bearing message stream is encoded continuous fashion by continuously interleaving information bits and error control bits. Example: Viterbi algorithm EETE 4220 Digital Communication & Switching -E&E Section. Dept. of Engine Dec 24, 26 ering. ACT 2024 ELEMENTS OF DIGITAL COMMUNICATION SYSTEM MODULATOR  The modulator converts the input bit stream into an electrical waveform suitable for transmission over the communication channel.  Digital modulation is a process that impresses a digital symbol onto a signal suitable for transmission.  For short distance transmissions, baseband modulation is usually used. Baseband modulation is often called line coding.  Line codes are sequence of digital symbols used to create a square pulse waveform with certain features which represent each type of symbol without ambiguity so that they can be recovered upon reception.  Example of Line codes: NRZ, RZ, Manchester, Miller Codes EETE 4220 Digital Communication & Switching -E&E Section. Dept. of Engine Dec 24, 27 ering. ACT 2024 ELEMENTS OF DIGITAL COMMUNICATION SYSTEM MODULATOR  For long distance and wireless transmissions, passband modulation is usually used.  Passband modulation is also called carrier modulation. A sequence of digital symbols is used to alter the parameters of a high-frequency sinusoidal signal called carrier.  Examples of digital passband modulations are amplitude shift keying (ASK), frequency shift keying (FSK), phase shift keying (PSK) and quadrature amplitude modulation (QAM).  Modulator can be effectively used to minimize the effects of channel noise, to match the frequency spectrum of transmitted signal with channel characteristics, to provide the capability to multiplex many signals. EETE 4220 Digital Communication & Switching -E&E Section. Dept. of Engine ering. ACT Dec 24, 20 24 28 ELEMENTS OF DIGITAL COMMUNICATION SYSTEM COMMUNICATION CHANNEL  path for the transmission of signal.  the physical medium used to send the signal.  provides the electrical connection between the source and destination.  have only finite bandwidth, non-ideal frequency response.  The signal often suffers amplitude and phase distortion as it travels over the channel.  Also, the signal power decreases due to the attenuation of the channel.  The signal is corrupted by noise. EETE 4220 Digital Communication & Switching -E&E Section. Dept. of Engine Dec 24, 29 ering. ACT 2024 ELEMENTS OF DIGITAL COMMUNICATION SYSTEM COMMUNICATION CHANNEL  Five channels are considered in the digital communication, namely: (a) telephone channel (b) coaxial cables (c) optical fibers (d) microwave radio (e) satellite channels EETE 4220 Digital Communication & Switching -E&E Section. Dept. of Engine Dec 24, 30 ering. ACT 2024 ELEMENTS OF DIGITAL COMMUNICATION SYSTEM COMMUNICATION CHANNEL The important parameters of the channel are as follows: Signal to Noise power Ratio (SNR) Usable bandwidth Amplitude and phase response Statistical properties of noise. The modulation and coding used in a digital communication system depend on the characteristics of the channel. Modulation schemes are chosen or designed according to channel characteristic in order to optimize their performance. EETE 4220 Digital Communication & Switching -E&E Section. Dept. of Engine Dec 24, 31 ering. ACT 2024 ELEMENTS OF DIGITAL COMMUNICATION SYSTEM COMMUNICATION CHANNEL Channel may be classified in different ways:  A channel may be linear or non-linear. A wireless radio channel is linear, whereas a satellite channel is usually non- linear.  A channel may be time invariant or time varying. An optical fiber is time invariant, while mobile radio channel is typically time varying.  A channel may be bandwidth limited or power limited (limited in the available transmitted power). A telephone channel is bandwidth limited, whereas an optical fiber and satellite channel are both power limited. EETE 4220 Digital Communication & Switching -E&E Section. Dept. of Engine Dec 24, 32 ering. ACT 2024 Communication Channel Models (a) Additive white Gaussian noise (AWGN) channel Universal channel model for analyzing modulation schemes. In this model, the channel does nothing but add a white Gaussian noise to the signal passing through it. This implies that the channel's amplitude frequency response is flat and phase frequency response is linear for all frequencies so that modulated signals pass through it without any amplitude loss and phase distortion of frequency components. Fading does not exist. The only distortion is introduced by the AWGN. EETE 4220 Digital Communication & Switching -E&E Section. Dept. of Engine Dec 24, 33 ering. ACT 2024 Communication Channel Models (a) Additive white Gaussian noise (AWGN) channel Strictly speaking, the AWGN channel does not exist since no channel can have an infinite bandwidth. However, when the signal bandwidth is smaller than the channel bandwidth, many practical channels are approximately an AWGN channel. Line-of-sight (LOS) radio channels, including fixed terrestrial microwave links and fixed satellite links, are approximately AWGN channels when the weather is good. Wideband coaxial cables are also approximately AWGN channels since there is no other interference except the Gaussian noise. r(t) = s(t) + n(t) EETE 4220 Digital Communication & Switching -E&E Section. Dept. of Engine Dec 24, 34 ering. ACT 2024 Communication Channel Models (b) Bandlimited channel When the channel bandwidth is smaller than the signal bandwidth, the channel is bandlimited. Severe bandwidth limitation causes Intersymbol Interference (ISI) (i.e. digital pulses will extend beyond their transmission duration (symbol period Ts) and interfere with the next symbol or even more symbols. The ISI causes an increase in the bit error probability (Pb) or bit error rate (BER). When increasing the channel bandwidth is impossible or not cost-efficient, Channel equalization techniques are used for combatting ISI. 1/Tb= bit rate fb EETE 4220 Digital Communication & Switching -E&E Section. Dept. of Engine Dec 24, 35 ering. ACT 2024 INTERSYMBOL INTERFERENCE (ISI) EETE 4220 Digital Communication & Switching -E&E Section. Dept. of Engine Dec 24, 36 ering. ACT 2024 Communication Channel Models (c) Fading channel Fading is a phenomena occurring when the amplitude and phase of a radio signal change rapidly over a short period of time or travel distance. Fading is caused by interference between two or more versions of the transmitted signal which arrive at the receiver at slightly different times. These waves, called multipath waves, combine at the receiver antenna to give a resultant signal EETE 4220 Digital Communication & Switching -E&E Section. Dept. of Engine Dec 24, 37 which can vary widely in amplitude ering. ACT 2024 Communication Channel Models (c) Fading channel In mobile communication channels, such as terrestrial mobile channel and satellite mobile channel, fading and multipath interference are caused by reflections from surrounding buildings and terrains. The motion of surrounding objects, such as vehicles, also induces a time-varying Doppler shift on multipath component. Fading causes amplitude fluctuations and phase variations in received signals. Multipath causes Intersymbol Interference (ISI). Doppler shift causes carrier frequency drift and signal bandwidth spread. EETE 4220 Digital Communication & Switching -E&E Section. Dept. of Engine Dec 24, 38 ering. ACT 2024 ELEMENTS OF DIGITAL COMMUNICATION SYSTEM DEMODULATOR  The extraction of the message from the information bearing waveform produced by the modulation is accomplished by the demodulator.  It removes the carrier signal and reverses the process of the modulator.  The output of the demodulator is coded bit stream. EETE 4220 Digital Communication & Switching -E&E Section. Dept. of Engine Dec 24, 39 ering. ACT 2024 ELEMENTS OF DIGITAL COMMUNICATION SYSTEM CHANNEL DECODER  The Channel decoder recovers the information bearing bits from the coded binary stream.  The added redundancy is taken away by the channel decoder.  Error detection and possible correction is also performed by the channel decoder.  The important parameters of channel coder and decoder are: method of coding, efficiency, error control capabilities and complexity of the circuit. EETE 4220 Digital Communication & Switching -E&E Section. Dept. of Engine Dec 24, 40 ering. ACT 2024 ELEMENTS OF DIGITAL COMMUNICATION SYSTEM SOURCE DECODER  The source decoder decompresses the binary output of the channel decoder its original format. DIGITAL TO ANALOG CONVERTER  The D/A Converter transform digital data from source decoder into an analog signal needed by information sink. INFORMATION SINK  Information sink can be human or computer that needs the information sent by the source. EETE 4220 Digital Communication & Switching -E&E Section. Dept. of Engine Dec 24, 41 ering. ACT 2024 ADVANTAGES OF DIGITAL COMMUNICATION SYSTEM Flexible Noise hardware Ease of Ease of Immunity implementati Regeneration Multiplexing on Digital circuits are more Error Signal reliable and detection and Processing cheaper correction Capability compared to capability analog circuits. EETE 4220 Digital Communication & Switching -E&E Section. Dept. of Engine Dec 24, 42 ering. ACT 2024 DISADVANTAGES OF DIGITAL COMMUNICATION SYSTEM 1. Large 2. System System Synchronizati Bandwidth on EETE 4220 Digital Communication & Switching -E&E Section. Dept. of Engine Dec 24, 43 ering. ACT 2024 REFERENCES Text Book Frenzel, L., Principles of Electronic Communication Systems, 3rd Ed. McGrawHill, 2008 Book References Sklar, Benard, “Digital Communications: Fundamentals and Application”, Prentice Hall, USA, 2001 Glover, Ian A / Grant, Peter M, “ Digital Communications”, Prentice Hall, UK, 2004 Proakis,John G,“ Digital Communications”, McGraw Hill Inc, USA, 2000 Wesolowski,Krzysztof, “Introduction to Digital Communication Systems”,2009 Web References\ e-library(s) https://ebookcentral.proquest.com/lib/momp/detail.action?docID=3011330 571, CD/DVD, Electronic Communication Systems: A complete Course, ACT Store  THAN K YOU EETE 4220 Digital Communication & Switching -E&E Section. Dept. of Engine Dec 24, 45 ering. ACT 2024