Note for Digital Communication Techniques - DCT by Thamarai M

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LECTURE NOTES ON A ll JN TU W or l d DIGITAL COMMUNICATIONS

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UNIT – I ELEMENTS OF DIGITAL COMMUNICATION SYSTEMS source to a user destination via a communication channel. W or l MODEL OF A COMMUNICATION SYSTEM d The purpose of a Communication System is to transport an information bearing signal from a TU Fig. 1.1: Block diagram of Communication System The three basic elements of every communication systems are Transmitter, 2. Receiver and 3. Channel. JN 1. The Overall purpose of this system is to transfer information from one point (called Source) to another point, the user destination. The message produced by a source, normally, is not electrical. Hence an input transducer is used for converting the message to a time – varying electrical quantity called message signal. Similarly, at the destination point, another transducer converts the electrical waveform to the A ll appropriate message. The transmitter is located at one point in space, the receiver is located at some other point separate from the transmitter, and the channel is the medium that provides the electrical connection between them. The purpose of the transmitter is to transform the message signal produced by the source of information into a form suitable for transmission over the channel.

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The received signal is normally corrupted version of the transmitted signal, which is due to channel imperfections, noise and interference from other sources.The receiver has the task of operating on the received signal so as to reconstruct a recognizable form of the original d message signal and to deliver it to the user destination. Communication Systems are divided into 3 categories: analog modulation methods. or l 1. Analog Communication Systems are designed to transmit analog information using 2. Digital Communication Systems are designed for transmitting digital information using digital modulation schemes, and 3. Hybrid Systems that use digital modulation schemes for transmitting sampled and W quantized values of an analog message signal. ELEMENTS OF DIGITAL COMMUNICATION SYSTEMS: The figure 1.2 shows the functional elements of a digital communication system. Source of Information: 1. Analog Information Sources. 2. Digital Information Sources. TU Analog Information Sources → Microphone actuated by a speech, TV Camera scanning a scene, continuous amplitude signals. Digital Information Sources → These are teletype or the numerical output of computer which consists of a sequence of discrete symbols or letters. An Analog information is transformed into a discrete information through the process of JN sampling and quantizing. A ll Digital Communication System Fig. 1.2: Block Diagram of Digital Communication System

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SOURCE ENCODER / DECODER: The Source encoder ( or Source coder) converts the input i.e. symbol sequence into a binary sequence of 0s and 1s by assigning code words to the symbols in the input d sequence. For eg. :-If a source set is having hundred symbols, then the number of bits used to represent each symbol will be 7 because 27=128 unique combinations are block size, code or l available. The important parameters of a source encoder are word lengths, average data rate and theefficiency of the coder (i.e. Actual output data rate compared to the minimum achievable rate) At the receiver, the source decoder converts the binary output of the channel decoder into a symbol sequence. The decoder for a system using fixed – length code words will be very complex. Aim of W words is quite simple, but the decoder for a system using variable – length code the source coding is to remove the redundancy in the transmitting information, so that bandwidth required for transmission is TU minimized. Based on the probability of the symbol code word is assigned. Higher the probability, shorter is the codeword. Ex: Huffman coding. CHANNEL ENCODER / DECODER: Error control is accomplished by the channel coding operation that consists of JN systematically adding extra bits to the output of the source coder. These extra bits do not convey any information but helps the receiver to detect and / or correct some of the errors in the information bearing bits. There are two methods of channel coding: 1. Block Coding: The encoder takes a block of k information bits from A ll the source encoder and adds r error control bits, where r is dependent on k and error control capabilities desired. 2. Convolution Coding: The information bearing message stream is encoded in a continuous fashion by continuously interleaving information bits and error control bits.