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Digital Communication Techniques

by Anna Superkings
Type: NoteInstitute: Anna university Specialization: Electronics and Communication EngineeringDownloads: 5Views: 163Uploaded: 20 days agoAdd to Favourite

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Anna Superkings
Anna Superkings
Fatima Michael College of Engineering & Technology An ISO 9001:2008 Certified Institution EC6501-Digital Communication Fatima Michael College of Engineering & Technology 1
Fatima Michael College of Engineering & Technology Unit-1: Sampling & Quantization The purpose of a Communication System is to transport an information bearing signal from a source to a user destination via a communication channel. MODEL OF A COMMUNICATION SYSTEM(ANALOG) Information Source and Input I/P Signal TRANSMITTER CHANNEL Destination and Output Transducer O/P Signal RECEIVER Fig. 1.1: Block diagram of Communication System. The three basic elements of every communication systems are Transmitter, Receiver and Channel. 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 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. 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 Fatima Michael College of Engineering & Technology 2
Fatima Michael College of Engineering & Technology as to reconstruct a recognizable form of the original message signal and to deliver it to the user destination. Communication Systems are divided into 3 categories: 1. Analog Communication Systems are designed to transmit analog information using analog modulation methods. 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 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. Analog Information Sources → Microphone actuated by a speech, TV Camera scanning a scene, continuous amplitude signals. Digital Information Sources → These are teletype or the numericaloutput 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 sampling and quantizing. Digital Communication System Source of Information Source Encoder Channel Encoder Modulator Channel Received Signal User of Information Source Decoder Channel Decoder Demodulator Fatima Michael College of Engineering & Technology 3
Fatima Michael College of Engineering & Technology Fig 1.2: Block Diagram of a Digital Communication System SOURCE ENCODER / DECODER: The Source encoder ( or Source coder) converts the input i.e. symbol sequence into a binary sequence of 0‟ s and 1‟ s by assigning code words to the symbols in the input 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 available. The important parameters of a source encoder are block size, code word lengths, average data rate and the efficiency 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 is quite simple, but the decoder for a system using variable – length code words will be very complex. Aim of the source coding is to remove the redundancy in the transmitting information, so that bandwidth required for transmission is 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 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 the source encoder and adds „ r‟ error control bits, where „ r‟ is dependent on„ k‟ and error controlcapabilities desired. 2. Convolution Coding: The information bearing message stream is encoded in a continuous fashion by continuously interleaving information bits and error control bits. Fatima Michael College of Engineering & Technology 4

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