16-10-2012, 04:05 PM
Digital communication system
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INTRODUCTION
Digital communication system is used to transport an information bearing signal from the source to a user destination via a communication channel. The information signal is processed in a digital communication system to form discrete messages which makes the information more reliable for transmission. Channel coding is an important signal processing operation for the efficient transmission of digital information over the channel. It was introduced by Claude E. Shannon in 1948 by using the channel capacity as an important parameter for error - free transmission. In channel coding the number of symbols in the source encoded message is increased in a controlled manner in order to facilitate two basic objectives at the receiver: error detection and error correction.
Error detection and error correction to achieve good communication is also employed in electronic devices. It is used to reduce the level of noise and interferences in electronic medium. The amount of error detection and correction required and its effectiveness depends on the signal to noise ratio (SNR). Every information signal has to be processed in a digital communication system before it is transmitted so that the user at the receiver end receives an error – free information. A digital communication system has three basic signal processing operations: source coding, channel coding and modulation.
In source coding, the encoder maps the digital generated at the source output into another signal in digital form. The objective is to eliminate or reduce redundancy so as to provide an efficient representation of the source output. Since the source encoder mapping is one-to – one, the source decoder on the other end simply performs the inverse mapping, thereby delivers to the user a reproduction of the original digital source output.
In channel coding, the objective for the encoder is to map the incoming digital signal into a channel input and for the decoder is to map the channel output into an output signal in such a way that the effect of channel noise is minimized. That is the combined role of the channel encoder and decoder is to provide for a reliable communication over a noisy channel. Thus in source coding, redundant bits are removed whereas in channel coding, redundancy is introduced in a controlled manner.
Then modulation is performed for the efficient transmission of the signal over the channel. Various digital modulation techniques could be applied for modulation such as Amplitude Shift Keying (ASK), Frequency- Shift Keying (FSK) or Phase – Shift Keying (PSK).
Generally error – correcting codes have been classified into block codes and convolutional codes. The distinguishing feature for the classification is the presence or absence of memory in the encoders for the two codes. To generate a block code, the incoming information stream is divided into blocks and each block is processed individually by adding redundancy in accordance with a prescribed algorithm. The decoder processes each block individually and corrects errors by exploiting redundancy.
Many of the important block codes used for error – detection are cyclic codes. These are also called cyclic redundancy check codes. In a convolutional code, the encoding operation may be viewed as the discrete – time convolution of the input sequence with the impulse response of the encoder. The duration of the impulse response equals the memory of the encoder. The encoder for a convolutional code operates on the incoming message sequence, using a “sliding window” equal in duration to its own memory. Hence in a convolutional code, unlike a block code where codewords are produced on a block- by – block basis, the channel encoder accepts message bits as continuous sequence and thereby generates a continuous sequence of encoded bits at a higher rate.
ERROR CONTROL CODING
INTRODUCTION
The designer of an efficient digital communication system faces the task of providing a system which is cost effective and gives the user a level of reliability. The information transmitted through the channel to the receiver is prone to errors. These errors could be controlled by using Error- Control Coding which provides a reliable transmission of data through the channel. In this chapter, a few error control coding techniques are discussed that rely on systematic addition of redundant symbols to the transmitted information. Using these techniques, two basic objectives at the receiver are facilitated: Error Detection and Error Correction.
ERROR DETECTION AND ERROR CORRECTION
When a message is transmitted or stored it is influenced by interference which can distort the message. Radio transmission can be influenced by noise, multipath propagation or by other transmitters. In different types of storage, apart from noise, there is also interference which is due to damage or contaminant sin the storage medium. There are several ways of reducing the interference. However, some interference is too expensive or impossible to remove. One way of doing so is to design the messages in such ways that the receiver can detect if an error has occurred or even possibly correct the error too. This can be achieved by Error – Correcting Coding. In such coding the number of symbols in the source encoded message is increased in a controlled manner, which means that redundancy is introduced .
To make error correction possible the symbol errors must be detected. When an error has been detected, the correction can be obtained in the following ways:
(1) Asking for a repeated transmission of the incorrect codeword (Automatic repeat Request
(ARQ)) from the receiver.
(2) Using the structure of the error correcting code to correct the error (Forward Error
Correction (FEC)).
It is easier to detect an error than it is to correct it. FEC therefore requires a higher number of check bits and a higher transmission rate, given that a certain amount of information has to be transmitted within a certain time and with a certain minimum error probability. The reverse is also true; if the channel offers a certain possible transmission rate, ARQ permits a higher information rate than FEC, especially if the channel has a low error rate. FEC however has the advantage of not requiring a reply channel. The choice in each particular case therefore depends on the properties of the system or on the application in which the error – correcting is to be introduced. In many applications, such as radio broadcasting or Compact Disc (CD), there is no reply channel. Another advantage of the FEC is that the transmission is never completely blocked even if the channel quality falls below such low levels that ARQ system would have completely asked for retransmission. In a system using FEC, the receiver has no real- time contact with the transmitter and can not verify if the data was received correctly. It must make a decision about the received data and do whatever it can to either fix it or declare an alarm.