Intro to DC.textMark

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INTRODUCTION TO DIGITAL COMMUNICATIONS

The purpose of a Communication System is to transfer an informatio n bearing signa l from a

source to a user destination via a communication channel.

MODEL OF A COMMUNICATION SYSTEM (ANALOG)

Fig 1.1 Block diagram of Communication System.

The three basic elements of 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 timevarying 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 divides 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 as to reconstruct a recognizable form of the original message signal and to deliver it to the user destination.

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Communication Systems are divided into 3categories:

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.

ELEMENTSOFDIGITALCOMMUNICATIONSYSTEMS:

The figure 1.2 shows the functional elements of a digita l communication system.

Source of Information:

The source of information to be transmitted may be either analog or discrete. An analog

source produces time-continuous signals, while a discrete source produces sequences of

discrete symbols. Examples of signals from an analog source are speech signals, radar

outputs, and photographic scan data. Typical discrete sources are computer data files and

messages generated at teleprinter terminals.

1.Analog Information Sources.

2.Digital Information Sources.

Analog Information Sources Microphone actuated by a speech, TV Camera

scanninga scene, continuous amplitude signals. Digital Information SourcesThese are tele type or the numerical output o f

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.

DigitalCommunicationSystem

Fig1.2:BlockDiagramofa DigitalCommunicationSystem

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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 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.BlockCoding: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.

2.Convolution Coding:The information bearing message stream is encoded in a

continuous fashion by continuously interleaving information bits and error control

bits.

The Channel decoder recovers the information bearing bits from the coded binary stream.

Error detection and possible correction is also performed by the channel decoder. The

important parameters of coder / decoder are: Method of coding, efficiency, error control

capabilities and complexity of the circuit. MODULATOR: The Modulator converts the input bit stream into an electrical waveform s uita ble for

transmission over the co mmunication channel. Modulator can be effectively used to minimize the effects of channel noise, to match the frequency spectrum of transmitted

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signal with channel characteristics, to provide the capability to multiplex many signals. DEMODULATOR:

The extraction of the message fro m the information bearing waveform produced by the

modulation is accomplished by the demodulator. The output of the demodulator is bit

stream. The important parameter is the method of demodulation.

CHANNEL:

The Channel provides the electrical connection between the source and des tina tion.The

different channels are:

Pair of wires

Coaxial cable

Optical fibre

Radio channel

Satellite channel or combination of any o f these.

The communication channels have only finite Bandwidth, non- idea l frequency response,

the signal of ten 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 corrupte d by unwanted, unpredictable electrical signals referred

to as noise.

The important parameters of the channel are Signal to Noise power Ratio (SNR),

usable bandwidth, amplitude and phase response and the statistical properties of noise.

Advantages of Digital Communication

1. The effect of distortion, noise and interfe rence is less in a digita l communication

sys te m. T his is because the disturbance must be large enough to change the pulse

from one state to the other.

2. Regenerative repeaters can be used at fixed distance along the link, to identify and

regenerate a pulse before it is degraded to an ambiguous state.

3. Digital circuits are more reliable and cheaper compared to analo g circuits.

4. The Hardware implementation is more flexible than analog hardware because of the

use of microprocessors, VLSI chips etc.

5. Signal processing functions like encryption, compression can be employed to

maintain the secrecy of the information.

6. Error detecting and Error correcting codes improve the system performance by

reducing the probability of error.

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7. Combining digital signals using TDM is simpler than co mbining analog signals

us ing FDM. The different types of signals such as data, telephone, TV can be treated as identical signals in transmission and switching in a digital communication system.

8. We can avoid signal jamming using spread spectrum technique.

DisadvantagesofDigitalCommunication:

1. Large System Bandwidth:- Digital transmission requires a large system bandwidth

to communicate the same information in a digital format as compared to analog

format.

2. System Synchronization:-Digital detection requires system synchronization

where

as the analog signals generally have no such requirement.

PULSE MODULATION

Pulse Digital Modulation:

Transmitting digital signals has many advantages than transmitting analog signals over a

channel. In order to transmit the digital signals, analog signals are to be digitised. The scheme

that converts the analog signals to its corresponding digital form (analog to digital

conversion) is known as Pulse Digital Modulation. Various schemes/ techniques that are

employed to represent analog signals in digital format are known as Waveform Coding

Techniques.

WAVEFORM CODING TECHNIQUES:

Various types of modulation techniques are:

1) Pulse Code Modulation (PCM)

2) Differential Pulse Code Modulation (DPCM)

3) Delta Modulation (DM)

4) Adaptive Delta Modulation (ADM)

Note: Here, modulation doesnt reflect the meaning of conventional modulation in which the

parameter of message signal are varied with respect to carrier signal as done in the case of

Amplitude Modulation (AM), Frequency Modulation (FM), etc...These waveform coding

techniques are called baseband modulation techniques which imply that the signals are

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represented in digital form and no parameter of signal is changed. The output of these

techniques is a baseband signal.

Baseband Signal: Signals of low frequencies are baseband signals. In general, they are not

used for communication directly as they cant travel long distances. The output of the above

mentioned techniques is a baseband signal which is to be further processed (modulated) using

modulation techniques like ASK, PSK, FSK, etc... for transmitting over a channel effectively.

1) Pulse Code Modulation (PCM):

PCM is a method used to digitally represent sampled analog signals. The block diagram of

PCM is shown in the figure.

A PCM system has 3 main parts.

1) Transmitter

2) Transmission path

3) Receiver

PCM Transmitter:

It includes,

a) Low Pass Filter (LPF)

b) Sampler

c) Quantizer

d) Encoder

The blocks are as shown below.

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LPF: The analog signal is first passed into a LPF of cut-off frequency fm Hz which filters all

the frequencies above fm Hz. This implies that a signal is now band limited. It avoids aliasing

effect. So, it is called as anti-aliasing filter.

Sampler: Samples a continuous band limited analog signal at discrete intervals. The spacing

between the samples must follow the Nyquist Criteria i.e. Ts fm/2, where Ts is the sampling

time and fm is the highest signal frequency of band limited analog signal.

Quantizer: The sampled signal is now to be discretized in amplitude levels, called

quantization. This block quantizes the sampled band limited signal. The amplitude levels are

rounded off to nearest integer. This leads to an error called Quantization error. The process

is shown in figure. Below figure shows a 4-bit sampling and quantization.

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Quantization is of 2 types.

i) Uniform Quantization: The amplitude levels are discretized at regular intervals

having equal spacing between them i.e. the step size remains same throughout the

input range. n.

ii) Non-uniform Quantization: The amplitude levels are discretized at irregular

intervals having unequal spacing between them i.e. the step size varies according

to the input signal values. The use of non-uniform quantization is equivalent to

passing a baseband signal through a compressor and then applying compressed

signal to a uniform quantizer. Then the signal is then expanded and the whole

process is known as companding.

Companding = compressing + expanding

Companding can be achieved through 2 methods.

-law companding

A-law companding

-law companding:

It is defined as,

Where

m is the normalized input voltage

v is the normalized output voltage

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is a positive constant.

The practically used value of is 255. The case of uniform quantization arises when =0.

The graph of companding characteristics of a - law for 3 different values of is given in fig

(a) below. The reciprocal slope of the compression curve which defines the quantum steps is

given by derivative of |m| w.r.t |v| as,

We can hence observe that - law is neither strictly (except at =0) linear nor logarithmic

(except at =255). For |m| >1 it is

approximately logarithmic.

A-law companding:

It is defined as,

Where m and v are normalized input and output voltages and A is a positive constant. The

practically used value of A is 87.56. The case of uniform quantization arises when A=1.The

graph of companding characteristics of a A- law for 3 different values of A is given in fig (b)

above. The reciprocal slope of the compression curve which defines the quantum steps is

given by derivative of |m| w.r.t |v| as,

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At A=1 the characteristics are strictly linear corresponding to uniform quantization.

Quantizers are of 2 types.

i) Mid Tread Quantizer

ii) Mid Rise Quantizer

Note: Sampler and Quantizer are together known as Analog to Digital Converter (ADC).

Encoder: This converts the digital signal into a series of bit streams, usually called an n-bit

binary code word. In a binary code, each symbol may have either logic 0 or logic 1. There are

various formats to represent the binary sequence and are called line codes.

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PCM Transmission Path:

The path between PCM Tx and PCM Rx over which a PCM signal travels is called PCM

Transmission path. The important feature of PCM system lies in its ability to contro l the

effects of distortion and noise when PCM wave travels on channel. The capability is

accomplished by reconstructing PCM wave using regenerative repeaters located at

sufficiently close spacing along the transmission path.

The block diagram of a repeater is shown in the figure below.

3 basic functions of regenerative repeater are:

Equalization

Timing

Decision Making

The equalizer shapes the received pulses so as to compensate for the effects of amplitude and

phase distortions produced in the channel.

Timing circuit provides periodic pulse train derived from the received pulses for sampling the

equalized pulses at instants of time where SNR is maximum.

The decision device is enabled when the amplitude of equalized pulse (along with noise)

exceeds a predetermined voltage level. The signal is reconstructed when the amplitude

exceeds the predetermined level.

PCM Receiver:

Operations of a PCM receiver are,

Regeneration

Decoding

Reconstruction

The block diagram is as shown below.

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The received pulse is reshaped (regenerated). The clean pulses are regrouped into code words

and decoded into a quantized PAM signal. In the reconstruction part the decoded signal is

passed into a low pass reconstruction filter whose cut-off frequency is equal to bandwidth

(W) of the message, to recover the analog signal.

1) Effect of noise is reduced.

2) PCM permits the use of pulse regeneration.

3) Multiplexing of various PCM signals is possible