L21-Introduction to Digital MOdulation

7/31/2019 L21-Introduction to Digital MOdulation

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Introduction to DigitalModulation


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Simplified block diagram ofelectronic communication system


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Digital to analog conversion


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Digital transmission is Pure digital


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Digital radio = (analog + digital)


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Why not pure digital?

Communication channels are analog

systems by nature.

An analog waveform representing digital

information is created for transmission The analog waveforms representing the

digital information are optimized for

transmission over the communicationsystem.


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Digital modulation

Digital modulation is the transmittal ofdigitally modulated analog signals (carriers)

between two or more points in acommunication system. Also referred as digital radio.

Digital modulation is similar to analogmodulation, the only difference is themodulating signal is a digital signal.

In the cases where an analog signal needs to betransmitted it is first converted into a digital signalbefore it is use to modulated the carrier.

At the receiver the analog signal is regenerated.


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Where it is used?

Broadband Digital Subscriber Lines (DSL)

Digital Microwave and SatelliteCommunication Systems.

Cellular Phones


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Advantages

Digital transceiver is cheaper, faster, and more

power efficient than analog transceiver

Digital modulation is more powerful than analogmodulation in a number of aspects, including

Higher spectral efficiency

Powerful error correction techniques, i.e. it is lesssusceptible to noise and fading

Resistance to channel impairments, e.g. SpreadSpectrum techniques can resist interference

More efficient multiple access strategies, e.g. TDMAand CDMA

Better security and privacy, i.e. it is easy to encrypt


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Information Capacity, Bits, and

Bit Rate Information capacity represents the number of

independent symbols that can be carriedthrough a system in a given unit of time.

The most basic digital symbol used to represent

information is the binary digit, or bit. Therefore, it is often convenient to express the

information capacity of a system as a bit rate.

Bit rate is simply the number of bits transmittedduring one second and is expressed in bits persecond (b/s).


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Bandwidth

Bandwidth : Range of frequencies that can

be transmitted with minimum distortion

Bandwidth : Rate of information transfer

Amount of information that is communicatedbetween source and destination in the giventime

Bandwidth : Measure of the transmissioncapacity of the communication medium


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Hartleys law

Information that can

be transmitted in agiven time is directlyproportional to

bandwidth Information that can

be transmitted in a

given bandwidth isdirectly proportional totransmission time

kBtI =

I=amount of informationthat can be transmitted

(bits per second)B=bandwidth (hertz)

t=time (seconds)

K=constant depends ontype of modulation


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Crude understanding

You want to transfer books from one

shelf to another shelf


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You hire some persons (B) for this

purpose for some time (t) No of books transferred (I) is proportional to

no of persons hired as well as amount of time

you hired

I B t


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Shannon limit for informationcapacity

I=information

capacity (bits persecond)

B = bandwidth

(hertz) S/N= signal to noise

ratio (unit less)

+=

N

SBI 1log2


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Crude understanding You are a book dealer

You want to transfer a lot of books from oneplace to another place (may be to a customer)

You hire some persons for this (B)

But these persons are not very skilled

They may spoil some books while transporting

Let no of good books transported is S

Let no of spoiled books is N


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If you want to maximize the no books

transported to your customer then Hire more persons

Hire skilled persons i.e. persons with goodS/N ratio


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Digital radio


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Digital radio In transmitter

precoder performs Level conversion

Encodes

Output of precoder isapplied to modulator

Modulates the analog

carrier Output of modulator is

amplified and filtered

In receiver incoming

signal is filtered amplified

Amplifier output is

applied to demodulator Produces original

information

Clock and carrierrecovery circuits removecarrier and clockinformation from the

received signal


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Different ways of sending binary

data using analog signal


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Change the frequency


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Change the phase


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Sending multiple bits

simultaneously Each of the three modulation techniques can be

refined to send more than one bit at a time. It is

possible to send two bits on one wave by definingfour different amplitudes.


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Bit and baud rates Bit rate is the number of bits per

second Baud rate is the number of signal

elements per second

Bit rate = Baud rate no of bits perbaud


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Example An analog signal carries 4 bits per

signal element (baud). If 1000 signalelements (bauds) are sent per second,find the bit rate.

Answer

Bit rate=baud rate x no of bits per baud

=1000 x 4 = 4000 bits/sec


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Example An analog signal has a bit rate of 8000

bps and a baud rate of 1000 baud. Howmany data elements (bits) are carriedby each signal element (baud)?

Answer

No of data elements per signal=8000/1000=8

bits/baud


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Simplest case of AM

is where carrier is justturned on or off.


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Digital amplitude modulationSimplest digital modulation

Basically amplitude modulation

Difference: modulating signal is

digital

( )

+= t

Atvtv

cmamcos

2)](1[)(


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( )

+= t

Atvtv

cmamcos

2)](1[)(

Messagesignal

Carrier frequency

Carrier amplitude


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( )( )

( )

0

cos2

]11[)(

cos.

cos2]11[)(

=

=

=

+=

tA

tv

and

tA

tA

tv

cam

c

cam

When message is a normalized binary wave form

with vm(t)= +1 V for logic 1, vm(t)= -1 V for logic 0

Carrier is either on or offOn-off keying (OOK modulation)


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Demodulation Digitally amplitude modulated waves can

be demodulated either coherently or non-coherently

Used in low cost, low quality digital radiosystems

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L21-Introduction to Digital MOdulation

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