7/31/2019 L21-Introduction to Digital MOdulation
1/34
Introduction to DigitalModulation
7/31/2019 L21-Introduction to Digital MOdulation
2/34
7/31/2019 L21-Introduction to Digital MOdulation
3/34
Simplified block diagram ofelectronic communication system
7/31/2019 L21-Introduction to Digital MOdulation
4/34
Digital to analog conversion
7/31/2019 L21-Introduction to Digital MOdulation
5/34
Digital transmission is Pure digital
7/31/2019 L21-Introduction to Digital MOdulation
6/34
Digital radio = (analog + digital)
7/31/2019 L21-Introduction to Digital MOdulation
7/34
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.
7/31/2019 L21-Introduction to Digital MOdulation
8/34
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.
7/31/2019 L21-Introduction to Digital MOdulation
9/34
Where it is used?
Broadband Digital Subscriber Lines (DSL)
Digital Microwave and SatelliteCommunication Systems.
Cellular Phones
7/31/2019 L21-Introduction to Digital MOdulation
10/34
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
7/31/2019 L21-Introduction to Digital MOdulation
11/34
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).
7/31/2019 L21-Introduction to Digital MOdulation
12/34
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
7/31/2019 L21-Introduction to Digital MOdulation
13/34
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
7/31/2019 L21-Introduction to Digital MOdulation
14/34
Crude understanding
You want to transfer books from one
shelf to another shelf
7/31/2019 L21-Introduction to Digital MOdulation
15/34
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
7/31/2019 L21-Introduction to Digital MOdulation
16/34
Shannon limit for informationcapacity
I=information
capacity (bits persecond)
B = bandwidth
(hertz) S/N= signal to noise
ratio (unit less)
+=
N
SBI 1log2
7/31/2019 L21-Introduction to Digital MOdulation
17/34
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
7/31/2019 L21-Introduction to Digital MOdulation
18/34
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
7/31/2019 L21-Introduction to Digital MOdulation
19/34
Digital radio
7/31/2019 L21-Introduction to Digital MOdulation
20/34
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
7/31/2019 L21-Introduction to Digital MOdulation
21/34
Different ways of sending binary
data using analog signal
7/31/2019 L21-Introduction to Digital MOdulation
22/34
Change the frequency
7/31/2019 L21-Introduction to Digital MOdulation
23/34
Change the phase
7/31/2019 L21-Introduction to Digital MOdulation
24/34
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.
7/31/2019 L21-Introduction to Digital MOdulation
25/34
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
7/31/2019 L21-Introduction to Digital MOdulation
26/34
7/31/2019 L21-Introduction to Digital MOdulation
27/34
7/31/2019 L21-Introduction to Digital MOdulation
28/34
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
7/31/2019 L21-Introduction to Digital MOdulation
29/34
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
7/31/2019 L21-Introduction to Digital MOdulation
30/34
Simplest case of AM
is where carrier is justturned on or off.
7/31/2019 L21-Introduction to Digital MOdulation
31/34
Digital amplitude modulationSimplest digital modulation
Basically amplitude modulation
Difference: modulating signal is
digital
( )
+= t
Atvtv
cmamcos
2)](1[)(
7/31/2019 L21-Introduction to Digital MOdulation
32/34
( )
+= t
Atvtv
cmamcos
2)](1[)(
Messagesignal
Carrier frequency
Carrier amplitude
7/31/2019 L21-Introduction to Digital MOdulation
33/34
( )( )
( )
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)
7/31/2019 L21-Introduction to Digital MOdulation
34/34
Demodulation Digitally amplitude modulated waves can
be demodulated either coherently or non-coherently
Used in low cost, low quality digital radiosystems