Digital Modulation for TEC Dated 25.04.2011

7/30/2019 Digital Modulation for TEC Dated 25.04.2011

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DIGITAL MODULATION


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Developers of communications systems face these constraints w.r.tRF spectrum and power:

Available bandwidth

Permissible power

Inherent noise level of the system

The RF spectrum must be shared, yet every day there are more users

for that spectrum as demand for communications services increases.

Digital modulation schemes have greater capacity to convey large

amounts of information than analog modulation

Basic Concepts


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There is a fundamental tradeoff in communication systems. Simple

hardware can be used in transmitters and receivers to communicateinformation. However, this uses a lot of spectrum which limits the

number of users.

Alternatively, more complex transmitters and receivers can be used to

transmit the same information over less bandwidth. The transition to

more and more spectrally efficient transmission techniques requires moreand more complex hardware. Complex hardware is difficult to design,

test, and build. This tradeoff exists whether communication is over air or

wire, analog or digital.

Basic Concepts


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Basic Concepts

A communication system has mainly 3 entities

Information (Base-Band)

Medium

Carrier


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Basic Concepts

Modulation:

The process which places the signal information on to sine wave

carriers is called Modulation.

Definition

The process by which some characteristics of the carrier i.e.amplitude, frequency or phase is varied in accordance with the

instantaneous value of the modulating signal.

Modulation may be

Analog Modulation Digital Modulation


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Signal characteristics that can be modified

There are only three characteristics of a signal that can be changed over

time:Amplitude

Phase

Frequency

However, phase and frequency are just different ways to view or

measure the same signal change.

Transmitting information: To transmit a signal over the air, there are

three main steps:

1. A pure carrier is generated at the transmitter.2. The carrier is modulated with the information to be transmitted. Any

reliably detectable change in signal characteristics can carry information.

3. At the receiver the signal modifications or changes are detected and

demodulated.


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Three types of modulation is:

Amplitude Modulation: In AM, the amplitude of a high-frequency carrier

signal is varied in proportion to the instantaneous amplitude of the modulatingmessage signal.

Frequency Modulation: FM is the most popular analog modulation technique

used in mobile communications systems. In FM, the amplitude of the

modulating carrier is kept constant while its frequency is varied by themodulating message signal.

Phase Modulation: In PM the phase is varied by the modulating message

signal.


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Polar display: Magnitude and phase represented together.

A simple way to view amplitude and phase is with the polar diagram. The

signal can be expressed in polar form as a magnitude and a phase. The

phase is relative to a reference signal, the carrier in most communication

systems. The magnitude is either an absolute or relative value. Both are

used in digital communication systems. Polar diagrams are the basis ofmany displays used in digital communications, although it is common to

describe the signal vector by its rectangular coordinates ofI(In-phase) and

Q (Quadrature).

Digital Modulation: Terminology


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Signal changes or modifications in polar form

Figure shows different forms of modulation in polar form. Magnitude isrepresented as the distance from the center and phase is represented as the angle.

In Amplitude modulation (AM) only the magnitude of the signal changes.

In Phase modulation (PM) only the phase of the signal Changes.

Note that Amplitude and Phase both can be changed together.

Frequency modulation (FM) looks similar to phase modulation.



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In digital communications, modulation is often expressed in terms ofI and Q.

This is a rectangularrepresentation of the polar diagram. On a polar diagram,

theI axis lies on the zero degree phase reference, and the Q axis is rotated by90 degrees. The signal vectors projection onto theI axis is its I component

and the projection onto the Q axis is its Q component.

Digital modulation maps the data to a number of discrete points on the I/Q

plane. These are known as constellation points.


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I and Q in a radio transmitter:

I/Q diagrams are particularly useful because they mirror the way most

digital communications signals are created using an I/Q modulator. In the

transmitter, I and Q signals are mixed with the same local oscillator (LO).A 90 degree phase shifter is placed in one of the LO paths. Signals that are

separated by 90 degrees are also known as being orthogonal to each other

or in quadrature. Signals that are in quadrature do not interfere with each

other. They are two independent components of the signal. When

recombined, they are summed to a composite output signal. There are twoindependent signals in I and Q that can be sent and received with simple

circuits.


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I and Q in a radio receiver

The composite signal with magnitude and phase (or I and Q) information

arrives at the receiver input. The input signal is mixed with the local oscillator

signal at the carrier frequency in two forms. One is at an arbitrary zero phase.The other has a 90 degree phase shift. The composite input signal (in terms of

magnitude and phase) is thus broken into an in-phase, I, and a quadrature, Q,

component. These two components of the signal are independent and

orthogonal.


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Digital Modulation: Types

The varoius types of digital modulation are:

Frequency Shift Keying (FSK)

Phase Shift Keying (PSK)

Minimum Shift Keying (MSK) Quadrature Amplitude Modulation (QAM)


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Frequency Shift Keying Frequency of the carrier is varied in accordance with the

amplitude of the modulating signal and the carrier

amplitude remains constant.

F1

F2

FSK

MODBB

Digital Modulation methods


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FSK


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Phase Shift Keying (PSK)

The phase of the carrier is varied in accordance with the

information.

PSK is divided into two level and multilevel systems (M-ary

schemes).

Types of PSK are:

Binary Phase Shift Keying (BPSK)

Quadrature Phase Shift Keying (QPSK)

Offset Quadrature Phase Shift Keying (O-QPSK)

8PSK



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PSK


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BPSK


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BPSK


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QPSK

The modulated output signal is shifted by four phases inaccordance with the input binary data.

QPSK requires two input bits for each phase shift.ie each

symbol carries 2 bits. Thereforesymbol rate in QPSK=Bitrate/2

In QPSK, four different phases are represented by 45deg,135deg, -45deg & +135 deg.

When the input digit changes from 00 to 11 or 01 to 10output phase changes by 180 degrees


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QPSK


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Binary InputQPSK o/p

phase

0 0

0 1

1 0

1 1

-135

-45

+135

+45

I Q

+/4 +3/4 -/4 -3/4


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QPSK


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O-QPSK (Offset QPSK)

O-QPSK (Offset QPSK)

A variant of QPSK.

The bit wave forms for the I and Q channels are offset orshifted in phase from each other by one half of the bit time.

The range of phase transitions is 0 degree & 90 degrees

(the possibility of phase shift of 180 degrees is eliminated)


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In QPSK, the I and Q bit streams are switched at the same time. The symbol

clocks are synchronized. In Offset QPSK (OQPSK), the I and Q bit streams

are offset in their relative alignment by one bit period (one half of a symbol

period).Since the transitions of I and Q are offset, at any given time only one of the

two bit streams can change values. This creates a dramatically different

constellation, even though there are still just two I/Q values.


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Minimum Shift Keying

The rectangular symbol pulse is replaced by a half cycle

sinusoidal symbol pulse.

In MSK, also the phase shifts can be detected with an I or

Q modulator.

At even numbered symbols, the polarity of I channel

conveys the transmitted data and at odd numbered symbols

the polarity of Q channel conveys the data.

A phase shift of + 90 degrees represents a data bit equal to

1 and a phase shift of90 degrees represents a data bit

equal to zero.

MSK with a gaussian filter is termed as GMSK

Q d t A lit d M d l ti


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Quadrature Amplitude Modulation

Another member of the digital modulation family is Quadrature

Amplitude modulation (QAM). QAM. The types of QAM are

16-QAM

32 QAM

64 QAM

256 QAM



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16-state Quadrature Amplitude Modulation (16QAM), there are fourI

values and four Q values. This results in a total of 16 possible states forthe signal. It can transition from any state to any other state at every

symbol time. The symbol rate is one fourth of the bit rate. So this

modulation format produces a more spectrally efficient transmission. It

is more efficient than BPSK, QPSK, or 8PSK. Note that QPSK is the

same as 4QAM.



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Another variation is 64QAM. In this case there are 8 I values and 8 Q

values. Since 26 =64, there are Six bits per symbol and the symbol rate

is one sixth of the bit rate.

The current practical limits are approximately 256QAM, though work is

underway to extend the limits to 512 or 1024 QAM. A 256QAM system

uses 16I-values and 16 Q-values, giving 256 possible states. Since 28 =

256, each symbol can represent eight bits. A 256QAM signal that cansend eight bits per symbol is very spectrally efficient.

However, the symbols are very close together and are thus more subject

to errors due to noise and distortion. Such a signal may have to be

transmitted with extra power (to effectively spread the symbols out

more) and this reduces power efficiency as compared to simpler

schemes.


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64 QAM Constellation


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Adaptive Modulation


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THANKYOU

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Digital Modulation for TEC Dated 25.04.2011