Digital Modulation Basics

N N Maurya

Why Digital Modulation ?

More information capacity

Compatibility with digital data services

Higher data security

Better quality communications

Digital modulation schemes have greater capacity to convey large amounts of information than analog modulation schemes

Constraints ?

Available bandwidth

Permissible power

Inherent noise level of the system

Trading off

There is a fundamental tradeoff in communication systems

This tradeoff exists whether communication is over air or wire, analog or digital

Spectrally efficient transmission techniques require more and more complex hardware

Trends in the Industry

Block Diagram of Digital Communication System

Classification of Communication System

Most communication systems can be classified into one of three different categories:– Bandwidth efficient

Ability of system to accommodate data within a prescribed bandwidth

– Power efficientReliable sending of data with minimal power

requirements– Cost efficient

System needs to be affordable in the context of its use

Types of Digital Modulation System

COHERENT

NON- COHERENT

Coherent (synchronous) detection: process receives signal with a local carrier of same frequency and phase

Non coherent (envelope) detection: requires no reference wave

TYPES OF DIGITAL MODULATION SYSTEM……

Coherent detection Receiver uses the carrier phase to detect signal

Cross correlate with replica signals at receiver

Match within threshold to make decision

Non coherent detection Does not exploit phase reference information

Less complex receiver, but worse performance

Hierarchy of digital modulation technique

Digital modulation techniques

Amplitude shift keying (ASK) Frequency shift keying (FSK) Phase shift keying (PSK) Quadrature phase shift keying (QPSK) Quadrature amplitude modulation (QAM)

Metrics for Digital Modulation

Power Efficiency Power efficiency is a measure of how much

signal power should be increased to achieve a particular BER for a given modulation scheme

Ability of a modulation technique to preserve the fidelity of the digital message at low power levels

Designer can increase noise immunity by increasing signal power

Signal energy per bit / noise power spectral density: Eb / N0

Metrics for Digital Modulation…

Bandwidth Efficiency Ability to accommodate data within a limited

bandwidth

Tradeoff between data rate and pulse width

Data rate per hertz: R/B bps per Hz

Shannon Limit: Channel capacity / bandwidth C = B log2(1 + S/N) OR

C/B = log2(1 + S/N)

Considerations in Choice ofModulation Scheme

High spectral efficiency High power efficiency Robust to multipath effects Low cost and ease of implementation Low carrier-to-cochannel interference

ratio Low out-of-band radiation Constant or near constant envelope

Constant: only phase is modulated Non-constant: phase and amplitude modulated

Amplitude Shift Keying

Digital

information

1 0 1 1 0 0 1 0 1 0

Carrier wave

ASK

modulated

signal

Carrier present Carrier absent

Amplitude varying-

frequency constant

ASK Generation

Lower Side band Upper Side band

Band width=2 X Modulating freq.

00

1)2cos()(

tfAts

cc

1 1 1 1 1 0 1 0 0 0 1 1 0 1 0 1 0 0 1 0 1 1 1

t

t

tf

tASK

tf tASK

tA cc sin

n

nn awhereT

nTtrectatf

0

1

tAtft ccASK sin

0

sin tAt

cc

ASK

(logic 1)

(logic 0)

ASK Generation…

ASK Detection

Necessity to shape the pulse

A pulse contains infinite number of harmonics and hence bandwidth

Frequency shift keying

Digital

information

1 0 1 1 0 0 1

Carrier 1

(frequency #1)

FSK

modulated

signal

Carrier 2

(frequency #2)

Frequency varying-

amplitude constant

FSK Generation

0)2cos(

1)2cos()(

2

1

btfA

btfAts

c

c

FSK Detection

Minimum Shift Keying

When the frequency of the separation becomes lowest it is known as minimum shift keying (MSK)

Phase Shift Keying

PSK Generation

1800 shift

0)2cos(

1)2cos()(

btfA

btfAts

cc

cc

PSK Generation…

n

nn awhereT

nTtrectatf

0

1

tAtft ccPSK cos

tf tPSK

tA cc cos

1 1 1 1 1 0 1 0 0 0 1 1 0 1 0 1 0 0 1 0 1 1 1

t

t

tf

tPSK

~

~tf02cos

tf02cos

Signal Vector Representation

Phase

S

0 degreesI

Q

I-Q

Plane

s(t) = Ac(t) cos (2 fct + θ(t))

fixed!!!

t =

0

t = t

θ =

90

θ = 0

S1

S2 I

QMagnitude

Change

S1

S2

I

QPhase

Change

S1

S2

I

QMagnitude

& Phase

Changes

I-Q Diagrams

or

Constellations

Signal Changes: Representation in the I-Q plane

Constellation

QPSK The only way to achieve high data rates with a

narrowband channel is to increase the number of bits/symbol

The most reliable way to do this is with a combination of amplitude and phase modulation called quadrature amplitude modulation (QAM)

Quadrature Phase Shift Keying is effectively twoindependent BPSK

QPSK systems (I and Q), exhibits the same performance but twice the bandwidth efficiency of that of BPSK.

Large envelope variations occur during phase transitions, thus requiring linear amplification.

QPSK constellation

Basic QPSK constellation QPSK constellation Shifted by 450

QPSK……

Carrier phase is changed by 450 ,1350, 2250, 3150

00 10 11 10 01

QPSK Generation

QPSK Detection

Types of QPSK Conventional QPSK has transitions through zero

(i.e. 180 phase transition). Highly linear amplifier required.

In Offset QPSK, the transitions on the I and Q channels are staggered.

Phase transitions are therefore limited to 90degrees.

In /4-QPSK the set of constellation points are toggled each symbol, so transitions through zero cannot occur. This scheme produces the lowest envelope variations.

All QPSK schemes require linear power amplifiers.

Offset QPSK

/4 QPSK

Multi-level (M-ary) Phase and Amplitude Modulation Amplitude and phase shift keying can be combined

to transmit several bits per symbol These modulation schemes are often referred to

as linear, as they require linear amplification Amplitude modulation on both quadrature carriers 2^n discrete levels, n = 2 same as QPSK 16-QAM has the largest distance between points,

but requires very linear amplification. 16-PSK has less stringent linearity requirements, but has less spacing between constellation points, and is therefore more affected by noise

M-ary schemes are more bandwidth efficient, but more susceptible to noise.

16-QAM

16-QAM Generation

16-QAM Detection

16QAM with different impairments

AWGN

Loss

of

Sync

Interference

Phase

Noise

Application

Bandwidth efficiency limits

Thank You!

Actual example

Here is a 16-level constellation which is reconstructed in the presence of noise

-2 -1.5 -1 -0.5 0 0.5 1 1.5 2-2

-1.5

-1

-0.5

0

0.5

1

1.5

2Eb/No=5 dB

Defining decision regions An easy detection method, is to compute “decision

regions” offline. Here are a few examples

decide s1

decide s2

s1s2

measurement

decide s1decide s2

decide s3 decide s4

s1s2

s3 s4

decide s1

s1

On-off keying (OOK)

Simplest/oldest form of modulation

Morse code (1837) – developed for telegraphy

Eye diagrams