Digital modulation basics(nnm)
Transcript of Digital modulation basics(nnm)
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
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
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
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…
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
Minimum Shift Keying
When the frequency of the separation becomes lowest it is known as minimum shift keying (MSK)
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
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.
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.
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.
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