DIGITAL MODULATION

Lecture 7

Ir. Muhamad Asvial, MEng., PhDCenter for Information and Communication Engineering Research

Electrical Engineering Department – University of Indonesia Kampus UI Depok, 16424 – Indonesia

asvial@ee.ui.ac.idhttp://www.ee.ui.ac.id

Digital Baseband TransmissionDigital Baseband Transmission• Why to apply digital transmission?• Symbols and bits• Binary PAM Formats• Baseband transmission

– Binary error probabilities in baseband transmission• Pulse shaping

– minimizing ISI and making bandwidth adaptation – maximizing SNR at the instant of sampling - matched filtering– optimal terminal filters

• Determination of transmission bandwidth as a function of pulse shape– Spectral density of Pulse Amplitude Modulation (PAM)

• Equalization - removing residual ISI

Why to Apply Digital Transmission?Why to Apply Digital Transmission?

• Digital communication withstands channel noise, interference and distortionbetter than analog system. For instance in PSTN inter-exchange STP*-links NEXT (Near-End Cross-Talk) produces several interference. For analog systems interference must be below 50 dB whereas in digital system 20 dB is enough. With this respect digital systems can utilize lower quality cabling than analog systems

• Regenerative repeaters are efficient. Note that cleaning of analog-signals by repeaters does not work as well

• Digital HW/SW implementation is straightforward• Circuits can be easily reconfigured and preprogrammed by DSP techniques

(an application: software radio)• Digital signals can be coded to yield very low error rates• Digital communication enables efficient exchanging of SNR to BW-> easy

adaptation into different channels• The cost of digital HW continues to halve every two or three years

DigitalDigitalTransmissionTransmission

Information:- analog:BW & dynamic range- digital:bit rate

Maximization of information transferred

Transmitted power;bandpass/baseband signal BW

Message protection & channel adaptation;convolution, block coding

M-PSK/FSK/ASK..., depends on channel BW & characteristics

wireline/wirelessconstant/variablelinear/nonlinear

Noise

Interference

Channel

Modulator

ChannelEncoder

Source encoder

Channel decoder

Source decoder

Demodulator

Information sink

Information source

Message Message estimate

Received signal(may contain errors)Transmitted

signal

Interleaving

Fights against burst errors

Deinterleaving

In baseband systemsthese blocks are missing

Formatting and transmission of baseband Formatting and transmission of baseband signalsignal

ec][symbols/s /1 TR [bits/sec] /1 bb TR

Sampling at rate

(sampling time=Ts)

Quantizing each sampledvalue to one of the

L levels in quantizer.

Encoding each q. value to bits

(Data bit duration Tb=Ts/l)

EncodePulse

modulateSample Quantize

Pulse waveforms(baseband signals)

Bit stream(Data bits)Format

Digital info.

Textual info.

Analog info.

source

Mapping every data bits to a symbol out of M symbols and transmitting

a baseband waveform with duration T

ss Tf /1 Ll 2log

Mm 2log

ec][symbols/s /1 TR [bits/sec] /1 bb TR

Sampling at rate

(sampling time=Ts)

Quantizing each sampledvalue to one of the

L levels in quantizer.

Encoding each q. value to bits

(Data bit duration Tb=Ts/l)

EncodePulse

modulateSample Quantize

Pulse waveforms(baseband signals)

Bit stream(Data bits)Format

Digital info.

Textual info.

Analog info.

source

Mapping every data bits to a symbol out of M symbols and transmitting

a baseband waveform with duration T

ss Tf /1 Ll 2log

Mm 2log

Sampling at rate

(sampling time=Ts)

Quantizing each sampledvalue to one of the

L levels in quantizer.

Encoding each q. value to bits

(Data bit duration Tb=Ts/l)

EncodePulse

modulateSample Quantize

Pulse waveforms(baseband signals)

Bit stream(Data bits)Format

Digital info.

Textual info.

Analog info.

source

Mapping every data bits to a symbol out of M symbols and transmitting

a baseband waveform with duration T

ss Tf /1 Ll 2log

Mm 2log

Channel ModelChannel Model

Discrete

Source

Series to

ParallelMapper Waveform

Selector

WaveformDetectorDemapperParallel to

Series

Discrete

sink

+

channels

Symbols and BitsSymbols and Bits

:

::

number of bits: number of levels Symbol durationBit duaration

b

nMDT

1 1 00 1 11 110 1 0

bi ( 1/ ) ts/sb b bT r Tbitrate ( 1/ )D r Dsymbol rate baud

2nM

2logn M

( )s t

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

1

0

10110100

10110100

111101100110

010011001000

Binary Modulation

Quaternary Modulation

Octonary Modulation

)(8 tx ASK

)()(2 txtx BPSKASK

)(4 tx ASK

)(8 tx ASK

Pulse Shaping and BandPulse Shaping and Band--limited Transmission limited Transmission

• In digital transmission signaling pulse shape is chosen to satisfy the following requirements:– yields maximum SNR at the time instance of decision (matched

filtering)– accommodates signal to channel bandwidth:

• rapid decrease of pulse energy outside the main lobe in frequency domain alleviates filter design

• lowers cross-talk in multiplexed systems

Demodulation and detectionDemodulation and detection

• Major sources of errors:– Thermal noise (AWGN)

• disturbs the signal in an additive fashion (Additive)• has flat spectral density for all frequencies of interest (White)• is modeled by Gaussian random process (Gaussian Noise)

– Inter-Symbol Interference (ISI)• Due to the filtering effect of transmitter, channel and receiver, symbols are

“smeared”.

Format Pulse modulate

Bandpassmodulate

Format Detect Demod.& sample

)(tsi)(tgiim

im̂ )(tr)(Tz

channel)(thc

)(tn

transmitted symbol

estimated symbol

Mi ,,1M-ary modulation

Digital Bandpass TransmissionDigital Bandpass Transmission• Detection techniques

– Coherent– Non-coherent– Differentially coherent

• Examples of coherent and non-coherent detection error rate analysis

• A method for ‘analyzing’ PSK error rates• Effect of synchronization and envelope distortion (PSK)• Comparison: Error rate describing

– reception sensitivity– bandwidth efficiency

Detection TypesDetection Types• Number of allocated signaling levels determines

constellation diagram (=lowpass equivalent of the applied digital modulation format)

• At the receiver, detection can be– coherent (carrier phase information used for detection)– non- coherent (no carrier phase used for detection)– differentially coherent (‘local oscillator’ synthesized

from received bits)

( ) ( )h t s t

0( ) ( ) ( )v t s t y d

( )v T

( )v T

NonNon--coherent Detectioncoherent Detection

2-ASK

2-FSK

• Base on filtering signal energy on allocated spectra and using envelope detectors

• Has performance degradation of about 1-3 dB when compared to coherent detection (depending on Eb/N0)

• Examples:

Error Rate ComparisonError Rate Comparison

a: Coherent BPSKb: DPSKc:Coherent OOKd: Noncoherent FSKe: noncoherent OOK

Comparison of Quadrature Modulation Comparison of Quadrature Modulation MethodsMethods

APK=MQASK