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EIE325: Telecommunication Technologies Maciej Ogorzalek, PolyU, EIE
Telecommunication Technologies
Week 7
Digital and Analogue Modulation
Quantisation

EIE325: Telecommunication Technologies Maciej Ogorzalek, PolyU EIE,
Š Digital data, digital signal
„ NRZ, Manchester, HDB3
Š Digital data, analog signal
„ ASK, FSK, PSK
Š Analog data, digital signal
„ PCM, DM, ADPCM
Š Analog data, analog signal
„ AM, FM, PM
Encoding Techniques

Digital Data, Analog Signal
Š Public telephone system
„ »300Hz to »3400Hz
„ Use modem (modulator-demodulator)
Š Amplitude shift keying (ASK)
Š Frequency shift keying (FSK)
Š Phase shift keying (PSK)

Modulation Techniques
ASK
FSK
PSK

Amplitude Shift Keying
Š Values represented by different amplitudes
of carrier
Š Usually, one amplitude is zero
„ i.e. presence and absence of carrier is used
Š Susceptible to sudden gain changes
Š Inefficient
Š Up to 1200bps on voice grade lines
Š Used over optical fiber

Representation
Š Often A2 = 0
Š Used in fibre optics (with multiple carrier
frequencies)

Bandwidth

Frequency Shift Keying
Š Values represented by different
frequencies (near carrier)
Š Less susceptible to error than ASK
Š Up to 1200bps on voice grade lines
Š High frequency radio
Š Even higher frequency on LANs using co-ax

Representation
Š Where typically ½ (f1+f2) = fc

Bandwidth for FSK

FSK on Voice Grade Line

Phase Shift Keying
Š Phase of carrier signal is shifted to
represent data
Š Differential PSK
„ Phase shifted relative to previous transmission
rather than some reference signal

PSK

PSK Constellation

Question…
3-PSK:
Draw the constellation diagram for this (fictitious)
3-PSK system.

Quadrature PSK
Š More efficient use by each signal element
representing more than one bit
„ e.g. shifts of π/2 (90o)
„ Each element represents two bits
„ Can use 8 phase angles and have more than
one amplitude
„ 9600bps modem use 12 angles, four of which
have two amplitudes

Representation
PSK:
QPSK:

4-PSK

4-PSK Characteristics

8-PSK Characteristics

PSK Bandwidth

QAM
Š PSK/ASK/FSK: switch between two states
to encode one bit
Š QPSK: switch between n states to encode
log2 n bits
Š QPSK: changes only one attribute of the
carrier wave (phase)
Š QAM: change two (phase and amplitude)

4-QAM and 8-QAM Constellations
0
0
1
1
0
1
1
0
101 100
011
010
000 001
110
111
4-QAM
1 amplitude, 4 phases
8-QAM
2 amplitude, 4 phases

8-QAM Signal

16-QAM Constellation
16-QAM
3 amplitudes, 12 phases
16-QAM

16-QAM Constellation
16-QAM
16-QAM

Data rate and modulation rate
(revisited)
Š Multiple bits are encoded in each signal
element
Š
„ D: modulation rate (baud)
„ R: data rate (bps)
„ L: number of distinct signal elements
„ b: bits per signal element

Performance of Digital to Analog
Modulation Schemes
Š Bandwidth
„ ASK and PSK bandwidth directly related to bit
rate
„ FSK bandwidth related to data rate for lower
frequencies, but to offset of modulated
frequency from carrier at high frequencies
„ See text (pg. 146-148) for example
Š In the presence of noise, bit error rate of
PSK and QPSK are about 3dB superior to
ASK and FSK

BER performance

Analog Data, Digital Signal
Š Digitization!
„ Conversion of analog data into digital data
„ Digital data can then be transmitted using NRZ-
L, or not
„ Digital data can then be converted to analog
signal
„ Analog to digital conversion done using a
codec
Š Pulse code modulation
Š Delta modulation

Pulse Code Modulation (PCM)
Š The Sampling Theorem: If a signal is sampled at regular
intervals at a rate higher than twice the highest signal
frequency, the samples contain all the information of the
original signal
Š Voice data limited to below 4000Hz
Š Require 8000 sample per second
Š Analog samples (Pulse Amplitude Modulation, PAM)
Š Each sample assigned a digital value

Pulse Code Modulation (PCM)
Š 4 bit system gives 16 levels
Š Quantized
„ Quantizing error or noise
„ Approximations mean it is impossible to recover original exactly
Š 8 bit sample gives 256 levels
Š Quality comparable with analog transmission
Š 8000 samples per second of 8 bits each gives 64kbps

Digital takes more bandwidth
Š Analogue: 4kHz
Š Digital: 4*2*8 = 64 bps requiring 32kHz!
Š Why?
Š So why go digital?
„ repeaters not amplifiers ) no additive noise
„ TDM not FDM ) no intermodulation noise
„ allow for (superior) digital switching

Quantifying quantisation noise
Š Quantising to n bits gives:
(approximately)
Š I.e. each additional bit gives 6 dB
improvement (4 times)

Nonlinear Encoding
Š Quantization levels not evenly spaced
Š Reduces overall signal distortion
Š Can also be done by companding

Companding
(Compressing/Expanding)
Š Instead of nonlinear
quantisation,
nonlinearly scale the
data
Š After companding,
data are more evenly
distributed and can be
quantised linearly
Š used in ⎧-law and A-
law encodings

Delta Modulation
Š Analog input is approximated by a staircase
function
Š Move up or down one level (δ) at each
sample interval
Š Binary behavior
„ Function moves up or down at each sample
interval
„ Function may not be stationary!

Delta Modulation - Performance
Š Good voice reproduction
„ PCM - 128 levels (7 bit)
„ Voice bandwidth 4khz
„ Should be 8000 x 7 = 56kbps for PCM
Š Data compression can improve on this
„ e.g. Interframe coding techniques for video
Š Bad for too fast or too slow changes

In general
Š DM is simpler
Š PCM gives better SNR for same data rate

Analog Data, Analog Signals
Š Why modulate analog signals?
„ Higher frequency can give more efficient
transmission
„ Permits frequency division multiplexing
Š Types of modulation
„ Amplitude
„ Frequency
„ Phase

Amplitude Modulation

Amplitude Modulation:
Algebraic representation
Š Represented by
where
„ m(t): input signal
„ na: modulation index
„ fc: carrier frequency

Example
Š Compute the bandwidth required to
transmit sinusoidal signal with AM
Š x(t) = cos 2πfmt and

Example

Question
Š What bandwidth is required to transmit the
first n harmonics of a periodic signal with
period fs modulated AM at frequency fm?

AM Bandwidth

AM Band Allocation

Sidebands
Š Spectrum is carrier (at fc) plus spectrum of
input signal translated to fc and reflected
about fc
Š Upper sideband: |f|>|fc|
Š Lower sideband: |f|<|fc|
Š Single sideband (SSB): sidebands are
duplicates, only send one

DSBSC, DSBTC and VSB
Š DSBTC: Double sideband transmitted
carrier
Š DSBSC: Double sideband suppressed
carrier
Š SSB: Single sideband
Š VSB: Vestigial sideband

DSBSC, DSBTC and VSB
Š DSBTC
Š DSBSC
Š SSB
Š VSB

FM and PM modulation
Š FM and PM modulation are actually very
similar
Š Indistinguishable without knowledge of the
modulation function
Š PM:
Š FM:

Comparing the phases
Š Compare the phases for PM and FM
Š PM: m(t) is proportional to the phase
Š FM: m(t) is proportional to the derivative of
phase (m΄(t) = 0)

FM Bandwidth

FM Band Allocation

Spread Spectrum
Š Analog or digital data
Š Analog signal
Š Objective: Spread data over wide bandwidth
Š Makes jamming and interception harder
Š Frequency hoping
„ Signal broadcast over seemingly random series of frequencies
Š Direct Sequence
„ Each bit is represented by multiple bits in transmitted signal
„ Chipping code

General model for SS

Spread Spectrum
Š Frequency hoping
„ Signal broadcast over seemingly random series
of frequencies
Š Direct Sequence
„ Each bit is represented by multiple bits in
transmitted signal
„ Chipping code

Direct Sequence

Question
Š What is the bandwidth of a digital data
stream encoded with direct sequence
spreading?

Generating noise
Š Need to generate same “noise” at source
and destination
Š Computers are deterministic – generating
true noise is not possible (i.e. no truly
random numbers)
Š May use
„ pseudo-random algorithm
„ predetermined sequences (e.g. Gold
sequences)
„ chaos

Test your knowledge of digital communication systems with our interactive quiz! Explore various aspects of communication technologies and enhance your understanding. Enjoy learning!"

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Test your knowledge of digital communication systems with our interactive quiz! Explore various aspects of communication technologies and enhance your understanding. Enjoy learning!"