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

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


Digital communication

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

2. EIE325: Telecommunication TechnologiesMaciej Ogorzalek, PolyU EIE,
ŠDigital data, digital signal „
NRZ, Manchester, HDB3
ŠDigital data, analog signal „
ASK, FSK, PSK
ŠAnalog data, digital signal „
PCM, DM, ADPCM
ŠAnalog data, analog signal „
AM, FM, PM
Encoding Techniques

3. EIE325: Telecommunication TechnologiesMaciej Ogorzalek, PolyU EIE,
Digital Data, Analog Signal
ŠPublic telephone system
„
»300Hz to »3400Hz
„
Use modem (modulator-demodulator)
ŠAmplitude shift keying (ASK)
ŠFrequency shift keying (FSK)
ŠPhase shift keying (PSK)

4. EIE325: Telecommunication TechnologiesMaciej Ogorzalek, PolyU EIE,
Modulation Techniques
ASK
FSK
PSK

5. EIE325: Telecommunication TechnologiesMaciej Ogorzalek, PolyU EIE,
Amplitude Shift Keying
ŠValues represented by different amplitudes
of carrier
ŠUsually, one amplitude is zero
„
i.e. presence and absence of carrier is used
ŠSusceptible to sudden gain changes
ŠInefficient
ŠUp to 1200bps on voice grade lines
ŠUsed over optical fiber

6. EIE325: Telecommunication TechnologiesMaciej Ogorzalek, PolyU EIE,
Representation
ŠOften A
2
= 0
ŠUsed in fibre optics (with multiple carrier
frequencies)

7. EIE325: Telecommunication TechnologiesMaciej Ogorzalek, PolyU EIE,
Bandwidth

8. EIE325: Telecommunication TechnologiesMaciej Ogorzalek, PolyU EIE,
Frequency Shift Keying
ŠValues represented by different
frequencies (near carrier)
ŠLess susceptible to error than ASK
ŠUp to 1200bps on voice grade lines
ŠHigh frequency radio
ŠEven higher frequency on LANs using co-ax

9. EIE325: Telecommunication TechnologiesMaciej Ogorzalek, PolyU EIE,
Representation
ŠWhere typically ½(
f
1
+
f
2
) =
f
c

10. EIE325: Telecommunication TechnologiesMaciej Ogorzalek, PolyU EIE,
Bandwidth for FSK

11. EIE325: Telecommunication TechnologiesMaciej Ogorzalek, PolyU EIE,
FSK on Voice Grade Line

12. EIE325: Telecommunication TechnologiesMaciej Ogorzalek, PolyU EIE,
Phase Shift Keying
ŠPhase of carrier signal is shifted to
represent data
ŠDifferential PSK
„
Phase shifted relative to previous transmission
rather than some reference signal

13. EIE325: Telecommunication TechnologiesMaciej Ogorzalek, PolyU EIE,
PSK

14. EIE325: Telecommunication TechnologiesMaciej Ogorzalek, PolyU EIE,
PSK Constellation

15. EIE325: Telecommunication TechnologiesMaciej Ogorzalek, PolyU EIE,
Question…
3-PSK:
Draw the constellation diagram for this (fictitious)
3-PSK system.

16. EIE325: Telecommunication TechnologiesMaciej Ogorzalek, PolyU EIE,
Quadrature PSK
ŠMore efficient use by each signal element
representing more than one bit
„
e.g. shifts of π/2 (90
o
)
„
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

17. EIE325: Telecommunication TechnologiesMaciej Ogorzalek, PolyU EIE,
Representation
PSK:
QPSK:

18. EIE325: Telecommunication TechnologiesMaciej Ogorzalek, PolyU EIE,
4-PSK

19. EIE325: Telecommunication TechnologiesMaciej Ogorzalek, PolyU EIE,
4-PSK Characteristics

20. EIE325: Telecommunication TechnologiesMaciej Ogorzalek, PolyU EIE,
8-PSK Characteristics

21. EIE325: Telecommunication TechnologiesMaciej Ogorzalek, PolyU EIE,
PSK Bandwidth

22. EIE325: Telecommunication TechnologiesMaciej Ogorzalek, PolyU EIE,
QAM
ŠPSK/ASK/FSK: switch between two states
to encode one bit
ŠQPSK: switch between
n
states to encode
log
2
n
bits
ŠQPSK: changes only one attribute of the
carrier wave (phase)
ŠQAM: change two (phase and amplitude)

23. EIE325: Telecommunication TechnologiesMaciej Ogorzalek, PolyU EIE,
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

24. EIE325: Telecommunication TechnologiesMaciej Ogorzalek, PolyU EIE,
8-QAM Signal

25. EIE325: Telecommunication TechnologiesMaciej Ogorzalek, PolyU EIE,
16-QAM Constellation
16-QAM
3 amplitudes, 12 phases
16-QAM
4 amplitudes, 8 phases

26. EIE325: Telecommunication TechnologiesMaciej Ogorzalek, PolyU EIE,
16-QAM Constellation
16-QAM
4 amplitudes, 8 phases
16-QAM
2 amplitudes, 8 phases

27. EIE325: Telecommunication TechnologiesMaciej Ogorzalek, PolyU EIE,
Data rate and modulation rate
(revisited)
ŠMultiple 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

28. EIE325: Telecommunication TechnologiesMaciej Ogorzalek, PolyU EIE,
Performance of Digital to Analog
Modulation Schemes
ŠBandwidth
„
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
ŠIn the presence of noise, bit error rate of
PSK and QPSK are about 3dB superior to
ASK and FSK

29. EIE325: Telecommunication TechnologiesMaciej Ogorzalek, PolyU EIE,
BER performance

30. EIE325: Telecommunication TechnologiesMaciej Ogorzalek, PolyU EIE,
ŠDigital data, digital signal „
NRZ, Manchester, HDB3
ŠDigital data, analog signal „
ASK, FSK, PSK
ŠAnalog data, digital signal „
PCM, DM, ADPCM
ŠAnalog data, analog signal „
AM, FM, PM
Encoding Techniques

31. EIE325: Telecommunication TechnologiesMaciej Ogorzalek, PolyU EIE,
Analog Data, Digital Signal
ŠDigitization!
„
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
ŠPulse code modulation
ŠDelta modulation

32. EIE325: Telecommunication TechnologiesMaciej Ogorzalek, PolyU EIE,
Pulse Code Modulation (PCM)
ŠThe 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
ŠVoice data limited to below 4000Hz
ŠRequire 8000 sample per second
ŠAnalog samples (Pulse Amplitude Modulation, PAM)
ŠEach sample assigned a digital value

33. EIE325: Telecommunication TechnologiesMaciej Ogorzalek, PolyU EIE,
Pulse Code Modulation (PCM)
Š4 bit system gives 16 levels
ŠQuantized
„
Quantizing error or noise
„
Approximations mean it is impossible to recover original exactly
Š8 bit sample gives 256 levels
ŠQuality comparable with analog transmission
Š8000 samples per second of 8 bits each gives 64kbps

34. EIE325: Telecommunication TechnologiesMaciej Ogorzalek, PolyU EIE,
Digital takes more bandwidth
ŠAnalogue: 4kHz
ŠDigital: 4*2*8 = 64 bps requiring 32kHz!
ŠWhy?
ŠSo why go digital?
„
repeaters not amplifiers )no additive noise
„
TDM not FDM )no intermodulation noise
„
allow for (superior) digital switching

35. EIE325: Telecommunication TechnologiesMaciej Ogorzalek, PolyU EIE,
Quantifying quantisation noise
ŠQuantising to n bits gives:
(approximately)
ŠI.e. each additional bit gives 6 dB
improvement (4 times)

36. EIE325: Telecommunication TechnologiesMaciej Ogorzalek, PolyU EIE,
Nonlinear Encoding
ŠQuantization levels not evenly spaced
ŠReduces overall signal distortion
ŠCan also be done by companding

37. Nonlinear Quantisation

38. EIE325: Telecommunication TechnologiesMaciej Ogorzalek, PolyU EIE,
Companding
(Compressing/Expanding)
ŠInstead of nonlinear
quantisation,
nonlinearly scale the
data
ŠAfter companding,
data are more evenly
distributed and can be
quantised linearly
Šused in ⎧-law and A-
law encodings

39. Companding

40. EIE325: Telecommunication TechnologiesMaciej Ogorzalek, PolyU EIE,
Delta Modulation
ŠAnalog input is approximated by a staircase
function
ŠMove up or down one level (δ) at each
sample interval
ŠBinary behavior
„
Function moves up or down at each sample
interval
„
Function may not
be stationary!

41. Delta Modulation -example

42. EIE325: Telecommunication TechnologiesMaciej Ogorzalek, PolyU EIE,
Delta Modulation -Performance
ŠGood voice reproduction
„
PCM -128 levels (7 bit)
„
Voice bandwidth 4khz
„
Should be 8000 x 7 = 56kbps for PCM
ŠData compression can improve on this
„
e.g. Interframe coding techniques for video
ŠBad for too fast or too slow changes

43. EIE325: Telecommunication TechnologiesMaciej Ogorzalek, PolyU EIE,
In general
ŠDM is simpler
ŠPCM gives better SNR for same data rate

44. EIE325: Telecommunication TechnologiesMaciej Ogorzalek, PolyU EIE,
ŠDigital data, digital signal „
NRZ, Manchester, HDB3
ŠDigital data, analog signal „
ASK, FSK, PSK
ŠAnalog data, digital signal „
PCM, DM, ADPCM
ŠAnalog data, analog signal „
AM, FM, PM
Encoding Techniques

45. EIE325: Telecommunication TechnologiesMaciej Ogorzalek, PolyU EIE,
Analog Data, Analog Signals
ŠWhy modulate analog signals?
„
Higher frequency can give more efficient
transmission
„
Permits frequency division multiplexing
ŠTypes of modulation
„
Amplitude
„
Frequency
„
Phase

46. Analog
Modulation

47. EIE325: Telecommunication TechnologiesMaciej Ogorzalek, PolyU EIE,
Amplitude Modulation

48. EIE325: Telecommunication TechnologiesMaciej Ogorzalek, PolyU EIE,
Amplitude Modulation:
Algebraic representation
ŠRepresented by
where
„
m
(
t
): input signal
„
n
a
: modulation index
„
f
c
: carrier frequency

49. EIE325: Telecommunication TechnologiesMaciej Ogorzalek, PolyU EIE,
Example
ŠCompute the bandwidth required to
transmit sinusoidal signal with AM
Š
x
(
t
) = cos 2π
f
m
t
and

50. EIE325: Telecommunication TechnologiesMaciej Ogorzalek, PolyU EIE,
Example

51. EIE325: Telecommunication TechnologiesMaciej Ogorzalek, PolyU EIE,
Question
ŠWhat bandwidth is required to transmit the
first
n
harmonics of a periodic signal with
period
f
s
modulated AM at frequency
f
m
?

52. EIE325: Telecommunication TechnologiesMaciej Ogorzalek, PolyU EIE,
AM Bandwidth

53. EIE325: Telecommunication TechnologiesMaciej Ogorzalek, PolyU EIE,
AM Band Allocation

54. EIE325: Telecommunication TechnologiesMaciej Ogorzalek, PolyU EIE,
Sidebands
ŠSpectrum is carrier (at
f
c
) plus spectrum of
input signal translated to f
c
and reflected
about
f
c
ŠUpper sideband: |
f
|>|
f
c
|
ŠLower sideband: |
f
|<|
f
c
|
ŠSingle sideband (SSB): sidebands are
duplicates, only send one

55. EIE325: Telecommunication TechnologiesMaciej Ogorzalek, PolyU EIE,
DSBSC, DSBTC and VSB
ŠDSBTC: Double sideband transmitted
carrier
ŠDSBSC: Double sideband suppressed
carrier
ŠSSB: Single sideband
ŠVSB: Vestigial sideband

56. EIE325: Telecommunication TechnologiesMaciej Ogorzalek, PolyU EIE,
DSBSC, DSBTC and VSB
ŠDSBTC
ŠDSBSC
ŠSSB
ŠVSB

57. EIE325: Telecommunication TechnologiesMaciej Ogorzalek, PolyU EIE,
FM and PM modulation
ŠFM and PM modulation are actually very
similar
ŠIndistinguishable without knowledge of the
modulation function
ŠPM:
ŠFM:

58. EIE325: Telecommunication TechnologiesMaciej Ogorzalek, PolyU EIE,
Comparing the phases
ŠCompare the phases for PM and FM
ŠPM:
m
(
t
) is proportional to the phase
ŠFM:
m
(
t
) is proportional to the derivative of
phase ( m
΄(
t
) = 0)

59. EIE325: Telecommunication TechnologiesMaciej Ogorzalek, PolyU EIE,
FM Bandwidth

60. EIE325: Telecommunication TechnologiesMaciej Ogorzalek, PolyU EIE,
FM Band Allocation

61. EIE325: Telecommunication TechnologiesMaciej Ogorzalek, PolyU EIE,
Spread Spectrum
ŠAnalog or digital data
ŠAnalog signal
ŠObjective: Spread data over wide bandwidth
ŠMakes jamming and interception harder
ŠFrequency hoping
„
Signal broadcast over seemingly random series of frequencies
ŠDirect Sequence
„
Each bit is represented by multiple bits in transmitted signal
„
Chipping code

62. EIE325: Telecommunication TechnologiesMaciej Ogorzalek, PolyU EIE,
General model for SS

63. EIE325: Telecommunication TechnologiesMaciej Ogorzalek, PolyU EIE,
Spread Spectrum
ŠFrequency hoping
„
Signal broadcast over seemingly random series
of frequencies
ŠDirect Sequence
„
Each bit is represented by multiple bits in
transmitted signal
„
Chipping code

64. EIE325: Telecommunication TechnologiesMaciej Ogorzalek, PolyU EIE,
Direct Sequence

65. EIE325: Telecommunication TechnologiesMaciej Ogorzalek, PolyU EIE,
Question
ŠWhat is the bandwidth of a digital data
stream encoded with direct sequence
spreading?

66. EIE325: Telecommunication TechnologiesMaciej Ogorzalek, PolyU EIE,
Generating noise
ŠNeed to generate same “noise”at source
and destination
ŠComputers are deterministic –generating
true noise is not possible (i.e. no truly
random numbers)
ŠMay use
„
pseudo-random algorithm
„
predetermined sequences (e.g. Gold
sequences)
„
chaos