information theory & fundamentals of digital communications
DESCRIPTION
Information Theory & Fundamentals of Digital Communications. Network/Link Design Factors. Transmission media Signals are transmitted over transmission media Examples: telephone cables, fiber optics, twisted pairs, coaxial cables Bandwidth ( εύρος ζώνης) - PowerPoint PPT PresentationTRANSCRIPT
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A primer on Information Theory &Fundamentals of Digital Communications
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Network/Link Design Factors Transmission media
Signals are transmitted over transmission media
Examples: telephone cables, fiber optics, twisted pairs, coaxial cables
Channel capacity and Bandwidth (εύρος ζώνης) Higher channel capacity (measured in Hz),
gives higher bandwidth (range of frequencies), gives higher data rate
Transmission impairments Attenuation (εξασθένηση) Interference (παρεμβολή)
Number of receivers In guided media More receivers (multi-point) introduce more
attenuation
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Channel Capacity and data rate Bandwidth
In cycles per second, or Hertz (e.g. 10 Mhz bandwidth)
Available bandwidth for signal transmission is constrained by transmitter and transmission medium makeup and capacity
Data rate Related to the bandwidth capabilities of the
medium in bits per second (the higher the bandwidth
the higher the rate) Rate at which data can be communicated Baud rate (symbols/sec) ≠ bit rate (bits/sec)
• Number of symbol changes made to the transmission medium per second
• One symbol can carry one or more bits of information
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Data Rate and Bandwidth Any transmission system has a limited
band of frequencies This limits the data rate that can be
carried E.g., telephone cables due to
construction can carry signals within frequencies 300Hz – 3400Hz
This has severe effect on what signals and what capacity in effect the channel has.
To understand this lets consider any complex signal and see if we can analyse its frequency content, and what effect the channel may have on the signal (distortion?) and in effect limit the rate of the transmitted signal (measured in bits/sec)
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Frequency content of signals http://
www.allaboutcircuits.com/vol_2/chpt_7/2.html
any repeating, non-sinusoidal waveform can be equated to a combination of DC voltage, sine waves, and/or cosine waves (sine waves with a 90 degree phase shift) at various amplitudes and frequencies.
This is true no matter how strange or convoluted the waveform in question may be. So long as it repeats itself regularly over time, it is reducible to this series of sinusoidal waves.
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Fourier series Mathematically,
any repeating signal can be represented by a series of sinusoids in appropriate weights, i.e. a Fourier Series.
http://en.wikipedia.org/wiki/Fourier_series
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The Mathematic Formulation
A periodic function is any function that satisfies
( ) ( )f t f t T
where T is a constant and is called the period of the function.
Note: for a sinusoidal waveform the frequency is the reciprocal of the period (f=1/T)
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Synthesis
Tntb
Tntaatf
nn
nn
2sin2cos2
)(11
0
DC Part Even Part Odd Part
T is a period of all the above signals
)sin()cos(2
)( 01
01
0 tnbtnaatfn
nn
n
Let 0=2/T.
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1122
00
dta
,2,1 0sin1cos22
00
nntn
ntdtan
,6,4,20
,5,3,1/2)1cos(1 cos1sin
21
00
nnn
nn
ntn
ntdtbn
2 3 4 5--2-3-4-5-6
f(t)1
Example (Square Wave)
-0.5
0
0.5
1
1.5
ttttf 5sin
513sin
31sin2
21)(
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Fourier series example Thus, square waves (and indeed and
waves) are mathematically equivalent to the sum of a sine wave at that same frequency, plus an infinite series of odd-multiple frequency sine waves at diminishing amplitude
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Another example
With 4 sinusoids we represent quite well a triangular waveform
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The ability to represent a waveform as a series of sinusoids can be seen in the opposite way as well:
What happens to a waveform if sent through a bandlimited (practical) channel
E.g. some of the higher frequencies are removed, so signal is distorted…
E.g what happens if a square waveform of period T is sent through a channel with bandwidth (2/T)?
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The Electromagnetic Spectrum
The electromagnetic spectrum and its uses for communication.
Note: frequency is related to wavelength (f=1/l)
Useful spectrum is limited, therefore its allocation/usage is managed
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Electromagnetic Spectrum
Note: frequency is related to wavelength (f=1/l) which is related to antenna length for radio, capacity, power and distance trade-offs exist, etc…
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Generally speaking there is a push into higher frequencies due to: efficiency in propagation, immunity to some forms of noise and
impairments as well as the size of the antenna required.
The antenna size is typically related to the wavelength of the signal and in practice is usually ¼ wavelength.
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Data and Signal: Analog or Digital Data
Digital data – discrete value of data for storage or communication in computer networks
Analog data – continuous value of data such as sound or image
Signal Digital signal – discrete-time signals
containing digital information Analog signal – continuous-time signals
containing analog information
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Periodic and Aperiodic Signals (1/4) Spectra of periodic analog signals: discrete
f1=100 kHz
400k Frequency
Amplitude
Time
100k
Amplitude
f2=400 kHz periodic analog signal
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Periodic and Aperiodic Signals (2/4) Spectra of aperiodic analog signals:
continous
aperiodic analog signal
f1
Amplitude
Amplitude
f2
Time
Frequency
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Periodic and Aperiodic Signals (3/4) Spectra of periodic digital signals: discrete
(frequency pulse train, infinite)
frequency = f kHzAmplitude periodic digital signal
Amplitude frequency pulse train
Time
Frequencyf 2f 3f 4f 5f...
...
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Periodic and Aperiodic Signals (4/4) Spectra of aperiodic digital signals:
continuous (infinite)aperiodic digital signalAmplitude
Amplitude
0
Time
Frequency...
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Sine Wave Peak Amplitude (A)
maximum strength of signal volts
Frequency (f) Rate of change of signal Hertz (Hz) or cycles per second Period = time for one repetition (T) T = 1/f
Phase () Relative position in time
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Varying Sine Waves
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Signal Properties
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Figure 1.8 Modes of transmission: (a) baseband transmission
Baseband Transmission
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Modulation (Διαμόρφωση) Η διαμόρφωση σήματος είναι μία
διαδικασία κατά την οποία, ένα σήμα χαμηλών συχνοτήτων (baseband signal), μεταφέρεται από ένα σήμα με υψηλότερες συχνότητες που λέγεται φέρον σήμα (carrier signal)
Μετατροπή του σήματος σε άλλη συχνότητα
Χρησιμοποιείται για να επιτρέψει τη μεταφορά ενός σήματος σε συγκεκριμένη ζώνη συχνοτήτων π.χ. χρησιμοποιείται στο ΑΜ και FM ραδιόφωνο
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Πλεονεκτήματα Διαμόρφωσης Δυνατότητα εύκολης μετάδοσης του
σήματος Δυνατότητα χρήσης πολυπλεξίας
(ταυτόχρονη μετάδοση πολλαπλών σημάτων)
Δυνατότητα υπέρβασης των περιορισμών των μέσων μετάδοσης
Δυνατότητα εκπομπής σε πολλές συχνότητες ταυτόχρονα
Δυνατότητα περιορισμού θορύβου και παρεμβολών
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Modulated Transmission
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Continuous & Discrete SignalsAnalog & Digital Signals
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Analog Signals Carrying Analog and Digital Data
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Digital Signals Carrying Analog and Digital Data
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Digital Data, Digital Signal
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Encoding (Κωδικοποίηση) Signals propagate over a physical
medium modulate electromagnetic waves e.g., vary voltage
Encode binary data onto signals binary data must be encoded before
modulation e.g., 0 as low signal and 1 as high signal
• known as Non-Return to zero (NRZ)Bits
NRZ
0 0 1 0 1 1 1 1 0 1 0 0 0 0 1 0
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Encodings (cont)Bits
NRZ
Clock
Manchester
NRZI
0 0 1 0 1 1 1 1 0 1 0 0 0 0 1 0
If the encoded data contains long 'runs' of logic 1's or 0's, this does not result in any bit transitions. The lack of transitions makes impossible the detection of the boundaries of the received bits at the receiver.This is the reason why Manchester coding is used in Ethernet.
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Other Encoding Schemes Unipolar NRZ Polar NRZ Polar RZ Polar Manchester and Differential
Manchester Bipolar AMI and Pseudoternary Multilevel Coding Multilevel Transmission 3 Levels RLL
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The Waveforms of Line Coding Schemes
1 1 1 1 1 10 0 0 0 0 0ClockData stream
Polar RZ
Polar NRZ-L
Manchester
Polar NRZ-I
Differential Manchester
AMI
MLT-3
Unipolar NRZ-L
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The Waveforms of Line Coding Schemes
1 1 1 1 1 10 0 0 0 0 0ClockData stream
Polar RZ
Polar NRZ-L
Manchester
Polar NRZ-I
Differential Manchester
AMI
MLT-3
Unipolar NRZ-L
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Bandwidths of Line Coding (2/3)• The bandwidth of Manchester.
• The bandwidth of AMI.1N 2N Frequncy
Power Bandwidth of Manchester Line Codingsdr=2, average baud rate = N (N, bit rate)
00
1.0
0.5
N/2 3N/2
1N 2N Frequncy
Power Bandwidth of AMI Line Codingsdr=1, average baud rate = N/2 (N, bit rate)
00
1.0
0.5
N/2 3N/2
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Bandwidths of Line Coding (3/3)
1N 2N Frequncy
Power Bandwidth of 2B1Q Line Codingsdr=1/2, average baud rate=N/4 (N, bit rate)
00
1.0
0.5
N/2 3N/2
• The bandwidth of 2B1Q
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Digital Data, Analog Signal After encoding of digital data, the
resulting digital signal must be modulated before transmitted
Use modem (modulator-demodulator) Amplitude shift keying (ASK) Frequency shift keying (FSK) Phase shift keying (PSK)
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Modulation Techniques
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Constellation Diagram (1/2) A constellation diagram: constellation
points with two bits: b0b1
+1-1
+1
-1
I
Amplitue
Amplitue of I component
Amplitue of Q component
PhaseIn-phase Carrier
QQuadrature Carrier
1101
1000
41Chapter 2: Physical Layer
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Amplitude Shift Keying (ASK) and Phase Shift Keying (PSK) The constellation diagrams of ASK and
PSK.
(a) ASK (OOK): b0 (b) 2-PSK (BPSK): b0 (c) 4-PSK (QPSK): b0b1 (d) 8-PSK: b0b1b2 (e) 16-PSK: b0b1b2
+1-1
+1
-1
Q
I
1101
1000
Q
I
110011
101000
111
100
001
010Q
I+1-1
Q
I
10
+1
Q
I0
10
42Chapter 2: Physical Layer
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The Circular Constellation Diagrams
The constellation diagrams of ASK and PSK.
(a) Circular 4-QAM: b0b1 (b) Circular 8-QAM: b0b1b2 (c) Circular 16-QAM: b0b1b2b3
Q
I+1-1
+1
-1
Q
I+1+ 3-1 - 3
+1+ 3
-1 - 3
+1-1
+1
-1
Q
I
1101
1000
43Chapter 2: Physical Layer
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The Rectangular Constellation Diagrams
(a) Alternative Rectangular 4-QAM: b0b1
(b) Rectangular 4-QAM: b0b1
(c) Alternative Rectangular 8-QAM: b0b1b2
(d) Rectangular 8-QAM: b0b1b2
(e) Rectangular 16-QAM: b0b1b2b3
+1 +3-3 -1
+1
-1
Q
I +1-1
+1
-1
Q
I +1 +3-3 -1
+1
+3
-1
-3
Q
I
101111110011 0111
101011100010 0110
100011000000 0100
100111010001 0101+1
+1
Q
I-1
-1+1
+1Q
I0
44Chapter 2: Physical Layer
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Quadrature PSK More efficient use if each signal
element (symbol) represents more than one bit e.g. shifts of /2 (90o) 4 different phase
angles Each element (symbol) represents
two bits• With 2 bits we can represent the 4 different
phase angles• E.g. Baud rate = 4000 symbols/sec and each
symbol has 8 states (phase angles). Bit rate=??
If a symbol has M states each symbol can carry log2M bits
Can use more phase angles and have more than one amplitude
• E.g., 9600bps modem use 12 angles, four of which have two amplitudes
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Modems (2)
(a) QPSK.(b) QAM-16.(c) QAM-64.
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Modems (3)
(a) V.32 for 9600 bps.(b) V32 bis for 14,400 bps.
(a) (b)
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Ak
Bk
16 “levels”/ pulse4 bits / pulse4W bits per second
Ak
Bk
4 “levels”/ pulse2 bits / pulse2W bits per second
2-D signal2-D signal
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Ak
Bk
4 “levels”/ pulse2 bits / pulse2W bits per second
Ak
Bk
16 “levels”/ pulse4 bits / pulse4W bits per second
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Analog Data, Digital Signal
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Signal Sampling and Encoding
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Digital Signal Decoding
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Alias generation due to undersampling
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Nyquist Bandwidth If rate of signal transmission is 2B then
signal with frequencies no greater than B is sufficient to carry signal rate
Given bandwidth B, highest signal (baud) rate is 2B
Given binary signal, data rate supported by B Hz is 2B bps (if each symbol carries one bit)
Can be increased by using M signal levels
C= 2B log2M
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Transmission Impairments Signal received may differ from signal
transmitted Analog degradation of signal quality Channel impairements Digital bit errors Caused by
Attenuation and attenuation distortion Delay distortion Noise
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Attenuation Signal strength falls off with distance Depends on medium Received signal strength:
must be enough to be detected must be sufficiently higher than noise to be
received without error Attenuation is an increasing function
of frequency
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Noise (1) Additional signals, exisiting or inserted,
between transmitter and receiver Thermal
Due to thermal agitation of electrons Uniformly distributed White noise
Intermodulation Signals that are the sum and difference of
original frequencies sharing a medium
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Noise (2) Crosstalk
A signal from one line is picked up by another
Impulse Irregular pulses or spikes e.g. External electromagnetic interference Short duration High amplitude
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signal noise signal + noise
signal noise signal + noise
HighSNR
LowSNR
SNR = Average Signal Power
Average Noise Power
SNR (dB) = 10 log10 SNR
t t t
t t t
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Shannon’s Theorem
Real communication have some measure of noise. This theorem tells us the limits to a channel’s capacity (in bits per second) in the presence of noise. Shannon’s theorem uses the notion of signal-to-noise ratio (S/N), which is usually expressed in decibels (dB):
)/(log10 10 NSdB
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Shannon’s Theorem – cont.
))/(1(log 2 NSBC
Kbps30)10001(log3000 2 C
Shannon’s Theorem:C: achievable channel rate (bps)B: channel bandwidth
For POTS, bandwidth is 3000 Hz (upper limit of 3300 Hz and lower limit of 300 Hz), S/N = 1000
For S/N = 100
Kbps5.19)1001(log3000 2 C