lecture i introduction to digital communications 1.overview of comm. channels and digital links...
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Lecture IIntroduction to DigitalCommunications
1. Overview of comm. channels and digital links2. Signal propagation through baseband PAM Links• (ch. 1 – part 0 “Notes”)
Following Lecture II next week:2. …signal propagation through baseband PAM Links3. Finite Energy Signal Space representations4. Matched Filtering in AWGN for PAM antipodal linksRead-ahead: Ch. 2 – part 0 “Notes”
“The communication technology of a sufficiently advanced civilization is
indistinguishable from magic”
Defining communications 1. What does it mean to communicate?
• Derived from the Latin communicare: to share to impart or to transmit
Communication is the activity associated with transfering, distributing or exchanging information (usually
spanning a distance)
– One-way (one-to-one / one-to-many)
– Two-way (one-to-one / many-to-many)
• Interactive
• Simultaneous
What do we communicate (information/messages)?
• Data / Text / Graphics / Voice / Music / Video
How do we communicate? electro-magnetic signals carry the messages:
1. Analog signals (waveforms = continuous functions of time,
assuming any value)
2. Digital signals (discrete-time and discrete-valued)
• binary bit-streams (…10011011010111001…)
• symbol-streams (……)
analogmedium
TX RX
Transmitter Receiver
Data S(t) R(t)Data
N(t) Noise
Digital Communication Link
1. Transmitter (TX) משדר
2. Medium (Channel ערוץ) תווך
3. Receiver (RX) מקלט
DA AD
Medium (Channel ערוץ) תווך• Wireless (Free space - vacuum / air);
• Radio, microwave, satellite, cellular, Wireless LAN
• Wireline:• Telephone wires (twisted pairs)
• LAN (twisted pairs)
• RF cable (coaxial)
• Fiber optics
• May include active repeaters
• Satellite transponders
• Fiber optic repeaters
A digital Communications Link: bitstream-> TX->Analog Medium->RX > bitstream
• All media in Nature are analog – – A purely digital medium exists only in math.
• “Underneath every digital communications link there resides an
analog medium”
• The TX: Digital->Analog
• The RX: Analog->Digital
• The objective of a communication link:Receiving a bitstream at the TX and faithfully reproducing it at the RX
at maximum rate and with minimum power
Bitstream: a finite or possibly infinite sequence of random bits out of the set {0,1}, representing the information to be carried
(see Appendix to Lesson 1)
Two-way (full-duplex) links with modems
TX
RX
RX
TX
Modem Modem
D
D
D
D
A
A
A
ATwo-WayChannel
Telephony – Alexander Graham Bell receives a patent for the telephone (1876)
the earliest form of “analog electrical communications”
Telegraphy (1843)the earliest form of “digital electrical
communications”
26-ARY messageStream SourceHk {A,B,C,..Z}
A
B
Z...
S (t)S (t)
S (t)
(0)
(1)
(M-1)
......THIS COURSE IS FUN...
What’s wrong with this picture?
Data sources and pulse modulators
P-MOD example: QPSK transmittermapping pairs of bits to one of four signals
Bitstream
2 bitblocks
00 cos(2t/T)
01 -cos(2t/T)
10 sin(2t/T)
11 -sin(2t/T)
P-MODx(t)
S (t)(0)
S (t)(1)
S (t)(2)
S (t)(3)
Example QPSK TX
-
The transmitter’s most important block: P-MOD
Low-pass vs. Bandpass Media• LowPass – Passes DC and low freq.
– LowPass media: Telephone, LAN wires (RJ11/45)
• HighPass/Bandpass –Passes a passband of freq.
– Wireless media don’t carry low freq.
• radiation efficiency negligible at low freq.
– Certain wireline media are bandpass
• e.g. coaxial amplifiers are AC coupled
LTI systemb(t) Impulse responseB() freq. response
+b(t) x s(t) R(t)
N(t) noise
s(t)
Model for LTI-AWGN medium
analogmedium
S(t) R(t)
N(t) Noise
AdditiveWhiteGaussianNoise
DelayT0 +
S(t) R(t)
N(t)
Ideal Medium
B0
Model for zero-distortion idealized AWGN medium
Bo
|B()|
"flat"
B()
Linear Phase
Frequency responseB
B
Ideal Medium
End2end digital communication link
Data Source CoderPulse
Modulator
Decoder DetectorData
Destination
TX
RX
Medium
Analogpulses
Symbols(coded bits)
uncoded bits (messages)
Symbols(coded bits)
uncoded bits (messages)
Analogwaveforms
bits2symbols
symbols2bits
symbols2pulses
waveforms2symbols
Complete digital communication link (more detail)
Data SourcePulse
Modulator
DataDestination
TX
RX
Medium
SourceCoder
ChannelCoder
Coder
SufficientStatisticsExtractor
Detector
SlicerSource
DecoderChannelDecoder
Coder
MessageStream
Symbol StreamAK
S(t)
R(t)
WaveformRecognition
Sufficient StatisticsVector Stream
EstimatedSymbol Stream
KA
A/DQUANTIZATION
Datacompression
Redundant check-bitsinsertion
analogmedium
DataSource
Coder PulseModulator
Datasink
DecoderPulseDemodulator
bitstream...0111.0100.1010.1111....
symbols/indexes
...74AF055BA... S(t) R(t)
Analog link
Message link
TX RX
s(t)=...p7(t)+p4(t-T)+p10(t-2T)+p15(t-3T)+...
bitstream...0111.0100.1010.1111....
symbols/indexes
...74AF055BA...
4bit 4bit
DATA LINK EXAMPLE
16 QAM waveform code
Digital transmission formats for a binary stream
ASK, PSK, FSKAmp./Phase/Freq. Shift-Keying
PSK FSK
ASK
t
0f
1f
t
0 ( )p t
1( )p t
pulsegenerator
0 ( )p t
pulsegenerator
1( )p t
+
"0"
"1"
trigger
trigger
bitstream 1 0 1 1 0 0 0 1
Pulse modulation
1 0 1 1 0( ) ( ) ( 2 ) ( 3 ) ( 4 ) ...p t p t T p t T p t T p t T
1 0 1 1 0
FSK transmitter
FSK ReceiverFSK receiver
BPF f0
f
BPF f1
ff1
f0
Choose indexof largest
signal
101100
energydet
energydet
Sufficientstatistics extractor
slicer
PAM – Pulse Amplitude Modulation
P-MOD:
PAM:
4-level PAM transmission
2 - BitSerial toParralel
b1 b2 A
0 1 -11 0 11 1 3
0 0 -3 Ak2 S(t)Input
Bits
A
0 T
PAM
0 T 2T 3T 4T 5T
1
2
3
-1
-2
-3
b0b1: 00 10 10 01 11Ak: -3 1 1 -1 3
S(t)
Pulse Amplitude Modulation (PAM)
tt
PAM
P(t)AK KA P(t KT)k
k
Figure 1.12:
PAM modulator output waveform
“Single shot” @ t=0 – Isolated Pulse Amplitude Modulation
t
PAM
P(t)A0
LTIA0
t
P(t)( )t
0 ( )A p t
0 ( )A p t
t
General PAM Link Analysis
SlicerAK S(t)CODER
PAM
g(t)
SAMP [T]Bits CH. Filter
b(t)
N(t)
RX Filter
f(t)
r(t) q(t) qK
TX Medium/Channel RX
Figure 1.17:
(“multiple shots” analysis in TA classes…)
מועד א'מערכת תקשורת ספרתית משדרת אחת מצורות הגל
cos 2
cos2
cos2 2
cos2 3
cos2 63
o
o
o
o
o
A f t
A f f t
A f f t
A f f t
A f f t
0למשך זמן t T T נמדד מחדש בתחילת כל קטע של t שניות )הזמן T , כל
שניות(.קצב הסיביות )מספר הביטים לשניה( המשודר ע"י המערכת הוא:
64
64 /
8
8/
6
6 /
4
4 /
a T
b T
c T
d T
e T
f T
g T
h T
הפקולטה להנדסת חשמל 8.7.2003)044115כיוונים בהנדסת חשמל (