1 5. Data Encoding. Prof. Sang-Jo Yoo 2 Contents Introduction Data and Signals Encoding(...
-
Upload
marian-dalton -
Category
Documents
-
view
227 -
download
0
Transcript of 1 5. Data Encoding. Prof. Sang-Jo Yoo 2 Contents Introduction Data and Signals Encoding(...
2
Prof. Sang-Jo Yoo
ContentsContents
Introduction Data and Signals Encoding( modulation) techniques Spread Spectrum
3
Prof. Sang-Jo Yoo
IntroductionIntroduction
Data and signals
Digital Data Digital Signal
Analog SignalAnalog Data
Less complex, less expensive than digital-analog modulation equipment
Use of modern digital transmission And switching equipment
Some transmission media will onlypropagate analog signals
Efficient use of transmission channel : FDM
4
Prof. Sang-Jo Yoo
IntroductionIntroduction Encoding and modulation techniques
Encoder Decoderg(t)
digitalor
analog
x(t)digital
x(t)
t
g(t)
Encoding onto a digital signal
Modulator Demodulatorm(t)digital
oranalog
s(t)analog
m(t)
S(f)
f
fc fcModulation onto an analog signal
m(t) = baseband signal or modulating signal fc = carrier signal s(t) = modulated signal
5
Prof. Sang-Jo Yoo
Digital Data, Digital SignalsDigital Data, Digital Signals digital data : an abstraction of information in digital form digital signal: an sequence of discrete, discontinuous volta
ge pulses. Each pulse is a signal element Key data transmission terms
mark : binary digit 1 / space : binary digit 0
6
Prof. Sang-Jo Yoo
Evaluation of various encoding techniques signal spectrum : less bandwidth, no dc component, shape of the
spectrum(powers should be in the middle of transmitted bandwidth)
clocking : synchronization based on transmitted signal
---> self-clocking error detection : useful to have error-detection capability in
physical encoding scheme signal inference and noise immunity : low bit error rate cost and complexity
7
Prof. Sang-Jo Yoo
Various encoding techniques
0 1 0 0 1 1 0 0 0 1 1 NRZ-L
NRZI
Bipolar-AMI
Pseudoternary
Manchester
Differentialmanchester
Digital signal encoding formats
MultilevelBinary
Biphase
8
Prof. Sang-Jo Yoo
Return to Zero(RZ) 0 : no pulse 1 : positive pulse signal returns to zero after each encoded bit
Nonreturn to Zero Level(NRZ-L) 0 : positive voltage during bit interval 1 : negative voltage during bit interval
Nonreturn to Zero,invert on ones(NRZI) 1 : transition at the beginning of bit interval 0 : no transition differential encoding advantage
more reliable to detect a transition in the presence of noise less bandwidth
+V 0-V
1 0 0 1
50% RZ
+V -V
1 0 0 1
+V -V
1 0 0 1
9
Prof. Sang-Jo Yoo
Limitation of NRZ presence of dc component lack of synchronization capability (long 0s or 1s and drift)
Multilevel Binary bipolar-AMI(alternate mark inversion)
three voltage level 0 : no line signal 1 : positive or negative voltage during bit interval binary 1 (mark) must alternate in polarity benefit : no loss of sync. capability if a long string of 1 occurs
no net dc component , less bandwidth than NRZ, simple
means of error detection
pseudoternary same as bipolar-AMI except that the representations of 1 and 0 are int
erchanged
10
Prof. Sang-Jo Yoo
Biphase Always a transition in the middle of each bit interval Manchester code
0 : positive to negative transition 1 : negative to positive transition
Differential Manchester code 0 : transition at the beginnig of a bit interval 1 : no transition at the beginnig of a bit interval
Advantage synchronization, no dc component, error detection
Modulation Rate date rate(bits per second) modulation rate( baud : signal elements per second)
L
R
b
RD
2log D : modulation rate(baud)
R : data rate(bps)L : number of different signal elementsb : number of bits per signal elements
11
Prof. Sang-Jo Yoo
Scrambling technique biphase encoding : although used in LAN(10Mbps), but not used i
n long-distance application because of high signaling rate relative to the data rate.
Conditions of codes for long-distance application constant voltage level on the line should be replaced by sufficient tra
nsitions for the receiver’s clock synchronization no dc component, no long sequences of zero-level line signals
no reduction in data rate, error-detection capability
12
Prof. Sang-Jo Yoo
Scrambling technique B8ZS(bipolar with 8-zeros substitution)
Based on a bipolar-AMI Drawback of AMI: long string of zeros (loss of synchronization). 8 consecutive zeros are encoded as either 000+-0-+ or 000-+0+-
(Positive 1) + 8 zeros = 000+-0-+ (negative 1) +8 zeros = 000-+0+-
receiver recognizes the pattern
HDB3(high-density bipolar-3 zeros) 4 consecutive zeros are encoded as either 000- , 000+, +00+, -00- Table 5.4 (stalling) fourth zeros are always a code violation and successive violations are
of alternate polarity( no dc component) In Europe and Japan.
13
Prof. Sang-Jo Yoo
Encoding rules for B8ZS and HDB3
1 1 0 0 0 0 0 0 0 0 1 1 0 0 0 0 0 1 0
Bipolar-AMI
B8ZS
HDB3
0 0 0 V B 0 V B
0 0 0 V B 0 0 V B 0 0 V
B = Valid bipolar signal, V = Bipolar violation
15
Prof. Sang-Jo Yoo
Digital Data, Analog SignalsDigital Data, Analog Signals
Encoding of digital data by continuous sinusoidal carrier signal ex) MODEM : transmitting digital data through PSTN
Encoding techniques : ASK, FSK, PSK
ASK
FSK
PSK
Digital Data
A cos(2fct )
0
A cos(2f1t )
A cos(2f2t )
A cos(2fct+ )
A cos(2fct )
Binary 1
Binary 0
Binary 1
Binary 0
Binary 1
Binary 0
16
Prof. Sang-Jo Yoo
QPSK(Quadrature PSK) more efficient use of bandwidth
PSK using 12 phase angles and two amplitude used in standard 9600 bps modem( modulation rate=2400 bauds)
cos(2fct + 45)cos(2fct + 135)cos(2fct + 225)cos(2fct + 315)
: Binary 11 : Binary 10 : Binary 00 : Binary 01
17
Prof. Sang-Jo Yoo
Analog Data, Digital SignalsAnalog Data, Digital Signals
Digitization process Digital data NRZ-L Digital data a code other than NRZ-L Digital data analog signal (modulation)
Digitizer Modulator
Analog Data(Voice)
Digital Data(NRZ-L) Analog Signal
(ASK)
18
Prof. Sang-Jo Yoo
Analog Data, Digital SignalsAnalog Data, Digital Signals
PCM(Pulse Code Modulation) based on the sampling theorem
If a signal f(t) is sampled at regular intervals of time and at a rate higher than twice the highest significant signal frequency, then the samples contain all the information of the original signal. The function f(t) may be reconstructed from these samples by the use of a low-pass filter
Appendix 5A
8000 samples * 8 bits per sample = 64 Kbps For 4 KHz voice channel
19
Prof. Sang-Jo Yoo
PAM pulses(Discrete Signal): discrete-time, continuous-amplitude
PCM pulses(Digital Signal): discrete-time, discrete-amplitude
PCM outputs
Origianl signalPAM sampler
Quantizer
Encoder
011001110001011110100
20
Prof. Sang-Jo Yoo
Quantization noise : S/N=6n-1.76 dB To reduce quantization noise
large n nonlinear encoding companding(compressing-expanding)
Effect of nonlinear coding
21
Prof. Sang-Jo Yoo
DM(Delta Modulation) an analog input is approximated by a staircase function that moves
up or down by one quantization level( ) at each sampling interval(Ts)
simpler hardware implementation than PCM
22
Prof. Sang-Jo Yoo
Delta Modulation
Comparator
One timeunit delay
+
1 = +0 = -
Analoginput Binary
output
+
One timeunit delay
Binaryinput
Reconstructedwaveform
Transmission
Reception
23
Prof. Sang-Jo Yoo
Analog Data, Analog SignalsAnalog Data, Analog Signals
Motivation it is impossible to transmit baseband signals : higher frequency is
needed (especially, for unguided transmission) modulation permits FDM(frequency-division multiplexing)
DSBTC(double-sideband transmitted carrier) spectrum of an AM signal
tftxnts ca 2cos)](1[)(
Spectrum ofmodulating signal
M(f)
0 fB 0
Discrete carrier termS(f)
ffcfc-B
Uppersideband
Lowersideband
Spectrum ofAM signal with carrier at fc
Fc+B-fc-fc-B -fc+B
Uppersideband
Lowersideband
24
Prof. Sang-Jo Yoo
na: modulation index
“1” : to prevents loss of information
Power ? example of DSBTC
tftxnts ca 2cos)](1[)(
25
Prof. Sang-Jo Yoo
DSBSC(double-sideband supressed carrier) saves power but uses as much bandwidth as DSBTC
SSB(single-sideband) send only one of the sidebands, eliminating the other sideband and
carrier only half of the bandwidth is required less power is required
tftxAts cc 2cos)()(
S(f)
ffcfc-B
UppersidebandLower
sideband
Spectrum ofDSBSC signal with carrier at fc
Fc+B-fc-fc-B -fc+B
Uppersideband
Lowersideband
26
Prof. Sang-Jo Yoo
Angle Modulation(FM and PM)
: PM(phase modulation) : FM(frequency modulation)
)](2cos[)( ttfAts cc
)()( tmnt p)()(' tmnt f
carrier
Modulating sine-wavesignal
DSBTC wave
PM wave
FM wave
27
Prof. Sang-Jo Yoo
Spread SpectrumSpread Spectrum
Developed initially for military and intelligence requirements
spread the information over a wider bandwidth in order to make jamming and interception more difficult
Two types FH(frequency hopping) and DS(direct sequence)
General model of spread spectrum digital communication system
28
Prof. Sang-Jo Yoo
Frequency-Hopping signal is transmitted over a seemingly random series of radio
frequencies, hopping from frequency to frequency at split-second interval
transmitter
receiver