1. Chapter 5. Data Encoding Digital Data, Digital Signals
Digital Data, Analog Signals
Analog Data, Digital Signals
Analog Data, Analog Signals
Spread Spectrum

2. Encoding and Modulation
x(t)
g(t)
digital or
analog
x(t)
g(t)
Encoder
Decoder
digital t
(a) Encoding onto a digital signal
S(f)
m(t)
digital or
analog
s(t)
m(t)
Modulator
Demodulator
analog f fc
(b) Modulating onto an analog signal

3. Digital Data, Digital Signals
a sequence of discrete, discontinuous voltage pulses
Encoding schemes
Nonreturn-to-Zero-Level (NRZ-L)
0 = high level
1 = low level
Nonreturn to Zero Inverted (NRZI)
0 = no transition at beginning of interval (one bit time)
1 = transition at beginning of interval

4. NRZ-L and NRZI Advantages of NRZI Main limitations of NRZ
May be more reliable to detect a transition in the presence of noise than to compare a value to a threshold
Still work when the leads are inverted
Main limitations of NRZ
Presence of a dc component
Lack of synchronization capability
E.g. long string of 1s or 0s for NRZ-L, or long string of 0s for NRZI

5. Multilevel Binary Schemes
Bipolar-AMI (alternate mark inversion)
0 = no line signal
1 = positive or negative level, alternating for successive ones
Pseudo-ternary
0 = positive or negative level, alternating for successive ones
1 = no line signal

6. Bipolar-AMI No loss of synchronization if a long string of 1s occurs
No net dc component
Not as efficient as NRZ
Three levels of signal, but each signal element, which could represent log23 = 1.58 bits of information, bears only one bit of information
Provides a simple means of error detection

7. Biphase Schemes Manchester Differential Manchester
0 = transition from high to low in middle of interval
1 = transition from low to high in middle of interval
Differential Manchester
Always a transition in middle of interval
0 = transition at beginning of interval
1 = no transition at beginning of interval

8. Adv. of Biphase Schemes Synchronization No dc component
Known as self-clocking codes
No dc component
Error detection
Absence of an expected transition can be used to detect errors

9. Examples 1 NRZ-L NRZI Bipolar-AMI Pseudo-ternary Manchester1
NRZ-L
NRZI
Bipolar-AMI
Pseudo-ternary
Manchester
Differential
Manchester

10. Scrambling Tech.: B8ZS, HDB3
Commonly used in long-distance transmission services
No dc component
Overcome the drawback of AMI code
Long string of zeros may result in loss of synchronization
No reduction in data rate
Error-detection capability

11. B8ZS and HDB3 (cont) 1 1 1 1 1 Bipolar-AMI V B V B B8ZS V B V B V1 1 1
Bipolar-AMI V B V B
B8ZS V B V B V
HDB3 (odd # of 1s since last substitution)
Code violation

12. Evaluating Encoding Schemes
Signal spectrum
Lack of high-frequency components  less bandwidth is required
Lack of a direct-current (dc) component is desirable
Concentrate the transmitted power in the middle of the transmission bandwidth
Clocking
Determine the beginning and end of each bit position
Separate clock or self-synchronization

13. Evaluating Encoding Schemes
Error detection
Data link level
Physical level
Signal interference and noise immunity
Cost and complexity
The higher the signaling rate, the greater the cost

14. Modulation Rate Data rate (bit rate) Modulation rate
Expressed in bits per second
1/tB, tB = bit duration
Modulation rate
Expressed in baud
The rate at which signal elements are generated
D = R/b, D = modulation rate, R = data rate, b = # of bits per signal element

15. Bit vs. Signal Element 1 Mbps 1 1 1 1 1 NRZI
1M Baud
1 bit = 1signal element = 1ms
Manchester
2M Baud
1 bit = 1ms
1 signal element = 0.5ms

16. Digital Data, Analog Signals
ASK: Amplitude-shift keying
E.g.
s(t) =
Acos(2p fct) binary 1
0 binary 0

17. Digital Data, Analog Signals (cont)
FSK: Frequency-shift keying
E.g.
s(t) =
Acos(2p f1t) binary 1
Acos(2p f2t) binary 0

18. Digital Data, Analog Signals (cont)
PSK: Phase-shift keying
E.g.
s(t) =
Acos(2p fct +p ) binary 1
Acos(2p fct) binary 0

19. Analog Data, Digital Signals
Digitization
(a) Original signal
6.2
5.9
4.1
(b) PAM pulse
3.0
2.8
1.4
1.3 Ts 6 6
(c) PCM pulse 4 3 3 1 1
(d) PCM output
011
001
110
001
011
110
100

20. Analog Data, Digital Signals (cont)
Analog-to-digital conversion
PAM
sampler
Quantizer
Encoder
Continuous-time
continuous-amplitude
(analog) input signal
Discrete-time
continuous-amplitude
signal (PAM pulses)
Discrete-time
discrete-amplitude
signal (PCM pulses)
Digital bit-stream
output signal
PAM: Pulse Amplitude Modulation
PCM: Pulse Code Modulation

21. Analog Data, Analog Signals
Amplitude Modulation
Phase Modulation
Frequency Modulation

22. QAM Quadrature Amplitude Modulation
Combination of amplitude and phase modulation
Send two different signals simultaneously on the same carrier frequency, by using two copies of the carrier frequency, one shifted by 90°
Each carrier is ASK modulated
S(t) = d1(t)cos2pfct + d2(t)sin2pfct

23. 2-bit Serial-to-parallel converter
QAM Modulator
d1(t) R/2 bps
cos2pfct
Binary input
Carrier oscillator
2-bit Serial-to-parallel converter 
QAM signal out S
d(t) R bps
s(t)
Phase shift
-p/2
sin2pfct
d2(t) R/2 bps

24. Spread Spectrum

25. Spread Spectrum (cont)
Frequency Hopping, Direct Sequence