Chapter 5 Data Encoding Digital Data Digital Signals
- Slides: 25
Chapter 5. Data Encoding • Digital Data, Digital Signals • Digital Data, Analog Signals • Analog Data, Digital Signals • Analog Data, Analog Signals • Spread Spectrum 1
Encoding and Modulation x(t) g(t) digital or analog Encoder x(t) digital Decoder g(t) t (a) Encoding onto a digital signal S(f) m(t) digital or analog Modulator s(t) m(t) Demodulator analog fc f (b) Modulating onto an analog signal 2
Digital Data, Digital Signals • Digital signal – 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 3
NRZ-L and NRZI • Advantages of NRZI – 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 1 s or 0 s for NRZ-L, or long string of 0 s for NRZI 4
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 5
Bipolar-AMI • No loss of synchronization if a long string of 1 s occurs • No net dc component • Not as efficient as NRZ – Three levels of signal, but each signal element, which could represent log 23 = 1. 58 bits of information, bears only one bit of information • Provides a simple means of error detection 6
Biphase Schemes • 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 7
Adv. of Biphase Schemes • Synchronization – Known as self-clocking codes • No dc component • Error detection – Absence of an expected transition can be used to detect errors 8
Examples 0 1 0 0 1 1 0 0 0 1 1 NRZ-L NRZI Bipolar-AMI Pseudo-ternary Manchester Differential Manchester 9
Scrambling Tech. : B 8 ZS, HDB 3 • 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 10
B 8 ZS and HDB 3 (cont) 1 1 0 0 0 0 1 1 0 0 0 1 0 Bipolar-AMI 0 0 0 V B B 8 ZS HDB 3 (odd # of 1 s since last substitution) 0 0 0 V B 0 0 V Code violation 11
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 12
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 13
Modulation Rate • Data rate (bit rate) – Expressed in bits per second – 1/t. B, t. B = 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 14
Bit vs. Signal Element 1 Mbps 1 1 1 NRZI 1 M Baud 1 bit = 1 signal element = 1 ms Manchester 2 M Baud 1 bit = 1 ms 1 signal element = 0. 5 ms 15
Digital Data, Analog Signals • ASK: Amplitude-shift keying – E. g. s(t) = Acos(2 p fct) binary 1 0 binary 0 16
Digital Data, Analog Signals (cont) • FSK: Frequency-shift keying – E. g. s(t) = Acos(2 p f 1 t) binary 1 Acos(2 p f 2 t) binary 0 17
Digital Data, Analog Signals (cont) • PSK: Phase-shift keying – E. g. s(t) = Acos(2 p fct +p ) binary 1 Acos(2 p fct) binary 0 18
Analog Data, Digital Signals • Digitization (a) Original signal 6. 2 (b) PAM pulse 3. 0 1. 4 Ts (c) PCM pulse 1. 3 6 4 3 3 011 001 4. 1 2. 8 6 1 (d) PCM output 5. 9 1 110 001 011 110 100 19
Analog Data, Digital Signals (cont) • Analog-to-digital conversion PAM sampler Quantizer Discrete-time Continuous-time continuous-amplitude (analog) input signal (PAM pulses) Encoder Discrete-time discrete-amplitude signal (PCM pulses) Digital bit-stream output signal PAM: Pulse Amplitude Modulation PCM: Pulse Code Modulation 20
Analog Data, Analog Signals • Amplitude Modulation • Phase Modulation • Frequency Modulation 21
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) = d 1(t)cos 2 pfct + d 2(t)sin 2 pfct 22
QAM Modulator d 1(t) R/2 bps Binary input d(t) R bps cos 2 pfct 2 -bit Serial-to-parallel converter Carrier oscillator QAM signal out S s(t) Phase -p/2 shift sin 2 pfct d 2(t) R/2 bps 23
Spread Spectrum 24
Spread Spectrum (cont) • Frequency Hopping, Direct Sequence 25
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