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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Analog Data, Analog Signals Amplitude Modulation Phase Modulation Frequency Modulation

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

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

Spread Spectrum

Spread Spectrum (cont) Frequency Hopping, Direct Sequence