# Materi 4 Data Encoding

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Chap. 4 Data Encoding

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Analog Data

adv :

for tx. efficiency & FDMuse

Analog Signal

Digital Data

of d igit al s w. &t ia x. e ed qui e m tx. als p om not ign s n s ca gital di Digital

Signal

Encoding and modulation techniquesx(t) g(t) digital or analog x(t) g(t) t

Encoder digital

Decoder

Encoding onto a digital signalS(f)

m(t) digital or analog

Modulator

s(t) m(t) Demodulator analog f

fcModulation onto an analog signal

fcfc = carrier signals(t) = modulated signal

m(t) = baseband signal or modulating signal

Chap. 4 Data Encoding

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1. Digital Data Digital Signals A digital signal is a sequence of discrete, discontinuous voltage pulses. Each pulse is a signal element Binary data are transmitted by encoding each data bit into signal elements Encoding scheme: Mapping from data bits to signal elements Key data transmission terms

Mark: binary digit 1; Space: binary digit 0

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Various encoding schemes Evaluation factors Signal spectrum: less bandwidth, no dc component, shape of spectrum (better to center in the middle of bandwidth) Clocking: self-clocking capability is desired for synchronization Error detection: better to have error-detection capability Signal interference and noise immunity: Cost and complexity:1 0 0 1

RZ (Return to Zero)

0: positive pulse 1: negative pulse Signal returns to zero after each encoded bit

+V 0 -V

NRZ (Nonreturn to Zero) Voltage level is constant during bit interval (no return to a zero voltage level) 1 0 0 1 NRZ-L (NRZ Level) 0: positive voltage 1: negative voltage +V -V

NRZ-I (NRZ Inverted) a form of differential encoding 1: transition at the beginning of bit interval 0: no transition

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NRZ is simple, and efficiently use bandwidth NRZ limitations presence of dc component lack of synchronization capability

Multilevel Binary Bipolar-AMI (Alternate Mark Inversion) Three voltage levels (positive, zero, negative) 0: zero voltage 1: alternately by positive and negative voltages Better synchronization than NRZ no dc component error detection capability

Pseudoternary Same as bipolar-AMI, except representation of 0 and 1 is interchanged

Biphase Always a transition at the middle of each bit interval Manchester 0: high to low transition 1: low to high transition

Differential Manchester 0: transition at the beginning of bit interval 1: no transition

Synchronization and error detection capability, and no dc component

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Definition of digital signal encoding formatsNonreturn-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 Bipolar-AMI: 0 = no line signal, 1 = positive or negative level, alternating for successive ones Pseudoternary: 0 = positive or negative level, alternating for successive zeros, 1 = no line signal 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 B8ZS: Same as bipolar AMI, except that any string of eight zeros is replaced by a string with two code violations HDB3: Same as bipolar AMI, except that any string of four zeros is replaced by a string with one code violation

0 1 0 0 1 1 0 0 0 1 1NRZ-L NRZI Bipolar-AMI Pseudoternary Manchester Differential manchester

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Modulation Rate Data rate (expressed in bps) = modulation rate (or signaling rate or signal transition rate)(expressed in baud) times the number of bits per signal elemetNormalized signal transition rate of various encoding schemes Minimum NRZ-L NRZI Binary-AMI Pseudoternary Manchester Diff Manchester 0 (all 0s or 1s) 0 (all 0s) 0 (all 0s) 0 (all 1s) 1.0 (1010) 1.0 (all 1s) 101010 1.0 0.5 1.0 1.0 1.0 1.5 Maximum 1.0 1.0 (all 1s) 1.0 1.0 2.0 (all 0s or 1s) 2.0 (all 0s)

Spectral density of various signal encoding schemes

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Scrambling Techniques For long-distance communications No dc component, good synchronization and error detection capability, without reduction in data rate B8ZS (Bipolar with 8-Zeros Substitution) Based on bipolar-AMI 8 consecutive zeros are encoded as either 000+-0-+ or 000-+0+-, s.t. two code violations always occur

HDB3 (High-Density Bipolar 3-Zeros) 4 zeros are encoded as either 000-, 000+, +00+, or -00 Substitution rule is s.t. the 4th bit is always a code violation, and successive violations are of alternate polarity (not to introduce dc component)1 1 0 0 0 0 0 0 0 0 1 1 0 0 0 0 0 1 0 Bipolar-AMI

0 0 0 V B 0 V B B8ZS

0 0 0 V B 0 0 V HDB3

B 0 0 V

B = Valid bipolar signal, V = Bipolar violation

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SummaryNRZ (NRZ-L, NRZI*) dc Comp.? BW Required Self-clocking? Error-detection? Application Yes B No No -Simple -Digital mag. recording Multilevel Binary Biphase Scrambling (Bipolar AMI, (Manchester, (B8ZS, Pseudo ternary) Diff. Man.*) HDB3) No B Yes, but x Yes No 2B Yes Yes LAN No B Better than multilevel Yes Long dist. comm.

* Differential encoding x A long string of 0s cause a problem in AMI A long string of 1s cause a problem inPseudoternary

2. Digital Data Analog Signals Encoding is by modulation of a continuous sinusoidal carrier signal. This involves alteration of some characteristics of the carrier signal amplitude, frequency, or phase. Various encoding techniques, ASK, FSK, PSK,...

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ASK

FSK

(Diff.) PSK A cos(2fct + 180) A cos(2fct)

Binary 1 A cos(2fct + c) A cos(2f1t + c) Binary 0 0 A cos(2f2t + c)

(ASK)

(FSK)

(PSK)

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QPSK(Quadrature PSK) Each signal element represents two bitsBinary Binary Binary Binary 11: 10: 00: 01: cos(2fct + 45) cos(2fct + 135) cos(2fct + 225) cos(2fct + 315)

PSK using 12 angles and two amplitudes 9,600 bps modem (2,400 baud x 4)

Those patterns showing legal combinations of amplitude and phase are called constellation patterns 14,400 bps modem 64 points constellation pattern 28,800 bps modem 128 points

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3. Analog Data Digital Signals PCM (Pulse Code Modulation) Based on the Nyquists Sampling Theorem: If a signal is periodically sampled at a rate twice the highest significant frequency component in the signal, then it can be reconstructed from the samples by using a low-pass filter

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Analog-to-digital conversionPAM sampler Analog input signal Quantizer PAM pulses PCM pulses Encoder output

Quantization noise: S/N = 6n + 1.8 dB, where n is # of bits used To reduce quantization noise large n or Nonlinear coding Companding

Effect of nonlinear coding

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DM (Delta Modulation) Uses n = 1, I.e., binary digital signal is produced; 0 stands for change of - and 1 for change of + . Higher sampling rate than PCM (Nyquists rate) is needed, but each sample uses only 1 bit instead of n. Implementation much simpler than PCM.

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Delta ModulationAnalog input Comparator 1 = + 0=- Binary output

+ One time unit delay Binary input + One time unit delay

TransmissionReconstructed waveform

Reception

4. AnalogData Analog Signals Motivation Low frequency analog signals cannot be transmitted on unguided media. (would require antennas with km diameters) higher frequency needed. For FDM (Frequency Division Multiplexing)

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AM (Amplitude Modulation)

s(t) = [1 + m(t)]cos2 f c t

Spectrum of an AM signalM(f) S(f) Discrete carrier term Lower sideband

Upper sideband

0

B

f

0

fc-B

fc

fc+B

f

Spectrum of modulating signal

Spectrum of AM signal with carrier at fc

DSBTC

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Angle Modulation (FM and PM) PM: (t) = npm(t)

s(t) = Acos[2 f c t + (t)]

FM: (t) = nfm(t)

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5. Spread Spectrum Developed initially and popular for military and intelligence application Spread the info signal over a wider bandwidth in order to make jamming and interception more difficult

General model of spread spectrum digital communication system

Two types: Frequency hopping and Direct sequence

Basis for CDMA (Code Division Multiple Access)

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Frequency-Hopping Signal is transmitted over a seemingly random series of frequencies, hopping from frequency to frequency at split-second intervals. A receiver, hopping between frequencies in synchronization with the transmitter, picks up the message

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Direct Sequence Each bit in the original signal is represented by multiple bits (chip code) in the transmitted signal The chipping code spreads the signal across a wider frequency band in direct proportion to the number of bits used

Example of direct sequence spread spectrum