formatting and baseband transmission -...

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College of Information & Communications

The world goes wireless! Prepared by Sung Ho Cho

Hanyang University

Formatting and Baseband Transmission

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Definitions

Formatting : Transformation from source information to digital symbols.

Source Coding :Additional data redundancy reduction (or data compression)Efficient description of source information

Baseband (or Lowpass) Signal :A signal whose spectrum extends from 0 to MHz

Digital Symbol : Formatted information

Baseband Modulation : Assign a pulse waveform to represent a symbol.The waveform will be transmitted over a cable.Not appropriate for propagation though most media.

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Baseband Systems

AnalogInformation

TextualInformation

DigitalInformation

InformationSource

PulseWaveform

DigitalInformation

TextualInformation

AnalogInformation

Sampler Quantizer Coder

WaveformEncoder

(Modulator)XMT

Channel

RCVWaveformEncoder

(Demodulator)

LPF Decoder

BinaryDigits

InformationSink

Format

Format

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Messages, Characters, and Symbols

Message: THINK(text)

Character coding:(6‐bit ASCII)

8‐ary digitals:(symbols)

8‐ary waveforms:

0 0 1 0 1 0 0 0 0 1 0 0 1 0 0 1 0 0 0 1 1 1 0 0 1 1 0 1 0 0

1 2 0 4 4 4 3 4 6 4

T H I N K

( )ts1 ( )ts2 ( )ts0 ( )ts4 ( )ts4 ( )ts4 ( )ts3 ( )ts4 ( )ts6 ( )ts4

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Formatting Analog InformationAnalog Sampling Quantizing Encoding Bit Stream

Ideal Sampling Theorem

: Highest frequency of the message: Sampling frequency

Nyquist Rate :

If

Then

where

is periodic with period .

Aliasing takes place if .

ms ff 2≥

mf

ms ff 2=

( ) ( ) ( ) ( )fXtxfXtx s

FT

s

FT↔↔ ,

( ) ( )∑∞

−∞=−=

ks

ss nffX

TfX 1

ss f

T 1=

( )fX s sf

ms ff 2<

sf

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Ideal Sampling

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Source of Corruption

Sampling EffectNatural sampling by switching waveformZero‐order sampling

Quantizing EffectRound‐off or truncation errorQuantizer saturation error (can be avoided by an AGC)

Timing Jitter

Channel NoiseThermal noiseInterferences from other usersAWGNFading

Intersymbol interference (ISI)

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Natural Sampling by Switching Waveform

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Zero‐order Sampling (Sample‐and‐hold)

Natural Sampling

Zero‐order Sampling

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Uniform vs. Non‐uniform Quantization

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Non‐uniform PCM: μ‐law vs. A‐law

μ‐law A‐law

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Baseband Transmission (1/4)

Waveform Representation

PCM Sequence

Pulse Representation of PCM

Pulse Waveform

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PCM WaveformsNon‐return‐to‐zero (NRZ)

NRZ‐L (level)NRZ‐M (mark, change at “1”)NRZ‐S (space, change at “0”)

Return‐to‐zero (RZ)Unipolar RZBipolar RZRZ‐AMI (alternate mark inversion)

Phase encodedBi‐phase‐L (Manchester)Bi‐phase‐MBi‐phase‐SDelay Modulation

Multilevel binaryDicode NRZDicode RZ

Telephone

Optical

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Baseband Transmission (3/4)Characteristics of PCM Waveforms

DC componentSelf‐clocking

Inherent synchronizationError detection

Detection of isolated errorsBandwidth compression

Bandwidth efficiency (e.g., multilevel codes)Differential encoding

Detection of polarity inversion of waveforms during transmissionNoise immunity

The NRZ has better error performance than the unipolar RZ.

NoteBit rate : the rate measured at which data are transmitted

Modulation rate : the rate at which signal level is changed

For M‐ary format,

bb T

R 1=

TR 1=

MTT b 2log=

(bps)

(sps :symbol per sec) or (bauds)

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Spectral Densities of PCM Waveforms

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