Digital Transmission (Line Coding) (Line Coding)

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Line Coding: Output of the multiplexer (TDM) is coded into electrical pulses or .... Line codes are used for digital base-band modulation in data communication applications,. ❑ Digital data stream is ..... Start-stop technique. ❑ Idle state: When  ...

Digital Transmission (Line Coding)

EE4367 Telecom. Switching & Transmission

Prof. Murat Torlak

Pulse Transmission  Source  Multiplexer  Line Coder  Line Coding: Output of the multiplexer (TDM) is coded into electrical pulses or waveforms for the purpose of transmission over the channel (baseband transmission)  Many possible ways, the simplest line code on-off  All digital transmission systems are design around some particular form of pulse response.

Nonreturn-to-zero (NRZ) Return-to-zero (RZ) EE4367 Telecom. Switching & Transmission

(a) (b) (c) (d) (e)

On-off (RZ) Polar (RZ) Bipolar (RZ) On-Off (NRZ) Polar (NRZ)

Prof. Murat Torlak

Pulse Transmission over a Channel

EE4367 Telecom. Switching & Transmission

Prof. Murat Torlak

Desirable Properties for Line Codes  Transmission Bandwidth: as small as possible  Power Efficiency: As small as possible for given BW and probability of error  Error Detection and Correction capability: Ex: Bipolar  Favorable power spectral density: dc=0  Adequate timing content: Extract timing from pulses  Transparency: Prevent long strings of 0s or 1s

EE4367 Telecom. Switching & Transmission

Prof. Murat Torlak

Review: Energy and Power Signals  An energy signal x(t) has 0 < E < ∞ for average energy

 A power signal x(t) has 0 < P < ∞ for average power

Can think of average power as average energy/time. An energy signal has zero average power. A power signal has infinite average energy. Power signals are generally not integrable so don’t necessarily have a Fourier transform.  We use power spectral density to characterize power signals that don’t have a Fourier transform.    

EE6390 Intro. to Wireless Comm. Systems

Prof. Murat Torlak

Review: TimeTime-Invariant Systems  Linear Time-Invariant Systems  System Impulse Response: h(t)  Filtering as Convolution in Time  Frequency Response: H(f)=|H(f)|ej∠H(f)

x(t)

h(t)

x(t)*h(t)

X(f)

H(f)X(f) H(f)

EE4367 Telecom. Switching & Transmission

Prof. Murat Torlak

Review: Distortion Distortionless Transmission  Output equals input except for amplitude scaling

and/or delay x(t)

h(t)=Kδ δ(t-ττ)

H(f)=Kej2ππfττ

X(f)

Kx(t-ττ) Kej2ππfττX(f)

Simple equalizers invert channel distortion  Can enhance noise power

Channel X(f)

H(f)

N(f) Equalizer +

1/H(f)

X(f)+N(f)/H(f)

Prof. Murat Torlak

Review: Ideal Filters  Low Pass Filter

A -B

B

 Band Pass Filter A

A -B2

-B1

B1

B2

Prof. Murat Torlak

Power Spectral Density  Power signals (P=Energy/t)  Distribution of signal power over frequency X T ( w) T -T/2

0

2

T/2

X ( w) S x ( w) = lim T T →∞ T

2

 Useful for filter analysis

Sx(f)

|H(f)|2Sx(f) H(f)

For Sx(f) bandlimited [–B,B], B