Digital Transmission (Line Coding) (Line Coding)

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