DIGITAL SIGNAL TRANSMISSION

BY

ANKITH J SHETTY AND YASHWIN

Dept of Electronics,

Mangalore University

INTRODUCTION

• Signal transmission occurs between a transmitter & receiver via

some medium.

• Guided medium

– Eg. Twisted pair, coaxial cable, optical fiber

• Unguided / wireless medium

– Eg. Air, water, vacuum

OPTICAL FIBER TRANSMISSION LINK

•Types•System consideration•Link Power budget•Rise-Time Budget

OPTICAL FIBER TRANSMISSION LINK

• Optical fiber transmission link have three types of links they are

• point-to-point

– only 2 devices share link

• direct link

– no intermediate devices

• multi-point

– more than two devices share the link

POINT-POINT LINK

• The simplest transmission link is a point-to-point line having a

transmitter at one and receiver on the other as shown in below.

(a) Emission wavelength(b) Spectral line width(c) Output power(d) Effective radiating area(e) Emission pattern

(a) Core size(b) Core index profile(c) BW or dispersion(d) Attenuation(e) NA or MFD

(a) Responsivity(b) Operating λ(c) Speed(d) Sensitivity

LED or laser MMF or SMF pin or APD

POINT-POINT LINK

• The design of an optical link involves fiber ,source ,and photo

detector . So that the link design and analysis may require several

iterations before they completed satisfactorily.

• performance and cost constraints are very important factors in fiber

optic communication links ,the designer must carefully choose the

components to ensure the desired performance level can be

maintain over the expected system lifetime without over specifying

the component characteristics.

• The key system requirements needed in analyzing a link are :

1. The desired(or possible) transmission distance

2. The data rate or channel bandwidth

3. The Bit Error Rate (BER)

SYSTEM CONSIDERATION

• Two analyses are usually carried out to ensure that the desired

system performance can be met; these are link power budget and

the system rise-time budget analysis.

• In carrying out a link power budget ,we first decide at which wave

length to transmit and then choose components operating in this

region.

• If the distance over which the data are to be transmitted is not too

far, we may decided to operate in the 800 – 900 nm region.

• On the other hand ,if the transmission distance is relatively long ,we

may want to take advantage of the lower attenuations and

dispersion that occurs at wavelengths around 1300 or 1550 nm.

LINK POWER BUDGET

• The optical power received at the photo detector depends on the

amount of light coupled into the fiber and the losses occurring in the

fiber and at the connectors splices. The link loss budget is derived

from the sequential loss contributions of each elements in the link.

loss =10 log(Pout / Pin)

Where Pout and Pin are the optical powers emanating into and

out of the loss element ,respectively

OPTICAL POWER-LOSS MODEL

ystem MarginT s R c sp fP P P ml nl L S

: Total loss; : Source power; : Rx sensitivity

connectors; splices

T s RP P P

m n

RISE-TIME BUDGET• A rise time budget analysis is a convenient method for determining

the dispersion limitation of an optical fiber link. In this approach the total rise time tsys of the link is the root-sum-square of the rise times from each contributor ti to the pulse rise-time degradation.

• The four basic elements that may significantly limit the systemspeed they are,

1. Transmitter rise time ttx2. Material dispersion rise time tmat

3. Modal dispersion rise time tmod

4. Receiver rise time trx

LINE CODING

•Principle

•Types

LINE CODING

• Arranging signal symbols in a particular pattern using a set of rules.

• Binary codes – widely used and advantageous.

PRINCIPLE OF LINE CODING

• Introduce redundancy into data stream.

• Thus minimizing errors resulting from channel interference effects.

• Depending on the amount of redundancy introduced, any degree of

error-free transmission of digital data can be archived, provided that

the data rate that includes this redundancy is less than the channel

capacity. This is called Shannon channel-coding theory.

LINE CODING EXAMPLES

TYPES OF LINE CODING

• There are 3 basic types of line coding, that is used for optical fiber

transmission links:

• NRZ(Non-Return-to-Zero) Format

• RZ(Return Zero) Format

• PE(Phase Encoded) Format

NZR CODES

a binary code in which 1s are represented by one significant

condition (positive voltage) and 0s are represented by some other

significant condition (negative voltage)

RETURN-TO-ZERO

RZ takes place even if a number of consecutive 0s or 1s occur in

the signal. The signal is self-clocking.

PE (PHASE ENCODED) FORMAT

• The mBnB code converts a block of m input bits to a block of n code

bits.

• Coding efficiency is determined by the ratio m/n, but generally n is

chosen as m+1.

EYE-PATTERN

EYE-PATTERN

• Simple but powerful measurement method for assessing the data-

handling capability of a digital transmission system.

• The eye pattern measurements are done in time domain.

• Also allow the effect of waveform distortion to be immediately shown

on the oscilloscope.

EYE-PATTERN

EYE-PATTERN

• The output from pseudorandom data pattern generator is applied to

the vertical input of an oscilloscope.

• Data rate is used to trigger the horizontal sweep.

• The resulting pattern resembles human eye.

• Random data signal is used because characteristics of data streams

are similar.

SIMPLIFIED EYE-PATTERN DIAGRAM

• The width of the eye opening – time interval in which signal can be sampled.

• Best time to sample – height of the eye opening is largest.

• Due to amplitude distortion – height of eye opening is reduced.

• Maximum distortion – vertical distance between the top of eye opening and the maximum signal level.

• Noise margin -