OPTICAL FIBER TRANSMISSION USING CDMA

TEACHNIQUE

Optical fiber

Introduction

An optical fiber is a glass or plastic fiber that carries light along its length.

Fiber optics is the overlap of applied science and engineering concerned with the design and application of optical fibers.

Optical fibers are widely used in fiber-optic communications.

Fibers are also used for illumination, and are wrapped in bundles so they can be used to carry images, thus allowing viewing in tight spaces.

Specially designed fibers are used for a variety of other applications, including sensors and fiber lasers.

Classification Of Optical Fiber

Multi-mode fibers (MMS): generally have a larger core diameter, and are used for

short-distance communication links and for applications where high power must be transmitted .

Single-mode fibers(SMF): They are using for most communication links longer than

550 meters (1,800 ft) .

MMF& SMF

AdvantagesOptical fibers are widely used in fiber-optic

communications, which permits transmission over longer distances and at higher bandwidths (data rates) than other forms of communications.

Fibers are used instead of metal wires because signals travel along them with less loss, and they are also immune to electromagnetic interference.

Objectives of our project: The well known advantages of optical fiber as :

Take advantage of optical fiber for obtaining more data rate.

Multiple access of users at same time & channel frequency distinguish between them by using PN codes.

Low cost, low loss, light weight.

Large bandwidth characteristics.

In addition to its transmission properties, the insensitivity of fiber optic cables to electromagnetic radiation .

CDMACDMA is a spread spectrum technology that uses digital code division ,not frequency or time division, to realize multiple access.

It is a multiple access technique that allow multiple user to share the same bandwidth channel by give every user a unique code.

Spread Spectrum

Divider1

LPF

Transmitter System

Divider2

Clock Circuit

Divider1 Divider2

AudioAmplifier

A/D PNCode1

User1Mic

Summing Circuit

AudioAmplifier

A/D PNCode2

User2Mic

E/OSignalO\P

Receiver System

Speaker

PN1

BPF

Comparator

D/A PowerAmplifier

O/EUser 1

PN2 D/A PowerAmplifier

User 2

Speaker

Signal

O\P

Divider2 Divider1

Divider2 Divider1

Clock FunctionIn a digital computer or controller, the device

or circuit that supplies timing pulses to pace the operation of the system.

Clock signal in digital circuit is responsible for synchronizing the transfer to the data between processing elements.

Clock generation circuit

Frequency DividerDivides the circuit input Frequency

The lower waveform is the generated signal from the clock circuit with a frequency (4 MHz) .

The upper waveform is the output signal from divider circuit with a frequency (1MHz) .

• The lower waveform is the generated signal from the clock circuit with a frequency (4 MHz) .

• The upper waveform is the output signal from divider circuit with a frequency ( 250KHz) .

Frequency Divider Schematic:

Function of PN Code :We use sequence of random like noise to spread

the signal and make it like noise level .

This techniques called CDMA .

Method of PN Code Generations: LFSRs sequence through L=(2N - 1) states, where N is the number of registers in the LFSR

Figure 1. Direct Sequence Spread Spectrum System

PN Circuit of user 1 :

PN Code Circuit Of user 2:

SN74LS164 Logic Diagram:

Output of circuit 1

Output of circuit 2

An active filters offers the following advantages over a passive filters:

Gain and frequency adjustment flexibility.

No loading problem.

Passive low pass filter

Active low pass filter

Low pass filter (LPF)Definition:A low-pass filter is a filter that passes low-frequency signals but attenuates (reduces the amplitude of) signals with frequencies higher than the

cutoff frequency.

Filter Design: 1 -Choose a value of high cutoff frequency fH

2 -select a value of C3 -Calculate the value of R using

R =

4 -select value of R1 and Rf dependent on the desired passband gain Af using

Af =1 +

LPF Experiment of( : 500KHz )

LPF for 500KHZ

Experiment Of LPF : (1MHz)

Band pass filter (BPF)Definition:A band-pass filter is a device that passes frequencies within a certain range and rejects (attenuates) frequencies outside that range.

Filter Design:1 -Choose a value of high cutoff frequency fH

and low cutoff frequency fL2 -select a value of C

3 -Calculate the value of R for LPF and R for HPF using

for LPF R=

for HPF R =

4 -select value of R1 and Rf dependent on the desired passband gain Af using

1 +Af =

Experiment of BPF: (500KHz)

BPF for 500KHZ

Experiment of BPF: (1MHz)

Electrical To Optical Conversion Circuit:

In optical communications, a form of Modulation in which the Optical Power output of a Source is varied in accordance with some characteristic of the modulating signal.

 

Buffer..

1 -Holding data .

2 -Matching Impedance.

Buffer circuit schematic

Driver circuit schematic:

In our project we used the common-emitter amplifier to drive the optical source and convert the signal from the electrical form into optical form using the intensity modulation.

The Transmission Over Optical Fiber Test: