APPLICATIONS OF SS SYSTEMS

THE FUTURE LIES WITHIN

QUICK LOOK BACK

• The main concern of a signal– The B.W and Energy being used effectively

• SS trades off between both but provides– The secure communication

• The application of SS lies within its capabilities:– Secure– Anti jamming capability– Multi path fading minimization– Rejection to interference

SPACE SYSTEMS

• Satellite communications are subjected to interference• SS Systems have proved effective in this regard

• Two major satellite based systems employ SS:– GPS

• Global Positioning Systems

– CDMA Systems• Code Division Multiple Access Systems

Global Positioning SystemsGPS

INTRODUCTION

• GPS is a satellite based navigation system• The GPS system consists of 27 orbiting satellites• At any one time at least 4 satellites are visible• GPS receiver locates the satellite and then calculates

the distance using a simple technique “trilateration “

TRILATERATION 2D

• Let’s consider the following• You are totally lost in US. Someone says:

– “You are 625 miles from Boise”

– “You are 690 miles from Minneapolis “

– “You are 615 miles from Tucson “

TRILATERATION 3D

• Now imagine spheres instead of circle• This time :

– “You are 10 miles from Satellite A”

– “You are 15 miles from Satellite B “

– “Earth is itself a sphere; the third one “

CODE DIVISION MULTIPLEXING

CDMA

TDMA and FDMA

• Time Division Multiplexing (TDMA) defines allocation of time slots– Frequency is same while channels are different

• Frequency Division Multiplexing (FDMA) allows user to have unique frequency for signal tranmission– Channel may be shared while frequency is different

CDMA definition

• Code Division Multiplexing (CDMA) solves the problem of different frequencies and limited time slots

• Users can transmit their signals using one frequency band.• Transmission depends upon the code assigned to each signal and

their respective occurence

Advantages

• A number of advantages are:– Power Spectral Density is getting very small, so other communications

systems do not suffer of communications

– Random Access can be dealt with, as a large number of codes can be generated a large number of users can be permitted.

– The maximal number of users is interference limited.

– Security: without knowing the spreading code, it is (nearly) impossible to recover the transmitted data.

– Fading rejection: as a large bandwidth is used the system is less susceptible to distortions.

WIRELESS LOCAL AREA NETWORKWLAN

Introduction

• WLAN is data communication system used as an alternative to wired Local Area Network

• WLANs combine data connectivity with user mobility and enable movable LANs.

Technique

• Most WLAN systems use SS technology• Both frequency hopping and direct sequence are used.• To operate a WLAN system using SS the receiver must know the

parameters of the SS signal being broadcast. • It creates a network by sending radio-frequency signals • There are type of network

– Bluetooth

– IrDA

– HomeRF (SWAP)

– WECA (Wi-Fi)

• SS means data is sent in small pieces hopping at different frequencies

Personal Communication SystemsPCS

Introduction

• Personal Telecommunication Systems (PCS) are targeted specifically at individuals.

• The advantages of spread spectrum systems like anti-interference, anti-multipath and anti jamming make this system inevitable for Telecom. Systems.

• CDMA implemented with direct sequence SS (DS-SS) signaling is among the most promising multiplexing technologies for cellular telecom. services

Why SS? For PCS

• Superior operation in multipath environments

• Flexibility in the allocation of channels

• Privacy and the ability to operate asynchronously

• Ability to share bandwidth with narrow band communication

SECURE COMMUNICATIONSANTI JAMMING ANTI INTERFERENCE

ANTI JAMMING

• Any signal trying to suppress the message signal is said to be jamming the signal

• SS systems offer resistance to jamming because they reject the signal that does not contain the code (key)

• In FHSS, it is very difficult to fix the jamming signal to a particular frequency

Resistance to interference

• During the process of spreading any signal causing interference will be mixed with the noise level

• While despreading it will be rejected because it won’t have key• This allow SS technology to be active over its whole bandwidth

Resistance to interception

• Unauthorized persons will not be able to decode the signal because of lack of the right key

• The signal is actually hidden under noise and seems like noise, therefore completely undetectable

Resistance to Fading

• Signal reaching the receiver from different locations cause time shift in signal call “FADING”

• SS systems allow multipath fading rejection• While in the process of despreading only the Direct (D) signal will

be considered while Reflected (R) will be rejected