APPLICATIONS OF SS SYSTEMS THE FUTURE LIES WITHIN

QUICK LOOK BACK The main concern of a signal –T–The B.W and Energy being used effectively SS trades off between both but provides –T–The secure communication The application of SS lies within its capabilities: –S–Secure –A–Anti jamming capability –M–Multi path fading minimization –R–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: –G–GPS Global Positioning Systems –C–CDMA Systems Code Division Multiple Access Systems

Global Positioning Systems GPS

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 –F–Frequency is same while channels are different Frequency Division Multiplexing (FDMA) allows user to have unique frequency for signal tranmission –C–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: –P–Power Spectral Density is getting very small, so other communications systems do not suffer of communications –R–Random Access can be dealt with, as a large number of codes can be generated a large number of users can be permitted. –T–The maximal number of users is interference limited. –S–Security: without knowing the spreading code, it is (nearly) impossible to recover the transmitted data. –F–Fading rejection: as a large bandwidth is used the system is less susceptible to distortions.

WIRELESS LOCAL AREA NETWORK WLAN

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 –B–Bluetooth –I–IrDA –H–HomeRF (SWAP) –W–WECA (Wi-Fi) SS means data is sent in small pieces hopping at different frequencies

Personal Communication Systems PCS

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 COMMUNICATIONS ANTI 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