CODE DIVISION MULTIPLE ACCESS

PRIYAL ZAVERI

WHY MULTIPLE ACCESS?

Users/Earth Stations Share the Transmission Resource i.e. Radio Spectrum Aim is to develop Efficient Techniques that Maximize System

Capacity through Dynamic Resource Allocation and Spectrum Reuse

Simple FDM/FM Satellite Systems become Inefficient is BW Utilization and Economically Impractical

Pre-Assigned or Demand-Assigned Channel Allocation In case of Pre-Assigned System, a given number of available voice-band channels from each earth station are assigned to a dedicated destination….Some-times wastage of Precious BW Resource In case of Demand-Assigned System, Resources allocation is on need basis, versatile and efficient usages of Radio Spectrum, but a Complex Mechanism is required at all Earth Stations/Users

A PRE-ASSIGNED/DEDICATED SYSTEM

• Each earth station requires two dedicated pairs of Tx/Rx frequencies to communicate with any other station

• As many communication partners, same number of transponders (RF-RF duplex translator/repeater)

• Transponder BW 36 MHz which is mostly wasted

THREE MULTIPLE ACCESS TECHNIQUES

Satellite Multiple Accessing/Destination means more than one users/earth stations can access to one or more Radio Channels (Transponders) on board

FDMA TDMA CDMA

FH-CDMA DS-CDMA

Wide-band CDMA, entire spectrum is used by each user all the time but with use of orthogonal codes. CDMA/FDD and CDMA/TDMA both configurations are possible.

• Consider an “International Cocktail Party”• FDMA – Large room divided up into small rooms.

Each pair of people takes turns speaking.• TDMA – Large room divided up into small rooms.

Three pairs of people per room, however, each pair gets 20 seconds to speak.

• CDMA – No small rooms. Everyone is speaking in different languages. If voice volume is minimized, the number of people is maximized.

Code Division Multiple Access (CDMA)-The Concept

No restrictions on any user/earth station on time and frequency slots usages, rather any user can use allocated BW or all system BW at any time, however, using a special chip code to spread its low-bandwidth signal over the entire allocated spectrum… Spread Spectrum Multiple Access

Code Division Multiple Access (CDMA)-The Concept (Cont’d)

Types Of CDMA Orthogonal Codes Correlation and Cross-Correlation How Spreading and De-Spreading is done? Processing Gain, G = Chip Rate/Date Rate

NDG Notes 8Wayne Tomasi-Ch 15

Correlation and Cross-Correlation

NDG Notes 10Wayne Tomasi-Ch 15

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FH-Spread Spectrum

NDG Notes 13Wayne Tomasi-Ch 15

NDG Notes 14Wayne Tomasi-Ch 15

DS-Spread Spectrum

NDG Notes 15Wayne Tomasi-Ch 15

Spread Spectrum Transmission

Highest power consumption

Highest potential data rates

Lowest aggregate capacity using multiple physical layers than frequency hopping

Smallest number of geographically separate radio cells due to limited channels

Greater range than frequency hopping

Slices transmission into small coded bits and spreads message across whole spectrum

Utilizes wide signal channel

Lower cost

Lowest power consumption

Most tolerant to signal interference

Lower potential data rates

Highest aggregate capacity using multiple physical layers

Less range than direct sequence

Concentrates power in very narrow spectrum

Hops in random pattern 100 times/sec

Spreads power across band instead of signal

Direct Sequence Frequency Hopping

NDG Notes 17Wayne Tomasi-Ch 15

NDG Notes 18Wayne Tomasi-Ch 15

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NDG Notes 20Wayne Tomasi-Ch 15

NDG Notes 21Wayne Tomasi-Ch 15

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