17.pptx
TRANSCRIPT
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
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