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multiple access techniques used in wireless communication



multiple access techniques

multiple access techniques



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    multiple access techniques used in wireless communication multiple access techniques used in wireless communication Presentation Transcript

    • Presented To: Dr Imran Presented By : Syed Sajid Ali Reg No: 11-MDTLC-0513 Class No: 37
    • INTRODUCTION • Multiple access schemes are used to allow many users to share simultaneously a finite amount of radio spectrum resources. • For high quality communication this sharing of spectrum should not degrade performance of the system  high performance  duplexing generally required  frequency domain  time domain
    • DUPLEXING  What is Duplexing?  to talk and listen simultaneously is called duplexing.  Classification of communication systems according to their connectivity.  Simplex  Half-duplex  Duplex A B A B A B
    •  Duplexing may be done using  frequency domain technique  time domain technique
    • Frequency division duplexing (FDD)  two bands of frequencies for every user  forward band ( for traffic from Base station to mobile unit)  reverse band (for traffic from mobile unit to Base station) • frequency separation between forward band and reverse band is constant throughout the system
    • Time division duplexing (TDD)  uses different time slots for forward and reverse link  forward time slot  reverse time slot  Communication is not full-duplex
    • TYPES OF MULTIPLE ACCESS  Frequency division multiple access (FDMA)  Time division multiple access (TDMA)  Code division multiple access (CDMA)
    • Frequency division multiple access FDMA  one phone circuit per channel  idle time causes wasting of resources  simultaneously and continuously transmitting  usually implemented in narrowband systems  Complexity of FDMA mobile systems is lower compared to TDMA  FDMA uses duplexers  for example: AMPS is a FDMA system with bandwidth of 30 kHz
    • FDMA power Example: Advanced Mobile Phone System
    • Time Division Multiple Access  Time slots  one user per slot  Buffer and burst method  Non-continuous transmission Advantage: Total bandwidth is utilized Disadvantage: Strict Burst Timing is required at the earth station
    • TDMA power Example: Global System for Mobile (GSM)
    • Code division multiple access (CDMA)  Narrowband signals is multiplied by a very large bandwidth signal called the spreading signal.  The spreading signal is pseudo noise code sequence that has a chip rate which is orders of magnitudes greater than data rate of the message  All users use the same carrier frequency and transmit simultaneously  Each user has its own pseudo random code word which is approximately orthogonal to other codewords.
    •  Receiver performs time co-relation  All other codewords appear as noise  Receiver needs to know the code word used by transmitter
    • CDMA power
    • Multiple Access Techniques in use Cellular System Multiple Access technique used Advanced Mobile Phone System (AMPS) FDMA/FDD Global System for Mobile (GSM TDMA/FDD US Digital Cellular (USDC) TDMA/FDD Digital European Cordless Telephone (DECT) FDMA/FDD US Narrowband Spread Spectrum (IS-95) CDMA/FDD
    • Q..??