hybrid ofdma csma based medium access control for next generation wireless lans
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Hybrid OFDMA/CSMA Based Medium Access Control for Next-Generation Wireless LANs. Yaser Pourmohammadi Fallah, Salman Khan, Panos Nasiopoulos, Hussein Alnuweiri IEEE ICC 2008 學生:黃志堅. Outline. Introduction OFDMA System Specifications Hybrid OFDMA/CSMA MAC Design

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hybrid ofdma csma based medium access control for next generation wireless lans

Hybrid OFDMA/CSMA Based Medium Access Control for Next-Generation Wireless LANs

Yaser Pourmohammadi Fallah, Salman Khan, Panos Nasiopoulos, Hussein Alnuweiri

IEEE ICC 2008

學生:黃志堅

outline
Outline
  • Introduction
  • OFDMA System Specifications
  • Hybrid OFDMA/CSMA MAC Design
  • Analysis And Performance Evaluation
  • Conclusion
introduction
Introduction
  • The efficiency and scalability of WLAN is greatly dependent on multiple access (MA)
  • Address low CSMA\CA MAC efficiency under heavy loading conditions in crowded WLANs
  • Support multimedia traffic and variable length packet transmission in the MAC
  • Use orthogonal frequency division multiple access (OFDMA) to solve
ofdma system specifications1
OFDMA System Specifications
  • Subchannels formed from grouping subcarriers together
    • may be adjacent
      • higher throughput in fixed, portable and low mobility
    • may be distributed
      • high throughput in mobility applications
hybrid ofdma csma mac design
HYBRID OFDMA/CSMA MAC DESIGN
  • Their proposed MAC uses a two stage frame delivery process
    • Transmission opportunity Request (TR) phase
    • Scheduled data Transmission (ST) phase
  • OFDMA is used in the TR phase and the ST phase uses controlled CSMA
mac operation timeline
MAC Operation Timeline
  • MIFS < CIFS < RIFS
to request tr phase 1 2
TO Request (TR) Phase 1/2
  • AP keeps track of the number of associated stations (Q) and active stations (n)
    • first assigns the active stations, then distributes the rest of the stations
  • Assign number of (more than one) station to each subchannel
  • To achieve even higher performance
    • AP can dynamically change the number of subchannels
    • assign stations to different subchannels
to request tr phase 2 2
TO Request (TR) Phase 2/2
  • Each subchannel uses its own separate CSMA/CA procedure
  • After a station transmits a TR message, it will wait for a response from the AP
    • a poll
    • position in the schedule
  • If no response is received, the station will interpret this as a collision (or lost packet)
    • doubles its contention window size
    • selects new random backoff number
scheduled transmission st phase 1 2
Scheduled Transmission (ST) Phase 1/2
  • All subchannels in the ST phase are assigned to only one station at a given time
  • The schedule and order of access are enforced
    • broadcast messages indicating the schedule
    • explicit poll (TO assignment) messages
quality of service and multimedia support
Quality of Service and Multimedia Support
  • To enable QoS and multimedia provisioning for the proposed MAC
    • prioritized random access
    • scheduled guaranteed access
  • Provide priority services, we specify different limits for contention window sizes
  • Also required that the access point schedule TOs for higher priority ahead of the others
802 11
802.11
  • CSMA/CA
  • CSMA/CA with RTS/CTS
conclusion
Conclusion
  • The MAC protocol they proposed combines OFDMA with CSMA/CA mechanisms
  • Compared to CDMA systems, combat fading with less complexity
  • Compared to TDMA systems, it suitable for a combination of data and multimedia traffic
  • Another research subject that can be based on the design of scheduling algorithms
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