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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. 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

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  1. 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

  2. Outline • Introduction • OFDMA System Specifications • Hybrid OFDMA/CSMA MAC Design • Analysis And Performance Evaluation • Conclusion

  3. 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

  4. OFDMA System Specifications

  5. 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

  6. 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

  7. MAC Operation Timeline • MIFS < CIFS < RIFS

  8. 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

  9. 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

  10. 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

  11. Scheduled Transmission (ST) Phase 2/2

  12. 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

  13. PARAMETERS USED FOR SIMULATION

  14. = of subchannel i Analytical Modeling Number of backoff

  15. Throughput

  16. 802.11 • CSMA/CA • CSMA/CA with RTS/CTS

  17. Model Validation

  18. CSMA/CA vs. OFDMA/CSMA

  19. Maximum Throughput

  20. Effect of Increasing The Load 32 stations

  21. 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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