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. 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. Analytical Modeling = of subchannel i 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