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MAC enhancements to 802.11a

MAC enhancements to 802.11a. EE206A Student Presentation #4 Yen-Cheng Kuan Chih-Chieh Han {yckuan, simonhan}@ee.ucla.edu. Outline. Access Mechanisms of 802.11. DCF – Distributed Coordination Function (Ad-hoc). PCF – Point Coordination Function (Access Point). Enhancements of 802.11.

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MAC enhancements to 802.11a

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  1. MAC enhancements to 802.11a EE206A Student Presentation #4 Yen-Cheng Kuan Chih-Chieh Han {yckuan, simonhan}@ee.ucla.edu

  2. Outline • Access Mechanisms of 802.11. • DCF – Distributed Coordination Function (Ad-hoc). • PCF – Point Coordination Function (Access Point). • Enhancements of 802.11. • Goodput in DCF – Adaptive Fragmentation & PHY mode selection. • Energy Efficiency in PCF – Adaptive TPC (Transmission Power Control) & PHY mode selection.

  3. STA_A STA_A STA_B STA_B DCF Timing Diagrams • Transmission Success. (Short Inter-frame Space) (Distributed Inter-frame Space) • Re – transmission. = SIFS + ACK + aSlotTime

  4. Link Adaptation in DCF • Goodput is a step function of SNR in a given PHY mode. • High SNR  high goodput. • Low SNR  low(zero) goodput. • PHY mode selection. • Higher PHY mode  higher SNR is needed  high goodput. • Lower PHY mode  low goodput. • Fragmentation may help. • More fragments  shorter packet size low packet error rate. less retransmission and high control overhead • Less fragments  more re-Tx and low control overhead.

  5. Goodput Analysis

  6. | L | Goodput Analysis (cont’d)

  7. Goodput Analysis (cont’d)

  8. Goodput Analysis (cont’d) : Bit error probability of X – octet long packet to be transmitted by PHY m mode X

  9. Goodput Analysis (cont’d) X • Upper bound of X X dfree : free distance of the convolution code selected in PHY mode m. ad : the total number of error events of weight d. : the bit error probability for the modulation scheme selected in PHY mode m.

  10. , Goodput Analysis (cont’d) • Bit Error Probability relates to SNR (Eav/N0) • M-ary QAM modulation, M = 4, 16, 64. PHY m = 5, 6, 7, 8. • BPSK modulation. PHY m = 1, 2. • QPSK modulation. PHY m = 3, 4. - the same as 4 – ary QAM.

  11. Goodput Analysis (cont’d) • Average Backoff interval after i consecutive unsuccessful transmission attempts.

  12. relates to PHY mode m and SNR (Eav/N0). X Goodput Analysis (cont’d) • Goodput relates to Number of Fragmentation, PHY mode, SNR.

  13. Simulation Results

  14. Simulation Results (cont’d)

  15. Simulation Results (cont’d)

  16. Implementation • The link adaptor is compatible to the existing network or higher layer applications. • No additional error correction codes need to be implemented by taking advantages of different modulation scheme and FEC provided by OFDM PHY. • Simple implementation.

  17. Point Coordination Function - PCF • Central Controlled Access Mechanism. (AP & STAs) • Polling – Based. - A wireless station (STA) can only transmit “aftrer” being polled by a Access Point (AP). STA0 STA4 AP STA1 STA3 STA2

  18. AP AP AP STA STA STA PCF Timing Diagrams • Successful Uplink. (Short Inter-frame Space) • Retransmission. FCS, CRC errors (Priority Inter-frame Space)

  19. Energy Efficient in PCF • Transmission power should not be constant.  Can use less power when Path Loss is low. • PHY mode should be dynamic.  The lower the Path Loss,  more bits can be transmitted,  higher PHY mode can be used.

  20. MAC/PHY Energy Consumption

  21. MAC/PHY Energy Consumption (cont’d) • Low efficiency PA. • Simplified Block Diagram of a WLAN Card • High efficiency PA. • High Efficiency PA.

  22. | L | MAC/PHY Energy Consumption (cont’d) , m : PHY mode

  23. X MAC/PHY Energy Consumption (cont’d) : Bit error probability of X – octet long packet to be transmitted by PHY m mode

  24. relates to PHY mode m and SNR (Eav/N0). X MAC/PHY Energy Consumption (cont’d) • SNR(dbm) = Pout – Path Loss – White Noise. • Energy Consumption relates to Transmission Power (Pout, TPC), PHY mode, Path Loss.

  25. Simulation Results

  26. Simulation Results (cont’d) • TPC (Transmission Power Control) is required.

  27. Goodput Constrains

  28. Simulation Results (cont’d)

  29. Implementation • AP incorporates the TXPWR_LEVEL (transmitting power level) into the service field in the packet. • STA can get TXPWR_LEVEL and RSSI (Received Signal Strength Indicator) when receiving the packet. • Path Loss = TXPWR – RSSI.

  30. References • Daji Qiao; Sunghyun Choi Goodput enhancement of IEEE 802.11a wireless LAN via link adaptation. ICC 2001. IEEE International Conference on Communications. Conference Record (Cat. No.01CH37240), (vol.7), ICC 2001. IEEE International Conference on Communications. • Daji Qiao, Sunghyun Choi, Amjad Soomoro, and Kang G. Shin. Energy-Efficient PCF Operation of IEEE 802.11a Wireless LAN. Proceedings of INFOCOM 2002, New York City, New York, June, 2002.

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