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Goodput Enhancement of IEEE 802.11a Wireless LAN via Link Adaptation

Goodput Enhancement of IEEE 802.11a Wireless LAN via Link Adaptation. Communications , 2001.ICC , CNF 2005/12/27 通訊所 , 徐文壕. Outline. Introduction System overview Error probability analysis Goodput performance Numerical results Conclusion. System overview. ˙DCF of IEEE 802.11 MAC

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Goodput Enhancement of IEEE 802.11a Wireless LAN via Link Adaptation

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  1. Goodput Enhancement of IEEE 802.11a Wireless LAN via Link Adaptation Communications , 2001.ICC , CNF 2005/12/27 通訊所 , 徐文壕

  2. Outline • Introduction • System overview • Error probability analysis • Goodput performance • Numerical results • Conclusion

  3. System overview ˙DCF of IEEE 802.11 MAC ˙IEEE 802.11a OFDM PHY

  4. Error probability analysis(1/3) ˙Bit error probability :  assuming AWGN channel  the symbol error probability for an M-ary QAM modulation is where is the symbol error probability for the PAM modulation

  5. Error probability analysis(2/3)  with Gray coding , the bit error probability for an M-ary QAM modulation can be approximated by  for M=4 , we assume that the error probability : 4-ary QAM = QPSK  for M=2 , we assume that bit error probability of BPSK is :

  6. Error probability analysis(3/3) ˙Packet error probability :  For an L-octet long packet to be transmitted using PHY mode m ( ), this error upper bound is :

  7. Goodput performance(1/10) ˙Assumptions : (1) Assume that two stations running the DCF are communicating with each other with no interfering stations nearby. (2) Only one station is transmitting and its queue is never empty.  no collisions (3) Assume that there is no retry limit for each frame. (4) No power control on the transmitting station. (5) Neglect the air propagation delays in our goodput analysis. (6) Assume that the ACK frame is transmitted at the same rate as the data frame which it is acknowledging.

  8. Goodput performance(2/10) ˙MAC/PHY layer overhrads :

  9. Goodput performance(3/10)

  10. Goodput performance(4/10)

  11. Goodput performance(5/10) For L-octet long information to be transmitted over the IEEE 802.11a physical layer using PHY mode m, the transmission duration is : The transmission duration for an ACK frame using PHY mode m is :

  12. Goodput performance(6/10) ˙Backoff/Contention phase : Let Tbkoff (i) denote the average backoff interval after i consecutive unsuccessful transmission attempts, and it can be calculated by :

  13. Goodput performance(7/10) ˙Goodput analysis : (1) The probability of a successful transmission cycle can be calculated by : where

  14. Goodput performance(8/10) (2) Let and n be the number of fragments and the number of consecutive unsuccessful transmission attempts before the current transmission, respectively. (3) Therefore, the average time space between contiguous MSDU transmissions is : (4) each successful fragment transmission duration is equal to the fragment transmission time, plus the ACK transmission time, and plus two SIFS times.

  15. Goodput performance(9/10) (5) Therefore , the average transmission duration for each fragment , , can be calculated by : where

  16. Goodput performance(10/10) (6) The average goodput can then be approximated by :

  17. Numerical results(1/4)

  18. Numerical results(2/4)

  19. Numerical results(3/4)

  20. Numerical results(4/4)

  21. Conclusion ˙The numerical results suggest that link adaptation, which performs both dynamic fragmentation and PHY mode selection, is an attractive way to enhance the goodput performance of an IEEE 802.11a wireless LAN. ˙Is it also good for the multi-station environment ? ˙How to design a link adaptation algorithm that can be embedded into the existing IEEE 802.11 MAC protocol ?

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