Goodput Enhancement of IEEE 802.11a Wireless LAN via Link Adaptation

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Wireless Access Tech. Lab.

CCU Wireless Access Tech. Lab.

Goodput Enhancement of IEEE 802.11a Wireless LAN via Link Adaptation

Communications , 2001.ICC , CNF

2005/12/27 通訊所 , 徐文壕

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Outline

IntroductionSystem overviewError probability analysisGoodput performanceNumerical resultsConclusion

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System overview

˙DCF of IEEE 802.11 MAC

˙IEEE 802.11a OFDM PHY

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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

0(0, )2NGaussian

21 (1 ) , 4,16,64M MP P M

0

1 32 (1 )* ( * )

1av

M

EP Q

M NM

M ary

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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 :

( )

2

1*

logM

b MP PM

20

2( )avE

P QN

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˙Packet error probability : For an L-octet long packet to be transmitted using

PHY mode m ( ), this error upper bound is :1 8m

8( ) 1 (1 )m m Le uP L P

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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.

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˙MAC/PHY layer overhrads :

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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 :

30.75( ) Pr *

( )m

data

LT L tPLCP eamble tPLCPHeader tSymbol

Bps m

16.75Pr *

( )m

ackT tPLCP eamble tPLCPHeader tSymbolBps m

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˙Backoff/Contention phase :

Let Tbkoff (i) denote the average backoff interval after i

consecutive unsuccessful transmission attempts, and it can be

calculated by :

2 *( min 1) 1* ,0 6

2max

* , 62

( )i aCW

aSlotTime ibkoff aCW

aSlotTime iT i

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˙Goodput analysis : (1) The probability of a successful transmission cycle can be

calculated by :

where

_ , ,( ) (1 ( ))*(1 )m m mgood cycle e data e ackP L P L P

_ ,( ) 1 ( )m mgood cycle e dataP L P L

1, ( ) 1 (1 (3))*(1 (30.75 ))m m

e data e eP L P P L

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(2) Let and n be the number of fragments and the numberof consecutive unsuccessful transmission attempts before thecurrent 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.

fragN

(0)bkoffmsdu aDIFSTime T

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(5) Therefore , the average transmission duration for each

fragment , , can be calculated by :

where

1

* ( ) ( )m mbkofffrag ack data

i frag

LD P n i aSIFSTime T aSlotTime T j T

N

fragD

2* ( )m mdata ack

frag

LaSIFSTime T T

N

_ _1 ( ) * ( )

i

m mgood cycle good cycle

frag frag

L LP n i P P

N N

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(6) The average goodput can then be approximated by :

8*( )

*msdu frag frag

Lg Mbps

N D aSIFSTime

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Numerical results(1/4)

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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 ?

Goodput Enhancement of IEEE 802.11a Wireless LAN via Link Adaptation

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