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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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Wireless Access Tech. Lab.
CCU Wireless Access Tech. Lab.
Outline
IntroductionSystem overviewError probability analysisGoodput performanceNumerical resultsConclusion
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Wireless Access Tech. Lab.
CCU Wireless Access Tech. Lab.
System overview
˙DCF of IEEE 802.11 MAC
˙IEEE 802.11a OFDM PHY
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Wireless Access Tech. Lab.
CCU Wireless Access Tech. Lab.
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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Wireless Access Tech. Lab.
CCU Wireless Access Tech. Lab.
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 :
( )
2
1*
logM
b MP PM
20
2( )avE
P QN
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Wireless Access Tech. Lab.
CCU Wireless Access Tech. Lab.
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 :1 8m
8( ) 1 (1 )m m Le uP L P
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Wireless Access Tech. Lab.
CCU Wireless Access Tech. Lab.
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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Wireless Access Tech. Lab.
CCU Wireless Access Tech. Lab.
Goodput performance(2/10)
˙MAC/PHY layer overhrads :
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Wireless Access Tech. Lab.
CCU Wireless Access Tech. Lab.
Goodput performance(3/10)
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CCU Wireless Access Tech. Lab.
Goodput performance(4/10)
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Wireless Access Tech. Lab.
CCU Wireless Access Tech. Lab.
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 :
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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Wireless Access Tech. Lab.
CCU Wireless Access Tech. Lab.
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 :
2 *( min 1) 1* ,0 6
2max
* , 62
( )i aCW
aSlotTime ibkoff aCW
aSlotTime iT i
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Wireless Access Tech. Lab.
CCU Wireless Access Tech. Lab.
Goodput performance(7/10)
˙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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Wireless Access Tech. Lab.
CCU Wireless Access Tech. Lab.
Goodput performance(8/10)
(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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Wireless Access Tech. Lab.
CCU Wireless Access Tech. Lab.
Goodput performance(9/10)
(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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Wireless Access Tech. Lab.
CCU Wireless Access Tech. Lab.
Goodput performance(10/10)
(6) The average goodput can then be approximated by :
8*( )
*msdu frag frag
Lg Mbps
N D aSIFSTime
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Wireless Access Tech. Lab.
CCU Wireless Access Tech. Lab.
Numerical results(1/4)
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Wireless Access Tech. Lab.
CCU Wireless Access Tech. Lab.
Numerical results(2/4)
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Wireless Access Tech. Lab.
CCU Wireless Access Tech. Lab.
Numerical results(3/4)
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Wireless Access Tech. Lab.
CCU Wireless Access Tech. Lab.
Numerical results(4/4)
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Wireless Access Tech. Lab.
CCU Wireless Access Tech. Lab.
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 ?