May 2001
doc.: IEEE 802.11-01/227
Throughput and Loss Packet
Performance of DCF with Variable
Transmit Power
Steven D. Gray and Venkatesh Vadde
[steven.gray, venkatesh.vadde]@nokia.com
Nokia Research Center
Irving, TX
Submission
Slide 1
S. Gray, Nokia
May 2001
doc.: IEEE 802.11-01/227
Outline
• Motivation
• CCA performance
– Probability of detection analytical model
– Simulation
• Test cases
• DCF simulation
– Throughput and delay
Submission
Slide 2
S. Gray, Nokia
May 2001
doc.: IEEE 802.11-01/227
Motivation for Analysis
• Proposals have been offered to allow STAs to adjust
power on an individual basis
– In discussion, many expressed the opinion that this will
cause “hidden terminals”
– Quantitative results are offered to illustrate the performance
(delay and throughput) of a networking using two different
power settings
• (E)DCF and PCF/HCF
– DCF STAs must transmit at a power level to achieve a target
packet loss rate AND keep others off the channel
– PCF/HCF must only transmit at a power level to achieve a
target packet loss rate
Submission
Slide 3
S. Gray, Nokia
May 2001
doc.: IEEE 802.11-01/227
CCA Probability of Detection
• IEEE802.11a states: “A start of a valid OFDM
transmission at receiver level equal or greater
than 6 Mbits/s sensitivity (-82dBm) shall
cause CCA to indicate Busy with probability
>90% within 4 us. If the preamble portion
was missed, the receiver shall hold the CS
signal Busy for any signal 20 dB above the
minimum 6 Mbits/s sensitivity (-62 dBm)”
Submission
Slide 4
S. Gray, Nokia
May 2001
doc.: IEEE 802.11-01/227
CCA Test: Analytical Model
• Many ways exist for doing CCA. One method
is to use a delay and correlate filter based upon
the periodicity of the PLCP Preamble
• General structure of a delay and correlate filter
Delay by D
Input
signal
Complex
conjugate
r*(-D), r*(-D+1) , ..., r*(-D+n)
r(0), r(1) , ..., r(n)
Moving average
Length L
r(0), r(1) , ..., r(n)
r (k )
D
Submission
P(k)=r(k+D).r*(k)+...
...+r(k+D+L-1).r*(k+L-1)
are the sampled time-series from the PLCP Preamble
the periodicity in the short symbols
Slide 5
S. Gray, Nokia
May 2001
doc.: IEEE 802.11-01/227
CCA Test: Analytical Model
• General approach for CCA is to define
M (k ) 
where
M (k )  0,1
Pnorm (k )
P( k )
Pnorm (k )
is a normalizing signal such that
Busy
CCA  
 Idle
Submission
Slide 6
M (k )  TH
M (k )  TH
S. Gray, Nokia
May 2001
doc.: IEEE 802.11-01/227
CCA Test: Analytical Model
Consider a specular channel
h(t )   ck  t  k   
k
Let the received signal be modeled as
r (t )   ck st  k     n(t )
k
is the transmitted OFDM signal
n(t ) is background noise
s (t )
Submission
Slide 7
S. Gray, Nokia
May 2001
doc.: IEEE 802.11-01/227
CCA Test: Analytical Model
• The essence of a delay and correlate filter is
P(n)   r (t )r * (t  D)dt
T
n  T , 2T , ...
where
P ( n )   ck
k
2
s(t  k   ) dt
T 

2
W LOG1
*
 OT

n
(
t
)
s
(t  k   )dt
 
Complex
T

Gaussian 
 (n)
by CLT
conditioned on a valid PLCP preamble being
sent
• If ck is complex Gaussian then
Submission
Slide 8
ck
2
is exponential
S. Gray, Nokia
May 2001
doc.: IEEE 802.11-01/227
CCA Test: Analytical Model
• Probability of detection can be defined by
considering the distribution of a sum of non
iid exponential random variables (CLT can not
typically be invoked because number of
multipaths are few in number i.e., five)
• For ease of analysis, two multipaths are
considered
Submission
Slide 9
S. Gray, Nokia
May 2001
doc.: IEEE 802.11-01/227
CCA Test: Analytical Model
If
ck
2
is distributed exponential with
then if
z  c1  c2
2
f Z ( z) 
Submission
2
E ck 
1
2
k
the pdf for z is
12

e  z  e  z 
2  1 
1
2
Slide 10
: z    , 2  1
S. Gray, Nokia
May 2001
doc.: IEEE 802.11-01/227
CCA Test: Analytical Model
• To simplify analysis, background noise is modeled as
a constant level
• The probability of detection can be defined as
Pd  Pr  z  TH  Noise  

f
Z
( z ) dz
TH
12  e  TH  Noise  e  TH  Noise 




2  1   1
2

1
Submission
Slide 11
2
: 1  2
S. Gray, Nokia
May 2001
doc.: IEEE 802.11-01/227
Test Case
PLCP Preamble Detection
SNRSTA1= 8 dB
SNRSTA2= 8 dB
AP
60 meters
OBS
20 meters
LOS
STA 1
STA 2
TX Power = 5 dBm
63 meters
OBS
TX Power = 23 dBm
• The signal power of STA 1 seen at STA2 = -11 dB
• The signal power of STA 2 seen at STA1 = 7 dB
Submission
Slide 12
S. Gray, Nokia
May 2001
doc.: IEEE 802.11-01/227
Test Case
• Line of Sight (LOS) path loss
10n1 log d   p1
1 d  d f


PLd   
d 
10
n
log
d  df
 d   10n1 log d f  p1
2

f 


• Obstucted Line of Sight (OBS) path loss
PL(d )  10n log d   p1
LOS
Transmitter Antenna
Height
OBS
n
n1
Low (3.7 m)
2.18
3.29
2.58
Medium (8.5 m)
2.17
3.36
2.56
High (13.3 m)
2.07
4.16
2.69
n2
df
Fresnel distanc
p1 Reference path
loss (1 meter)
Feuerstein, M.J., Blackard, K.L., Rappaport, T.S., Seidel, S.Y., and Xia, H.H., “Path Loss, Delay Spread and Outage Models as Func
Height for Microcellular System Design,” IEEE Transactions on Vehicular Technology, Vol. 43, No 3, pp. 487- 498 August 1994.
Submission
Slide 13
S. Gray, Nokia
May 2001
doc.: IEEE 802.11-01/227
Test Case
Probability of Detection Curve
1
0.9
0.8
0.7
d
0.6
P
• Two tap channel
 tap 1 = 0.7
 tap 2 = 0.3
• Probability of false
alarm at SNR = 6 dB
is approx 9 %
• Pd = 0.011 at
SNR = - 11dB
0.5
0.4
0.3
0.2
0.1
0
-10
Submission
-8
Slide 14
-6
-4
-2
0
SNR (dB)
2
4
6
8
10
S. Gray, Nokia
May 2001
doc.: IEEE 802.11-01/227
DCF Simulation Procedure
• Traffic evenly distributed between different users
• Poisson arrivals for traffic;
Uniformly distributed packet sizes [500, 1500] bits
• 12Mb/s PHY assumed, average packet length = 1000 bits
• Average PHY-related packet error rate = 1%
• Simulations averaged over 106 slot-times
Submission
Slide 15
S. Gray, Nokia
May 2001
doc.: IEEE 802.11-01/227
Hidden-node DCF Simulations
• Assume 10-30 users, randomly allocated power levels
• 2 power-levels defined: High and Low
• Probability{detection of low-power users} = 1%
Probability{detection of high-power users} = 90%
• Number of retransmissions allowed = 3
• Initial back-off value = 16 slot-times
(1 slot-time = 9 micro-sec)
Submission
Slide 16
S. Gray, Nokia
May 2001
doc.: IEEE 802.11-01/227
Throughput Results - 1
1
Fractional Throughput
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0
0L:100H
50L:50H
20
40
60
DCF Traffic Load (%)
80
10-30% of network throughput can be lost with hidden-nodes
Submission
Slide 17
S. Gray, Nokia
May 2001
doc.: IEEE 802.11-01/227
Throughput Results - 2
4
14
x 10
12
10
Packets
Arvls
8
Txmns,0L:100H
6
Txmns,50L:50H
4
2
0
10
Submission
20
30
40
50
60
DCF Traffic Load (%)
Slide 18
70
80
S. Gray, Nokia
May 2001
doc.: IEEE 802.11-01/227
Throughput of High-vs-Low
Power Packets
4
14
x 10
12
Packets
10
Total
Arrived pkts
8
6
High power pkts Txed
4
2
0
0
Low power pkts Txed
20
40
60
DCF Traffic Load (%)
80
Network composition assumed = 50%low:50% High
Conclusion: Low power STAs suffer poor throughput
as load is increased
Submission
Slide 19
S. Gray, Nokia
May 2001
doc.: IEEE 802.11-01/227
Dropped Packet Results
10
x 10
4
9
Dropped Packets
8
7
Arvls
50L:50H
6
5
4
Dropped
pkts
3
0L:100H
2
1
0
20
40
60
DCF Traffic Load (%)
80
Up to 70 % increase in dropped packets for network
Submission
Slide 20
S. Gray, Nokia
May 2001
doc.: IEEE 802.11-01/227
Conclusions
• Throughput is decreased when large
variations of power are used by STAs in a
BSS
• Low power STAs increase the number of
hidden terminals
– Multiple retransmission of packets required to
ensure success
– Battery saving desired with power reduction may
be lost
Submission
Slide 21
S. Gray, Nokia