May 2007
doc.: IEEE 802.19-07/0010r1
Clear Channel Assessment Energy Detection
(CCA-ED) in 802.11y
Date: 2007-05-29
Authors:
Name
Company Address
Steve Shellhammer Qualcomm
5775 Morehouse Dr
San Diego, CA 92121
Phone
E-mail
(858) 658-1874
Shellhammer@ieee.org
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Submission
Slide 1
Steve Shellhammer, Qualcomm
May 2007
doc.: IEEE 802.19-07/0010r1
Abstract
• This is an evaluation of clear channel assessment
energy detection (CCA-ED) in 802.11y
• The other system being considered with 802.11y is
802.16h
• Similar energy detection is being considered in 16h
for its listen-before-talk (LBT) protocol
Submission
Slide 2
Steve Shellhammer, Qualcomm
May 2007
doc.: IEEE 802.19-07/0010r1
Channel Bandwidths and Sensing Times
• There are three possible bandwidths for 802.11y and
for each bandwidth there is a different sensing time
Submission
Bandwidth
Sensing Time
20 MHz
4s
10 MHz
8s
5 MHz
16s
Slide 3
Steve Shellhammer, Qualcomm
May 2007
doc.: IEEE 802.19-07/0010r1
Energy Detection Test Statistic
• The energy detection CCA mechanism estimates the
power of the signal observed over a sensing time and
compares the estimate to a threshold
• This estimate of the power is the test statistic
1
T
M
M
 y(n) y *(n)
n 1
• M is the number of samples
Submission
Slide 4
Steve Shellhammer, Qualcomm
May 2007
doc.: IEEE 802.19-07/0010r1
Number of Samples in Estimate
• The number of samples used is the sampling rate (same
as bandwidth) times the sensing time
• The number of samples used is the same for all three
bandwidths
Submission
Bandwidth
Sensing Time
M
20 MHz
4s
80
10 MHz
8s
80
5 MHz
16s
80
Slide 5
Steve Shellhammer, Qualcomm
May 2007
doc.: IEEE 802.19-07/0010r1
Density Function of Test Statistic
• This is a binary hypothesis test
• Two hypotheses
– Noise only
– Signal plus noise
H 0 : y(n)  w(n)
H1 : y(n)  x(n)  w(n)
• x(t) signal
• w(t) noise
Submission
Slide 6
Steve Shellhammer, Qualcomm
May 2007
doc.: IEEE 802.19-07/0010r1
Parameters
B
Bandwidth
PS
Signal Power
PN
Noise Power
M
Number of samples
PN  174  10Log ( B)  NF
PN  174  10 Log (20 106 )  10  91 dBm
• For the 20 MHz bandwidth case
• Assume a conservative 10 dB receiver noise figure
Submission
Slide 7
Steve Shellhammer, Qualcomm
May 2007
doc.: IEEE 802.19-07/0010r1
Probability Density Function of T
• For large M the central limit theorem says the density
function of T is approximately Gaussian
• PDF of T under H0
T

PN 2 
N  PN ,

M


• PDF of T under H1
T
Submission

( PS  PN )2 
N  PS  PN ,

M


Slide 8
Steve Shellhammer, Qualcomm
May 2007
doc.: IEEE 802.19-07/0010r1
Detector Threshold
• In 802.11y there is a different threshold for each
bandwidth
• The draft [1] only specifies a maximum CCA-ED
threshold
Submission
Bandwidth
MAX Threshold
20 MHz
-72 dBm
10 MHz
-75 dBm
5 MHz
-78 dBm
Slide 9
Steve Shellhammer, Qualcomm
May 2007
doc.: IEEE 802.19-07/0010r1
Detector Threshold
• We will use typical values from [2]
• These are the same as the maximum threshold values
Submission
Bandwidth
Threshold
20 MHz
-72 dBm
10 MHz
-75 dBm
5 MHz
-78 dBm
Slide 10
Steve Shellhammer, Qualcomm
May 2007
doc.: IEEE 802.19-07/0010r1
Probability of Detection
Submission
Slide 11
Steve Shellhammer, Qualcomm
May 2007
doc.: IEEE 802.19-07/0010r1
Preliminary Observations
• The threshold is set so that there is a very low
probability of false alarm
pFA  10106358
• Due to the use of M samples the transition band is
small, about 2 dB.
– So we can approximate CCA-ED with an idealized detector that
detects above the threshold and does not detect below the
threshold
– The simulations will use the actual probability of detection curve,
but the results are not that much different that those of an ideal
detector
Submission
Slide 12
Steve Shellhammer, Qualcomm
May 2007
doc.: IEEE 802.19-07/0010r1
Significant Interference
• To simplify the analysis we will specify a level of
interference that we consider significant
• This is clearly an approximation
• It is used to allow for a simplified analysis to gain
insight into the operation of the energy detector for
CCA
• We define significant interference if the interference is
10 dB greater than the noise floor
• For a 20 MHz system this means,
PI  PN  10  91  10  81dBm
Submission
Slide 13
Steve Shellhammer, Qualcomm
May 2007
doc.: IEEE 802.19-07/0010r1
Simple Coexistence Scenario
• An 802.11y network and an 802.16h network
– Each network is only two stations
•
•
•
•
•
Both systems 20 MHz bandwidth
Co-channel operation
Path loss model
No shadow fading model
802.11y performing CCA-ED
– Due to symmetry the same conclusions should apply
to LBT in 802.16h
Submission
Slide 14
Steve Shellhammer, Qualcomm
May 2007
doc.: IEEE 802.19-07/0010r1
Simple Coexistence Scenario
• Simple two-node networks
802.11y
802.16h
yTX
Transmitter
hTX
Transmitter
yRX
Receiver
hRX
Receiver
Submission
Slide 15
Steve Shellhammer, Qualcomm
May 2007
doc.: IEEE 802.19-07/0010r1
Definition of Interference Events
Submission
Event Label
Description
Iy2h
Significant interference
from 802.11y at 802.16h
Ih2y
Significant interference
from 802.16h at 802.11y
Slide 16
Steve Shellhammer, Qualcomm
May 2007
doc.: IEEE 802.19-07/0010r1
Goal of CCA
• If Iy2h=Ih2y=False then CCA should be True
– We would like CCA to be True if a resulting
transmission would not cause interference and would
also be successfully received at its destination
• If Iy2h=True or Ih2y=True then CCA should be
False
– We would like CCA to be False if a resulting
transmission would cause interference or if the message
would not be properly received at its destination due to
interference
Submission
Slide 17
Steve Shellhammer, Qualcomm
May 2007
doc.: IEEE 802.19-07/0010r1
Enumeration of Error Events
• Error 1
– Iy2h = False and Ih2y= False and CCA = False
– In this case 11y could have completed a transmission
without jamming 16h but did not since the channel
appeared busy
– This results in lower throughput for 11y
– This is the “exposed node” problem [3]
• Error 2
–
–
–
–
Submission
Iy2h = True and Ih2y = False and CCA = True
11y transmits and jams 16h
This results in lower 16h throughput
This is the famous “hidden node” problem [4]
Slide 18
Steve Shellhammer, Qualcomm
May 2007
doc.: IEEE 802.19-07/0010r1
Error Events
• Error 3
–
–
–
–
Iy2h = False and Ih2y = True and CCA = True
11y transmits but due to interference it is unsuccessful
Does not harm 16h transmission
This is not a big deal. If CCA were False 11y would
not have transmitted anyway
• Error 4
– Iy2h = True and Ih2y = True and CCA = True
– This is a combination of 2 and 3. The one that counts is
the jamming of 16h
Submission
Slide 19
Steve Shellhammer, Qualcomm
May 2007
doc.: IEEE 802.19-07/0010r1
Error Events of Interest
• Exposed Node Event
– Iy2h = False and Ih2y= False and CCA = False
– CCA believes the channel is busy but if the STA did
transmit it would not cause interference and would have
been successfully received
• Hidden Node Event
– Iy2h = True and CCA = True
– CCA believes the channel is not busy so the station
transmits and jams 16h
• We want to evaluate under what conditions
these two events occur
Submission
Slide 20
Steve Shellhammer, Qualcomm
May 2007
doc.: IEEE 802.19-07/0010r1
Clear Channel Assessment
Interference Link
hTX
yRX
Sensing Link
yTX
Interference Link
hRX
Submission
Slide 21
Steve Shellhammer, Qualcomm
May 2007
doc.: IEEE 802.19-07/0010r1
Successful CCA-ED
Significant Interference Circle
CCA-ED Circle
yRX
hTX
-72 dBm
-82 dBm
yTX
-65 dBm
-81 dBm
• 802.11y CCAED detects busy
channel
• If CCA-ED had
not detected 16h
then 11y would
have jammed
16h
hRX
Submission
Slide 22
Steve Shellhammer, Qualcomm
May 2007
doc.: IEEE 802.19-07/0010r1
Exposed Node Event
Significant Interference Circle
yRX
CCA-ED Circle
-81 dBm
-72 dBm
-82 dBm
yTX
-65 dBm
hTX
• 802.11y CCAED detects busy
channel
• However if 11y
had transmitted
no error would
have occurred
• Based on
current ED
value this is an
unlikely event
hRX
Submission
Slide 23
Steve Shellhammer, Qualcomm
May 2007
doc.: IEEE 802.19-07/0010r1
Hidden Node Event
Significant Interference Circle
CCA-ED Circle
yRX
-72 dBm
yTX
-82 dBm
-81 dBm
• 802.11y CCAED does not
detect a busy
channel
• 11y transmits
and jams 16h
-65 dBm
hTX
hRX
Submission
Slide 24
Steve Shellhammer, Qualcomm
May 2007
doc.: IEEE 802.19-07/0010r1
Effect of Varying the CCA-ED Threshold
• If the CCA-ED threshold is reduced to a lower
value (more sensitive CCA-ED) then,
– The false alarm rate will increase
– The exposed node probability will increase
– The hidden node probability will decrease
• So there is a natural tradeoff between
– False alarm rate & exposed node probability
And
– Hidden node probability
Submission
Slide 25
Steve Shellhammer, Qualcomm
May 2007
doc.: IEEE 802.19-07/0010r1
Simulation Parameters
• 802.11y
–
–
–
–
–
–
Fixed TX Power = 40 dBm
Portable TX Power = 20 dBm
0 dBi Antenna
Noise Figure = 10 dB
Receiver Sensitivity = -82 dBm
CCA-ED Threshold = -72 dBm
• 802.16h
–
–
–
–
–
Submission
Fixed TX Power = 40 dBm
Portable TX Power = 20 dBm
0 dBi Antenna
Noise Figure = 10 dB
Receiver Sensitivity = -80 dBm
Slide 26
Steve Shellhammer, Qualcomm
May 2007
doc.: IEEE 802.19-07/0010r1
Simulation Parameters
• Path Loss Model
– Single break point at 100 m as in [5]
– Free space out to 100 m
– Exponent of 3.5 beyond 100m
41.76  20 Log (d ) d  100m
pl (d )  
11.76  35Log (d ) d  100m
Submission
Slide 27
Steve Shellhammer, Qualcomm
May 2007
doc.: IEEE 802.19-07/0010r1
Simulation Procedure
• Place an 802.11y and an 802.16 base station
– The separation between these base station is varied
• Randomly place a single 802.11y client station within
the coverage area
– Coverage area depends on transmit power of client station (which
is less than or equal to power of base station) and path loss model
• Randomly place a single 802.16h client station within
the coverage area
– Coverage area depends on transmit power of client station (which
is less than or equal to power of base station) and path loss model
• Run 105 trials for each 11y/16h base station separation
Submission
Slide 28
Steve Shellhammer, Qualcomm
May 2007
doc.: IEEE 802.19-07/0010r1
Simulation Scenarios
•
1.
2.
3.
4.
Four Possible Scenarios
802.11y BS performs CCA-ED while 16h Base Station is TX
802.11y BS performs CCA-ED while 16h Client Station is TX
802.11y STA performs CCA-ED while 16h Base Station is TX
802.11y STA performs CCA-ED while 16h Client Station is TX
•
Evaluate exposed node probability and hidden node probability
in each of these four cases
Submission
Slide 29
Steve Shellhammer, Qualcomm
May 2007
doc.: IEEE 802.19-07/0010r1
Statement About Probabilities
• These probabilities are dependent on spatial
randomness and not temporal randomness
• If the stations do not move the situation will not change
• You cannot necessarily solve any problems by waiting
for the situation to change because it is a spatial process
and not a temporal process
Submission
Slide 30
Steve Shellhammer, Qualcomm
May 2007
doc.: IEEE 802.19-07/0010r1
Two Sets of Simulations
• First set of simulations
– Fixed stations for both 11y and 16h
– High power fixed base stations
– High power fixed client stations
• Second set of simulations
– Portable client stations for both 11y and 16h
– High power fixed base stations
– Low power portable client stations
Submission
Slide 31
Steve Shellhammer, Qualcomm
May 2007
doc.: IEEE 802.19-07/0010r1
Fixed – Scenario 1
Submission
Slide 32
Steve Shellhammer, Qualcomm
May 2007
doc.: IEEE 802.19-07/0010r1
Fixed – Scenario 2
Submission
Slide 33
Steve Shellhammer, Qualcomm
May 2007
doc.: IEEE 802.19-07/0010r1
Fixed – Scenario 3
Submission
Slide 34
Steve Shellhammer, Qualcomm
May 2007
doc.: IEEE 802.19-07/0010r1
Fixed – Scenario 4
Submission
Slide 35
Steve Shellhammer, Qualcomm
May 2007
doc.: IEEE 802.19-07/0010r1
Observations
• The exposed node probability is low in all four
cases
– This is due to the rather high CCA-ED threshold of -72
dBm
• The hidden node probability can get quite high
– It depended on which of the four cases we are considering
• Once the base stations are separated enough, as
one would expect, the exposed node and hidden
node probabilities drop to zero
Submission
Slide 36
Steve Shellhammer, Qualcomm
May 2007
doc.: IEEE 802.19-07/0010r1
Portable – Scenario 1
Submission
Slide 37
Steve Shellhammer, Qualcomm
May 2007
doc.: IEEE 802.19-07/0010r1
Portable – Scenario 2
Submission
Slide 38
Steve Shellhammer, Qualcomm
May 2007
doc.: IEEE 802.19-07/0010r1
Portable – Scenario 3
Submission
Slide 39
Steve Shellhammer, Qualcomm
May 2007
doc.: IEEE 802.19-07/0010r1
Portable – Scenario 4
Submission
Slide 40
Steve Shellhammer, Qualcomm
May 2007
doc.: IEEE 802.19-07/0010r1
Observations
• Exposed node probabilities are still very low
• The hidden node problem is much worse in the
case of the base station performing CCA-ED
– The Base station CCA-ED region is a subset of its
interference region
– The Portable station CCA-ED region is larger than the
interference region
• For some values of base station separations the
hidden node probability is one!
Submission
Slide 41
Steve Shellhammer, Qualcomm
May 2007
doc.: IEEE 802.19-07/0010r1
Another Set of Simulations
• Modified the CCA-ED threshold to -82 dBm which
makes the CCA-ED much more sensitive
• PFA is still quite low
pFA  10836
• The sensing region is comparable to the interference
region
– A set of simulations for fixed high power clients
– A set of simulations for portable low power clients
Submission
Slide 42
Steve Shellhammer, Qualcomm
May 2007
doc.: IEEE 802.19-07/0010r1
Fixed – Scenario 1
Submission
Slide 43
Steve Shellhammer, Qualcomm
May 2007
doc.: IEEE 802.19-07/0010r1
Fixed – Scenario 2
Submission
Slide 44
Steve Shellhammer, Qualcomm
May 2007
doc.: IEEE 802.19-07/0010r1
Fixed – Scenario 3
Submission
Slide 45
Steve Shellhammer, Qualcomm
May 2007
doc.: IEEE 802.19-07/0010r1
Fixed – Scenario 4
Submission
Slide 46
Steve Shellhammer, Qualcomm
May 2007
doc.: IEEE 802.19-07/0010r1
Portable – Scenario 1
Submission
Slide 47
Steve Shellhammer, Qualcomm
May 2007
doc.: IEEE 802.19-07/0010r1
Portable – Scenario 2
Submission
Slide 48
Steve Shellhammer, Qualcomm
May 2007
doc.: IEEE 802.19-07/0010r1
Portable – Scenario 3
Submission
Slide 49
Steve Shellhammer, Qualcomm
May 2007
doc.: IEEE 802.19-07/0010r1
Portable – Scenario 4
Submission
Slide 50
Steve Shellhammer, Qualcomm
May 2007
doc.: IEEE 802.19-07/0010r1
Observations
• Increased CCA-ED sensitivity resulted in
– Increase in exposed node probability
– Decrease in hidden node probability
• The results were better in the fixed (equal
power) case
• In the portable (unequal power) case the was
still a significant problem when the high-power
11y base station was performing CCA-ED and
the low-power 16h client was transmitting
Submission
Slide 51
Steve Shellhammer, Qualcomm
May 2007
doc.: IEEE 802.19-07/0010r1
Conclusions
• A method for evaluating the CCA-ED was introduced
• With the maximum CCA-ED threshold the exposed
node probability is quite near-zero
• With the maximum CCA-ED threshold the hidden
node probability for CCA-ED can be quite high
• A more sensitive CCA-ED makes things better but
there is still a problem in the case of unequal power
• This hidden node cannot be addressed by using RTSCTS since 802.16h cannot send an 802.11y packet
• By symmetry LBT in 802.16h will have similar issues
Submission
Slide 52
Steve Shellhammer, Qualcomm
May 2007
doc.: IEEE 802.19-07/0010r1
Future Work
• If the 11y/16h/19 team decides on different
simulation parameter the simulation can be
updated
• Decide what is the right value for “Significant
Interference”
• This work could be integrated into a system
level simulation
Submission
Slide 53
Steve Shellhammer, Qualcomm
May 2007
doc.: IEEE 802.19-07/0010r1
References
1. Draft IEEE 802.11y, D2.0, March 2007
2. Paul Piggin, Parameters for simulation of Wireless
Coexistence in the US and Canada 3.65GHz band, IEEE
802.19-07/11r0, April 2007
3. http://en.wikipedia.org/wiki/Exposed_terminal_problem
4. http://en.wikipedia.org/wiki/Hidden_terminal_problem
5. V. Erceg, et. al., Channel Models for Fixed Wireless
Applications, IEEE 802.16.3c-01/29, January 2001
Submission
Slide 54
Steve Shellhammer, Qualcomm