May 2007
Steve Shellhammer, Qualcomm
Slide 1
doc.: IEEE 802.19-07/0010r0
Submission
Clear Channel Assessment Energy Detection (CCA-ED) in 802.11y
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Date: 2007-05-14
Name Company Address Phone E-mail Steve Shellhammer Qualcomm 5775 Morehouse Dr
San Diego, CA 92121 (858) 658-1874 [email protected]
Authors:
May 2007
Steve Shellhammer, Qualcomm
Slide 2
doc.: IEEE 802.19-07/0010r0
Submission
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
May 2007
Steve Shellhammer, Qualcomm
Slide 3
doc.: IEEE 802.19-07/0010r0
Submission
Channel Bandwidths and Sensing Times
• There are three possible bandwidths for 802.11y and for each bandwidth there is a different sensing time
Bandwidth Sensing Time
20 MHz 4s
10 MHz 8s
5 MHz 16s
May 2007
Steve Shellhammer, Qualcomm
Slide 4
doc.: IEEE 802.19-07/0010r0
Submission
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
• From [1] we have the formula for the test statistic
M
n
nynyM
BT
1
)(*)(
• B is the signal bandwidth
• M is the number of samples
May 2007
Steve Shellhammer, Qualcomm
Slide 5
doc.: IEEE 802.19-07/0010r0
Submission
Number of Samples in Estimate
• The number of samples used in the estimate is the sensing time the sample rate (also the bandwidth)
Bandwidth Sensing Time M
20 MHz 4s 80
10 MHz 8s 80
5 MHz 16s 80
May 2007
Steve Shellhammer, Qualcomm
Slide 6
doc.: IEEE 802.19-07/0010r0
Submission
Density Function of Test Statistic
• This is a binary hypothesis test
• Two hypotheses– Noise only
– Signal plus noise
)()(:0 nwnyH
)()()(:1 nwnxnyH
• x(t) signal
• w(t) noise
May 2007
Steve Shellhammer, Qualcomm
Slide 7
doc.: IEEE 802.19-07/0010r0
Submission
Parameters
B Bandwidth
N Noise Power Spectral Density
N= -174 + NF
N= -174 + 10 = -164 dBm/Hz
P Signal Power
M Number of samples
• Assume a conservative 10 dB receiver noise figure
May 2007
Steve Shellhammer, Qualcomm
Slide 8
doc.: IEEE 802.19-07/0010r0
Submission
Probability Density Function of T• For large M the central limit theorem says the density
function of T is approximately Gaussian
M
BNBNNtfT
2)(,)(
• PDF of T under H0
• PDF of T under H1
M
BNPNBPNtfT
2)(,)(
May 2007
Steve Shellhammer, Qualcomm
Slide 9
doc.: IEEE 802.19-07/0010r0
Submission
Detector Threshold
• In 802.11y there is a different threshold for each bandwidth
• The draft [2] only specifies a maximum CCA-ED threshold
Bandwidth MAX Threshold
20 MHz -72 dBm
10 MHz -75 dBm
5 MHz -78 dBm
May 2007
Steve Shellhammer, Qualcomm
Slide 10
doc.: IEEE 802.19-07/0010r0
Submission
Detector Threshold
Bandwidth Threshold
20 MHz -72 dBm
10 MHz -75 dBm
5 MHz -78 dBm
• We will use typical values from [3]
• These are the same as the maximum threshold values
May 2007
Steve Shellhammer, Qualcomm
Slide 11
doc.: IEEE 802.19-07/0010r0
Submission
Probability of Detection
May 2007
Steve Shellhammer, Qualcomm
Slide 12
doc.: IEEE 802.19-07/0010r0
Submission
Preliminary Observations
• The threshold is set so that there is a very low probability of false alarm
• 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
10635810FAp
May 2007
Steve Shellhammer, Qualcomm
Slide 13
doc.: IEEE 802.19-07/0010r0
Submission
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,
10 91 10 81I NP P dBm
May 2007
Steve Shellhammer, Qualcomm
Slide 14
doc.: IEEE 802.19-07/0010r0
Submission
Simple Coexistence Scenario
• An 802.11y network and an 802.16h network– Each network 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
May 2007
Steve Shellhammer, Qualcomm
Slide 15
doc.: IEEE 802.19-07/0010r0
Submission
Simple Coexistence Scenario
• Simple two-node networks
802.11y
yTX Transmitter
yRX Receiver
802.16h
hTX Transmitter
hRX Receiver
May 2007
Steve Shellhammer, Qualcomm
Slide 16
doc.: IEEE 802.19-07/0010r0
Submission
Definition of Interference Events
Event Label Description
Iy2h Significant interference from 802.11y at 802.16h
Ih2y Significant interference from 802.16h at 802.11y
May 2007
Steve Shellhammer, Qualcomm
Slide 17
doc.: IEEE 802.19-07/0010r0
Submission
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 in 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 result in interference or if the message would not be properly received at its destination
May 2007
Steve Shellhammer, Qualcomm
Slide 18
doc.: IEEE 802.19-07/0010r0
Submission
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 [4]
• Error 2– 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 [5]
May 2007
Steve Shellhammer, Qualcomm
Slide 19
doc.: IEEE 802.19-07/0010r0
Submission
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
May 2007
Steve Shellhammer, Qualcomm
Slide 20
doc.: IEEE 802.19-07/0010r0
Submission
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
May 2007
Steve Shellhammer, Qualcomm
Slide 21
doc.: IEEE 802.19-07/0010r0
Submission
Successful CCA-ED
• 802.11y CCA-ED detects busy channel
• If CCA-ED had not detected 16h then 11y would have jammed 16h
yTX
CCA-ED Circle
-81 dBm
-65 dBm
yRX
hTX
Significant Interference Circle
-72 dBm
-82 dBm
hRX
May 2007
Steve Shellhammer, Qualcomm
Slide 22
doc.: IEEE 802.19-07/0010r0
Submission
Exposed Node Event
• 802.11y CCA-ED detects busy channel
• However if 11y had transmitted no error would have occurred
• Based on current ED value this is an unlikely event
yTX
CCA-ED Circle
-81 dBm
-65 dBm
yRX
hTX
Significant Interference Circle
-72 dBm
-82 dBm
hRX
May 2007
Steve Shellhammer, Qualcomm
Slide 23
doc.: IEEE 802.19-07/0010r0
Submission
Hidden Node Event
yTX
CCA-ED Circle
-81 dBm
-65 dBm
yRX
hTX
Significant Interference Circle
-72 dBm
-82 dBm
hRX
• 802.11y CCA-ED does not detect a busy channel
• 11y transmits and jams 16h
May 2007
Steve Shellhammer, Qualcomm
Slide 24
doc.: IEEE 802.19-07/0010r0
Submission
Effect of Varying the CCA-ED Threshold
• If the CCA-ED 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
May 2007
Steve Shellhammer, Qualcomm
Slide 25
doc.: IEEE 802.19-07/0010r0
Submission
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– Fixed TX Power = 40 dBm– Portable TX Power = 20 dBm– 0 dBi Antenna– Noise Figure = 10 dB– Receiver Sensitivity = -80 dBm
May 2007
Steve Shellhammer, Qualcomm
Slide 26
doc.: IEEE 802.19-07/0010r0
Submission
Simulation Parameters
• Path Loss Model– Single break point at 100 m as in [6]
– Free space out to 100 m
– Exponent of 3.5 beyond 100m
41.76 20 ( ) 100( )
11.76 35 ( ) 100
Log d d mpl d
Log d d m
May 2007
Steve Shellhammer, Qualcomm
Slide 27
doc.: IEEE 802.19-07/0010r0
Submission
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
• Ran 105 trials for each 11y/16h base station separation
May 2007
Steve Shellhammer, Qualcomm
Slide 28
doc.: IEEE 802.19-07/0010r0
Submission
Simulation Scenarios
• Four Possible Scenarios
1. 802.11y BS performs CCA-ED while 16h BS is TX
2. 802.11y BS performs CCA-ED while 16h STA is TX
3. 802.11y STA performs CCA-ED while 16h BS is TX
4. 802.11y STA performs CCA-ED while 16h STA is TX
• Evaluate exposed node probability and hidden node probability in each of these four cases
May 2007
Steve Shellhammer, Qualcomm
Slide 29
doc.: IEEE 802.19-07/0010r0
Submission
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
May 2007
Steve Shellhammer, Qualcomm
Slide 30
doc.: IEEE 802.19-07/0010r0
Submission
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
May 2007
Steve Shellhammer, Qualcomm
Slide 31
doc.: IEEE 802.19-07/0010r0
Submission
Fixed – Scenario 1
May 2007
Steve Shellhammer, Qualcomm
Slide 32
doc.: IEEE 802.19-07/0010r0
Submission
Fixed – Scenario 2
May 2007
Steve Shellhammer, Qualcomm
Slide 33
doc.: IEEE 802.19-07/0010r0
Submission
Fixed – Scenario 3
May 2007
Steve Shellhammer, Qualcomm
Slide 34
doc.: IEEE 802.19-07/0010r0
Submission
Fixed – Scenario 4
May 2007
Steve Shellhammer, Qualcomm
Slide 35
doc.: IEEE 802.19-07/0010r0
Submission
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 could 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 hidden node probability drops to zero
May 2007
Steve Shellhammer, Qualcomm
Slide 36
doc.: IEEE 802.19-07/0010r0
Submission
Portable – Scenario 1
May 2007
Steve Shellhammer, Qualcomm
Slide 37
doc.: IEEE 802.19-07/0010r0
Submission
Portable – Scenario 2
May 2007
Steve Shellhammer, Qualcomm
Slide 38
doc.: IEEE 802.19-07/0010r0
Submission
Portable – Scenario 3
May 2007
Steve Shellhammer, Qualcomm
Slide 39
doc.: IEEE 802.19-07/0010r0
Submission
Portable – Scenario 4
May 2007
Steve Shellhammer, Qualcomm
Slide 40
doc.: IEEE 802.19-07/0010r0
Submission
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 the
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!
May 2007
Steve Shellhammer, Qualcomm
Slide 41
doc.: IEEE 802.19-07/0010r0
Submission
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
83610FAp
• 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
May 2007
Steve Shellhammer, Qualcomm
Slide 42
doc.: IEEE 802.19-07/0010r0
Submission
Fixed – Scenario 1
May 2007
Steve Shellhammer, Qualcomm
Slide 43
doc.: IEEE 802.19-07/0010r0
Submission
Fixed – Scenario 2
May 2007
Steve Shellhammer, Qualcomm
Slide 44
doc.: IEEE 802.19-07/0010r0
Submission
Fixed – Scenario 3
May 2007
Steve Shellhammer, Qualcomm
Slide 45
doc.: IEEE 802.19-07/0010r0
Submission
Fixed – Scenario 4
May 2007
Steve Shellhammer, Qualcomm
Slide 46
doc.: IEEE 802.19-07/0010r0
Submission
Portable – Scenario 1
May 2007
Steve Shellhammer, Qualcomm
Slide 47
doc.: IEEE 802.19-07/0010r0
Submission
Portable – Scenario 2
May 2007
Steve Shellhammer, Qualcomm
Slide 48
doc.: IEEE 802.19-07/0010r0
Submission
Portable – Scenario 3
May 2007
Steve Shellhammer, Qualcomm
Slide 49
doc.: IEEE 802.19-07/0010r0
Submission
Portable – Scenario 4
May 2007
Steve Shellhammer, Qualcomm
Slide 50
doc.: IEEE 802.19-07/0010r0
Submission
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
May 2007
Steve Shellhammer, Qualcomm
Slide 51
doc.: IEEE 802.19-07/0010r0
Submission
Conclusions
• A method for evaluating the CCA-ED was introduced
• With the current CCA-ED threshold the exposed node probability is quite near-zero
• With the current CCA-ED threshold the hidden node probability for CCA-ED can be quite high
• This hidden node cannot be addressed by using RTS-CTS since 802.16h cannot send an 802.11y packet
• This high hidden node probability can result in significant interference
• By symmetry LBT in 802.16h will have similar issues
May 2007
Steve Shellhammer, Qualcomm
Slide 52
doc.: IEEE 802.19-07/0010r0
Submission
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
May 2007
Steve Shellhammer, Qualcomm
Slide 53
doc.: IEEE 802.19-07/0010r0
Submission
References
1. Steve Shellhammer, Performance of the Power Detector, IEEE 802.22-06/75r0, May 2006
2. Draft IEEE 802.11y, D2.0, March 2007
3. Paul Piggin, Parameters for simulation of Wireless Coexistence in the US and Canada 3.65GHz band, IEEE 802.19-07/11r0, April 2007
4. http://en.wikipedia.org/wiki/Exposed_terminal_problem
5. http://en.wikipedia.org/wiki/Hidden_terminal_problem
6. V. Erceg, et. al., Channel Models for Fixed Wireless Applications, IEEE 802.16.3c-01/29, January 2001