doc.: ieee 802.19-07/0010r0 submission may 2007 steve shellhammer, qualcommslide 1 clear channel...

May 2007

Steve Shellhammer, Qualcomm

Slide 1

doc.: IEEE 802.19-07/0010r0

Submission

Clear Channel Assessment Energy Detection (CCA-ED) in 802.11y

Notice: This document has been prepared to assist IEEE 802.19. It is offered as a basis for discussion and is not binding on the contributing individual(s) or organization(s). The material in this document is subject to change in form and content after further study. The contributor(s) reserve(s) the right to add, amend or withdraw material contained herein.

Release: The contributor grants a free, irrevocable license to the IEEE to incorporate material contained in this contribution, and any modifications thereof, in the creation of an IEEE Standards publication; to copyright in the IEEE’s name any IEEE Standards publication even though it may include portions of this contribution; and at the IEEE’s sole discretion to permit others to reproduce in whole or in part the resulting IEEE Standards publication. The contributor also acknowledges and accepts that this contribution may be made public by IEEE 802.19.

Patent Policy and Procedures: The contributor is familiar with the IEEE 802 Patent Policy and Procedures <http:// ieee802.org/guides/bylaws/sb-bylaws.pdf>, including the statement "IEEE standards may include the known use of patent(s), including patent applications, provided the IEEE receives assurance from the patent holder or applicant with respect to patents essential for compliance with both mandatory and optional portions of the standard." Early disclosure to the TAG of patent information that might be relevant to the standard is essential to reduce the possibility for delays in the development process and increase the likelihood that the draft publication will be approved for publication. Please notify the Chair <[email protected]> as early as possible, in written or electronic form, if patented technology (or technology under patent application) might be incorporated into a draft standard being developed within the IEEE 802.19 TAG. If you have questions, contact the IEEE Patent Committee Administrator at <[email protected]>.

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


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


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


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


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


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


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


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


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


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


Slide 11

doc.: IEEE 802.19-07/0010r0

Submission

Probability of Detection

May 2007


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


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


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


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


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


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


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


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


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


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


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


-72 dBm

-82 dBm

hRX

May 2007


Slide 23

doc.: IEEE 802.19-07/0010r0

Submission

Hidden Node Event

yTX

CCA-ED Circle

-81 dBm

-65 dBm

yRX

hTX


-72 dBm

-82 dBm

hRX

• 802.11y CCA-ED does not detect a busy channel

• 11y transmits and jams 16h

May 2007


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


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


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


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


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


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


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


Slide 31

doc.: IEEE 802.19-07/0010r0

Submission

Fixed – Scenario 1

May 2007


Slide 32

doc.: IEEE 802.19-07/0010r0

Submission


May 2007


Slide 33

doc.: IEEE 802.19-07/0010r0

Submission


May 2007


Slide 34

doc.: IEEE 802.19-07/0010r0

Submission


May 2007


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


Slide 36

doc.: IEEE 802.19-07/0010r0

Submission

Portable – Scenario 1

May 2007


Slide 37

doc.: IEEE 802.19-07/0010r0

Submission


May 2007


Slide 38

doc.: IEEE 802.19-07/0010r0

Submission


May 2007


Slide 39

doc.: IEEE 802.19-07/0010r0

Submission


May 2007


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


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


Slide 42

doc.: IEEE 802.19-07/0010r0

Submission


May 2007


Slide 43

doc.: IEEE 802.19-07/0010r0

Submission


May 2007


Slide 44

doc.: IEEE 802.19-07/0010r0

Submission


May 2007


Slide 45

doc.: IEEE 802.19-07/0010r0

Submission


May 2007


Slide 46

doc.: IEEE 802.19-07/0010r0

Submission


May 2007


Slide 47

doc.: IEEE 802.19-07/0010r0

Submission


May 2007


Slide 48

doc.: IEEE 802.19-07/0010r0

Submission


May 2007


Slide 49

doc.: IEEE 802.19-07/0010r0

Submission


May 2007


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


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


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


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

doc.: ieee 802.19-07/0010r0 submission may 2007 steve shellhammer, qualcommslide 1 clear channel...

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