doc.: IEEE 802.11-13/1012r4

Submission

Nov 2013

Dynamic Sensitivity ControlV2

Date: 2013-11

Authors:

Name Company Address Phone email Graham Smith DSP Group 1037 Suncast

Lane, Ste 112, El Dorado Hills, CA95762

916 358 8725 Graham.smith@dspg.com

Graham

Smith, DSP

Group

Slide 1

doc.: IEEE 802.11-13/1012r4

Submission

Background• 802.11 uses CSMA/CA carrier sense multiple access with collision

avoidance.• STA listens before transmitting• Two methods of sensing the medium

– Physical Carrier Sense Is there RF energy present?

– Virtual Carrier SenseIs there an 802.11 signal present?

• Clear Channel Assessment (CCA)– OFDM transmission => minimum modulation and coding rate sensitivity

(6Mbps)(-82dBm for 20MHz channel, -79dBm for 40MHz channel)

– If no detected header, 20 dB higher, i.e. -62dBm

Nov 2013

Graham

Smith, DSP

Group

Slide 2

doc.: IEEE 802.11-13/1012r4

Submission

Example – background to idea

• AP1 to STA A -50dBm, (also AP2 to STA B)• STA B is 4x as far from AP 1 as STA A.

• Therefore AP1 receives STA B at -80dBm (50 + 20* +10 wall) *10dB per octave

• STA A receives TX from STA B at -70dBm (50 +10* +10wall)Note: AP1 receives AP2 <-82dBm so CCA is not exerted

STA A and STA B could both transmit successfully to their APs at the same timeBUT each is prevented by CCA.

CCA was designed for greatest rangeGraha

m Smith,

DSP Group

Slide 3

Nov 2013

doc.: IEEE 802.11-13/1012r4

Submission

Dynamic Sensitivity Control - DSC• Imagine a scheme where STA measures the RSSI of the AP Beacon

(R dBm)• Then sets its RX Sensitivity Threshold at (R – M) dBm,

where M is the “Margin”• Hence, for example:

– STA receives Beacon at -50dBm, with Margin = 20dBSTA sets RX Sensitivity Threshold to -70dBm.

• Also set an Upper Limit, L, to Beacon RSSI at, say, -30 or -40dBm to cater for case when STA is very close to AP. – Need to ensure that all the STAs in the wanted area do see each other. Hence

if one STA very close to AP, then it could set RX Sensitivity too high and we get hidden STAs.

Graham Smith, DSP Group

Nov 2013

Slide 4

doc.: IEEE 802.11-13/1012r4

Submission

L = Upper Limit M = Margin R = Received RSSI

RX Sensitivity, RxS

Reff = MIN (RxS, L)

RxS = (Reff – M)

Example, FOR L = -40dBm and M = 20dB

CCA Threshold/RX Sensitivity

Nov 2013

Graham

Smith, DSP

Group

Slide 5

RSSI, R dBm Reff Rx Sensitivity, dBm

-30 -40 -60

-40 -40 -60

-50 -50 -70

-60 -60 -80

doc.: IEEE 802.11-13/1012r4

Submission

Apartments

Graham

Smith, DSP

Group

Slide 6

Nov 2013

AP

-46dBm

-35dBm

-30dBm -37dBm

UPPER LIMIT = -30dBmMARGIN = 20dB

AP CCA Threshold = -50dBmAP CCA Threshold = -60dBm

35ft

20ft

Consider an apartmentUsing DSC complete apartment coverage but overlap is confined mostly to direct neighbors

Only ‘worse case’ STA overlap Extends into 2nd neighbor

doc.: IEEE 802.11-13/1012r4

Submission

Apartment Block

Graham

Smith, DSP

Group

Slide 7

Nov 2013

No DSC 45 Overlapping

With DSC7 to 8 overlapping

NOTE: Dense apartment block is a priority Use Case

doc.: IEEE 802.11-13/1012r4

Submission

Terrace/Town Houses

Graham

Smith, DSP

Group

Slide 8

Nov 2013

-45dBm

-60dBm

-66dBm

Upper Linmit = -30dBmMargin = 20dB STA 1 CCA

Threshold coverage on ground floor

STA 1 CCA Threshold

coverage on second floor

30.5ft

doc.: IEEE 802.11-13/1012r4

Submission

Terrace/Town Houses

Graham

Smith, DSP

Group

Slide 9

Nov 2013

20ft

30ft

-30dBm

-42dBm

-40dBm

Setting AP CCA Threshold = -50dBmcovers entire house wherever AP is located.

AP and STAs on same floor in each House

AP

AP Alt

Alt AP with CCA Threshold -50dBm

CCA Thresholds

Only STAs in furthest corner of House 1 will have overlap with some STAs in House 3

DSC Upper Limit = -30dBmDSC Margin = 20dB

HOUSE 1 HOUSE 2

HOUSE 3

doc.: IEEE 802.11-13/1012r4

Submission

Terrace/Townhouse

Graham

Smith, DSP

Group

Slide 10

Nov 2013

20ft

30ft

-30dBm

-42dBm

-40dBm

AP

AP Alt

HOUSE 1

No ‘hidden’ STAs if within

this area

Margin = 20dBUpper Limit = -30dBm

No ‘hidden’ STAs in garden

doc.: IEEE 802.11-13/1012r4

Submission Graham

Smith, DSP

Group

Slide 11

Nov 2013

1

7

6

3

2 '

5

1 '

2

3 '

4 ' 5 '

AP 4

STA A

STA B

r

Assume 3dB Obstruction loss

per cell wall

Distance between STA and STA B is 2.64rUsing distance loss = 35 log D and 3dB loss per cell wallSTA A receives signal from STA B at -24.7 dBm rel. AP4Hence, with Margin = 20dB NO INTERFERENCE between STA A and STA B

If r = 20ft, STA A signal is -36dBm . Hence could set Upper Limit to -20dBm and AP CCA threshold to -40dBm. Covers complete cell with very little AP overlap BUT does not change STA A area.

With Upper Limit set to -30dBm, and AP CCA Threshold set to- 50dBm AP covers all cell radii up to 60ft.

For r = 20ftSTA A -36dBmAP CCA = -50dBmThis is coverage(all yellow)

For r = 30ftAP CCA = -50dBmThis is coverage

STA A CCA Coverage area,

No hidden STAs within cell or surrounding cells

Enterprise/Hotspots

doc.: IEEE 802.11-13/1012r4

Submission

Enterprise and Hotspots

Graham Smith, DSP Group

Note if STA A moves, thenit loses the DSC protection and then it is encouraged to switch channels as now has lower throughput.

Note that this type of cell cluster is impossible without TPC or DSC. TPC fails if any one not complyingBut also would make TX at highest data rates difficult.DSC ensures highest data rates used.

Slide 12

Nov 2013

doc.: IEEE 802.11-13/1012r4

Submission

Hidden STAs – Fixed CCA/Sensitivity

Graham

Smith, DSP

Group

Slide 13

Nov 2013

FIXED CCA e.g. -82dBm

Hidden STAs for Green

Hidden STAs for Blue

Hidden STAs for Purple

Hidden STAs for Black

doc.: IEEE 802.11-13/1012r4

Submission

Hidden STAs – Dynamic CCA/Sensitivity

Graham

Smith, DSP

Group

Slide 14

Nov 2013

AP CCA Threshoild -50dBmHidden area for Green only

Perfect match if AP CCA is -40dBmNo hidden area

30ft

Upper Limit -30dBmMargin 20dB

doc.: IEEE 802.11-13/1012r4

Submission Graham

Smith, DSP

Group

Slide 15

Nov 2013

Upper Limit -40dBmIncrease Margin to 25dB, but keep AP Threshold at -60dBm

60ft

NO HIDDEN STAsParameters can be adjusted to suit conditions and desired coverage

doc.: IEEE 802.11-13/1012r4

Submission

Coverage and Capacity - Conventional

Graham

Smith, DSP

Group

Slide 16

Nov 2013

2

3

4

64 QAM Limit

16 QAM Limit

HIDDEN STA CELLS

37 Cells in CCA AreaN STAs per cell

N STAs in Cell16N STAs in Cells 212N STAs in Cells 318N STAs in Cellls 4

0 Hidden cells for Cell17 Hidden cells for Cells 214 Hidden cells for Cells 321 Hidden cells for Cells 4

Cells 1, 2 and 3 64 QAMCells 4 16 QAM

~24 extra cells @ QPSK

-82dBm CCA Limit

doc.: IEEE 802.11-13/1012r4

Submission

Coverage and Capacity - DSC

Graham

Smith, DSP

Group

Slide 17

Nov 2013

1

7

2

3

4

4

5

6 3

5

4

6

3

7

6

3

7

3

2

5

1

2

6

1

3

7

1

4

2

1

5

3

1

6

4

1

7

37 Cells covered by fixed CCA (-82dBm)Can use 7 separate APs

Using DSC and 7 segment cell pattern, 37 separate networks

Cell radius 40ft(3dB per cell wall)

doc.: IEEE 802.11-13/1012r4

Submission

Assume 11n 2SS, 16K aggregation– 117Mbps throughput is 81.2Mbps max (74.5Mbps with RTS/CTS)– 52Mbps throughput is 39.7Mbps max (37.7Mbps with RTS/CTS)

Fixed CCA• 19/37 cells @ 117Mbps and 18/37 cells at 52Mbps

– Throughput is 19/37*74.5 + 18.37*37.7 = 56.6Mbps

• Assume 7 APs on different channels covering the area• Throughput over total 37 cells is 396.5Mbps (56.6 x 7)

DSC• All traffic at 117Mbs, • Throughput over 37 cells is 3004.8Mbps (81.2 x 37)

An improvement of 7.58 in capacity

Capacity Estimate for Fixed vs DSC CCA

Nov 2013

Graham

Smith, DSP

Group

Slide 18

doc.: IEEE 802.11-13/1012r4

Submission

The Margin needs to be set :

1. Large enough to provide adequate SNR– A STA at edge of CCA transmits at same time. The Margin is the

worse possible effective SNR (from a single simultaneous TX).

2. Large enough to account for sudden changes in reception of Beacon signal– If STA goes behind obstruction, RSSI will drop. If the drop is

higher than the Margin, then the AP Beacon is lost.

Suggested Margin is in the order of 20dB to 25dB.

See slide on Algorithm for setting Threshold

Setting the Margin

Nov 2013

Graham

Smith, DSP

Group

Slide 19

doc.: IEEE 802.11-13/1012r4

Submission

Flexibility

• Upper Limit and Margin can be adjusted to suit the application for an optimum result (AP can control)– 20dB Margin suggested as 20dB is approx required SNR for

higher data rates

• AP then sets its own Sensitivity or CCA – Based upon the Margin and Upper Limit

Graham Smith, DSP Group

Nov 2013

Slide 20

doc.: IEEE 802.11-13/1012r4

Submission

• The Beacon RSSI will vary as the STA moves, for example. Therefore, the calculation of the CCA threshold or Receive Sensitivity is a continuous one.

• A suggested algorithm outline is:1. Start a timer T

2. Record RSSI of each Beacon• Check if > Upper Limit, if so RSSI = Upper Limit

3. Calculate average RSSI • Use a moving average such that last reading has higher influence

4. Check if T > Update period (e.g. 1 second)• No, continue, get next beacon• Yes, Convert the Averaged RSSI to CCA Threshold (or RX Sensitivity)

– CCA Threshold = Ave RSSI – Margin.

5. Back to 1

In addition, check if a Beacon or consecutive Beacons are missed, and if so decrement the average RSSI by a set amount

Hence, every 1 second the CCA Threshold is reset, (reset immediately if beacon(s) missed.)

Algorithm for setting CCA/RX sensitivity

Nov 2013

Graham

Smith, DSP

Group

Slide 21

doc.: IEEE 802.11-13/1012r4

Submission

Summation of interference Worse case analysis

Nov 2013

Graham

Smith, DSP

Group

Slide 22

1

7

6

3

2 '

5

1 '

2

3 '

4 ' 5 '

AP 4

r

Rel signal strength is -31dB6 simultaneous TX = 7.8dBEffective SNR = -23.2dBm

-30dBm

2

6

3

-61dBm

-61dBm

4

1

5

4

-61dBm

4-61dBm

Upper Limit -30dBm

Assume 3dB Obstruction loss per cell wall

Assume 20 ft radius

There could be 6 other STAs, on same channel, TX at same time.This is 7.8dB addition.

Resultant SNR still > 23dB

doc.: IEEE 802.11-13/1012r4

Submission

AP Considerations• AP can set the Upper Limit and Margin parameters for STAs

– Advertises settings (similar to EDCA parameters)

• AP bases its own CCA on the DSC parameters it advertises – Based upon advertised settings– Based upon desired coverage

• AP can issue “No DSC” to be used– For large area coverage outdoors, for example.

• AP could learn OBSS situation while simply listening to Beacons from other network(s). Set Upper Limit accordingly.– Part of Channel Selection process (as per 11aa)– Sets Upper Limit so that OBSS is mitigated– Could be dynamic with periodic scans

All could be covered in 802.11 Standard now

(increase 11n throughput by >7 times)

Directly applicable to HEW SG as it improves the effective throughput in an area

Graham Smith, DSP Group

Nov 2013

Slide 23

doc.: IEEE 802.11-13/1012r4

Submission

• DSC Parameter Set Element• The DSC Parameter Set element provides information needed by STAs for

operation of dynamic sensitivity control that is used to control the thresholds for CCA

Additions to the Standard

Nov 2013

Graham

Smith, DSP

Group

Slide 24

 

Element ID Length DSC MarginDSC Upper

Limit

octets 1 1 1 1

Add to Table 8-104 – Capabilities field

Bit Information Notes TBA DSC Supported The DSC Support subfield indicates support for DSC as defined in

10.xx. When dot11DynamicSensitivityControlImplemented is true, this field is set to 1 to indicate support for DSC. The field is set to 0 otherwise to indicate that DSC is not supported

TBA DSC Prohibited The DSC Prohibited subfield indicates whether the use of DSC is prohibited. The field is set to 1 to indicate that DSC is prohibited and to 0 to indicate that DSC is allowed.

doc.: IEEE 802.11-13/1012r4

Submission

Legacy STAs – No problem if in separate network

In each of the cases considered, Apartments, Houses, Cell Cluster, the legacy STA is UNAFFECTED• If the Legacy STA is in a separate network,

– If STA does not use DSC then:• If already started to TX it will complete (DSC STA can TX at same time) • If Legacy STA not started to TX it will hold off with CCA in the normal

fashion if DSC STA is TX – no difference

• DSC simply allows the STA using it to TX at the same time.

• Legacy network performance improves as need not wait so long for DSC network to TX (simultaneous TX)

Graham Smith, DSP Group

Nov 2013

Slide 25

doc.: IEEE 802.11-13/1012r4

Submission

Legacy STA – Same Network• If any STA is outside the coverage area set by the DSC, then it is at a

disadvantage as its TX could be stepped on by the DSC STA that is close to the AP. This is the same situation as “hidden STA”. – “Hidden STA” situation exists now so nothing new– Number of “Hidden STAs” reduced by DSC

• Note examples, possibility of hidden legacy or DSC STA is remote.• Consider also need to keep high data rates hence want to restrict range.

(Especially if using 40MHz channels or higher).

Finally• If outdoor and large area coverage required, DSC could be disabled by AP

IE.

Finally, Finally• There is a huge encouragement to enact DSC, unlike TPC.

Graham Smith, DSP Group

Nov 2013

Slide 26

doc.: IEEE 802.11-13/1012r4

Submission

In the examples studied:• DSC has a significant impact on area throughput

– Frequency reuse is increased by a significant factor– In cell cluster example, 2SS 20MHz BW total area throughput is

increased from 396.5Mbps to 3004.8Mbps (7 APs vs 37 APs)!

• DSC reduces or eliminates chance of hidden STAs• Legacy STAs are not disadvantaged• DSC is easy to implement and does not require every

network to comply, (as does TPC).

Conclusions

Nov 2013

Graham

Smith, DSP

Group

Slide 27

doc.: IEEE 802.11-13/1012r4

Submission

Discussion• We can expand the examples to specific enterprise, office environments.

– Network coverage is NOT simple circles. It is bounded by walls, floors, obstructions such that the propagation is not dB linear it suffers from jumps, e.g. 10dB per outside wall, 3 – 6dB inside walls.

– Network coverage can be made ‘cell like’ so as to improve the overall coverage.

• If only one network uses DSC it does not impact performance on other network – in fact it lessens impact as now TX simultaneously so other network does not need to wait so long.

• DSC Limit can be set to cover desired network area. – Correct choice of Upper Limit and Margin

• DSC can be combined with channel selection and mitigate OBSS. • DSC can improve overall Wi-Fi throughput in an area.• AP can control settings – see next slide

Graham Smith, DSP Group

Nov 2013

Slide 28

doc.: IEEE 802.11-13/1012r4

Submission

1. Do you think that DSC merits consideration for existing PHYs?– Yes – No

2. Do you think that DSC merits consideration for HEW?– Yes – No

Straw Polls

Nov 2013

Graham

Smith, DSP

Group

Slide 29