Nov 2013
doc.: IEEE 802.11-13/1012r4
Dynamic Sensitivity Control
V2
Date: 2013-11
Authors:
Name
Company Address
Phone
email
Graham Smith
DSP Group
916 358 8725
[email protected]
Submission
1037 Suncast
Lane, Ste 112,
El Dorado Hills,
CA95762
Slide 1
Graham Smith, DSP Group
Nov 2013
doc.: IEEE 802.11-13/1012r4
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 Sense
Is 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
Submission
Slide 2
Graham Smith, DSP Group
Nov 2013
doc.: IEEE 802.11-13/1012r4
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)
• STA A receives TX from STA B at -70dBm (50 +10* +10wall)
*10dB per octave
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 time
BUT each is prevented by CCA.
CCA was designed for greatest range
Submission
Slide 3
Graham Smith, DSP Group
Nov 2013
doc.: IEEE 802.11-13/1012r4
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 = 20dB
STA 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.
Submission
Slide 4
Graham Smith, DSP Group
Nov 2013
doc.: IEEE 802.11-13/1012r4
CCA Threshold/RX Sensitivity
L = Upper Limit
M = Margin
RX Sensitivity, RxS
Reff = MIN (RxS, L)
RxS = (Reff – M)
Example,
RSSI, R dBm
Submission
R = Received RSSI
FOR L = -40dBm and M = 20dB
Reff
Rx Sensitivity, dBm
-30
-40
-60
-40
-40
-60
-50
-50
-70
-60
-60
-80
Slide 5
Graham Smith, DSP Group
Nov 2013
doc.: IEEE 802.11-13/1012r4
Apartments
Consider an apartment
Using DSC complete apartment coverage
but overlap is confined mostly to direct neighbors
Only ‘worse case’ STA overlap
Extends into 2nd neighbor
20ft
-46dBm
35ft
-35dBm
-30dBm
-37dBm
AP
UPPER LIMIT = -30dBm
MARGIN = 20dB
Submission
AP CCA Threshold = -50dBm
AP CCA Threshold = -60dBm
Slide 6
Graham Smith, DSP Group
Nov 2013
doc.: IEEE 802.11-13/1012r4
Apartment Block
No DSC
45 Overlapping
With DSC
7 to 8
overlapping
NOTE: Dense apartment block is a priority Use Case
Submission
Slide 7
Graham Smith, DSP Group
Nov 2013
doc.: IEEE 802.11-13/1012r4
Terrace/Town Houses
Upper Linmit = -30dBm
Margin = 20dB
-66dBm
STA 1 CCA
Threshold
coverage on
second floor
STA 1 CCA
Threshold
coverage on
ground floor
30.5ft
-45dBm
-60dBm
Submission
Slide 8
Graham Smith, DSP Group
Nov 2013
doc.: IEEE 802.11-13/1012r4
Terrace/Town Houses
AP and STAs on same floor in each House
DSC Upper Limit = -30dBm
DSC Margin = 20dB
Alt AP with CCA
Threshold -50dBm
20ft
AP
AP Alt
CCA Thresholds
-30dBm
30ft
HOUSE 3
-40dBm
HOUSE 1
-42dBm
HOUSE 2
Setting AP CCA Threshold = -50dBm
covers entire house wherever AP is located.
Submission
Slide 9
Only STAs in furthest corner of House
1 will have overlap with some STAs in
House 3
Graham Smith, DSP Group
Nov 2013
doc.: IEEE 802.11-13/1012r4
Terrace/Townhouse
No ‘hidden’ STAs in garden
No ‘hidden’
STAs if within
this area
20ft
AP
AP Alt
-30dBm
Margin = 20dB
Upper Limit = -30dBm
30ft
-40dBm
HOUSE 1
Submission
-42dBm
Slide 10
Graham Smith, DSP Group
Nov 2013
doc.: IEEE 802.11-13/1012r4
Assume 3dB
Obstruction loss
per cell wall
Enterprise/Hotspots
1
2
3
r
AP 4
No hidden STAs within cell or
surrounding cells
For r = 30ft
AP CCA = -50dBm
This is coverage
STA A
5
For r = 20ft
STA A -36dBm
AP CCA = -50dBm
This is coverage
(all yellow)
6
7
1'
2'
STA A CCA Coverage area,
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.
3'
STA B
4'
Submission
Distance between STA and STA B is 2.64r
Using distance loss = 35 log D and 3dB loss per cell wall
STA A receives signal from STA B at -24.7 dBm rel. AP4
Hence, with Margin = 20dB
NO INTERFERENCE between STA A and STA B
5'
With Upper Limit set to -30dBm, and AP CCA Threshold set to
- 50dBm AP covers all cell radii up to 60ft.
Slide 11
Graham Smith, DSP Group
Nov 2013
doc.: IEEE 802.11-13/1012r4
Enterprise and Hotspots
Note if STA A moves, then
it 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 complying
But also would make TX at highest data rates difficult.
DSC ensures highest data rates used.
Submission
Slide 12
Graham Smith, DSP Group
Nov 2013
doc.: IEEE 802.11-13/1012r4
Hidden STAs – Fixed CCA/Sensitivity
FIXED CCA
e.g. -82dBm
Hidden STAs for
Blue
Hidden STAs for
Green
Hidden STAs for
Purple
Hidden STAs for
Black
Submission
Slide 13
Graham Smith, DSP Group
Nov 2013
doc.: IEEE 802.11-13/1012r4
Hidden STAs – Dynamic CCA/Sensitivity
Upper Limit -30dBm
Margin 20dB
AP CCA Threshoild
-50dBm
Hidden area for Green
only
30ft
Perfect match if AP
CCA is -40dBm
No hidden area
Submission
Slide 14
Graham Smith, DSP Group
Nov 2013
doc.: IEEE 802.11-13/1012r4
Upper Limit -40dBm
Increase Margin to
25dB,
but keep AP Threshold
at -60dBm
60ft
NO HIDDEN STAs
Parameters can be adjusted to suit conditions and desired coverage
Submission
Slide 15
Graham Smith, DSP Group
Nov 2013
doc.: IEEE 802.11-13/1012r4
Coverage and Capacity - Conventional
-82dBm CCA
Limit
HIDDEN STA
CELLS
37 Cells in CCA Area
N STAs per cell
N STAs in Cell1
6N STAs in Cells 2
12N STAs in Cells 3
18N STAs in Cellls 4
0 Hidden cells for Cell1
7 Hidden cells for Cells 2
14 Hidden cells for Cells 3
21 Hidden cells for Cells 4
2
3
Cells 1, 2 and 3 64 QAM
Cells 4
16 QAM
4
~24 extra cells @ QPSK
64 QAM Limit
16 QAM Limit
Submission
Slide 16
Graham Smith, DSP Group
Nov 2013
doc.: IEEE 802.11-13/1012r4
Coverage and Capacity - DSC
7
1
4
5
2
3
6
7
2
4
6
3
7
5
1
1
6
1
3
3
5
4
6
4
3
3
6
7
3
1
1
2
7
5
Cell radius 40ft
(3dB per cell wall)
2
1
4
37 Cells covered by fixed CCA (-82dBm)
Can use 7 separate APs
Submission
Using DSC and 7 segment cell pattern,
37 separate networks
Slide 17
Graham Smith, DSP Group
Nov 2013
doc.: IEEE 802.11-13/1012r4
Capacity Estimate for Fixed vs DSC CCA
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
Submission
Slide 18
Graham Smith, DSP Group
Nov 2013
doc.: IEEE 802.11-13/1012r4
Setting the Margin
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
Submission
Slide 19
Graham Smith, DSP Group
Nov 2013
doc.: IEEE 802.11-13/1012r4
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
Submission
Slide 20
Graham Smith, DSP Group
Nov 2013
doc.: IEEE 802.11-13/1012r4
Algorithm for setting CCA/RX sensitivity
•
•
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.
2.
Start a timer T
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.)
Submission
Slide 21
Graham Smith, DSP Group
Nov 2013
doc.: IEEE 802.11-13/1012r4
Summation of interference
Worse case analysis
Assume 3dB Obstruction loss
per cell wall
Assume 20 ft radius
1
Rel signal strength is -31dB
6 simultaneous TX = 7.8dB
Effective SNR = -23.2dBm
2
4
3
-61dBm
r
3
5
AP 4
-30dBm
4
-61dBm
5
6
6
1
7
There could be 6 other STAs,
on same channel,
TX at same time.
This is 7.8dB addition.
Upper Limit -30dBm
1'
2'
Resultant SNR still > 23dB
-61dBm
2
4
3'
-61dBm
4'
Submission
5'
Slide 22
Graham Smith, DSP Group
Nov 2013
doc.: IEEE 802.11-13/1012r4
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
Submission
Slide 23
Graham Smith, DSP Group
Nov 2013
doc.: IEEE 802.11-13/1012r4
Additions to the Standard
Add to Table 8-104 – Capabilities field
•
Bit
TBA
Information
DSC Supported
TBA
DSC Prohibited
Notes
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
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.
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
DSC Upper
Element ID
Length
DSC Margin
Limit
octets
Submission
1
1
1
Slide 24
1
Graham Smith, DSP Group
Nov 2013
doc.: IEEE 802.11-13/1012r4
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)
Submission
Slide 25
Graham Smith, DSP Group
Nov 2013
doc.: IEEE 802.11-13/1012r4
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.
Submission
Slide 26
Graham Smith, DSP Group
Nov 2013
doc.: IEEE 802.11-13/1012r4
Conclusions
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).
Submission
Slide 27
Graham Smith, DSP Group
Nov 2013
doc.: IEEE 802.11-13/1012r4
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
Submission
Slide 28
Graham Smith, DSP Group
Nov 2013
doc.: IEEE 802.11-13/1012r4
Straw Polls
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
Submission
Slide 29
Graham Smith, DSP Group