Sep 2009

Hart (Cisco Systems)Slide 1

doc.: IEEE 802.11-09-0943r1

Submission

Impact of the Fluorescent Light EffectUpdate from 09/816r2

Date: 2009-09-01

Name Affiliation Address Phone Email Brian Hart Cisco Systems 170 West Tasman Drive,

San Jose, CA 95134 +1-408-526-3346 brianh@cisco.com

Hajime Suzuki (source of much data but not editorially responsible)

CSIRO PO Box 76 Epping, NSW 1710, Australia

Hajime.Suzuki@csiro.au

Graham Daniels (source of much data but not editorially responsible)

CSIRO PO Box 76 Epping, NSW 1710, Australia

Graham.Daniels@csiro.au

Sep 2009

Hart (Cisco Systems)Slide 2

doc.: IEEE 802.11-09-0943r1

Submission

Impact of the Fluorescent Light EffectOverview

Situation

• The Functional Requirements (09/451) assume no fluorescent light effect because recent measurements have not replicated this effect− Likely because fluorescent lights using electronic ballasts do not create the fluorescent

light effect

Complication

• Buildings with fluorescent lights using magnetic ballasts do create the fluorescent light effect.

• Although magnetic ballast sales are now much reduced and magnetic ballasts have a finite lifetime, it takes time for the installed base to taper to zero

• We estimate that in 2012, 35% of educational buildings, tapering to 0% by 2018, are likely to suffer from the fluorescent light effect.

• We show that the measured fluorescent light effect was significant for 10% of locations, and has a greater effect on DL-MU-MIMO than on SDM, TXBF or UL-MU-MIMO

Key Lines

• The fluorescent light effect will not be predominant yet it will have an appreciable effect in about 10% of locations in 35% of educational buildings (tapering to 0% over 6 years)

• This should be kept in mind by implementers but the effect may not be significant enough to warrant its own channel model or simulation scenario

Summary

Sep 2009

Hart (Cisco Systems)Slide 3

doc.: IEEE 802.11-09-0943r1

Submission

Technical Summary of Magnetic and Electronic ballasts in fluorescent lights*

• Since fluorescent tubes cannot effectively regulate their current use, ballasts are connected in series to appropriately limit current

• Fluorescent lights have traditionally used a large inductor as a ballast: hence “magnetic ballast”

− The applied voltage, varying at 50 or 60 Hz, flows through the inductor and tube, and creates oscillating periods of reflection and absorption^

− Results presented in 09/816r2 were of magnetic ballasts

• Modern fixtures use electronic ballasts that rectify mains power and modulate it, usually to 20 kHz or higher

− Above 20kHz, tubes are about 9% more efficient, and flicker is eliminated

− The period of any variation is 6.25 OFDM symbols

Sep 2009

Hart (Cisco Systems)Slide 4

doc.: IEEE 802.11-09-0943r1

Submission

USA Market Data, Regulations and LifetimesMarket Data+ (see figure)

Regulations *

• After 2006, manufacturers cannot make new fixtures with magnetic ballasts

• “For replacement only” magnetic ballasts are to be phased out after 2010

Ballast Lifetimes

• 50000 hours is the common age for ballasts^, leading to various lifetimes

− Average^: 3400 hours/year: 15 years

− Homesx (kitchen&, garage): 3 hours/day, 7 days/week: 46 years

− Education+: 10 hours/day, 5 days/week: 19 years

− Enterprise+: 11.5 hours/day, 6 days/week: 14 years

− Hospitals+: 16 hours/day, 7 days/week: 9 years

− Always: 24 hours/day, 7 days/week: 6 years

&For aesthetic reasons, these are likely hidden above cupboards, with reduced contribution to multipath

Sep 2009

Hart (Cisco Systems)Slide 5

doc.: IEEE 802.11-09-0943r1

Submission

Evolution of Installed Base (simulated): Mostly OK, except for education

• Assume 0% installed base in 1986

• Assume 1/Mean#Years ballasts fail every year

• Assume replacements are in proportion to sales ratio (using previous slide)

• This is a model; reality is likely to differ (worse/better?)

− Passes a basic check: 2/3 non-problematic buildings in 2009

• For the average building after 2012, 11ac is mostly OK and getting better

• For education (a big wireless user), fluorescent lights with magnetic ballasts are a concern for many buildings until 2018

&For aesthetic reasons, these are likely hidden above cupboards, with reduced contribution to multipath

Sep 2009

Hart (Cisco Systems)Slide 6

doc.: IEEE 802.11-09-0943r1

Submission

Subcarrier Capacity Impact from the Fluorescent Light Effect (1)• The large amplitude or phase changes in 09/816r2 don’t tell us about the

impact on capacity

• Therefore let’s analyze capacity as per Perahia and Stacey’s book, for SDM, TXBF and DL-MU-MIMO

• SDM: Y[2x1] = H[2x2].X[2x1] + N[2x1]− 2x2 MMSE equalizer− For the measurement set, SDM is also the best model for UL-MU-MIMO

• TXBF: Y[2x1] = H[2x2] Ve[2x2].X[2x1] + N[2x1]− Use the first channel as an estimate, He, for the next 130 ms.− X is precoded with Ve, from SVD He = Ue.Se.Ve’− 2x2 MMSE equalizer

• DL-MU-MIMO: y[1x1] = H[1x2] De[2x2].X[2x1] + n[1x1]− Considering 1 user w/l/o/g, then doubling capacity− Use first channel as an estimate, He, for the next 130 ms. − X is precoded with De, the channel’s “MMSE inverse” with a power constraint− De0 = (rho/NTX.He‘.He+PhiZ)-1*He‘ // MMSE inverse,

– Where rho = signal power, PhiZ = noise autocorr, normalized to 1− De = De0 .* √(NTX ./ mean(abs(De0(:)).^2)) // Power constraint− 1x1 MMSE equalizer

Sep 2009

Hart (Cisco Systems)Slide 7

doc.: IEEE 802.11-09-0943r1

Submission

Subcarrier Capacity Impact from the Fluorescent Light Effect (2)• For SDM, TXBF, e[2x1] = MMSE estimate of X, minus X[2x1]

• For DL-MU-MIMO, e[1x1] = MMSE estimate of x, minus x[1x1]

• J = E(ee’), so diagonal entries equal the error power per stream

• SNRi = (1-J(i,i))/J(i,i)

• C = Σi log2(1+SNRi)

• A capacity is claimed if it is available to 90% subcarriers (from the pool of all subcarriers and all locations)

− i.e. we’re reporting the 10% worst subcarriers

• SDM and TXBF calculations of equalizer coefficient W and error autocorrelation J as per Perahia and Stacey’s book

• DL-MU-MIMO calculations of MMSE equalizer coefficient and error autocorrelation:

− w= √(rho/NTX).[1,0].(H.De)'.(rho/NTX.(H.De)(H.De)'+Phiz)-1 − J = rho/NTX.w.(H.De)(H.De)'.w' + w.Phiz.w' -

2√(rho/NTX).Re(w.H.De.[1; 0]) + 1− A ML design likely could improve upon the performance of this

Sep 2009

Hart (Cisco Systems)Slide 8

doc.: IEEE 802.11-09-0943r1

Submission

0 20 40 60 80 100 120 1400

1

2

3

4

5

6

7

8

9

10

Time (ms)

Cap

acity

(b/

sym

/sub

carr

ier)

TXBF without lightsTXBF with lights

DL-MU-MIMO without lights

DL-MU-MIMO with lights

SDM without lightsSDM with lights

TXBF

SDM

DL-MU-MIMO

The DL-MU-MIMO 90% subcarrier capacity is sensitive to the fluorescent light effect

• With lights− TXBF and DL-MU-MIMO variation every 10 ms is clearly visible

−The variation is large for DL-MU-MIMO, and capacity can drop slightly below simple SDM performance

−The variation for TXBF is much less significant

−As expected since null steering is much more sensitive to a poor channel estimate than TXBF.

−SDM shows no variation since there is no channel prediction

• Without lights, −No variation −TXBF > DL-MU-MIMO > SDM−This is as expected since SVD > “MMSE inverse” > I

Sep 2009

Hart (Cisco Systems)Slide 9

doc.: IEEE 802.11-09-0943r1

Submission

But it’s not so bad looking across a 20MHz channel and considering all the deciles

• This time:−Average the capacity across 20 MHz so isolated bad subcarriers get averaged out−Divide the data into deciles and report the mean within each decile −Only plot DL-MU-MIMO with lights on

• The worst 10% of channels remain bad, but the other deciles only show small degradations

0 20 40 60 80 100 120 1400

2

4

6

8

10

12

14

Time (ms)

Mea

n C

apac

ity a

cros

s 20

MH

z by

dec

ile (

b/sy

m/s

ubca

rrie

r)

0-10%10-20%

20-30%

30-40%

40-50%50-60%

60-70%

70-80%

80-90%90-100%

Sep 2009

Hart (Cisco Systems)Slide 10

doc.: IEEE 802.11-09-0943r1

Submission

Summary

• By 802.11ac’s expected ratification, the fluorescent light effect will not be predominant, yet it will have an appreciable effect in about 10% of locations in an estimated 35% of educational buildings (tapering to 0% of educational buildings by 2018)

• DL-MU-MIMO is a valuable technology yet is more sensitive to channel variation than SDM, TXBF, or UL-MU-MIMO since a 1x1 receiver cannot perform spatial processing and so multi-user-interference with a MMSE MIMO equalizer appears as additional noise.

−Likely a ML equalizer could improve upon this

• The fluorescent light effect should be kept in mind by implementers yet the effect may not be significant enough to warrant its own channel model or simulation scenario

Sep 2009

Hart (Cisco Systems)Slide 11

doc.: IEEE 802.11-09-0943r1

Submission

Backup Slides

Sep 2009

Hart (Cisco Systems)Slide 12

doc.: IEEE 802.11-09-0943r1

Submission

Effect of Patch Orientation

• Some concern was expressed that the ceiling-directed client patch had unrealistic vertical gain and this exacerbated the measured effect

• Let’s try to assess this.

• (1) LOS was 8deg above the horizon

• (2) The ceiling reflection was 22deg above the horizon; here the patch had ~1.5dB greater gain than LOS.

− 1.5 dB is not a great difference, and clients should have consistent gain across 8-22deg above horizon, so this suggests the measurement was not unduly affected by the patch orientation

• (3,4) However, the lights immediately above TX and RX had angles of 45 and 51deg above the horizon; here the patch had ~5.5dB greater gain than LOS.

− This may be significant, yet there is no obvious path from TX to RX via these lights.

• At most, there is some weak evidence for concern.

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