1st December 2009
doc.: IEEE 802. 15-09-0827-00-0007
Project: IEEE P802.15 Working Group for Wireless Personal Area Networks (WPANs)
Submission Title: Colour-stable CCM
Date Submitted: 1st December 2009
Source: Joachim W. Walewski Company Siemens AG, Corporate Technology, Communication
Technologies Address Otto-Hahn-Ring 6, DE-81739 Munich, Germany
Voice: +49-89-636-45850, FAX: +49-89-636-51115, E-Mail: [email protected]
Re: N/A
Abstract: I propose an extension of the channel adaptation promoted in P802.15-09-0786-00-0007. By aid
of this extension unavoidable shift in the centre point of the constellation, i.e. it’s average colour, can be
compensated. Such shifts can be caused by aging of the LED chips and different temperature sensitivities of
the LEDs’ quantum yields.
Purpose: Helping the 802.15 to ensure lighting functionality in synergetic VLC use cases.
Notice: This document has been prepared to assist the IEEE P802.15. 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 acknowledges and accepts that this contribution becomes the property of IEEE
and may be made publicly available by P802.15.
Submission to TG IEEE 802.15.7
Slide 1
J. W. Walewski, Siemens AG, Corporate Technology
1st December 2009
doc.: IEEE 802. 15-09-0827-00-0007
Colour-stable CCM
Joachim W. Walewski
Siemens AG
Corporate Technology
Communication Technologies
Munich, Germany
Submission to TG IEEE 802.15.7
Slide 2
J. W. Walewski, Siemens AG, Corporate Technology
1st December 2009
doc.: IEEE 802. 15-09-0827-00-0007
Objective
• Issue addressed
• Nomenclature
• Introduction of CCM link and pertinent
quantities
• Current drift-compensation scheme
• Understanding colour drift
• Proposed compensation scheme
Submission to TG IEEE 802.15.7
Slide 3
J. W. Walewski, Siemens AG, Corporate Technology
1st December 2009
doc.: IEEE 802. 15-09-0827-00-0007
Issue addressed
• CCM as proposed in the baseline draft relies on
LEDs emitting three elementary colours
• Due to different aging and temperature dependency
of the LED quantum efficiency the will drift optical
output will drift and thus its colour
• The current CCM modulation scheme compensates
this shift by aid of training modules
• For lighting applications the colour drift has to be
compensated
• How to do this with minimum effort?
Submission to TG IEEE 802.15.7
Slide 4
J. W. Walewski, Siemens AG, Corporate Technology
1st December 2009
doc.: IEEE 802. 15-09-0827-00-0007
Nomenclature 1
•
•
•
•
•
•
a: one-dimensional quantitiy
a: column vector
A: matrix
Tx: transmitter
Rx: receiver
r, g, b: signals for red, green and blue
channel
Submission to TG IEEE 802.15.7
Slide 5
J. W. Walewski, Siemens AG, Corporate Technology
1st December 2009
doc.: IEEE 802. 15-09-0827-00-0007
CCM: link block diagram
 rTx(b) 
 (b) 
s Tx(b) :  gTx

 b(b) 
 Tx 
r(b)Tx
Colour
(b)
Coding y Tx
r
D/A
x(b)Tx
Data
ir,Tx
xy
to
RGB
g(b)Tx
LED
pr,Tx
pr,Rx
ir,Rx
r
Trg
g
b
pb,Tx
i Tx
Tbb
Data
RGB
to xy
D/A
ib,Rx
b
x(b)Rx
g(b)Rx
ig,Rx
ib,Tx
D/A
r(b)Rx
D/A
pg,Rx
g
D/A
b(b)Tx
Trr
pg,Tx
ig,Tx
(b)
 rRx



(b)
(b)
s Rx
:  gRx

 b(b) 
 Rx 
 pr,Rx 


p Rx :  pg,Rx 
p 
 b,Rx 
 pr,Tx 


p Tx :  pg,Tx 
p 
 b,Tx 
y(b)Rx
Colour
Decoding
b(b)Rx
D/A
pb,Rx
 trr trg trb 
 i r,Tx 




:  i g,Tx  T :   t gr tgg tgb 
t t

i 
 b,Tx 
 br bg tbb 
PD
 ir,Rx 


i Rx :  i g,Rx 
i 
 b,Rx 
• Assumption: flat frequency response
Submission to TG IEEE 802.15.7
Slide 6
J. W. Walewski, Siemens AG, Corporate Technology
1st December 2009
doc.: IEEE 802. 15-09-0827-00-0007
CCM: Nomenclature 2
•
•
•
•
•
•
s: signal
□(b): binary representation of □
i: current [A]
p: optical power [W]
T: transmittance
tkl: transmittance from LED k to photodetector
l
• PD: photodetector
Submission to TG IEEE 802.15.7
Slide 7
J. W. Walewski, Siemens AG, Corporate Technology
1st December 2009
doc.: IEEE 802. 15-09-0827-00-0007
CCM: Matrix abbreviations
i Tx  A s Tx(b) D/A
D/A
x(b)Tx
Data
Colour
(b)
Coding y Tx
xy
to
RGB
pRx TpTx
r
r
D/A
x(b)Rx
Data
D/A
g
g
D/A
D/A
b
b
D/A
 qr 0 0 


Q :   0 qg 0 
0 0 q 
b

Submission to TG IEEE 802.15.7
RGB
to xy
y(b)Rx
Colour
Decoding
i Rx  SpRx
p Tx  Qi Tx
 r 0 0 


A :   0 g 0 
0 0  
b

(b)
s Rx
 Bi Rx A/D
 srr

S :   sgr
s
 br
Slide 8
srg
sgg
sbg
srb 

sgb 
sbb 
 r

B :  0
0

0
g
0
0

0
b 
J. W. Walewski, Siemens AG, Corporate Technology
1st December 2009
doc.: IEEE 802. 15-09-0827-00-0007
CCM: Nomenclature 3
• k: D/A scaling factor for colour k
• qk~ quantum efficiency for LED of colour
k [W/A]
• skl: sensitivity of photo detector for
colour l when receiving colour k [A/W]
• k: A/D scaling factor for colour l
Submission to TG IEEE 802.15.7
Slide 9
J. W. Walewski, Siemens AG, Corporate Technology
1st December 2009
doc.: IEEE 802. 15-09-0827-00-0007
CCM: channel matrix H
(b)
D/A
i Tx  A s Tx
r
D/A
Data Colour
Coding
xy
to
RGB
(b)
s Rx
 BiRx A/D
pRx TpTx
r
D/A
g
g
D/A
b
b
D/A
p Tx  Qi Tx
i Rx  SpRx
D/A
RGB
to xy
Colour Data
Decoding
ib,Tx
D/A
What is the relation between transmitted
and received signals?
s
(b)
Rx

  
 BSTQA s
(b)
Tx D/A A/D
Where: H:=BSTQA
Submission to TG IEEE 802.15.7
Slide 10
J. W. Walewski, Siemens AG, Corporate Technology
1st December 2009
doc.: IEEE 802. 15-09-0827-00-0007
Current drift compensation
• Matrix H estimated by aid of orthogonal
training sequences sent in the header of
each frame
• In case of change  update H
• Notice: No compensation on transmitter
side
Submission to TG IEEE 802.15.7
Slide 11
J. W. Walewski, Siemens AG, Corporate Technology
1st December 2009
doc.: IEEE 802. 15-09-0827-00-0007
Understanding colour drift
• Change in LED quantum efficiency due to
drift of chip temperature and aging
• qk  q’k
• If driving currents not changed optical output
changes too: pk,Tx = qk ik
 p’k,Tx = q’k ik
• Thus: Q  Q’ and H  H’ = BSTQ’A
• Accordingly:
Submission to TG IEEE 802.15.7
s '  BSTQ ' A s
(b)
Rx
Slide 12
 
(b)
Tx D/A A/D
J. W. Walewski, Siemens AG, Corporate Technology
1st December 2009
doc.: IEEE 802. 15-09-0827-00-0007
Proposed compensation scheme
Increase value of transmitted signal so
that received signal the same, i.e.
s
(b)
k ,Tx
c s
(b)
k
(b)
k ,Tx
so that
s
(b)
Rx
 H' C s


(b)
(b)
(b)
Tx
H (b)
Thus:
H
(b)
 H' C
Submission to TG IEEE 802.15.7
(b)
(b)
 C
Slide 13
(b)
 H'
H
(b) -1
(b)
J. W. Walewski, Siemens AG, Corporate Technology
1st December 2009
doc.: IEEE 802. 15-09-0827-00-0007
Proposed compensation scheme, contd.
Salient steps
• Notice: Works only for duplex link
• Estimate H for reference setting  H0
• Recurring measurements of H yield Hj
• If significant difference between Hj and
H0  transmit diagonal elements of
H 
(b) 1
j
Submission to TG IEEE 802.15.7
H
(b)
back to Tx
Slide 14
J. W. Walewski, Siemens AG, Corporate Technology
1st December 2009
doc.: IEEE 802. 15-09-0827-00-0007
Proposed compensation scheme, contd.
Implementation guidelines
• Decision about update can be made by aide
of C(b): if diagonal elements significantly
different from 1  force update
• In order to avoid damage of LEDs or bit-level
saturation one should introduce a-priori
maximum levels for the three values in sTx(b).
These maxima a correlated to LED driving
currents via Q.
Submission to TG IEEE 802.15.7
Slide 15
J. W. Walewski, Siemens AG, Corporate Technology
1st December 2009
doc.: IEEE 802. 15-09-0827-00-0007
Proposed compensation scheme, contd.
Implementation guidelines
• Change that affects all channels similarly, e.g.
change in link length, can be identified by
comparing all diagonal values of C(b). Same
relative change  do not send diag(C(b)) back
to Tx but update H0
• If centre of colour gravity needs to be moved
 disable transmission of C(b) during move;
update H0 after move
Submission to TG IEEE 802.15.7
Slide 16
J. W. Walewski, Siemens AG, Corporate Technology
1st December 2009
doc.: IEEE 802. 15-09-0827-00-0007
Proposed compensation scheme, contd.
In case of non-flat channel
• Replace all quantities in previous equations
with their Fourier transforms F{.}: H  HF
• Equate HF at modulation frequency for which
convex (or concave) function of sRx(b) is
maximised.
• Example for convex function:
F{s r,Rx}2 + F{s g,Rx}2 + F{s b,Rx}2
Submission to TG IEEE 802.15.7
Slide 17
J. W. Walewski, Siemens AG, Corporate Technology
1st December 2009
doc.: IEEE 802. 15-09-0827-00-0007
Proposed compensation scheme, contd.
Add-on bonus
• Also works for nonlinear E-O characteristics
of LEDs (as long as the dependence is
(localy) strictly increasing or decreasing with i
• Proposed compensation scheme can be used
for colour stabilisation even if no payload data
is sent: only send preamble containing
training signals
Submission to TG IEEE 802.15.7
Slide 18
J. W. Walewski, Siemens AG, Corporate Technology