Colour-stable CCM

1. 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: joachim.walewski@siemens.com 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. J. W. Walewski, Siemens AG, Corporate Technology

2. Colour-stable CCM Joachim W. Walewski Siemens AG Corporate Technology Communication Technologies Munich, Germany J. W. Walewski, Siemens AG, Corporate Technology

3. Objective • Issue addressed • Nomenclature • Introduction of CCM link and pertinent quantities • Current drift-compensation scheme • Understanding colour drift • Proposed compensation scheme J. W. Walewski, Siemens AG, Corporate Technology

4. 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? J. W. Walewski, Siemens AG, Corporate Technology

5. 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 J. W. Walewski, Siemens AG, Corporate Technology

6. CCM: link block diagram • Assumption: flat frequency response LED r(b)Tx ir,Tx pr,Tx pr,Rx ir,Rx r(b)Rx Trr Colour Coding xy to RGB RGB to xy Colour Decoding r r D/A D/A x(b)Tx x(b)Rx Trg g(b)Tx pg,Tx pg,Rx ig,Tx ig,Rx g(b)Rx Data Data g g D/A D/A y(b)Rx y(b)Tx b(b)Tx ib,Tx ib,Rx b(b)Rx b b D/A D/A pb,Tx Tbb pb,Rx PD J. W. Walewski, Siemens AG, Corporate Technology

7. 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 J. W. Walewski, Siemens AG, Corporate Technology

8. CCM: Matrix abbreviations Colour Coding Colour Decoding xy to RGB RGB to xy r r D/A D/A x(b)Tx x(b)Rx Data Data g g D/A D/A y(b)Rx y(b)Tx b b D/A D/A J. W. Walewski, Siemens AG, Corporate Technology

9. 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 J. W. Walewski, Siemens AG, Corporate Technology

10. CCM: channel matrix H Colour Coding xy to RGB RGB to xy Colour Decoding What is the relation between transmitted and received signals? r r D/A D/A Data Data g g D/A D/A ib,Tx b b D/A D/A Where: H:=BSTQA J. W. Walewski, Siemens AG, Corporate Technology

11. 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 J. W. Walewski, Siemens AG, Corporate Technology

12. 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: J. W. Walewski, Siemens AG, Corporate Technology

13. Proposed compensation scheme Increase value of transmitted signal so that received signal the same, i.e. Thus: J. W. Walewski, Siemens AG, Corporate Technology

14. Proposed compensation scheme, contd. Salient steps • Notice: Works only for duplex link • Estimate H for reference setting  H0 • Recurring measurements of H yieldHj • If significant difference between Hj and H0  transmit diagonal elements of back to Tx J. W. Walewski, Siemens AG, Corporate Technology

15. 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. J. W. Walewski, Siemens AG, Corporate Technology

16. 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 J. W. Walewski, Siemens AG, Corporate Technology

17. 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 J. W. Walewski, Siemens AG, Corporate Technology

18. 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 J. W. Walewski, Siemens AG, Corporate Technology