## Reverse polarity optical-OFDM (RPO-OFDM): dimming compatible OFDM for gigabit VLC links |

Optics Express, Vol. 21, Issue 20, pp. 24288-24299 (2013)

http://dx.doi.org/10.1364/OE.21.024288

Acrobat PDF (784 KB)

### Abstract

Visible light communications (VLC) technology permits the exploitation of light-emitting diode (LED) luminaries for simultaneous illumination and broadband wireless communication. Optical orthogonal frequency-division multiplexing (O-OFDM) is a promising modulation technique for VLC systems, in which the real-valued O-OFDM baseband signal is used to modulate the instantaneous power of the optical carrier to achieve gigabit data rates. However, a major design challenge that limits the commercialization of VLC is how to incorporate the industry-preferred pulse-width modulation (PWM) light dimming technique while maintaining a broadband and reliable communication link. In this work, a novel signal format, reverse polarity O-OFDM (RPO-OFDM), is proposed to combine the fast O-OFDM communication signal with the relatively slow PWM dimming signal, where both signals contribute to the effective LED brightness. The advantages of using RPO-OFDM include, (1) the data rate is not limited by the frequency of the PWM signal, (2) the LED dynamic range is fully utilized to minimize the nonlinear distortion of the O-OFDM communication signal, and (3) the bit-error performance is sustained over a large fraction of the luminaire dimming range. In addition, RPO-OFDM offers a practical approach to utilize off-the-shelf LED drivers. We show results of numerical simulations to study the trade-offs between the PWM duty cycle, average electrical O-OFDM signal power, radiated optical flux as well as human perceived light.

© 2013 OSA

## 1. Introduction

1. M. H. Crawford, “LEDs for solid-state lighting: performance challenges and recent advances,” IEEE J. Sel. Top. Quantum Electron. **15**(4), 1028–1040 (2009). [CrossRef]

*i.e.*color mixing. Besides the several advantages of LEDs, they also have a fast response time making them excellent light sources for high-speed VLC links in which data is transmitted wirelessly from LED-based luminaries through subtle intensity variations [4

4. H. Elgala, R. Mesleh, and H. Haas, “Indoor optical wireless communication: potential and state-of-the-art,” IEEE Commun. Mag. **49**(9), 56–62 (2011). [CrossRef]

*i.e.*, intensity modulation with direct detection (IM/DD) [5

5. H. Elgala, R. Mesleh, and H. Haas, “Indoor broadcasting via white LEDs and OFDM,” IEEE Trans. Consumer Electron. **55**(3), 1127–1134 (2009). [CrossRef]

*i.e.*based on existing power-line infrastructure [7].

13. Z. Wang, W.-D. Zhong, C. Yu, J. Chen, C. P. S. Francois, and W. Chen, “Performance of dimming control scheme in visible light communication system,” Opt. Express **20**(17), 18861–18868 (2012). [CrossRef] [PubMed]

14. G. Ntogari, T. Kamalakis, J. Walewski, and T. Sphicopoulos, “Combining illumination dimming based on pulse-width modulation with visible-light communications based on discrete multitone,” J. Opt. Commun. Netw. **3**(1), 56–65 (2011). [CrossRef]

*i.e.*inter-carrier interference (ICI) for slower PWM rates. This constraint diminishes the feasibility of industry compatible dimmed broadband VLC link as PWM frequencies of off-the-shelf LED drivers are in KHz.

*i.e.*, the data throughput is not limited by the PWM frequency, (2) to use the full LED dynamic range of operation to minimize the nonlinear distortion (mainly clipping) of the O-OFDM signal,

*i.e.*, DC biasing is not required and the linear range of operation is extended, and (3) to maintain a high link capacity for a wide dimming range,

*i.e.*the signal-to-noise ratio (SNR) is independent on the dimming level within that wide range.

## 2. Dimming and O-OFDM modulation

*i.e.*a digitally modulated pulse train drives the LED at a constant current level. Pulse width modulation (PWM) is the simplest example of digital dimming modulation techniques. The time period of the PWM signal is fixed, whereas the duty cycle

*D*varies proportionally to the required dimming percentage. Since PWM reduces light intensity more linearly than AM and induces less of a chromaticity shift, PWM is preferred in industry solutions [15

15. Y. Gu, N. Narendran, T. Dong, and H. Wu, “Spectral and luminous efficacy change of high-power LEDs under different dimming methods,” Proc. SPIE **6337**, 63370J–63370J-7 (2006). [CrossRef]

*i*(

_{PWM}*t*) with pulse width

*T*and period

*T*is shown in Fig. 1 and can be expressed as, where

_{PWM}*I*is the high level ”on-state” LED current,

_{H}*I*is the low level ”off-state” LED current and

_{L}*D*=

*T/T*.

_{PWM}16. J. Armstrong and B. J. Schmidt, “Comparison of asymmetrically clipped optical OFDM and DC-biased optical OFDM in AWGN,” IEEE Commun. Lett. **12**(5), 343–345 (2008). [CrossRef]

*i.e.*

*S*,

_{n}*n*= 0, ··· ,

*N*− 1 are the input data symbols of the

*n*sub-carrier,

^{th}*i*,

_{k}*k*= 0, ··· ,

*N*− 1 are the

*N*time-domain output current samples,

*i*(

_{OFDM}*t*) is the O-OFDM time-domain signal current,

*T*is the time-domain O-OFDM symbol period, and

_{OFDM}*ω*= 2

_{n}*πn/T*. A normalized ACO-OFDM time-domain symbol is also shown in Fig. 2.

_{OFDM}## 3. The proposed RPO-OFDM

*i*(

_{OFDM}*t*) with

*i*(

_{PWM}*t*) as follows: where,

*i*(

_{LED}*t*) is the LED drive current and

*m*is a real-valued scaling factor of the O-OFDM modulating signal,

*i.e.*sets the average electrical O-OFDM signal power

*P*.

_{OFDM}*i*(

_{PWM}*t*) pulsating between

*I*and

_{L}*I*is shown.

_{H}*I*is assumed to correspond to the maximum allowed

_{H}*i*(

_{LED}*t*) and

*I*corresponds to the minimum

_{L}*i*(

_{LED}*t*) according to the LED data sheet,

*i.e.*the LED dynamic range can be denoted by

*I*-

_{H}*I*. In the proposed system,

_{L}*i*(

_{OFDM}*t*) is superimposed on

*i*(

_{PWM}*t*) after setting a proper polarity of the individual ACO-OFDM symbols using a RPO-OFDM modulator depending on whether the symbol is being transmitted on

*I*or

_{H}*I*during

_{L}*T*. To explain the idea of RPO-OFDM with an example,

_{PWM}*D*= 20%, 10 ACO-OFDM symbols,

*i*= 64 and

_{k}*T*= 10 ×

_{PWM}*T*are assumed. Consequently, the polarity of the first two ACO-OFDM symbols is reversed,

_{OFDM}*i.e.*−ve polarity, then transmitted on the

*I*followed by 8 ACO-OFDM symbols,

_{H}*i.e.*+ve polarity, transmitted on

*I*. At the receiver side, and after time-synchronization, all 640 samples are extracted and the polarity of the first 128 samples is re-adjusted. Figure 4 shows ACO-OFDM symbols that are sequentially transmitted at

_{L}*D*= 20% and

*D*= 70%.

16. J. Armstrong and B. J. Schmidt, “Comparison of asymmetrically clipped optical OFDM and DC-biased optical OFDM in AWGN,” IEEE Commun. Lett. **12**(5), 343–345 (2008). [CrossRef]

*i.e. D*, the polarity inverter is activated.

*i*(

_{PWM}*t*) supplied by the LED driver (assuming PWM as shown in Fig. 6(b)) using the polarity-adjusted O-OFDM analog signal current (

*i*(

_{OFDM}*t*)) available after the RPO-OFDM modulator according to a dimming set point,

*i.e. D*. There are different circuit topologies to combine

*i*(

_{PWM}*t*) and

*i*(

_{OFDM}*t*) to obtain

*i*(

_{LED}*t*). It is also possible to realize a circuit topology to directly drive the LED using

*i*(

_{OFDM}*t*) (see Fig. 6(c)). However, these topics are not the scope of the paper and will be shown in a latter work. It is also worth pointing out that RPO-OFDM can also be applied to any unipolar O-OFDM version or the bipolar O-OFDM version,

*i.e.*DCO-OFDM. For example, the same modulation-demodulation sequence is valid for a bipolar DCO-OFDM, however using two consecutive PWM periods.

## 4. Signal quality and perceived brightness

*i.e.*the transmitted OFDM signal shape is preserved. Preserving the OFDM signal shape corresponds to less induced clipping noise allowing the use of high-order constellations (better spectral efficiency) or the establishment of more robust links (better BER). The proposed approach decouples dimming for a wide-range from the performance,

*i.e.*assuming optimum ACO-OFDM signal power to maintain a target quality of service (QoS).

*D*does not contribute to SNR improvement. The SNR can be improved only through maximizing

*i.e.*maximum

*POFDM*determined by the value of the scaling factor

*m*. However, to ensure that

*i*stays within the dynamic range of the LED, a maximum

_{k}*m*with absolute value of

*I*(

_{H}/max*i*) is required. At a certain

_{k}*P*, If the magnitude of any

_{OFDM}*i*sample is beyond the dynamic range of the LED, and assuming

_{k}*I*= 0,

_{L}*i*(

_{LED}*t*) will be clipped and the clipped signal

*i′*(

_{LED}*t*) is given by

*O*can be controlled by adjusting

_{Avg}*D*or jointly adjusting

*D*and

*m*. An optimum

*m*is determined based on the available LED dynamic range, the target dimming range and the M-QAM order. Over one PWM period,

*O*corresponds to an average LED current denoted by

_{Avg}*I*and described as, where,

_{Avg}*i*is the number of the O-OFDM symbols transmitted during the ”on-state” and

*j*is the number of the O-OFDM symbols transmitted during the ”off-state”. Assuming the LED input-output characteristic as shown in Fig. 3,

*I*= 0 and

_{L}*D*= 50%, it is clearly observed that

*I*is always equal to half the

_{Avg}*I*and independent on

_{H}*m*,

*i.e. P*.

_{OFDM}*O*by enlarging the pupil, allowing more light to enter the eye. This response results in a non-linear relation between measured and perceived light levels that is captured by the following formula:

_{Avg}## 5. Simulation results

*D*and

*P*on the radiated optical power,

_{OFDM}*i.e.*optical dimming, as well as the perceived LED brightness,

*i.e.*perceived dimming. The dimming level is defined as a percentage of the maximum brightness,

*i.e.*100% dimming is full brightness.

*I*= 1

_{H}*A*,

*I*= 0

_{L}*A*and

*P*are calculated over one ACO-OFDM symbol. Modulating signal current values above 1A are clipped. Figure 7 shows the obtained optical dimming and perceived dimming as a function of

_{OFDM}*D*. Curves are obtained at different values of

*P*that is incremented starting 10dBm up to 24dBm in steps of 2dBm. The numerical results confirm that the optical dimming can be linearly adjusted by varying

_{OFDM}*D*for different values of

*P*. The O-OFDM signal is the sum of N independent sub-carriers. For large values of N (N>10), the ACO-OFDM envelope can be accurately modeled as a Gaussian random process (due to the central limit theorem) with zero mean. Thus at

_{OFDM}*D*= 50%,

*P*has no effect on the dimming level. It is also highlighted that the human eye has better sensitivity at low luminance than high luminance. The non-linear relation between optical dimming and perceived dimming is clearly observed. For instance, decreasing the optical power by 50% achieves only a 70% reduction in brightness.

_{OFDM}*P*increases. At any

_{OFDM}*P*value, the range between the two vertical circles represents the optical dimming range, however the range between the two vertical squares represents the perceived dimming range. For low

_{OFDM}*P*,

_{OFDM}*e.g.*at

*P*= 10

_{OFDM}*dBm*, the full dimming range is slightly reduced by 10%,

*i.e.*from 95% up to 5%. At

*P*= 16

_{OFDM}*dBm*, the highest brightness level that can be achieved is limited to 90% and the highest dimming level that can be reached is 10%. Further increase of

*P*narrows the dimming range around 50% dimming.

_{OFDM}*P*on the system bit-error performance is also investigated. In these Monte-carlo simulations, un-coded 8-QAM, 16-QAM, and 32-QAM modulation schemes, −3dBm AWGN,

_{OFDM}*I*= 1

_{H}*A*,

*I*= 0

_{L}*A*and perfect synchronization between the transmitter and the receiver are assumed. Typical receiver sensitivity is around −30dBm, however a high AWGN value,

*i.e.*−3dBm, is chosen in order to be able to illustrate the full behavior of the bit-error ratio (BER) curves,

*i.e.*avoid BER values below 10

^{−6}that are difficult to obtain using simulations. For a target BER of 10

^{−3}, it is clearly confirmed that for all three curves,

*P*= 10

_{OFDM}*dBm*is not sufficient to achieve the target BER. Above 14dBm and below 21dBm, 8-QAM and 16-QAM curves show BER values better than 10

^{−3}. However, for wider dimming range

*P*= 14

_{s}*dBm*is recommended as illustrated in Fig. 8. To achieve BER of 10

^{−3}using 32-QAM,

*P*must be set at 18dBm. The SNR and BER are improved, as expected, with the increase of

_{OFDM}*P*reaching an optimal value for a specific noise power. By further increasing

_{OFDM}*P*, the SNR starts to deteriorate as a result of induced non-linear distortions caused by the LED limited dynamic range (clipping at 1A),

_{OFDM}*i.e.*AWGN noise dominates at low

*P*values and clipping distortion dominates at high

_{OFDM}*P*values. The optimum

_{OFDM}*P*values for 8-QAM, 16-QAM, and 32-QAM are 16dBm, 17dBm and 18dBm, respectively.

_{OFDM}*x*-axis), for example, at 8-QAM, the lowest BER of 10

^{−6}is achieved at

*P*= 16

_{OFDM}*dBm*and is maintained independent on the dimming range that is between 90% and 10%. However, BER of 10

^{−3}, requires only

*P*= 12

_{OFDM}*dBm*and is maintained independent on the dimming range that is between 92% and 7%. Thus

*P*has to be optimized for maximum data-rate, minimum BER and/or target dimming range.

_{OFDM}^{−3}and AWGN of −3dBm, Fig. 10 shows the possible modulation order as a function of the optical dimming as well as the perceived dimming. It is clearly noticed how a wide dimming range of operation is supported,

*i.e.*between 88% and 12% for optical dimming and between 93% and 35% for perceived dimming, while maintaining 32-QAM,

*i.e.*high order modulation corresponds to high spectral efficiency. Beyond this range,

*e.g.*8-QAM, is maintained between 93% and 7% for optical dimming and between 97% and 26% for perceived dimming.

*P*for 8-QAM, 16-QAM, and 32-QAM. For example, using

_{OFDM}*P*= 18

_{OFDM}*dBm*jointly maximizes the spectral efficiency,

*i.e.*32-QAM, and minimizes the BER,

*i.e.*about 10

^{−3}. This BER is maintained independent on the dimming range that is between 87% and 14%. The dimming range can be extended by reducing

*P*(loss in BER) up to 1dB (

_{OFDM}*P*= 17

_{OFDM}*dBm*) while maintaining

*BER*= 10

^{−3}. To further extend the dimming range, a switch to 16-QAM (loss in spectral efficiency) is required. Using 16-QAM, the dimming range can be further extended by reducing

*P*up to 3dB (

_{OFDM}*P*= 14

_{OFDM}*dBm*) while maintaining BER below 10

^{−3}. The corresponding values of

*D*to obtain certain dimming set-points at different values of

*P*are shown in Fig. 7. Using Fig. 11 and Fig. 7 (having the same

_{OFDM}*x*-axis), for example, at optimum

*P*,

_{OFDM}*D*= 8% and

*D*= 12% are required to obtain 20% optical dimming for 32-QAM and 16-QAM, respectively.

## 6. Conclusion

*i.e.*DC bias, to maximize the dimming range while maintaining a good SNR to fulfill a target data rate. We anticipate describing implementation details of how the PWM and RPO-OFDM will be achieved in practical systems in future works.

## Acknowledgments

## References and links

1. | M. H. Crawford, “LEDs for solid-state lighting: performance challenges and recent advances,” IEEE J. Sel. Top. Quantum Electron. |

2. | C. DiLouie, Advanced lighting controls: energy savings, productivity, technology and applications(The Fairmont Press, Inc., 2006). |

3. | E. F. Schubert, T. Gessmann, and J. K. Kim, |

4. | H. Elgala, R. Mesleh, and H. Haas, “Indoor optical wireless communication: potential and state-of-the-art,” IEEE Commun. Mag. |

5. | H. Elgala, R. Mesleh, and H. Haas, “Indoor broadcasting via white LEDs and OFDM,” IEEE Trans. Consumer Electron. |

6. | M. B. Rahaim, A. M. Vegni, and T. D. Little, “A hybrid radio frequency and broadcast visible light communication system,” in |

7. | H. Ma, L. Lampe, and S. Hranilovic, “Integration of indoor visible light and power line communication systems,” in |

8. | H. Le Minh, Z. Ghassemlooy, D. O’Brien, and G. Faulkner, “Indoor gigabit optical wireless communications: challenges and possibilities,” in |

9. | J. Vucic and K.-D. Langer, “High-speed visible light communications: State-of-the-art,” in |

10. | R. Mesleh, H. Elgala, and H. Haas, “Performance analysis of indoor OFDM optical wireless communication systems,” in |

11. | E. Pisek, S. Rajagopal, and S. Abu-Surra, “Gigabit rate mobile connectivity through visible light communication,” |

12. | P. Apse-Apsitis, A. Avotins, and L. Ribickis, “Wirelessly controlled LED lighting system,” in |

13. | Z. Wang, W.-D. Zhong, C. Yu, J. Chen, C. P. S. Francois, and W. Chen, “Performance of dimming control scheme in visible light communication system,” Opt. Express |

14. | G. Ntogari, T. Kamalakis, J. Walewski, and T. Sphicopoulos, “Combining illumination dimming based on pulse-width modulation with visible-light communications based on discrete multitone,” J. Opt. Commun. Netw. |

15. | Y. Gu, N. Narendran, T. Dong, and H. Wu, “Spectral and luminous efficacy change of high-power LEDs under different dimming methods,” Proc. SPIE |

16. | J. Armstrong and B. J. Schmidt, “Comparison of asymmetrically clipped optical OFDM and DC-biased optical OFDM in AWGN,” IEEE Commun. Lett. |

17. | N. Fernando, Y. Hong, and E. Viterbo, “Flip-OFDM for optical wireless communications,” in |

18. | A. Nuwanpriya, A. Grant, S.-W. Ho, and L. Luo, “Position modulating OFDM for optical wireless communications,” in |

19. | D. Tsonev, S. Sinanovic, and H. Haas, “Novel unipolar orthogonal frequency division multiplexing (U-OFDM) for optical wireless communication,” in |

**OCIS Codes**

(060.4080) Fiber optics and optical communications : Modulation

(060.4510) Fiber optics and optical communications : Optical communications

(200.4560) Optics in computing : Optical data processing

(230.3670) Optical devices : Light-emitting diodes

**ToC Category:**

Fiber Optics and Optical Communications

**History**

Original Manuscript: July 26, 2013

Revised Manuscript: September 12, 2013

Manuscript Accepted: September 14, 2013

Published: October 3, 2013

**Citation**

Hany Elgala and Thomas D. C. Little, "Reverse polarity optical-OFDM (RPO-OFDM): dimming compatible OFDM for gigabit VLC links," Opt. Express **21**, 24288-24299 (2013)

http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-21-20-24288

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### References

- M. H. Crawford, “LEDs for solid-state lighting: performance challenges and recent advances,” IEEE J. Sel. Top. Quantum Electron.15(4), 1028–1040 (2009). [CrossRef]
- C. DiLouie, Advanced lighting controls: energy savings, productivity, technology and applications(The Fairmont Press, Inc., 2006).
- E. F. Schubert, T. Gessmann, and J. K. Kim, Light emitting diodes (Wiley Online Library, 2005).
- H. Elgala, R. Mesleh, and H. Haas, “Indoor optical wireless communication: potential and state-of-the-art,” IEEE Commun. Mag.49(9), 56–62 (2011). [CrossRef]
- H. Elgala, R. Mesleh, and H. Haas, “Indoor broadcasting via white LEDs and OFDM,” IEEE Trans. Consumer Electron.55(3), 1127–1134 (2009). [CrossRef]
- M. B. Rahaim, A. M. Vegni, and T. D. Little, “A hybrid radio frequency and broadcast visible light communication system,” in GC Wkshps, 792–796, (IEEE, 2011).
- H. Ma, L. Lampe, and S. Hranilovic, “Integration of indoor visible light and power line communication systems,” in ISPLC, 291–296, (IEEE, 2013).
- H. Le Minh, Z. Ghassemlooy, D. O’Brien, and G. Faulkner, “Indoor gigabit optical wireless communications: challenges and possibilities,” in ICTON, 1–6, (IEEE, 2010).
- J. Vucic and K.-D. Langer, “High-speed visible light communications: State-of-the-art,” in OFC/NFOEC, 1–3, (IEEE, 2012).
- R. Mesleh, H. Elgala, and H. Haas, “Performance analysis of indoor OFDM optical wireless communication systems,” in WCNC, 1005–1010, (IEEE, 2012).
- E. Pisek, S. Rajagopal, and S. Abu-Surra, “Gigabit rate mobile connectivity through visible light communication,” in ICC, 3122–3127, (IEEE, 2012).
- P. Apse-Apsitis, A. Avotins, and L. Ribickis, “Wirelessly controlled LED lighting system,” in ENERGYCON, 952–956, (IEEE, 2012).
- Z. Wang, W.-D. Zhong, C. Yu, J. Chen, C. P. S. Francois, and W. Chen, “Performance of dimming control scheme in visible light communication system,” Opt. Express20(17), 18861–18868 (2012). [CrossRef] [PubMed]
- G. Ntogari, T. Kamalakis, J. Walewski, and T. Sphicopoulos, “Combining illumination dimming based on pulse-width modulation with visible-light communications based on discrete multitone,” J. Opt. Commun. Netw.3(1), 56–65 (2011). [CrossRef]
- Y. Gu, N. Narendran, T. Dong, and H. Wu, “Spectral and luminous efficacy change of high-power LEDs under different dimming methods,” Proc. SPIE6337, 63370J–63370J-7 (2006). [CrossRef]
- J. Armstrong and B. J. Schmidt, “Comparison of asymmetrically clipped optical OFDM and DC-biased optical OFDM in AWGN,” IEEE Commun. Lett.12(5), 343–345 (2008). [CrossRef]
- N. Fernando, Y. Hong, and E. Viterbo, “Flip-OFDM for optical wireless communications,” in ITW, 5–9, (IEEE, 2011).
- A. Nuwanpriya, A. Grant, S.-W. Ho, and L. Luo, “Position modulating OFDM for optical wireless communications,” in GC Wkshps, 1219–1223, (IEEE, 2012).
- D. Tsonev, S. Sinanovic, and H. Haas, “Novel unipolar orthogonal frequency division multiplexing (U-OFDM) for optical wireless communication,” in VTC-Spring, 1–5, (IEEE, 2012).

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