## Performance improvement by tilting receiver plane in M-QAM OFDM visible light communications |

Optics Express, Vol. 19, Issue 14, pp. 13418-13427 (2011)

http://dx.doi.org/10.1364/OE.19.013418

Acrobat PDF (1437 KB)

### Abstract

We propose a scheme to improve the SNR distribution as well as the spectral efficiency of M-QAM OFDM signal for indoor visible light communication by tilting the receiver plane. Newton method is employed for the photo-detector to receive maximum power by finding the optimal tilting angle. This method is a fast algorithm that only three searching steps are needed. The simulation results show that in the case of one LED source, the maximum spectral efficiency improvement is 0.44bit/s/Hz when the launching power of LED source is 12W; while in the case of four LED sources, the maximum spectral efficiency improvement is 0.21bit/s/Hz when the total launching power of the four LED sources is 12W.

© 2011 OSA

## 1. Introduction

1. T. Komine and M. Nakagawa, “Fundamental analysis for visible-light communication system using LED lights,” IEEE Trans. Consum. Electron. **50**(1), 100–107 (2004). [CrossRef]

3. M. Zhang, Y. Zhang, X. Yuan, and J. Zhang, “Mathematic models for a ray tracing method and its applications in wireless optical communications,” Opt. Express **18**(17), 18431–18437 (2010). [CrossRef] [PubMed]

1. T. Komine and M. Nakagawa, “Fundamental analysis for visible-light communication system using LED lights,” IEEE Trans. Consum. Electron. **50**(1), 100–107 (2004). [CrossRef]

1. T. Komine and M. Nakagawa, “Fundamental analysis for visible-light communication system using LED lights,” IEEE Trans. Consum. Electron. **50**(1), 100–107 (2004). [CrossRef]

6. J. Armstrong, “OFDM for optical communications,” J. Lightwave Technol. **27**(3), 189–204 (2009). [CrossRef]

## 2. Principle of tilting receiver plane

*φ*be the angle of irradiance from the LED. As in [8,9], the generalized Lambertian radiation pattern in Eq. (1) is used to model LED radiant irradiance:where

*m*is the order of Lambertian emission which is defined by the transmitter’s semi-angle at half power

*φ*,

_{1/2}*m*= ln(1/2)/ln(cos(

*φ*)).The channel direct current (DC) gain is given as [1

_{1/2}**50**(1), 100–107 (2004). [CrossRef]

*d*is the distance between the source and the receiver,

*A*is the physical area of photo-detector, and

*θ*is the angle of incidence. Angles

*φ*and

*θ*are associated with the positions of both source and receiver. Let [

*X*,

_{S}*Y*,

_{S}*Z*] and [

_{S}*X*,

_{R}*Y*,

_{R}*Z*] be the locations of source and receiver respectively, thenwhere

_{R}*X*. Equation (3) indicates that the irradiance angle

*φ*is constant for a particular source and receiver. However the situation of the angle of incidence

*θ*is different. The value of

*θ*is determined not only by the locations of source and receiver, but also by the dihedral angle between the receiver plane and desk where the receiver locates. Let

**and**

*V*_{RS}**be the vector from the receiver to the source and the vector of receiver respectively, thenwhere (**

*V*_{R}**,**

*V*_{RS}**) is the inner product of**

*V*_{R}**and**

*V*_{RS}**. So the channel DC gain in Eq. (2) becomesThe recovered electrical signal after photo-detection is denoted as**

*V*_{R}*s*(

*t*) =

*R**

*P*(1 +

_{rx}*M**

_{I}*f*(

*t*)), where the average received power

*P*=

_{rx}*H*(0)*

*P*;

_{tx}*P*is the launching power of LED;

_{tx}*R*is responsivity of photo-detector;

*M*is the modulation index [10

_{I}10. I. Neokosmidis, T. Kamalakis, J. Walewski, B. Inan, and T. Sphicopoulos, “Impact of nonlinear LED transfer function on discrete multitone modulation: analytical approach,” J. Lightwave Technol. **27**(22), 4970–4978 (2009). [CrossRef]

*f*(

*t*) is the normalized modulating OFDM signal. Hence, the signal to noise ratio (SNR) of a particular receiver position is given by [8],where DC component of the recovered electrical signal is blocked, the parameters to determine shot noise and thermal noise are the same as those in [1

**50**(1), 100–107 (2004). [CrossRef]

*z*= 0.85

*m*) is shown in Fig. 2(a) , where the launching power of LED is 12W and the LED locates in the center of the ceiling. From Fig. 2(a) we find that, the maximum SNR is 36.48dB when the receiver is right below the source LED, while the minimum SNR is 13.83dB when the receiver is in the corners of the room. Hence the peak-to-valley value of SNR is 22.65dB, which is caused by the non-normal incidence of the light from LED to the receiver, deteriorating the average system performance in the whole room. Note that 12W-LED is safe for human eyes [11].

**and**

*V*_{R}**. Note that the vector**

*V*_{RS}**is always perpendicular to the receiver plane. The vector**

*V*_{R}**is also constant for a particular source and receiver. It is easy to find that the value of**

*V*_{RS}*cosθ*in Eq. (4) is maximized when the two vectors (

**,**

*V*_{RS}**) are parallel to each other, i.e., the receiver plane faces to the source. In case the receiver does not locate on the desk right below the source on the ceiling, especially when the receiver is in the corners of the room, we cannot get the maximum channel DC gain in terms of**

*V*_{R}*cosθ*. After tilting the receiver plane towards the source in order to make the two vectors (

**,**

*V*_{RS}**) parallel, the value of**

*V*_{R}*cosθ*reaches its maximum-unity, thus maximum channel DC gain of a particular position could be obtained, which is only associated with the transmission distance

*d*.

**in Eq. (4) could be expressed as**

*V*_{RS}**= [**

*V*_{RS}*a*,

*b*,

*c*] = [

*X*,

_{R}*Y*,

_{R}*Z*] - [

_{R}*X*,

_{S}*Y*,

_{S}*Z*], which is also constant for a particular source and receiver. Here we assume that tiling the receiver plane will not affect the position of the receiver. In spherical coordinate system, the location of receiver is selected as the origin. Before tilting the receiver plane, the vector

_{S}**is [0, 0, 1], which means that the receiver plane points to the ceiling; after tilting the receiver plane towards the source on the ceiling, the vector**

*V*_{R}**becomes [**

*V*_{R}*sinβ*·

*cosα*,

*sinβ*·

*sinα*,

*cosβ*], where

*β*is the inclination angle [12] which is the same as the tilting angle, as shown in Fig. 3 and the azimuth angle

*α*is determined by the positions of the receiver as well as the source projection on the desk. In the Cartesian coordinate system with receiver as the origin, the value of angle

*α*is expressed in Eq. (7) and also depicted in Fig. 4 , which takes the first quadrant for example.

*cosθ*in Eq. (4) becomesThe channel DC gain after tilting is denoted as

*f (β)*,The initial inclination angle

*β*is zero, i.e., the receiver locates on the desk and the angle-tilting is implemented by an electrical machinery. When the inclination angle is increased after tilting the receiver plane, the two vectors

**and**

*V*_{R}**tend to be parallel to each other. More and more optical power is collected. The electrical machinery will not stop changing the tilting angle**

*V*_{RS}*β*until no more optical power could be received. This searching method is known as Newton method-a fast algorithm to find the maximum of

*f (β)*[13]-which is defined aswhere

*f*and

^{(1)}(β)*f*are the first and second order derivative of

^{(2)}(β)*f (β)*. After finding the optimum tilting angle by Newton method, the maximum optical power is obtained in each receiver position. The improved SNR distribution is shown in Fig. 2 (b), where the maximum SNR remains the same 36.48dB while the minimum SNR in the corners of the room increases to 19.51dB. So there is a 5.68dB improvement of peak-to-valley SNR value when there is only one LED source on the ceiling.

*α*. When the receiver is equidistance to two LEDs, it faces to the middle of them. The total channel DC gain is again denoted as

*f (β)*in order to find the optimum tilting angle

*β*.

## 3. Adaptive M-QAM OFDM system setup and discussion

6. J. Armstrong, “OFDM for optical communications,” J. Lightwave Technol. **27**(3), 189–204 (2009). [CrossRef]

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

_{S}/N

_{0}corresponds to better SER performance. From Fig. 2 and Fig. 5, we find that the distribution of SNR in a particular room is not uniform. Thus, adaptive modulation could be employed to improve the whole system performance [7

7. A. Svensson, “An introduction to adaptive QAM modulation schemes for known and predicted channels,” Proc. IEEE **95**(12), 2322–2336 (2007). [CrossRef]

^{−3}as the benchmark SER. As shown in Fig. 8 , when the M-QAM OFDM optical signal comes to the photo-detector, its power is detected and sent back to the sources on the ceiling via infrared (IR) feedback channel after tiling the receiver plane. In the places where SNR is low, the small value of

*M*is selected to guarantee that 10

^{−3}SER could be achieved; while in the places where SNR is high, the modulation format is changed, i.e., advanced M-QAM (large

*M*) is applied for high data rate performance. In optical communication, the value of M-QAM OFDM signal should be real by applying Hermitian symmetry which reduces the total spectral efficiency by half [2,6

6. J. Armstrong, “OFDM for optical communications,” J. Lightwave Technol. **27**(3), 189–204 (2009). [CrossRef]

*M*is varied according to the SNR. The average SE iswhere

*p*(

*M*) is probability of

_{i}*M*which is associated with the distribution of SNR in a particular room as described in Fig. 2 and Fig. 5 and Table 2.

## 4. Conclusion

## Acknowledgment

## References and links

1. | T. Komine and M. Nakagawa, “Fundamental analysis for visible-light communication system using LED lights,” IEEE Trans. Consum. Electron. |

2. | M. Z. Afgani, H. Haas, H. Elgala, D. Knipp, and W. Hirt, “Visible light communication using OFDM,” in |

3. | M. Zhang, Y. Zhang, X. Yuan, and J. Zhang, “Mathematic models for a ray tracing method and its applications in wireless optical communications,” Opt. Express |

4. | J. Vucic, C. Kottke, K. Habel, and K.-D. Langer, “803Mbit/s visible light WDM link based on DMT modulation of a single RGB LED luminary,” in |

5. | S. K. Hashemi, Z. Ghassemlooy, L. Chao, and D. Benhaddou, “Orthogonal frequency division multiplexing for indoor optical wireless communications using visible light LEDs,” in |

6. | J. Armstrong, “OFDM for optical communications,” J. Lightwave Technol. |

7. | A. Svensson, “An introduction to adaptive QAM modulation schemes for known and predicted channels,” Proc. IEEE |

8. | J. R. Barry, |

9. | L. Zeng, D. O’Brien, H. Le-Minh, K. Lee, D. Jung, and Y. Oh, “Improvement of data rate by using equalization in an indoor visible light communication system,” in |

10. | I. Neokosmidis, T. Kamalakis, J. Walewski, B. Inan, and T. Sphicopoulos, “Impact of nonlinear LED transfer function on discrete multitone modulation: analytical approach,” J. Lightwave Technol. |

11. | |

12. | C. H. Edwards and D. E. Penney, |

13. | M. T. Heath, |

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

15. | H. Nguyen and E. Shwedyk, |

16. | U. S. Jha and R. Prasad, |

17. | J. Proakis, |

**OCIS Codes**

(060.4080) Fiber optics and optical communications : Modulation

(060.4230) Fiber optics and optical communications : Multiplexing

(060.4510) Fiber optics and optical communications : Optical communications

(230.3670) Optical devices : Light-emitting diodes

**ToC Category:**

Fiber Optics and Optical Communications

**History**

Original Manuscript: March 29, 2011

Revised Manuscript: June 2, 2011

Manuscript Accepted: June 4, 2011

Published: June 27, 2011

**Citation**

Zixiong Wang, Changyuan Yu, Wen-De Zhong, and Jian Chen, "Performance improvement by tilting receiver plane in M-QAM OFDM visible light communications," Opt. Express **19**, 13418-13427 (2011)

http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-19-14-13418

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### References

- T. Komine and M. Nakagawa, “Fundamental analysis for visible-light communication system using LED lights,” IEEE Trans. Consum. Electron. 50(1), 100–107 (2004). [CrossRef]
- M. Z. Afgani, H. Haas, H. Elgala, D. Knipp, and W. Hirt, “Visible light communication using OFDM,” in International Conference on Testbeds and Research Infrastructures for the Development on Networks and Communities, 129–134 (2006).
- M. Zhang, Y. Zhang, X. Yuan, and J. Zhang, “Mathematic models for a ray tracing method and its applications in wireless optical communications,” Opt. Express 18(17), 18431–18437 (2010). [CrossRef] [PubMed]
- J. Vucic, C. Kottke, K. Habel, and K.-D. Langer, “803Mbit/s visible light WDM link based on DMT modulation of a single RGB LED luminary,” in Proc. OFC, Los Angeles, CA, OWB6 (2011).
- S. K. Hashemi, Z. Ghassemlooy, L. Chao, and D. Benhaddou, “Orthogonal frequency division multiplexing for indoor optical wireless communications using visible light LEDs,” in International Symposium on Communication Systems, Networks and Digital Signal Processing, 174–178 (2008).
- J. Armstrong, “OFDM for optical communications,” J. Lightwave Technol. 27(3), 189–204 (2009). [CrossRef]
- A. Svensson, “An introduction to adaptive QAM modulation schemes for known and predicted channels,” Proc. IEEE 95(12), 2322–2336 (2007). [CrossRef]
- J. R. Barry, Wireless Infrared Communications (Kluwer Academic Publishers, 2006).
- L. Zeng, D. O’Brien, H. Le-Minh, K. Lee, D. Jung, and Y. Oh, “Improvement of data rate by using equalization in an indoor visible light communication system,” in International Conference on Circuits and Systems for Communications, 678–682 (2008).
- I. Neokosmidis, T. Kamalakis, J. Walewski, B. Inan, and T. Sphicopoulos, “Impact of nonlinear LED transfer function on discrete multitone modulation: analytical approach,” J. Lightwave Technol. 27(22), 4970–4978 (2009). [CrossRef]
- http://www.effled.com/15W-high-power-led-p-58.html
- C. H. Edwards and D. E. Penney, Calculus (Prentice Hall, 2002).
- M. T. Heath, Scientific Computing—An Introductory Survey (McGraw-Hill, 2002).
- H. Elgala, R. Mesleh, and H. Haas, “Indoor broadcasting via white LEDs and OFDM,” IEEE Trans. Consum. Electron. 55(3), 1127–1134 (2009). [CrossRef]
- H. Nguyen and E. Shwedyk, A First Course in Digital Communications (Cambridge University Press, 2009).
- U. S. Jha and R. Prasad, OFDM towards Fixed and Mobile Broadband Wireless Access (Artech House, 2007).
- J. Proakis, Digital Communications (McGraw-Hill, 2008).

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