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Optics Express

  • Editor: Andrew M. Weiner
  • Vol. 22, Iss. 9 — May. 5, 2014
  • pp: 11107–11118
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Design of dual-link (wide- and narrow-beam) LED communication systems

Thomas C. Shen, Robert J. Drost, Christopher C. Davis, and Brian M. Sadler  »View Author Affiliations


Optics Express, Vol. 22, Issue 9, pp. 11107-11118 (2014)
http://dx.doi.org/10.1364/OE.22.011107


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Abstract

We explore the design of an LED-based communication system comprising two free space optical links: one narrow-beam (primary) link for bulk data transmission and one wide-beam (beacon) link for alignment and support of the narrow-beam link. Such a system combines the high throughput of a highly directional link with the robust insensitivity to pointing errors of a wider-beam link. We develop a modeling framework for this dual-link configuration and then use this framework to explore system tradeoffs in power, range, and achievable rates. The proposed design presents a low-cost, compact, robust means of communication at short- to medium-ranges, and calculations show that data rates on the order of Mb/s are achievable at hundreds of meters with only a few LEDs.

© 2014 Optical Society of America

1. Introduction

2. Link model

Fig. 1 Diagram of an optical link with an LED transmitter and photodiode (PD) receiver. The transmitter beam is described by its half-power half-beamwidth Φ1/2 and its pointing error ϕ. A distance d separates the transmitter and receiver. In this model, the field of view of the receiver is defined by the concentrator half-angle Ψc. The receiver pointing error is ψ. In this diagram, the angles described by ϕ and ψ are coplanar, but the derived link model is generally valid.

For a receiver placed at a location defined by (d, ϕ), the received optical signal power PRx is given by
PRx=IsAeff,
(3)
and the corresponding excited photocurrent is
Ip=RPRx.
(4)
Here, R [A/W] is the responsivity of the photodiode and Aeff [m2] is the effective area of the receiver. In general, Aeff is a function of the angle-of-incidence of the transmitted light at the receiver, which we define as ψ (see Fig. 1). The case of ψ = 0 corresponds to a receiver that is perfectly pointed at the transmitter. For a receiver that is composed of a photodiode of active area A, an optical filter described by the parameter Ts(ψ), and an optical concentrator of gain g(ψ), the effective area is
Aeff(ψ)=g(ψ)Ts(ψ)Acos(ψ).
(5)
For a given spectrum of LED emission incident on the receiver at an angle ψ, Ts(ψ) is the fraction of incident optical signal power allowed through the filter. If we assume that the concentrator is ideal, then its gain g is [24

24. X. Ning, R. Winston, and J. O’Gallagher, “Dielectric totally internally reflecting concentrators,” Applied Optics 26, 300–305 (1987). [CrossRef] [PubMed]

]
g(ψ)={n2/sin2(Ψc),if|ψ|Ψc0,if|ψ|>Ψc.
(6)
Here, the concentrator index of refraction is n and its half-angle field-of-view is Ψc. Practical concentrators often approach this ideal gain relation [23

23. J. Kahn and J. Barry, “Wireless infrared communications,” in Proceedings of the IEEE, (IEEE, 1997), pp. 265–298. [CrossRef]

]. The case of no optical concentrator corresponds to a case of a concentrator with n = 1 (free space) and Ψc = 90°, yielding a gain of g = 1.

In OW systems, there are many potential sources of noise, including thermal noise in the receiver, artificial lighting [25

25. C. Chow, C. Yeh, Y. Liu, and P. Huang, “Mitigation of optical background noise in light-emitting diode (LED) optical wireless communication systems,” IEEE Photonics Journal 5, 7900307 (2013). [CrossRef]

, 26

26. A.J. Moreira, R.T Valadas, and A. de Oliveira Duarte, “Optical interference produced by artificial light,” Wireless Networks 3, 131–140, 1997. [CrossRef]

], and shot noise from the ambient sunlight [9

9. J. Barry, Wireless Infrared Communications (Springer, 1994). [CrossRef]

]. Often in OW systems, and especially for outdoor systems, the dominant noise source is shot noise from isotropic ambient light [3

3. M. Wolf and D. Kreß, “Short-range wireless infrared transmission: the link budget compared to RF,” IEEE Wireless Communications 10(2), 8–14 (2003). [CrossRef]

, 23

23. J. Kahn and J. Barry, “Wireless infrared communications,” in Proceedings of the IEEE, (IEEE, 1997), pp. 265–298. [CrossRef]

, 26

26. A.J. Moreira, R.T Valadas, and A. de Oliveira Duarte, “Optical interference produced by artificial light,” Wireless Networks 3, 131–140, 1997. [CrossRef]

28

28. D. OBrien, M. Katz, P. Wang, K. Kalliojarvi, S. Arnon, M. Matsumoto, R.J. Green, and S. Jivkova, “Short-range optical wireless communications,” Wireless World Research Forum, 1–22 (2005).

]. To reduce the ambient optical power Pn [W] that is received by the photodiode, an optical passband filter can be placed on the receiver. In calculating the effect of this filter on the noise level, we model it as a an ideal “boxcar” passband filter of spectral width Δλ [nm]. The filter has a transmittance Tn within the passband and zero outside the passband. A practical filter may have an angularly depedendent transmittance, but can be approximately modeled as a “boxcar” filter of effective passband width Δλ. We also assume that the ambient background noise incident on the receiver is “white” (constant within the pass-band), and define its spectral irradiance (power per unit photodetector area per unit spectrum) as pbg [W/nm-cm2]. With an ideal optical concentrator of index of refraction n, the ambient optical power incident on the photodiode is [23

23. J. Kahn and J. Barry, “Wireless infrared communications,” in Proceedings of the IEEE, (IEEE, 1997), pp. 265–298. [CrossRef]

]
Pn=pbgΔλTnAn2.
(7)

This ambient light creates shot noise in the receiver, which is typically modeled as introducing zero-mean additive white Gaussian noise (AWGN) to the received photocurrent, where the variance σshot2 [A2] of the AWGN can be approximated by [9

9. J. Barry, Wireless Infrared Communications (Springer, 1994). [CrossRef]

]
σshot2=2qRPnB.
(8)
Here, −q [C] is the charge of an electron, B [bits/s] is the bit rate of the signal, and R [A/W] is the responsivity of the photodiode.

We use this noise model to calculate the signal-to-noise ratio (SNR) at the receiver, which we define as
SNR=Ip2σshot2=(RPRx)2σshot2.
(9)
For a given SNR, the bit-error rate (BER) is
BER=Q(SNR),
(10)
where Q(·) is the tail probability of the standard normal distribution [23

23. J. Kahn and J. Barry, “Wireless infrared communications,” in Proceedings of the IEEE, (IEEE, 1997), pp. 265–298. [CrossRef]

]. We can relate the bit rate B, ambient shot noise level, average transmitted power, range, beamwidth, and BER. Combining Eq. (8)(10) and solving for B yields the rate
B=R2PRx22qRPn[Q1(BER)]2.
(11)
To solve for the range, we substitute Eqs. (1) and (3) into Eq. (11), yielding
d=(12qRPnB)1/4[RAeffP(m+1)cosm(ϕ)2πQ1(BER)]1/2.
(12)

3. Design of wide beam/narrow-beam dual link system

3.1. Defining the role of the beacon link

With this optimal beamwidth, beacon connectivity (BbBb0) is guaranteed to any receiver that lies d0 or less away from the transmitter, within the angular range −θaϕbθa. Note that connectivity at ranges greater than d0 can be established for |ϕb| < θa, as well as for ranges less than d0 for |ϕb| > θa. A diagram that illustrates the geometry of the angular range |ϕb| ≤ θa and distance d0 is shown in Fig. 2. In practice, a single node can employ several beacons to “cover” a wider range of azimuthal and/or elevation angle, building on angle-diversity schemes that have been explored [31

31. M. Bilgi and Y. Murat, “Multi-element free-space-optical spherical structures with intermittent connectivity patterns,” in INFOCOM Workshops, (IEEE, 2008), pp. 1–4.

, 32

32. M. Yuksel, J. Akella, S. Kalyanaraman, and P. Dutta, “Free-space-optical mobile ad hoc networks: Auto-configurable building blocks,” Wireless Networks 15(3), 295–312 (2009). [CrossRef]

]. However, the analysis in this work will focus on the use of a single beacon per node.

Fig. 2 Diagram of beacon-link coverage range and the primary link beamwidth. A receiver positioned in the angular range |ϕb| ≤ θa is guaranteed a beacon connection (BbBb0) if its range is less than or equal to d0. The beacon beamwidth is Φ1/2,b (not shown), while the primary link beamwidth is Φ1/2,p.

3.2. Exploring reasonable beacon rates and ranges

Fig. 3 Spatial maps of beacon-link bit rates, with diagrams of receiver geometries for (a) perfect receiver alignment (ψb = 0) and (b) poor receiver alignment (ψb = 45°). In both cases, θa is chosen to be 45°, and the concentrator field of view Ψc,b is chosen to match θa (i.e., Ψc,b = θa = 45°). The LED transmitter is assumed to be at (X,Y)=(0,0) and pointing in the positive Y-direction. The contours represent the logarithm of the bit rate in bits/s. For example, “3” represents Bb = 1 kb/s. The calculations assume Pb = 0.3 W, 2θa = 90°, pbg = 5.8 μW/nm/cm2, Δλb = 100 nm, R = 0.6 A/W, n = 1.5, Ab = 1 cm2, Ts,b = Tn,b = 0.8, and BER = 10−4.

For the purposes of acquisition and feedback control, assuming a minimum beacon rate of Bb0 = 1 kb/s is reasonable. The calculations in Fig. 3(a) show that for an aligned receiver (ψb = 0), this required rate is achievable at d0 ≈ 85 m. If both the the transmitter and receiver are pointed perfectly (i.e., the receiver lies along the line X = 0, where ϕb = 0), then Bb = 1 kb/s is achievable at d ≈ 133 m. In the case of poor receiver alignment (ψb = θa = 45°), shown in Fig. 3(b), d0 is roughly 71 m.

In general, the sensitivity of d0 to the receiver pointing angle ψb depends on the optical concentrator gain [gb(ψb)], optical filter [Ts,b(ψb)], and a geometrical factor cos(ψb) [see Eqs. (5) and (12)]. Specifically, d0 is proportional to the square root of these factors. In the calculations presented in Fig. 3, the concentrator gain g is considered constant within its field of view defined by ψb < Ψc,b = θa. We also assume that Ts,b(ψb) is invariant in ψb for the beacon link, which is consistent with the behavior of an absorptive colored filter. Thus, in these calculations, the only dependence of d0 on the receiver misalignment ψb is the geometrical factor (cosψb)1/2. For the two receiver alignments examined here, [cos(ψb)]1/2 = 1 for the well-aligned receiver [Fig. 3(a)], and [cos(ψb)]1/2 ≈ 0.84 for the poorly aligned case [Fig. 3(b)]. Thus the ratio of the values of d0 in Fig. 3(a) and Fig. 3(b) is (71 m)/(85 m) ≈ 0.84.

3.3. Jointly designing the beacon and primary link

Fig. 4 (a) Plot of the range d0 of the beacon link as a function of beacon transmitter power Pb for several values of 2θa, and for receiver orientations ψb = 0 and ψb = θa. The three colors correspond to three values of 2θa: blue (2θa = 90°), green (2θa = 60°) and red (2θa = 40°). Unless stated otherwise, other parameter values are the same as those used in Fig. 3. (b) Plot of data rates Bp of the primary link as a function of Pp/Pb, assuming perfect primary-transmitter pointing (ϕp = 0) and perfect primary-receiver alignment (ψp = 0). The color-coding used here is the same as in (a). Three curves (one of each pair) correspond to to a narrow beamwidth of Φ1/2,p = 10°, and three curves correspond to Φ1/2,p = 20°. In this plot we assume that the primary link detector area is Ap = 1 mm2 and that the primary link concentrator field-of-view half-angle is 5° for all curves.

Figure 4(a) defines a pair of curves for d0 as a function of beacon power Pb, one for ψb = 0 (well-aligned receiver, greater d0) and one for ψb = θa (misaligned receiver, shorter d0); this pair of curves is presented for three values of 2θa. Thus, for a given power Pb, θa, and receiver alignment ψb, the plot defines a range d0. This is the distance from the transmitter at which a data rate of Bb = 1 kb/s can be guaranteed within the angular range −θaϕbθa. Taken alone, Fig. 4(a) is a design space only for the beacon link.

4. Conclusion

Acknowledgments

Work at the University of Maryland was supported by a grant from the U.S. Army Research Office (ARO). The third author acknowledges support by ARO under grant number W911NF-13-1-0003.

References and links

1.

F.R. Gfeller and U. Bapst, “Wireless in-house data communication via diffuse infrared radiation,” in Proceedings of the IEEE (IEEE, 1979), pp. 1474–1486. [CrossRef]

2.

D.K. Borah, A.C. Boucouvalas, C.C. Davis, S. Hranilovic, and K. Yiannopoulos, “A review of communication-oriented optical wireless systems,” EURASIP Journal on Wireless Communications and Networking 1, 1–28 (2012).

3.

M. Wolf and D. Kreß, “Short-range wireless infrared transmission: the link budget compared to RF,” IEEE Wireless Communications 10(2), 8–14 (2003). [CrossRef]

4.

A.K. Majumdar and J.C. Ricklin, Free-space Laser Communications: Principles and Advances (Springer, 2008, vol. 2). [CrossRef]

5.

J. Rzasa, S. Milner, and C.C. Davis, “Design and implementation of pan-tilt FSO transceiver gimbals for real-time compensation of platform disturbances using a secondary control network,” SPIE Optical Engineering+ Applications , 8162, 81620E (2011).

6.

F.G. Walther, G. A. Nowak, S. Michael, R. Parenti, J. Roth, J. Taylor, W. Wilcox, R. Murphy, J. Greco, and J. Peters, “Air-to-ground lasercom system demonstration,” in Military Comm. Conference, (IEEE, 2010), pp. 1594–1600.

7.

J. Rzasa, M. C. Ertem, and C. C. Davis., “Pointing, acquisition, and tracking considerations for mobile directional wireless communications systems,” SPIE Optical Engineering+ Applications, 88740C (2013).

8.

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

9.

J. Barry, Wireless Infrared Communications (Springer, 1994). [CrossRef]

10.

Jelena Grubor, Sebastian Randel, Klaus-Dieter Langer, and Joachim W Walewski, “Broadband information broadcasting using LED-based interior lighting,” Journal of Lightwave Technology 26, 3883–3892 (2008). [CrossRef]

11.

D. O’Brien, G. Parry, and P. Stavrinou, “Optical hotspots speed up wireless communication,” Nature Photonics 1, 245–247 (2007). [CrossRef]

12.

H. Chowdhury and M. Katz, “Data download on move in indoor hybrid (radio-optical) WLAN-VLC hotspot coverages,” in Vehicular Technology Conference, (IEEE, 2013), 1–5.

13.

T. Ho, S. Trisno, A. Desai, J. Llorca, S. Milner, and C. C. Davis, “Performance and analysis of reconfigurable hybrid FSO/RF wireless networks,” Proc. SPIE 119, 119–130 (2005). [CrossRef]

14.

B. Epple, “Using a GPS-aided inertial system for coarse-pointing of free-space optical communication terminals,” Proc. SPIE 6304, 630418 (2006). [CrossRef]

15.

L. Stotts, P. Cherry, P. Klodzy, R. Phillips, and D. Young, “Hybrid optical RF airborne communications,” in Proceedings of the IEEE (IEEE, 2009), pp. 1109–1127. [CrossRef]

16.

S. Milner and C.C. Davis, “Hybrid free space optical/RF networks for tactical operations,” in Military Communications Conference, (IEEE, 2004), pp. 409–415.

17.

H. Izadpanah, T. Elbatt, V. Kukshya, F. Dolezal, and B. K. Ryu, “High-availability free space optical and RF hybrid wireless networks,” IEEE Wireless Communications 10, 45–53 (2003). [CrossRef]

18.

Y. Tang and M. Brandt-Pearce, “Link allocation, routing and scheduling of FSO augmented RF wireless mesh networks,” in International Conference on Communications, (IEEE, 2012), pp. 3139–3143.

19.

I. S. Ansari, M.S. Alouini, and F. Yilmaz, “On the performance of hybrid RF and RF/FSO fixed gain dual-hop transmission systems,” in Electronics, Communications and Photonics Conference, (SIECPC, 2013), pp. 1–6.

20.

W. Zhang, S. Hranilovic, and C. Shi, “Soft-switching hybrid FSO/RF links using short-length raptor codes: design and implementation,” IEEE Journal on Selected Areas in Communications 27, 1698–1708 (2009). [CrossRef]

21.

M. Shur and R. Zukauskas, “Solid-state lighting: toward superior illumination,” in Proceedings of the IEEE, (IEEE, 2005), pp. 1691–1703. [CrossRef]

22.

S. Pimputkar, J. Speck, S. DenBaars, and S. Nakamura, “Prospects for LED lighting,” Nature Photonics 3, 180–182 (2009). [CrossRef]

23.

J. Kahn and J. Barry, “Wireless infrared communications,” in Proceedings of the IEEE, (IEEE, 1997), pp. 265–298. [CrossRef]

24.

X. Ning, R. Winston, and J. O’Gallagher, “Dielectric totally internally reflecting concentrators,” Applied Optics 26, 300–305 (1987). [CrossRef] [PubMed]

25.

C. Chow, C. Yeh, Y. Liu, and P. Huang, “Mitigation of optical background noise in light-emitting diode (LED) optical wireless communication systems,” IEEE Photonics Journal 5, 7900307 (2013). [CrossRef]

26.

A.J. Moreira, R.T Valadas, and A. de Oliveira Duarte, “Optical interference produced by artificial light,” Wireless Networks 3, 131–140, 1997. [CrossRef]

27.

J. Kahn, J. Barry, M. Audeh, J. Carruthers, W. Krause, and G. Marsh, “Non-directed infrared links for high-capacity wireless LANs,” IEEE Personal Communications1, 12–25 (1994). [CrossRef]

28.

D. OBrien, M. Katz, P. Wang, K. Kalliojarvi, S. Arnon, M. Matsumoto, R.J. Green, and S. Jivkova, “Short-range optical wireless communications,” Wireless World Research Forum, 1–22 (2005).

29.

J. Grubor, S.C.J. Lee, K. Langer, T. Koonen, and J. Walewski, “Wireless high-speed data transmission with phosphorescent white-light LEDs,” in European Conference and Exhibition of Optical Communication-Post-Deadline Papers, (ECOC, 2008), pp. 1–2.

30.

D. O’Brien and M. Katz, “Optical wireless communications within fourth-generation wireless systems,” Journal of Optical Networking 4, 312–322 (2005). [CrossRef]

31.

M. Bilgi and Y. Murat, “Multi-element free-space-optical spherical structures with intermittent connectivity patterns,” in INFOCOM Workshops, (IEEE, 2008), pp. 1–4.

32.

M. Yuksel, J. Akella, S. Kalyanaraman, and P. Dutta, “Free-space-optical mobile ad hoc networks: Auto-configurable building blocks,” Wireless Networks 15(3), 295–312 (2009). [CrossRef]

OCIS Codes
(060.4510) Fiber optics and optical communications : Optical communications
(230.3670) Optical devices : Light-emitting diodes
(060.2605) Fiber optics and optical communications : Free-space optical communication

ToC Category:
Optical Communications

History
Original Manuscript: February 20, 2014
Revised Manuscript: April 5, 2014
Manuscript Accepted: April 8, 2014
Published: May 1, 2014

Citation
Thomas C. Shen, Robert J. Drost, Christopher C. Davis, and Brian M. Sadler, "Design of dual-link (wide- and narrow-beam) LED communication systems," Opt. Express 22, 11107-11118 (2014)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-22-9-11107


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References

  1. F.R. Gfeller, U. Bapst, “Wireless in-house data communication via diffuse infrared radiation,” in Proceedings of the IEEE (IEEE, 1979), pp. 1474–1486. [CrossRef]
  2. D.K. Borah, A.C. Boucouvalas, C.C. Davis, S. Hranilovic, K. Yiannopoulos, “A review of communication-oriented optical wireless systems,” EURASIP Journal on Wireless Communications and Networking 1, 1–28 (2012).
  3. M. Wolf, D. Kreß, “Short-range wireless infrared transmission: the link budget compared to RF,” IEEE Wireless Communications 10(2), 8–14 (2003). [CrossRef]
  4. A.K. Majumdar, J.C. Ricklin, Free-space Laser Communications: Principles and Advances (Springer, 2008, vol. 2). [CrossRef]
  5. J. Rzasa, S. Milner, C.C. Davis, “Design and implementation of pan-tilt FSO transceiver gimbals for real-time compensation of platform disturbances using a secondary control network,” SPIE Optical Engineering+ Applications, 8162, 81620E (2011).
  6. F.G. Walther, G. A. Nowak, S. Michael, R. Parenti, J. Roth, J. Taylor, W. Wilcox, R. Murphy, J. Greco, J. Peters et al., “Air-to-ground lasercom system demonstration,” in Military Comm. Conference, (IEEE, 2010), pp. 1594–1600.
  7. J. Rzasa, M. C. Ertem, C. C. Davis., “Pointing, acquisition, and tracking considerations for mobile directional wireless communications systems,” SPIE Optical Engineering+ Applications, 88740C (2013).
  8. T. Komine, M. Nakagawa, “Fundamental analysis for visible-light communication system using LED lights,” IEEE Transactions on Consumer Electronics 50(1), 100–107 (2004). [CrossRef]
  9. J. Barry, Wireless Infrared Communications (Springer, 1994). [CrossRef]
  10. Jelena Grubor, Sebastian Randel, Klaus-Dieter Langer, Joachim W Walewski, “Broadband information broadcasting using LED-based interior lighting,” Journal of Lightwave Technology 26, 3883–3892 (2008). [CrossRef]
  11. D. O’Brien, G. Parry, P. Stavrinou, “Optical hotspots speed up wireless communication,” Nature Photonics 1, 245–247 (2007). [CrossRef]
  12. H. Chowdhury, M. Katz, “Data download on move in indoor hybrid (radio-optical) WLAN-VLC hotspot coverages,” in Vehicular Technology Conference, (IEEE, 2013), 1–5.
  13. T. Ho, S. Trisno, A. Desai, J. Llorca, S. Milner, C. C. Davis, “Performance and analysis of reconfigurable hybrid FSO/RF wireless networks,” Proc. SPIE 119, 119–130 (2005). [CrossRef]
  14. B. Epple, “Using a GPS-aided inertial system for coarse-pointing of free-space optical communication terminals,” Proc. SPIE 6304, 630418 (2006). [CrossRef]
  15. L. Stotts, P. Cherry, P. Klodzy, R. Phillips, D. Young, “Hybrid optical RF airborne communications,” in Proceedings of the IEEE (IEEE, 2009), pp. 1109–1127. [CrossRef]
  16. S. Milner, C.C. Davis, “Hybrid free space optical/RF networks for tactical operations,” in Military Communications Conference, (IEEE, 2004), pp. 409–415.
  17. H. Izadpanah, T. Elbatt, V. Kukshya, F. Dolezal, B. K. Ryu, “High-availability free space optical and RF hybrid wireless networks,” IEEE Wireless Communications 10, 45–53 (2003). [CrossRef]
  18. Y. Tang, M. Brandt-Pearce, “Link allocation, routing and scheduling of FSO augmented RF wireless mesh networks,” in International Conference on Communications, (IEEE, 2012), pp. 3139–3143.
  19. I. S. Ansari, M.S. Alouini, F. Yilmaz, “On the performance of hybrid RF and RF/FSO fixed gain dual-hop transmission systems,” in Electronics, Communications and Photonics Conference, (SIECPC, 2013), pp. 1–6.
  20. W. Zhang, S. Hranilovic, C. Shi, “Soft-switching hybrid FSO/RF links using short-length raptor codes: design and implementation,” IEEE Journal on Selected Areas in Communications 27, 1698–1708 (2009). [CrossRef]
  21. M. Shur, R. Zukauskas, “Solid-state lighting: toward superior illumination,” in Proceedings of the IEEE, (IEEE, 2005), pp. 1691–1703. [CrossRef]
  22. S. Pimputkar, J. Speck, S. DenBaars, S. Nakamura, “Prospects for LED lighting,” Nature Photonics 3, 180–182 (2009). [CrossRef]
  23. J. Kahn, J. Barry, “Wireless infrared communications,” in Proceedings of the IEEE, (IEEE, 1997), pp. 265–298. [CrossRef]
  24. X. Ning, R. Winston, J. O’Gallagher, “Dielectric totally internally reflecting concentrators,” Applied Optics 26, 300–305 (1987). [CrossRef] [PubMed]
  25. C. Chow, C. Yeh, Y. Liu, P. Huang, “Mitigation of optical background noise in light-emitting diode (LED) optical wireless communication systems,” IEEE Photonics Journal 5, 7900307 (2013). [CrossRef]
  26. A.J. Moreira, R.T Valadas, A. de Oliveira Duarte, “Optical interference produced by artificial light,” Wireless Networks 3, 131–140, 1997. [CrossRef]
  27. J. Kahn, J. Barry, M. Audeh, J. Carruthers, W. Krause, G. Marsh, “Non-directed infrared links for high-capacity wireless LANs,” IEEE Personal Communications1, 12–25 (1994). [CrossRef]
  28. D. OBrien, M. Katz, P. Wang, K. Kalliojarvi, S. Arnon, M. Matsumoto, R.J. Green, S. Jivkova, “Short-range optical wireless communications,” Wireless World Research Forum, 1–22 (2005).
  29. J. Grubor, S.C.J. Lee, K. Langer, T. Koonen, J. Walewski, “Wireless high-speed data transmission with phosphorescent white-light LEDs,” in European Conference and Exhibition of Optical Communication-Post-Deadline Papers, (ECOC, 2008), pp. 1–2.
  30. D. O’Brien, M. Katz, “Optical wireless communications within fourth-generation wireless systems,” Journal of Optical Networking 4, 312–322 (2005). [CrossRef]
  31. M. Bilgi, Y. Murat, “Multi-element free-space-optical spherical structures with intermittent connectivity patterns,” in INFOCOM Workshops, (IEEE, 2008), pp. 1–4.
  32. M. Yuksel, J. Akella, S. Kalyanaraman, P. Dutta, “Free-space-optical mobile ad hoc networks: Auto-configurable building blocks,” Wireless Networks 15(3), 295–312 (2009). [CrossRef]

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