This paper presents a three dimensions (3D) model of optical power distribution in a diffused cellular indoor visible light communication (VLC) system. To achieve an ideal system which has a maximum coverage area with a minimum power consumption, both hexagon geometric structure and holographic light shaping diffuser (LSD) are employed. We analysed the mathematical models for both square and hexagonal structures with and without using LSD. In addition, the practical system consisting of a (Luxeon Star/O) royal blue LED as a transmitter is used to verify and evaluate the system performance. The system operates at a date rate of 5 Mb/s using the on-off keying non-return-to-zero (OOK-NRZ) modulation format. The simulation results show that using hexagon geometry and a 30o holographic LSD diffuser, the received optical power distribution becomes uniform. The coverage area of the cellular link is therefore significantly extended by 343%. In addition the experimental results for a single cell system are also presented

Ghassemlooy, Zabih

Le Minh, Hoa

Rajbhandari, Sujan

Wu, Dehao

English

Northumbria Research Link

Power Distribution Investigation of a Diffused Cellular Indoor Visible Light Communication System  D. Wu, Z. Ghassemlooy, H. Le Minh, S. Rajbhandari  Optical Communication Research Group,School of CEIS, Northumbria University, Newcastle Upon Tyne, UK E-mail: dehao.wu@unn.ac.uk       Abstract- This paper presents a three dimensions (3D) model of optical power distribution in a diffused cellular indoor visible light communication (VLC) system. To achieve an ideal system which has a maximum coverage area with a minimum power consumption, both hexagon geometric structure and holographic light shaping diffuser (LSD) are employed. We analysed the mathematical models for both square and hexagonal structures with and without using LSD. In addition, the practical system consisting of a (Luxeon Star/O) royal blue LED as a transmitter is used to verify and evaluate the system performance. The system operates at a date rate of 5 Mb/s using the on-off keying non-return-to-zero (OOK-NRZ) modulation format. The simulation results show that using hexagon geometry and a 30o holographic LSD diffuser, the received optical power distribution becomes uniform. The coverage area of the cellular link is therefore significantly extended by 343%. In addition the experimental results for a single cell system are also presented.  Keywords- Visible light communication, LED, Optical Power distribution, light shaping diffuser  I.    INTRODUCTION   In the last decades, solid-state light emitting diodes (LEDs) have been widely used in color displays, traffic signals, and illumination applications. Moreover, LEDs can also be used for high speed data communications due to the fast dynamic response in the order of a few megahertz [1-3].  With the availability of highly efficient white LEDs, created by combing the prime colours (red, green, and blue) or used a blue emitter in combination with a phosphor, the dual application of illumination and data communications can be obtained [4, 5].     There are several possible link configurations for indoor optical wireless communication (OWC) system. The diffuse system which originally expounded by Gfeller and Bapst [6], displays excellent mobility, however at the cost of lower channel bandwidth due to higher path losses and the multipath induced intersymbol interference (ISI). Another scheme employs the direct line of sight (LOS), where the beam is confined within a narrow field of view (FOV). This approach offers a much wider channel bandwidth [7]. However, LOS links have limitation on the coverage area and require alignment or tracking systems to maintain the link availability. Alternatively, non-directed LOS links can be used offering both mobility and higher data rates. In this work, a diffused cellular system is proposed.   In cellular systems, there should in principle be minimum overlapping between coverage areas to achieve the optimum power efficiency. There are a number of cell shapes that could be adopted such as: circular, square, equilateral triangle, and the hexagon. For a given distance between the center of a polygon and its farthest points, the hexagon has the largest area of the three [8] with no un-covered regions between cells. In this paper, we have adopted the hexagon shape to ensure uniform distribution of the optical radiation. The rest of the paper is organized as follow. In section II, the characteristics of transmitter, diffuser, channel model and receiver are described. In section III, the experiment work is outlined followed by results and discussion. Finally, the conclusion and future work are given in the last section IV.   II.   SYSTEM DESCRIPTION  A.    System Overview The proposed indoor cellular VLC system is shown in Fig.1. The LEDs, used for lighting as well as communications, are mounted at the centre of the ceiling to achieve a hexagonal shape coverage area. At the receiving end a wide FOV photoreceiver mounted on to a mobile terminal ensure seamless connectivity as well as alleviating the need for using pointing and tracking systems.  Fig. 2 depicts the proposed practical cellular indoor VLC link. The metallic frame has a dimension of 1.8×1.5×1 m3, which is divided into four cells each containing an LED, a holographic LSD and an optical receiver. The LEDs are mounted at the ceiling 1 m above the receiver at the floor level.     Fig. 1. Proposed indoor cellular VLC system      Fig. 2. Practical setup of indoor VLC system    Fig. 3. A single cell VLC system diagram    B.    Transmitter     Without using any beam shaping lens, the Led sources can be essentially considered as Lambertian [3]. However, in many applications there are requirements for specific radiation distributions to optimise the system performance. Here we have used LEDs with an integrated optics lens. With the transmitter having a full width at half maximum angle (FWHM) of 7o [9], the receiving cell radius is 12.2 cm. This is equivalent to a coverage area of 474 cm2. To achieve a wider coverage area with a uniform radiation distribution, the Luminit holographic LSD (100, 200, and 300) is employed. Fig. 3 shows the system block diagram of a signal cell VLC system.  C.    Model of Holographic LSD Using the holographic LSD, the effective divergence angle of the transmitter can be extended given by [6]:                                     (1)  where θoutput is the effective divergence angle of the light, θFOV is the divergence angle of light source, and θLSD is the angle of LSD [7].    The hologram is a two level surface relief diffractive element that affects only the phase of light passing through it [10]. The far-field radiation pattern passing through the hologram is approximately the Fourier transform of the surface relief structure [11]. In order to simplify the calculating of the beam intensity through the holographic LSD, see Fig. 4, we have divided LSD into an array of „pixels‟. A simulation tool is used to generate the beam profile for every „pixel‟. For each beam, the intensity of light can be considered as uniform after passing through the single „pixel‟. Finally, overall footprint of beams can be obtained by combining the multiple beam profiles.    Fig. 4. Multiple beams profile after holographic LSD   D.    Optical Wireless Channel    Following the LOS link configuration adopted in [12], assuming no optical filters and optical concentrators , the LOS link DC gain is given by:     (2)     where  is the order of Lambertian radiant which is related to the transmitter semiangle φ1/2, (at half power). The order m of the LED which we used is 110. A is the photodetector surface area, ϕ is the incidence angle, φ is irradiance angle,  is the FOV (semiangle) at the receiver and H is the distance between transmitter and receiver. The received power for a single transmitter is given by:                                                                 (3)     where  is the transmitted optical power of LED,  is the received optical power at receiving plane.     E.    Receiver    Fig. 2 illustrates the designed surface-mount receiver PCB with a size of 5 cm × 5 cm, which consists of a commercial PIN junction photodetector and a trans-impedance amplifier.    TABLE I SPECIFICATION FOR INDOOR VLC SYSTEM   LED Product number  LXHL-NRR8 Bandwidth 3.8 MHz Wavelength  455 nm Output optical power 40 mW Field of view (FWHM) 14o Diffuser Luminit LSD 10o, 20o and 300 Channel distance  1 m   Photodectector Product number OSD-15T Detection area 15 mm2 Responsivity 0.21 A/W (@ λ = 436 nm) Photodectector rise time 12 ns Preamplifier product number AD8015 3-dB bandwidth 240 MHz  III.   RESULTS     The system specifications and parameters are given in Table I.  A.    Simulated Results      We used Matlab to simulate the mathematical optical power distribution model of the indoor cellular OWC system. Using (2) and (3), the normalized power distribution for seven cells (see Fig. 1) is illustrated in Fig. 5. Fig. 5(a) illustrates power concentration near the centre of each cell decreasing sharply towards the cell edges. Fig. 5(b) is the contour of the power density at the receiving plane. In a 7-cell configuration with a circular footprint, the area within dotted line circles, marked in Fig. 5(b), is the 3-dB power attenuation area from the centre of a cell. The rest is defined as no coverage area or the „dead zones‟ with no optical illumination.  The over 3-dB coverage area (which means power attenuation is less than 3-dB in this area) for 7 cells is around 4900cm2 which is given by:                                                             (4)                          The total area of receiving plane is given by:                                                  (5)  where,  is the „dead zones‟ area, and  is the overlapping area. To achieve a uniform distribution within each cell, we have used a 30o LSD. Figs. 6 (a) and (b) display predicted power distribution and the power density contours for a 7-cell structure. The over 3-dB coverage area is marked in Fig. 6(b). The coverage area with the 3 dB variation of intensity is around 16800 cm2. Comparing with the a 7-cell model without diffusers, the over 3-dB coverage area is increased by 343%.    Fig. 5 (a). Predicted normalized power distribution at the receiving plane without LSD  Fig. 5 (b). Predicted power contours at the receiving plane without LSD   Fig. 6(a). Predicted normalized power distribution at the receiving plane using 30o LSD  Fig. 6 (b). Predicted power contour plot at the receiving plane using 30o LSD  B.    Practical Measurements    Fig. 7 outlines the measured optical power distribution with and without holographic LSDs for a 1-cell link. With LSDs, the power density becomes more uniform when larger angles LSDs are used. Comparing the power density profiles of a link with no holographic LSD with a 30o holographic LSD (Figs. 7(a) and (d)), it can be seen that a 3-dB transmission boundary has increased from 8 cm to 20 cm (i.e. 625% increasing in the coverage area).    Fig. 7. Spatial distribution of received power a) without LSD, (b) with 10o LSD, (c) with 20o LSD, and (d) with 30o LSD  IV.   CONCLUSION        In this paper, we modeled, simulated and measured the received power distribution for a practical indoor VLC link employing a holographic diffuser. The investigation showed that, using holographic LSD with an appropriate angle, a uniform power distribution can be obtained and thus increase the coverage area in the indoor VLC environment. This is a step forward to design a VLC cellular system.  ACKNOWLEDGMENT          I would like to acknowledge Dr David Johnston and Mr Andrew Burton for support to build the practical system.  REFERENCES  [1]   J.K. Kim and E.F. Schubert, “Transcending the replacement paradigm of solid-state lighting,”  Optics Express, vol. 16, no. 26, pp. 21835-21842, December 2008. [2]   Y. Tanaka, S. Haruyama, and M. Nakagawa, “Wireless optical transmissions with white colored LED for wireless home links,” Proc. IEEE International Symposium on Personal, Indoor and Mobile Radio Communications, vol. 2, pp. 1325-1329, 2000. [3]  T. Komine and M. Nakagawa, “Fundamental analysis for visible light communication system using LED lights,” IEEE Trans. on consumer Electronics, vol. 50, pp. 100-107, 2004. [4]   T. Komine, J.H. Lee, S. Haruyama and M. Nakagawa, “Adaptive equalization system for visible light wireless communication utilizing multiple white LED lighting equipment,” IEEE Trans. on wireless communications, vol. 8, no. 6, pp. 1536-1276, 2009. [5]   H.L. Minh, et al, “100-Mb/s NRZ visible light communication using a post-equalized white LED,” IEEE Photonics Technology Letters, vol. 21, no. 15, August 1, 2009. [6]   F.R. Gfeller., and U. Bapst., “Wireless in-house data communication via diffuse infra-red radiation,” Proc. IEEE, Vol. 67, No. 11, Nov. 1979. [7]   D.R. Wisely, “A 1 Gbit/s Optical Wireless Tracked Architecture for ATM Delivery,” IEE Colloquium on Optical Free Space Communication Links, London, UK, 1996. [8]   T.David and V. Pramod, “Fundamentals of wireless communication,” pp. 120-126, Cambridge Press, 2005. [9] Luminit,  “Light Shaping Diffusers,” Available at: http://www.luminitco.com/files/u1/Technical_Data_Sheet_rev_9-28-10.pdf (Accessed: 15 Jan 2011). [10]   P.L. Eardley, D.R. Wisely, D. Wood and P. McKee, “Holograms for Optical Wireless LANs,” IEE Proc. Optoelectron., Vol. 143, No. 6, December 1996. [11]   M.Born and E. Wolf, “Principles of optics,” Pergamon Press, 1986. [12]   J.M. Kahn and J.R. Barry, “Wireless infrared communication,” Proc. IEEE, vol. 85, no. 2 pp. 265-298, 1997.  

100-Mb/s NRZ visible light communication using a post-equalized white LED,”

Adaptive equalization system for visible light wireless communication utilizing multiple white LED lighting equipment,”

Fundamental analysis for visible light communication system using

Fundamentals of wireless communication,”

Holograms for Optical Wireless LANs,”

Light Shaping Diffusers,” Available at: http://www.luminitco.com/files/u1/Technical_Data_Sheet_rev_9-28-10.pdf (Accessed: 15

Principles of optics,”

Transcending the replacement paradigm of solid-state lighting,”

Wireless in-house data communication via diffuse infra-red radiation,”

Wireless infrared communication,”

Wireless optical transmissions with white colored LED for wireless home links,”

Power distribution investigation of a hexagonal diffused cellular indoor visible light communications system

Power distribution investigation of a hexagonal diffused cellular indoor visible light communications system

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