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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 19, Iss. 22 — Oct. 24, 2011
  • pp: 21321–21332
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Impact of background light induced shot noise in high-speed full-duplex indoor optical wireless communication systems

Ke Wang, Ampalavanapillai Nirmalathas, Christina Lim, and Efstratios Skafidas  »View Author Affiliations


Optics Express, Vol. 19, Issue 22, pp. 21321-21332 (2011)
http://dx.doi.org/10.1364/OE.19.021321


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Abstract

The use of infrared radiation to provide high speed indoor wireless communication has attracted considerable attention for over a decade. In previous studies we proposed a novel full-duplex indoor optical wireless communication system with high-speed data transmission and limited mobility can be provided to users. When it is incorporated with localization function, gigabit mobile communication can be provided over the entire room. In this paper we theoretically analyze the limiting factor of our proposed system – background light induced shot noise. A theoretical model that allows the receiver sensitivity and the corresponding power penalty is proposed and the model is validated by experiments. Experimental results show that for both down-link and up-link transmission the background light will result in several dB power penalty and it is more dominant in lower speed links. As the bit rate increases, the preamplifier induced noise becomes larger and eventually dominates the noise process.

© 2011 OSA

1. Introduction

Indoor optical wireless communication systems have been widely studied for over a decade to provide high speed access to end users. The large unregulated bandwidth resource together with the desire for extremely high bit-rate transmission has fuelled the optical wireless communication technology. Another advantage of optical wireless technique is its immunity to electro-magnetic interference which enables it to be used in radio frequency (RF) hostile environments such as hospitals. Despite the numerous advantages, indoor optical wireless systems also have drawbacks such as the interference from strong background light and the limited transmission power due to laser eye and skin safety regulations [1

1. F. R. Gfeller and U. Bapst, “Wireless in-house data communication via diffuse infrared radiation,” Proc. IEEE 67(11), 1474–1486 (1979). [CrossRef]

4

4. D. C. O’Brien and M. Katz, “Optical wireless communications within fourth-generation wireless systems,” J. Opt. Netw. 4(6), 312–322 (2005). [CrossRef]

] that need to be addressed.

There are generally two kinds of indoor optical wireless communication systems, namely the direct line-of-sight (LOS) systems and the diffused beam systems. Compared with the conventional direct LOS system, the diffused beam systems do not require strict alignment between the transceivers so the users can move freely over the entire room. In addition, it is more robust to the physical shadowing which explains why almost all studies are focused on this scheme. The diffused beam systems on the other hand, are limited by multipath dispersion as a result of multiple diffusive reflections which in turn limits the maximum achievable bit rates. To overcome this, multiple advanced techniques have been proposed and demonstrated. These include the use of angle diversity receiver [5

5. G. Yun and M. Kavehrad, “Spot-diffusing and fly-eye receivers for indoor infrared wireless communications,” in Proceedings of IEEE International Conference on Selected Topics in Wireless Communications (London, 1992), pp. 262–265.

7

7. K. L. Sterckx, J. M. H. Elmirghani, and R. A. Cryan, “Pyramidal fly-eye detection antenna for optical wireless systems,” in Proceedings of IEE Colloquium on Optical Wireless Communications (London, 1999), pp. 1–5.

], the use of imaging receiver instead of non-imaging receiver [8

8. P. Djahani and J. M. Kahn, “Analysis of infrared wireless links employing multibeam transmitters and imaging diversity receivers,” IEEE Trans. Commun. 48(12), 2077–2088 (2000). [CrossRef]

], the multiple-transmitter technique such as the widely used line-strip multi-spot (LSMS) transmitter configuration [9

9. A. G. Al-Ghamdi and J. M. H. Elmirghani, “Spot diffusing technique and angle diversity performance for high speed indoor diffuse infra-red wireless transmission,” IEE Proc., Optoelectron. 151(1), 46–52 (2004). [CrossRef]

11

11. S. T. Jovkova and M. Kavehard, “Multispot diffusing configuration for wireless infrared access,” IEEE Trans. Commun. 48(6), 970–978 (2000). [CrossRef]

], the adaptive power distribution technique [12

12. F. E. Alsaadi and J. M. H. Elmirghani, “Mobile multigigabit indoor optical wireless systems employing multibeam power adaptation and imaging diversity receivers,” J. Opt. Commun. Netw. 3(1), 27–39 (2011). [CrossRef]

] and the adaptive angle distribution technique [13

13. F. E. Alsaadi and J. M. H. Elmirghani, “Performance evaluation of 2.5 Gbit/s and 5 Gbit/s optical wireless systems employing a two dimensional adaptive beam clustering method and imaging diversity detection,” IEEE J. Sel. Areas Comm. 27(8), 1507–1519 (2009). [CrossRef]

]. A remarkable error-free 1.25 Gbps indoor cellular optical wireless communication system with 1-D angle diversity receiver has been experimentally demonstrated recently [14

14. H. Le Minh, D. O’Brien, G. Faulkner, O. Bouchet, M. Wolf, L. Grobe, and J. Li, “A 1.25Gb/s Indoor Cellular Optical Wireless Communications Demonstrator,” IEEE Photon. Technol. Lett. 22(21), 1598–1600 (2010). [CrossRef]

]. However, the angle diversity receiver used is complicated since it requires three separate receiving elements and each of the elements requires a separate optical concentrator which makes the whole receiver bulky and costly. In addition, this system is based on diffused beam which will ultimately limit the bit rate and also mobility is only provided over a limited coverage area [15

15. J. Fadlullah and M. Kavehard, “Indoor high-bandwidth optical wireless links for sensor networks,” J. Lightwave Technol. 28(21), 3086–3094 (2010).

].

2. Theoretical analysis and simulations

As mentioned before the dominant noises in our system are the background light induced shot noise and the receiver preamplifier induced noise. The background light induced shot noise only exists in optical wireless communication systems because of the free space transmission. Here we propose a theoretical model to investigate the impact of background light noise.

In our proposed system, we use on-off-keying (OOK) modulation format and it is found in [21

21. J. B. Carruthers, “Multipath channels in wireless infrared communications: modeling, angle diversity and estimation,” Ph.D. dissertation (Univ. of California, Berkeley, 1997).

] that the signal dependent noise is very small and can be neglected. Therefore the noise variance σ02 and σ12 associated with the transmitted signal “0” and “1” are the same and can be given by:
σ02=σ12=σ2=σpr2+σbn2
(1)
where σpr2 represents the preamplifier induced noise variance component and σbn2 represents the background light induced shot noise variance. The preamplifier used in our system is a field-effect-transistor (FET) trans-impedance receiver proposed in [22

22. F. Alsaadi and J. M. H. Elmirghani, “Adaptive mobile line strip multibeam MC-CDMA optical wireless system employing imaging detection in a real indoor environment,” IEEE J. Sel. Areas Comm. 27(9), 1663–1675 (2009). [CrossRef]

]. The principle noise sources in this preamplifier are Johnson noise associated with the FET channel conductance, Johnson noise from the load or feedback resistor, shot noise arising from gate leakage current and 1/f noise. The preamplifier shot noise variance is then given by [23

23. B. Leskovar, “Optical receivers for wide band data transmission systems,” IEEE Trans. Nucl. Sci. 36(1), 787–793 (1989). [CrossRef]

]:
σpr2=(4kTRF+2eIL)I2B+4kTΓgm(2πCT)2AFfcB2+4kTΓgm(2πCT)2I3B3
(2)
where B is the electrical bandwidth, AF is the weighting function and for the non-return-to-zero (NRZ) coding format AF = 0.184, IL is the total leakage current (FET gate current and dark current of photodiode), gm is the FET trans-conductance, Γ is a noise factor associated with channel thermal noise and gate induced noise in the FET, CT is the total input capacitance consisting of photodiode and stray capacitance, fc is the 1/f corner frequency of the FET, I2 and I3 are the weighting functions which are dependent only on the input optical pulse shape to the receiver and the equalized output pulse shape, RF is the feedback resistance, k is the Boltzmann’s constant, T is the absolute temperature, and e is the electron charge. For simplicity, the FET gate leakage and 1/f noise can be neglected [23

23. B. Leskovar, “Optical receivers for wide band data transmission systems,” IEEE Trans. Nucl. Sci. 36(1), 787–793 (1989). [CrossRef]

]. Therefore the preamplifier induced noise variance can be further simplified to:

σpr2=4kTRFI2B+4kTΓgm(2πCT)2I3B3
(3)

In addition to the preamplifier induced noise, the background light induced shot noise can be calculated by [23

23. B. Leskovar, “Optical receivers for wide band data transmission systems,” IEEE Trans. Nucl. Sci. 36(1), 787–793 (1989). [CrossRef]

]:
σbn2=2eRPbnI2B
(4)
where R is the photodiode responsivity (R is supposed to be 0.8 A/W) and Pbn is the received background light power. This background light originates from the lamps within the room and here we assume four 100 W tungsten floodlights to create a well-illuminated environment. These lamps can be modeled as a generalized Lambertian source [21

21. J. B. Carruthers, “Multipath channels in wireless infrared communications: modeling, angle diversity and estimation,” Ph.D. dissertation (Univ. of California, Berkeley, 1997).

] and the radiant intensity (W / Sr) is:
I(φ)=n+12π×Pt×cosn(φ)
(5)
where Pt is the total transmitted optical power radiated by the lamp, φ is the angle of incidence with respect to the transmitter’s surface normal, and n is the mode number describing the shape of the transmitted beam. In our system, the lamp has a mode n = 2.0 and an optical spectral density of Plamp = 0.037 W/nm. To reduce the received background light power, an optical band-pass filter with a bandwidth of Bfilter = 30 nm based on thin film is utilized in front of the concentrator at the receiver end. Therefore, the received background light power in Eq. (4) is given by:
Pbn=i=14n+12π×Plamp×cosn(φi)×Bfilter×Rreceiver
(6)
where Rreceiver is the receiver area.

In many houses, florescent lamps are widely used and this type of lamps can also be modelled as a Lambertian source [21

21. J. B. Carruthers, “Multipath channels in wireless infrared communications: modeling, angle diversity and estimation,” Ph.D. dissertation (Univ. of California, Berkeley, 1997).

]. However, the mode number associated is n = 31. Therefore the optical power is more evenly distributed over the entire room and at positions directly under the lamps, smaller power will be collected by the receiver in comparison to the cases of tungsten floodlights being used. Consequently, the impact of background light from fluorescent lamps is less pronounced and we only considered tungsten floodlights for the worst case scenario in this paper.

The system performance can be quantified by the received signal to noise ratio (SNR). The SNR for OOK modulation format is defined as [23

23. B. Leskovar, “Optical receivers for wide band data transmission systems,” IEEE Trans. Nucl. Sci. 36(1), 787–793 (1989). [CrossRef]

]:
SNR=(R×(Ps1Ps0)σ0+σ1)2
(7)
where Ps0 and Ps1 are the powers associated with signal “0” and “1” respectively, and Ps0- Ps1 accounts for the eye opening at the sampling instant. For the system without free space transmission (optical fiber communication system), there is no background light induced noise so the SNR can be estimated to be:

SNR=(R×(Ps1Ps0)2σ)2=(R×(Ps1Ps0)2σpr)2
(8)

To achieve the same SNR in the system with free space transmission, a larger received power is required as a result of the additional background light induced noise. Here we define the difference in the required received optical power as the power penalty due to background light induced noise and it can be calculated from Eq. (8) as follows:

PowerPenalty(dB)=5×log10σpr2+σbn2σpr2
(9)

3. Experiments and discussions

In the experiments, the wavelength used is 1550.12nm and we change the transmitted optical power to measure the receiver sensitivity. The receiver sensitivity with and without free space transmission are measured. For the experiment without free space transmission, the transmitter and receiver ends are directly connected with a fiber. The receiver sensitivity of our system is defined as the minimum average received power for a specific bit rate required by the receiver to achieve an error-free operation and the error-free operation is set as a bit-error-rate < 10−9. The experimental results for receiver sensitivity as a function of transmission bit-rates are shown in Fig. 5
Fig. 5 Experimental results on receiver sensitivity when the free space transmission distances are 63cm, 104cm and 243cm respectively.
and Fig. 6
Fig. 6 Experimental results on receiver sensitivity with and without free space transmission.
.

In the experiment, the overhead lamps are turned on and the received background light power is measured to be −27.33 dBm. This power is measured with an optical power-meter with a free space detection head just after the CPC when the transmission signal is turned off. The photo-sensitive area of the detection head is similar to the exit area of the CPC. Therefore almost all the background light collected by CPC can be measured. In Fig. 5 we measured the receiver sensitivity when the free space transmission distances are 63 cm, 104 cm and 243 cm respectively. The different distances between the transmitter and receiver are achieved by moving the transmitter end since we want to fix the receiver at the same position to keep the received background light power constant for all scenarios and the coupling system in the receiver is sensitive to any movement. From the measured results it can be seen that the receiver sensitivity does not depend on the free space transmission distance. This is because the 1550nm band is one the atmospheric transmission windows which has a typical propagation loss of ~3-5dB/km in clear weather conditions. Therefore for indoor scenarios with a transmission distance of only several meters, the propagation loss is negligible. Furthermore, for a higher speed system, the receiver requires higher received power for error-free operation. This is due to the higher preamplifier induced noise which increases with the communication bandwidth while the background light induced noise remains almost constant.

Shown in Fig. 6 are the experimental results of receiver sensitivity of the systems with and without free space transmission. The free space transmission distance is fixed at 243 cm for the system with free space transmission. We can see that the system without free space transmission always has better receiver sensitivity than the system with free space transmission. The difference between the receiver sensitivities can be attributed to the background light induced noise. Furthermore, it can also be seen that the power penalty due to background noise reduces with increasing bit rate. This can be attributed to the fact that for a higher speed system, the pre-amplifier induced noise becomes larger according to Eq. (3) while the background light induced noise remains almost constant. Therefore, according to Eq. (9) the power penalty due to received background light decreases with bit rate.

Figure 7
Fig. 7 Experimental and simulation results of power penalty due to background light induced noise when the overhead lamps are turned on.
shows the experimental results of the measured power penalty due to background light induced noise as a function of transmission bit-rates. In this investigation the overhead lamps were turned on and the received background light was measured to be −27.33 dBm. The simulation results are also shown in Fig. 7. It can be seen that the simulation results agree well with the experimental ones and the difference is well within 0.4 dB. In addition, we can see that when the bit rate is higher the power penalty due to background light is smaller and this is consistent with the receiver sensitivity results. This reduction is again due to the increased preamplifier noise as discussed before.

When the overhead lamps are turned off, we have also measured the receiver sensitivity and power penalty. The results are shown in Fig. 8
Fig. 8 Experimental results of receiver sensitivity with and without free space transmission. The results when the overhead lamps are turned on and off are both shown.
and Fig. 9
Fig. 9 Experimental and simulation results of power penalty due to background light induced noise when the overhead lamps are turned off.
, respectively. The theoretical results in this case are also shown. When the lamps are turned off the received background light power is measured to be −34.5 dBm. It is clear from Fig. 8 that without the overhead lamps, the system has better receiver sensitivity and comparable to that without free space transmission. Figure 9 shows the power penalty measurements as a function of bit-rates. The theoretical results of the power penalty again agree well with the experimental ones. Without the overhead lamps, the power penalty is always smaller than 1 dB compared to when the lamps are turned on as plotted in Fig. 7.

4. Conclusion

It should be noted that in typical outdoor OW systems, the free space propagation loss is considerably important and is one of the system performance limiting factors. However, in our proposed indoor OW system the performance is almost independent of the transmission distance. This is mainly due to the more stable environmental condition in the indoor system which results in a smaller propagation loss and is further improved by the short propagation distance of a few meters to tens of meters. Therefore the transmission loss is always <0.1dB and the system performance is almost independent on the transmission distance.

Acknowledgments

This work was supported in part by NICTA. NICTA is funded by the Australian Government as represented by the Department of Broadband, Communications and the Digital Economy and the Australian Research Council through the ICT Centre of Excellence program.

References and links

1.

F. R. Gfeller and U. Bapst, “Wireless in-house data communication via diffuse infrared radiation,” Proc. IEEE 67(11), 1474–1486 (1979). [CrossRef]

2.

J. M. Kahn, J. R. Barry, M. D. Audeh, J. B. Carruthers, W. J. Krause, and G. W. Marsh, “Non-directed infrared links for high-capacity wireless LANs,” IEEE Personal Commun. 1(2), 12–25 (1994). [CrossRef]

3.

J. M. Kahn and J. R. Barry, “Wireless infrared communications,” Proc. IEEE 85(2), 265–298 (1997). [CrossRef]

4.

D. C. O’Brien and M. Katz, “Optical wireless communications within fourth-generation wireless systems,” J. Opt. Netw. 4(6), 312–322 (2005). [CrossRef]

5.

G. Yun and M. Kavehrad, “Spot-diffusing and fly-eye receivers for indoor infrared wireless communications,” in Proceedings of IEEE International Conference on Selected Topics in Wireless Communications (London, 1992), pp. 262–265.

6.

J. B. Carruther and J. M. Kahn, “Angle diversity for nondirected wireless infrared communication,” IEEE Trans. Commun. 48(6), 960–969 (2000). [CrossRef]

7.

K. L. Sterckx, J. M. H. Elmirghani, and R. A. Cryan, “Pyramidal fly-eye detection antenna for optical wireless systems,” in Proceedings of IEE Colloquium on Optical Wireless Communications (London, 1999), pp. 1–5.

8.

P. Djahani and J. M. Kahn, “Analysis of infrared wireless links employing multibeam transmitters and imaging diversity receivers,” IEEE Trans. Commun. 48(12), 2077–2088 (2000). [CrossRef]

9.

A. G. Al-Ghamdi and J. M. H. Elmirghani, “Spot diffusing technique and angle diversity performance for high speed indoor diffuse infra-red wireless transmission,” IEE Proc., Optoelectron. 151(1), 46–52 (2004). [CrossRef]

10.

A. G. Al-Ghamdi and J. M. H. Elmirghani, “Line strip spot-diffusing transmitter configuration for optical wireless systems influenced by background noise and multipath dispersion,” IEEE Trans. Commun. 52(1), 37–45 (2004). [CrossRef]

11.

S. T. Jovkova and M. Kavehard, “Multispot diffusing configuration for wireless infrared access,” IEEE Trans. Commun. 48(6), 970–978 (2000). [CrossRef]

12.

F. E. Alsaadi and J. M. H. Elmirghani, “Mobile multigigabit indoor optical wireless systems employing multibeam power adaptation and imaging diversity receivers,” J. Opt. Commun. Netw. 3(1), 27–39 (2011). [CrossRef]

13.

F. E. Alsaadi and J. M. H. Elmirghani, “Performance evaluation of 2.5 Gbit/s and 5 Gbit/s optical wireless systems employing a two dimensional adaptive beam clustering method and imaging diversity detection,” IEEE J. Sel. Areas Comm. 27(8), 1507–1519 (2009). [CrossRef]

14.

H. Le Minh, D. O’Brien, G. Faulkner, O. Bouchet, M. Wolf, L. Grobe, and J. Li, “A 1.25Gb/s Indoor Cellular Optical Wireless Communications Demonstrator,” IEEE Photon. Technol. Lett. 22(21), 1598–1600 (2010). [CrossRef]

15.

J. Fadlullah and M. Kavehard, “Indoor high-bandwidth optical wireless links for sensor networks,” J. Lightwave Technol. 28(21), 3086–3094 (2010).

16.

K. Wang, A. Nirmalathas, C. Lim, and E. Skafidas, “High-speed duplex optical wireless communication system for indoor personal area networks,” Opt. Express 18(24), 25199–25216 (2010). [CrossRef] [PubMed]

17.

K. Wang, A. Nirmalathas, C. Lim, and E. Skafidas, “High-speed 4×12.5Gbps WDM optical wireless communication systems for indoor applications,” in Proceedings of Optical Fiber Communication Conference and Exposition and the National Fiber Optic Engineers Conference (OFC/NFOEC, Los Angeles, 2011), pp. JWA081.

18.

K. Wang, A. Nirmalathas, C. Lim, and E. Skafidas, “High-speed optical wireless communication system for indoor applications,” IEEE Photon. Technol. Lett. 23(8), 519–521 (2011). [CrossRef]

19.

P. J. Winzer and W. R. Leeb, “Fiber coupling efficiency for random light and its applications to lidar,” Opt. Lett. 23(13), 986–988 (1998). [CrossRef] [PubMed]

20.

K. Wang, A. Nirmalathas, C. Lim, and E. Skafidas, “High-speed full-duplex optical wireless communication system for indoor applications,” in Proceedings of Conference of Lasers and Opto-Electronics (CLEO, Baltimore, 2011), pp. CFH6.

21.

J. B. Carruthers, “Multipath channels in wireless infrared communications: modeling, angle diversity and estimation,” Ph.D. dissertation (Univ. of California, Berkeley, 1997).

22.

F. Alsaadi and J. M. H. Elmirghani, “Adaptive mobile line strip multibeam MC-CDMA optical wireless system employing imaging detection in a real indoor environment,” IEEE J. Sel. Areas Comm. 27(9), 1663–1675 (2009). [CrossRef]

23.

B. Leskovar, “Optical receivers for wide band data transmission systems,” IEEE Trans. Nucl. Sci. 36(1), 787–793 (1989). [CrossRef]

24.

K. Wang, A. Nirmalathas, C. Lim, and E. Skafidas, “Gigabit optical wireless communication system for indoor applications,” in Proceedings of Asia Communication and Photonics Conference and Exhibition (ACP, Shanghai, 2010), pp. 453–454.

25.

K. Wang, A. Nirmalathas, C. Lim, and E. Skafidas, “Indoor gigabit optical wireless communication system for personal area networks,” in Proceedings of 23rd IEEE Photonics Society Annual Meeting (Denver, 2010), pp. 224–225.

OCIS Codes
(060.0060) Fiber optics and optical communications : Fiber optics and optical communications
(060.2360) Fiber optics and optical communications : Fiber optics links and subsystems
(060.2605) Fiber optics and optical communications : Free-space optical communication

ToC Category:
Fiber Optics and Optical Communications

History
Original Manuscript: July 27, 2011
Revised Manuscript: September 15, 2011
Manuscript Accepted: September 19, 2011
Published: October 12, 2011

Citation
Ke Wang, Ampalavanapillai Nirmalathas, Christina Lim, and Efstratios Skafidas, "Impact of background light induced shot noise in high-speed full-duplex indoor optical wireless communication systems," Opt. Express 19, 21321-21332 (2011)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-19-22-21321


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References

  1. F. R. Gfeller and U. Bapst, “Wireless in-house data communication via diffuse infrared radiation,” Proc. IEEE67(11), 1474–1486 (1979). [CrossRef]
  2. J. M. Kahn, J. R. Barry, M. D. Audeh, J. B. Carruthers, W. J. Krause, and G. W. Marsh, “Non-directed infrared links for high-capacity wireless LANs,” IEEE Personal Commun.1(2), 12–25 (1994). [CrossRef]
  3. J. M. Kahn and J. R. Barry, “Wireless infrared communications,” Proc. IEEE85(2), 265–298 (1997). [CrossRef]
  4. D. C. O’Brien and M. Katz, “Optical wireless communications within fourth-generation wireless systems,” J. Opt. Netw.4(6), 312–322 (2005). [CrossRef]
  5. G. Yun and M. Kavehrad, “Spot-diffusing and fly-eye receivers for indoor infrared wireless communications,” in Proceedings of IEEE International Conference on Selected Topics in Wireless Communications (London, 1992), pp. 262–265.
  6. J. B. Carruther and J. M. Kahn, “Angle diversity for nondirected wireless infrared communication,” IEEE Trans. Commun.48(6), 960–969 (2000). [CrossRef]
  7. K. L. Sterckx, J. M. H. Elmirghani, and R. A. Cryan, “Pyramidal fly-eye detection antenna for optical wireless systems,” in Proceedings of IEE Colloquium on Optical Wireless Communications (London, 1999), pp. 1–5.
  8. P. Djahani and J. M. Kahn, “Analysis of infrared wireless links employing multibeam transmitters and imaging diversity receivers,” IEEE Trans. Commun.48(12), 2077–2088 (2000). [CrossRef]
  9. A. G. Al-Ghamdi and J. M. H. Elmirghani, “Spot diffusing technique and angle diversity performance for high speed indoor diffuse infra-red wireless transmission,” IEE Proc., Optoelectron.151(1), 46–52 (2004). [CrossRef]
  10. A. G. Al-Ghamdi and J. M. H. Elmirghani, “Line strip spot-diffusing transmitter configuration for optical wireless systems influenced by background noise and multipath dispersion,” IEEE Trans. Commun.52(1), 37–45 (2004). [CrossRef]
  11. S. T. Jovkova and M. Kavehard, “Multispot diffusing configuration for wireless infrared access,” IEEE Trans. Commun.48(6), 970–978 (2000). [CrossRef]
  12. F. E. Alsaadi and J. M. H. Elmirghani, “Mobile multigigabit indoor optical wireless systems employing multibeam power adaptation and imaging diversity receivers,” J. Opt. Commun. Netw.3(1), 27–39 (2011). [CrossRef]
  13. F. E. Alsaadi and J. M. H. Elmirghani, “Performance evaluation of 2.5 Gbit/s and 5 Gbit/s optical wireless systems employing a two dimensional adaptive beam clustering method and imaging diversity detection,” IEEE J. Sel. Areas Comm.27(8), 1507–1519 (2009). [CrossRef]
  14. H. Le Minh, D. O’Brien, G. Faulkner, O. Bouchet, M. Wolf, L. Grobe, and J. Li, “A 1.25Gb/s Indoor Cellular Optical Wireless Communications Demonstrator,” IEEE Photon. Technol. Lett.22(21), 1598–1600 (2010). [CrossRef]
  15. J. Fadlullah and M. Kavehard, “Indoor high-bandwidth optical wireless links for sensor networks,” J. Lightwave Technol.28(21), 3086–3094 (2010).
  16. K. Wang, A. Nirmalathas, C. Lim, and E. Skafidas, “High-speed duplex optical wireless communication system for indoor personal area networks,” Opt. Express18(24), 25199–25216 (2010). [CrossRef] [PubMed]
  17. K. Wang, A. Nirmalathas, C. Lim, and E. Skafidas, “High-speed 4×12.5Gbps WDM optical wireless communication systems for indoor applications,” in Proceedings of Optical Fiber Communication Conference and Exposition and the National Fiber Optic Engineers Conference (OFC/NFOEC, Los Angeles, 2011), pp. JWA081.
  18. K. Wang, A. Nirmalathas, C. Lim, and E. Skafidas, “High-speed optical wireless communication system for indoor applications,” IEEE Photon. Technol. Lett.23(8), 519–521 (2011). [CrossRef]
  19. P. J. Winzer and W. R. Leeb, “Fiber coupling efficiency for random light and its applications to lidar,” Opt. Lett.23(13), 986–988 (1998). [CrossRef] [PubMed]
  20. K. Wang, A. Nirmalathas, C. Lim, and E. Skafidas, “High-speed full-duplex optical wireless communication system for indoor applications,” in Proceedings of Conference of Lasers and Opto-Electronics (CLEO, Baltimore, 2011), pp. CFH6.
  21. J. B. Carruthers, “Multipath channels in wireless infrared communications: modeling, angle diversity and estimation,” Ph.D. dissertation (Univ. of California, Berkeley, 1997).
  22. F. Alsaadi and J. M. H. Elmirghani, “Adaptive mobile line strip multibeam MC-CDMA optical wireless system employing imaging detection in a real indoor environment,” IEEE J. Sel. Areas Comm.27(9), 1663–1675 (2009). [CrossRef]
  23. B. Leskovar, “Optical receivers for wide band data transmission systems,” IEEE Trans. Nucl. Sci.36(1), 787–793 (1989). [CrossRef]
  24. K. Wang, A. Nirmalathas, C. Lim, and E. Skafidas, “Gigabit optical wireless communication system for indoor applications,” in Proceedings of Asia Communication and Photonics Conference and Exhibition (ACP, Shanghai, 2010), pp. 453–454.
  25. K. Wang, A. Nirmalathas, C. Lim, and E. Skafidas, “Indoor gigabit optical wireless communication system for personal area networks,” in Proceedings of 23rd IEEE Photonics Society Annual Meeting (Denver, 2010), pp. 224–225.

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