Impact of background light induced shot noise in ... - OSA Publishing

Impact of background light induced shot noise in high-speed full-duplex indoor optical wireless communication systems Ke Wang,1,2,* Ampalavanapillai Nirmalathas,1,2 Christina Lim,2 and Efstratios Skafidas1,2 2

1 National ICT Australia-Victoria Research Laboratory (NICTA-VRL), Melbourne, VIC 3010, Australia Department of Electrical and Electronic Engineering, The University of Melbourne, Melbourne, VIC 3010, Australia *[email protected]

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 Optical Society of America OCIS codes: (060.0060) Fiber optics and optical communications; (060.2360) Fiber optics links and subsystems; (060.2605) Free-space optical communication.

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). 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). 3. J. M. Kahn and J. R. Barry, “Wireless infrared communications,” Proc. IEEE 85(2), 265–298 (1997). 4. D. C. O’Brien and M. Katz, “Optical wireless communications within fourth-generation wireless systems,” J. Opt. Netw. 4(6), 312–322 (2005). 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). 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). 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). 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). 11. S. T. Jovkova and M. Kavehard, “Multispot diffusing configuration for wireless infrared access,” IEEE Trans. Commun. 48(6), 970–978 (2000). 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).

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Received 27 Jul 2011; revised 15 Sep 2011; accepted 19 Sep 2011; published 12 Oct 2011

24 October 2011 / Vol. 19, No. 22 / OPTICS EXPRESS 21321

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

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 [1–4] 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 [5–7], the use of imaging receiver instead of non-imaging receiver [8], the multiple-transmitter technique such as the widely used line-strip multi-spot (LSMS) transmitter configuration [9–11], the adaptive power distribution technique [12] and the adaptive angle distribution technique [13]. 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]. However, the angle diversity receiver used is complicated since it requires three separate receiving elements and each of the

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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].

Fig. 1. System architecture.

To achieve higher communication speed, in previous studies we have proposed a novel indoor optical wireless communication system incorporating localization function and the basic architecture is shown in Fig. 1. In this system, the ceiling mounted fiber end serves as the transmitter so a separate laser is no longer needed. The fiber end is connected to a central office (CO) via an optical fiber distribution network and multiple rooms can be served by the same CO to share the cost. Furthermore, a steering mirror attached to the fiber end is used to change the orientation of light beam according to the users' location information and comparatively wider light beam is used to cover a certain service area surrounding that position. Therefore direct LOS link is available for high-speed data transmission and limited mobility can be provided. The users' location functionality can be achieved with our recently proposed novel optical wireless based localization system [17]. When the user moves out the area initially covered by signal light, which can be identified by the localization system, the steering mirror then dynamically guides the signal light to the new position. Therefore mobility can be provided to the users over the entire room. At the receiver end, the simplest single wide field-of-view (FOV) non-imaging receiver is used and this receiver is composed of a compound parabolic concentrator (CPC) followed by a low-cost PIN photodiode. Proofof-concept experiments have been carried out and up to 12.5 Gbps error-free data transmission has been successfully demonstrated [18]. In addition to the down-link system, we have also proposed an indoor optical wireless uplink system [16]. In the up-link system, since the localization information is also available, comparatively wider beam is used as well for limited mobility purpose. In addition, instead of direct detection, we proposed to couple the up-link signal back into the fiber and transmitted back to the CO for further signal processing. A coupling efficiency better than 20% can be easily achieved by using multiple lenses [19]. Such a centralized architecture can reduce the cost and complexity of the ceiling mounted fiber transceiver. We have also carried out experiments and simultaneous error-free transmission of a 10 Gbps down-link and a 500 Mbps up-link with a reasonable beam footprint has been demonstrated [20].

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However, to further understand the limiting factors and design rules of our proposed system, substantial theoretical study still needs to be done. From previous studies it was found that the dominant noise sources in indoor optical wireless communications system are the background light induced shot noise and the receiver preamplifier induced noise [3,19]. Different from the preamplifier induced noise, the noise due to background light is solely due to the free space transmission. As a comparison to the conventional optical fiber transmission systems, the optical wireless link will have poorer receiver sensitivity and increased power penalty as a result of this background noise. Therefore this noise is unique and needs a thorough investigation. To the best of our knowledge the impact of background noise on the optical-wireless system performance has never been studied. Therefore in this paper we propose a theoretical model that enables the receiver sensitivity and power penalty due to background noise in both up-link and down-link transmission to be estimated. This analysis is of great importance to understand the principle limitations of indoor OW communication system, for practical system design and for further system optimization. Experiments are carried out to verify our theoretical analysis and it is shown that the experimental results agree well with the theoretical prediction. Results indicate that the free space transmission generally induces a power penalty of several dB in our proposed system when the operation bit rate is low. As the bit rate increases, the power penalty due to free space transmission becomes smaller while the preamplifier induced noise increases and eventually dominates the noise process. 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] 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:

σ 0 = σ 1 = σ = σ pr + σ bn 2

2

2

2

2

(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]. 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]:

 4kT



4kT Γ

4kT Γ

σ pr2 =  + 2eI L  I 2 B + (2π CT ) 2 AF f c B 2 + (2π CT ) 2 I 3 B 3 gm gm  RF 

(2)

where B is the electrical bandwidth, AF is the weighting function and for the non-return-tozero (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

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simplicity, the FET gate leakage and 1/f noise can be neglected [23]. Therefore the preamplifier induced noise variance can be further simplified to:

σ pr = 2

4 kT

I2B +

4kT Γ

(2π CT ) I 3 B 2

3

(3) RF gm In addition to the preamplifier induced noise, the background light induced shot noise can be calculated by [23]:

σ bn2 = 2eRPbn I 2 B

(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] and the radiant intensity (W / Sr) is:

I (ϕ ) =

n +1 2π

× Pt × cos (ϕ ) n

(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:

n +1 × Plamp × cosn (ϕ i ) × B filter × Rreceiver i =1 2π 4

Pbn = ∑

(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]. 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]:

 R × ( Ps 1 − Ps 0 )  SNR =    σ 0 + 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: 2 R × ( Ps1 − Ps 0 )   R × ( Ps1 − Ps 0 )  SNR =   =  2σ 2σ pr    

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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:

Power − Penalty ( dB ) = 5 × log10

σ 2pr + σ bn2 σ 2pr

(9)

Based on Eqs. (1)–(9), the power penalty due to the background light induced noise with respect to the received background light power for down-link transmission can be estimated for transmission bit rates of 1 Gbps, 2.5 Gbps, 5 Gbps and 10 Gbps. The results are plotted in, Fig. 2. From Fig. 2 it is obvious that the power penalty increases with received background light, but decreases with bit rate for a fixed received background light power. According to Eq. (3), the preamplifier induced noise variance increases with the bandwidth of the system. Therefore, for higher speed systems, the preamplifier induced noise becomes larger and dominates over the background light induced noise.

Fig. 2. Simulation result of the power penalty due to the background light for 1Gbps, 2.5Gbps, 5Gbps and 10Gbps system.

Shown in Fig. 3 is the simulated power penalty due to background light induced noise plotted as a function of down-link bit rates for different received background light power (−34.5 dBm, −30 dBm and −27 dBm). As shown in [15], even when the user is directly under a strong background lamp, the received background light power is typically smaller than −27 dBm. Therefore from Fig. 3 it can be seen that the power penalty due to background light induced noise in our proposed system is always smaller than 4 dB for a bit rate of < 2 Gbps or smaller than 1.5 dB when the bit rate > 7 Gbps. Furthermore, when the overhead lamps are turned off and the room is illuminated with ambient light, the received background light power is typically less than −30 dBm [15]. In this case the power penalty is less than 2 dB in most scenarios. In addition, with advanced receivers such as the previously mentioned anglediversity receiver and imaging receiver that can reject most of the background light, the received background light power can be further reduced. Consequently the power penalty can be further reduced to < than 1 dB.

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Fig. 3. Simulated results of power penalty due to background light induced noise for different received background light power.

From the simulation results and discussions above, it is obvious that for lower bit rate (