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
*KeDesmond.Wang@nicta.com.au
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).
#151771 - $15.00 USD
(C) 2011 OSA
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
#151771 - $15.00 USD
(C) 2011 OSA
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 21322
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].
#151771 - $15.00 USD
(C) 2011 OSA
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 21323
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
#151771 - $15.00 USD
(C) 2011 OSA
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 21324
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

 

#151771 - $15.00 USD
(C) 2011 OSA
2
(8)
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 21325
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.
#151771 - $15.00 USD
(C) 2011 OSA
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 21326
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 (<5
Gbps) down-link system, the noise is dominated by the background light induced noise.
However, the power penalty is typically <4 dB so high-speed indoor optical wireless
communication systems are still feasible with proper power budget.
Fig. 4. Experimental setup. The CPC here at the receiver end stands for compound parabolic
concentrator.
3. Experiments and discussions
We have carried out experiments to verify the theoretical model discussed in the previous
section. The experimental setup is shown in Fig. 4 which is similar to the setup used in our
previous studies [24,25]. In the system we proposed to use the CO/access points (APs)
structure which is widely used in the integrated fiber-wireless communication systems and
such a structure can be easily connected to the existing backbone networks. The signal is
#151771 - $15.00 USD
(C) 2011 OSA
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 21327
modulated with a MZM in the CO (induces ~3.6dB loss) before being fed from the CO to the
APs via an optical fiber distribution network which is emulated by a 5.6 km standard singlemode fiber (SMF) in the experiment. This fiber distribution network induces an extra ~1.3dB
loss. In a typical access network scenario, the distance between CO and AP may be up to 1020km. However, our system is used for indoor applications, so the fiber distribution network
is usually an in-building network which only caters for at most a few kilometers. Therefore, in
the experiment we only use 5.6km fiber to emulate this in-building network. Then in the AP
the signal goes through a lens to increase its divergence before propagating in the free space.
At the receiver end, the signal together with background light is captured by a CPC with thin
film bandpass filter. Then the light is coupled onto a photodiode (PD) with a small photosensitive area with a coupling system consisting of multiple lenses and a fiber collimator.
Each of the lenses as well as the fiber collimator is mounted on a triple divide XYZ axis
translation stage for fine tuning and a coupling loss smaller than 0.5 dB has been achieved.
Although using a PD with a large photo-sensitive area just after the CPC can simplify the
system structure, the bandwidth of such a PD is currently limited to low bit rates. After
detection the signal is amplified, characterized and measured with a 12.5 Gbps bit-error-rate
tester (BERT) and a wideband digital communication analyzer (DCA).
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 biterror-rate < 10−9. The experimental results for receiver sensitivity as a function of
transmission bit-rates are shown in Fig. 5 and Fig. 6.
Fig. 5. Experimental results on receiver sensitivity when the free space transmission distances
are 63cm, 104cm and 243cm respectively.
#151771 - $15.00 USD
(C) 2011 OSA
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 21328
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 errorfree 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.
#151771 - $15.00 USD
(C) 2011 OSA
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 21329
Fig. 7. Experimental and simulation results of power penalty due to background light induced
noise when the overhead lamps are turned on.
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.
Figure 7 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 and Fig. 9, 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.
#151771 - $15.00 USD
(C) 2011 OSA
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 21330
Fig. 9. Experimental and simulation results of power penalty due to background light induced
noise when the overhead lamps are turned off.
Fig. 10. Up-link experimental results on receiver sensitivity when the free space transmission
distances are 63cm, 104cm and 243cm respectively.
In addition to the down-link experiments, we have also experimentally studied the receiver
sensitivity and power penalty due to the background light induced noise in the up-link system.
The experimental setup is the same to that used in [20]. A laser diode is directly modulated by
up-link data and at the receiver end the signal is coupled into the fiber by multiple lenses.
Similar to the down-link experiments, we have also measured the receiver sensitivity when
the free space transmission distances were 63 cm, 104 cm and 243 cm, respectively. Since the
up-link bit rate does not need to be very high, in the experiments the investigated bit rate
ranges from 100 Mbps to 500 Mbps. The results are shown in Fig. 10. It is clear that the free
space transmission distance has little impact on the receiver performance.
The power penalty in the up-link due to free space transmission is shown in Fig. 11. The
measurements were carried out with and without the overhead lamps. The simulation results
are also shown. When the lamps are turned on, the background light power coupled into the
fiber is ~-33.4 dBm and when the lamps are off, this power is ~-37.6 dBm. It can be seen from
Fig. 11 that the theoretical results agree well with the experimental results and the power
penalty due to free space transmission decreases with increasing bit rate. This trend is the
same as that in the down-link transmission and is attributed to the larger pre-amplifier induced
noise in the higher speed system.
#151771 - $15.00 USD
(C) 2011 OSA
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 21331
Fig. 11. Experimental and simulation results of power penalty due to background light induced
noise in the up-link system.
4. Conclusion
In this paper we theoretically studied the receiver sensitivity and power penalty due to free
space transmission for indoor OW communication systems. With free space transmission,
background light will be collected by the receiver and this introduces shot noise into the
system. A theoretical model has been devised to assess this impact and the theoretical results
were verified experimentally. It is shown that free space transmission distance has no impact
on the link power penalty and the experimental results agree well with the theoretical
calculations. Therefore link budget of the OW system can be calculated with this theoretical
model to facilitate practical system design.
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.
#151771 - $15.00 USD
(C) 2011 OSA
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 21332