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Experimental analysis of a triple-hop relay-assisted FSO system with turbulence
Author’s Accepted Manuscript Experimental Analysis of a Triple-Hop RelayAssisted FSO System with Turbulence Norhanis Aida Mohd Nor, Zabih Ghassemlooy, Stanislav Zvanovec, Mohammad-Ali Khalighi, Manav R. Bhatnagar, Jan Bohata, Matej Komanec www.elsevier.com/locate/osn
PII: DOI: Reference:
S1573-4277(17)30158-3 https://doi.org/10.1016/j.osn.2017.11.002 OSN462
To appear in: Optical Switching and Networking Received date: 19 July 2017 Revised date: 16 October 2017 Accepted date: 6 November 2017 Cite this article as: Norhanis Aida Mohd Nor, Zabih Ghassemlooy, Stanislav Zvanovec, Mohammad-Ali Khalighi, Manav R. Bhatnagar, Jan Bohata and Matej Komanec, Experimental Analysis of a Triple-Hop Relay-Assisted FSO System with Turbulence, Optical Switching and Networking, https://doi.org/10.1016/j.osn.2017.11.002 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Experimental Analysis of a Triple-Hop RelayAssisted FSO System with Turbulence Norhanis Aida Mohd Nora*, Zabih Ghassemlooya,e*, Stanislav Zvanovecb, Mohammad-Ali Khalighic, Manav R. Bhatnagard, Jan Bohatab, Matej Komanecb a
Optical Communications Research Group, NCRLab, Faculty of Engineering and Environment, Northumbria University, Newcastle Upon Tyne, UK b Department of Electromagnetic Fields, Faculty of Electrical Engineering, Czech Technical University in Prague, Prague, Czech Republic c Institut Fresnel, UMR CNRS 7249, Ecole Centrale Marseille, Marseille, France d Department of Electrical Engineering, Indian Institute of Technology Delhi, Hauz Khan, IN-110016 New Delhi, India e QIEM, Haixi Institutes, Chinese Academy of Sciences, China *Corresponding authors: Norhanis Aida Mohd Nor, and Zabih Ghassemlooy
Abstract This paper outlines experimental investigation of the performance of the wireless optical network based on an all-optical triple-hop free space optical (FSO) communications employing amplify-and-forward relaying under the influence of atmospheric turbulence. We present new results for the bit error rate (BER) performance for seven possible turbulence network scenarios for relay-assisted FSO link and validate them with numerical simulations based on Gamma-Gamma turbulence model showing a good agreement between them. We also show results, which elucidate the impact of non-homogeneous turbulence along the entire transmission link span for the multiple-hop relay-assisted FSO system. More specifically we show that the BER performance considerably deteriorates for the case where turbulence is near to the receiver end. We outline that for a target BER of 10-4 the signal-tonoise ratio penalty can be as high as 9 dB compared to the case with no turbulence.
Keywords: All-optical relaying, atmospheric turbulence, BER, free space optics, multi-hop FSO network, relay-assisted FSO.
1. Introduction The rapid increase in the number of internet and mobile users together with higher data-rates demanding applications have necessitated the quest for alternative complementary wireless network technologies in order to circumvent the spectrum congestion problem currently being experienced in radio frequency (RF) based systems [1]. One possible viable solution is the free space optical (FSO) communications technology, which offers several advantages such as higher bandwidth, no RF induced electromagnetic interference, license free, low cost of installation and maintenance, and inherently secure due to a highly directional and narrow beam profile [2]. FSO systems used in the last mile access networks offer high data rates, with typical commercial products supporting up to 10 Gbps over a link span of few kilometres [3], increasing to a data rate of 1.6 Tbps over a much short link span of less than a few hundred meters using the wavelength division multiplexing technique in research laboratories [4]. However, the FSO technologies face a number of challenges including (i) the link availability under all weather conditions i.e., fog, rain, smoke, smog in big cities, and turbulence [5], and (ii) longer transmission spans in urban areas. Within this context, the relay-assisted FSO networks can address these challenges even under the influence of the diverse atmospheric channel conditions by extending the link span, ensuring higher link availability (i.e., 99.999 %) and improving performance (small-scale fading statistics) using a number of relays in place of a single long range span. A number of researchers have reported the reliability of relay-assisted FSO systems to broaden the coverage areas as well as to reduce the turbulence induced fading effects. Performance analyses for amplify-and-forward (AF) and decode-and-forward (DF) parallel or
serial relay-assisted FSO schemes were reported in [6] and [7]. More specifically, in [8] it was shown that relay-assisted transmission allows an increase in the power margin by ~19 dB and ~25 dB when using one and two (serial) relays between the source and destination, respectively, assuming a refractive index structure parameter
m-2/3 and a total
link span of 5 km in weak turbulence. In [9], a practically interesting approach
was
introduced, which employed an all-optical relay, that resulted in reduced delay latency and with significant simplification of the opto-electonic devices and signal processing at the relays. The bit error rate (BER) analysis of all-optical AF and DF relaying systems over lognormal fading was investigated in [9] using Monte Carlo simulations. It was shown that for a dual-hop system at a target BER of 10-5,
m-2/3, and a link span of 1 km, the total
transmission span can be increased by 0.9 km and 1.9 km using AF and DF schemes, respectively. In [10], a comprehensive and accurate outage probability Pout analysis of practical and exact models for dual- and multi-hop systems based on all-optical dual-hop AF relaying employing the erbium-doped fiber amplifier (EDFA) was presented showing Pout of 10-6 for
m-2/3 over a link span of 5 km. This configuration was further
analyzed in [11] for a dual-hop AF relaying system considering the effect of amplified spontaneous emission (ASE) and optical degree of freedom (DoF) of EDFA. It was shown that for a target Pout of 10-6, the performance gain (with respect to the direct link) reduced from 14.4 to 10.3 dB when DoF was increased from 10 to 1000 for
m-2/3
compared to a direct transmission at a 1550 nm wavelength over a link span of 10 km with no turbulence. Note that, all the analyses in [9], [10], and [11] were carried out the weak turbulence regime.
In [12], the detailed outage probability analysis of the all-optical AF
relaying system with subcarrier intensity modulation (SIM) based all-optical FSO networks
over Gamma-Gamma fading channel was reported where both the variable AF gain and the pointing error effect were considered. Recently a hybrid RF-FSO scheme was considered to further enhance the system performance when adopted between source-to-relay and from relay-to-destination, respectively [6], [13]. However, almost all works reported on this topic are theoretical in nature with very little or no experimental verifications. In [14], we experimentally investigated the performance of a dual-hop 10 Gbps FSO with turbulence using all-optical switching in the relay terminal. More recently, in [15] we, for the first time, compared the BER performance of a triple-hop AF relaying FSO link with both direct and dual-hop links. Nevertheless, in these works we considered identical turbulence conditions over the entire hops. However in real scenarios, it is highly probable that each hop will experience different turbulence regimes. This is because turbulence can be localized within a small area due to the different thermal flows in close vicinity of buildings, in open and semi-open urban and suburban areas, heating from air conditioning units etc. Therefore, in this paper we considerably extend our previous investigations by considering each hop as part of the longer transmission span in an optical wireless based network, which experiences different turbulence regimes, and consequently evaluate the system performance by means of experimental investigation and numerical simulations. To the best of our knowledge, such an investigation has not been reported in the literature before. The remainder of the paper is organized as follows: Section 2 describes the system and turbulence channel model, Section 3 enlightens the experimental campaign, then experimental results are explained in Section 4 and finally Section 5 concludes the paper.
2. System and Channel Models 2.1 System Model
The proposed all-optical AF relay-based system includes at least one intermediate terminal (the relaying node) between the source S and destination D, which amplifies and retransmits the optical signal. Let us denote the number of relay terminals between S and D by ( ),
M (i.e., R1, R2, …., RM) and the transmitted signal from the mth relay by {
}. The mth hop represents the channel between the mth and the (m+1)th relays. For ,
( ) denotes the transmitted signal from S, whereas the received signal at
can be
expressed as [9]: ( ) where
( )
( ) represents the received noise at
background radiations, and
( ),
(1)
, which is assumed to be mainly due to the
is the channel loss from (m-1)th to the mth terminal, which is
defined in terms of both the atmospheric attenuation
and turbulence
and is given by
[8]: (2)
.
Following amplification at the mth relay node within network, the re-transmitted signal is given as [9]: ( ) where and
( )
( ),
(3)
( ) is the ASE noise of the EDFA (modeled as an additive Gaussian white noise), is the gain of the mth optical amplifier. In most previous works reported on the relay-
assisted FSO networks, Gm was assumed to be fixed [9]. However, in this work we define it for the mth optical amplifier relay (m = 1, 2, 3) as [10]: √
(4) where
and
are the transmit and received powers at S and relay/destination, respectively.
Note that, we assume that the relay transmits the signal under a maximum total power constraint and Gm is self-adjusted in order to ensure a fixed output power with varying input
powers. Note that, this is typical for real optical networks, which is comparable to theoretical works based on assumptions made from the RF domain. The received signal at the jth receiving terminal j=1,2,…, M+1 where j=M+1 denotes the destination, can hence be formulated as follows [9]: ( ) ( )
(∏ ∑
)
( )
∏
( )
∏
(∑
( ))
(
( ).
5)
Note that, since in this work we consider a triple-hop network, therefore
.
2.2 Turbulence Model The atmospheric turbulence is mainly characterized by turbulence strength [2], [16].
, which is a measure of the
defined in terms of both the atmospheric pressure P and the
absolute temperature T, given as [16]: ( where
and
,
(6)
is related to the universal 2/3 power law of temperature variations by [16]: 〈(
where
)
and and
)〉
{
(7)
are the temperatures at two points separated by the propagation distance
,
stand for the inner and outer scales of the small-temperature fluctuations,
respectively. Another important parameter that is commonly used to determine the turbulence regime is Rytov variance
, which is defined as [16]: (
, 8)
where
is the wave number (or the propagation constant), and
propagation distance between the ( Gamma (
)
and the
is the
relays. Here, we use Gamma-
) distribution, which applies to all turbulence regimes, to describe the intensity
fluctuations where the probability density function is given by [16]: ( ) Here, ( ) and
( ) ( ) ( )
(
)
( √
)
(9)
( ) are the Gamma function and the modified Bessel function of the 2nd
kind of order (α-β), respectively. The parameters α and β are related to the turbulence conditions, for details see [16].
3. Experimental Setup Figure 1 shows the schematic block diagram of the experimental setup for the proposed network. It consists of three independent FSO hops each having a length of 2.2 m forming a total multi-hop link span of 6.6 m. As shown in Fig.1, we have used a total of three EDFAs with the last one acting as a booster amplifier. At the source, a BER tester (VeEX VePAL TX300 model) was used to generate a 10 Gbps non-to-return-to-zero on-off-keying (NRZ-OOK) data for intensity modulation (IM) of a laser source. The intensity modulated light beam is launched into the free space channel via a single mode fibre (SMF), a gradient index (GRIN) lens (Thorlabs 50-1550A-APC) and a plano-convex lens (N-BK7). Plano-convex lenses were used to ensure collimated optical beam transmission within the atmospheric chambers. Optical digital attenuators were used to control the level of the transmit power. At the other end of the channel, the incoming optical beam is launched into a SMF via a combination of plano-convex and GRIN lenses. At the receiving end of each chamber (i.e., hop) an EDFA is used to boost the signal level prior to transmission to the next hop. At the destination node, we used a BER tester and an optical
spectrum analyser (OSA) to assess the link performance in terms of the BER and signal-tonoise ratio (SNR), respectively. (Please note that, BER and SNR defined in this paper refer to the mean measured values). In order to generate different turbulence scenarios, external heating fans were used to blow hot air perpendicular to the propagating optical beam within the chamber. 20 remotely controlled temperature sensors were equally spaced along the propagation path at intervals Lp of ~0.28 cm in order to continuously monitor the temperature profile. We considered seven different turbulent scenarios within the optical wireless network, as illustrated in Fig. 2, and used the no turbulence case as a benchmark. Note that, with no turbulence the temperature along the propagation path was kept constant at the laboratory room temperature. We organized our investigation into three main categories: (i) a single hop with turbulence, see Figs. 2(a-c) with
=
m-2/3; (ii) two hops with turbulence, see Figs. 2(d) and m-2/3 for the links 1 and 2 (link 3 with no turbulence) and
(e), with
m-2/3 for the links 2 and 3 (link 1 with no turbulence); and (iii) three hops with turbulence with for the link 1 and
m-2/3, see Fig. 2(f), and with
m-2/3
m-2/3 for the links 2 and 3, see Fig. 2(g). Note that, we
used plastic dividers n order to isolate each section within a network and ensure a specific temperature gradient within the chambers. 4. Results and Discussions This section presents the measured BER results for the triple-hop all-optical relayassisted FSO network for the seven aforementioned turbulence conditions. The experimental results are also validated by means of numerical simulations. We furthermore evaluated the worst case scenario for each hop of 500 m long, as a typical practical case study e.g., as in the last mile network access. Table 1 shows all the key parameters adopted in the experiment work.
Table 1 The experimental setup parameters Parameter Data rate (NRZ-OOK) Laser source Wavelength Transmit power Aperture Gradient index lens Focal length f Convex lenses Diameter Max. output power EDFA Noise figure Sensitivity PIN photodiode Responsivity Total link distance
Value 10 Gbps 1550 nm 0.91 dBm 0.18 cm 10 cm 2.54 cm 21 dBm <7.5 dBm -16 to -1 dBm 0.6 A/W 6.6 m
4.1 One Link with Turbulence We begin with the case where only a single link within network is experiencing turbulence, see Figs. 2(a)-(c). Fig. 3(a) depicts the measured and simulated BER performance against the optical SNR for uncoded NRZ-OOK triple-hop FSO relay-assisted system with turbulence. Also shown for reference is the measured BER for the case with no turbulence. As we can see, the plot (c) displays the worst performance compared to the plots (a) and (b). For example, at a target BER of 10-4, which is lower then the forward error correction limit of 10-3, the SNR penalties are ~ 2.3, 3.5, and 4.5 dB for the plots (a), (b), and (c), respectively compared with the case with no turbulence. Note that, there is a good match between the measured and predicted plots, and the slight mismatch between them could be due to optical components losses and the ASE noise of EDFAs. These results show that in multi-hop FSO links the location of turbulence along the propagation path have a major impact on the entire link performance. The link performance is significantly affected when turbulence occurs within the vicinity of the receiver (Rx). 4.2 Two Links with Turbulence Now let us consider the case where two of the links (i.e., hops) within the network are experiencing the same turbulence levels. Fig. 3(b) depicts the BER against the optical SNR
for the uncoded NRZ-OOK FSO system and for the configurations shown in Figs. 2(d) and (e), where there is a good match between the experimental and simulated results. Here, we notice that the plot (e) display the worst case scnario, where turbulence is introduced close to the Rx end. In line with with the results shown in Fig. 3(a), higher SNR penalties are incurred with turbulence taking place closer to the Rx. For example, for a target BER of 10-4 the SNR penalties are ~3.7 and 5.3 dB for the plots (d) and (e), respectively compared with the case with no turbulence. Note that, the SNR penalties are also higher a single link span with turbulence, see Fig. 3(a). 4.3 All Links with Turbulence Finally, we consider the most common case where all hops of the optical wireless network are experiencing some level of turbulence, see Figs. 2(f) and (g), where the BER results are shown in Fig. 3(c). For fair comparison, we have also included a simulated plot (denoted as h), which is the inverse turbulence condition of (g) with for the link 1 and
m-2/3
m-2/3 for the links 2 and 3. As expected, the plot (h) with
the most significant turbulence effect around the Rx end has the worst performance. Compared to the no turbulence case, at a BER of 10-4 we have SNR penalties of about 3.5, 6.3, and 7.4 dB for plots (f), (g), and (h), respectively. Higher SNR penalties are due to reduced captured light at end of each hope because of turbulence induced beam scattering. Introduced EDFA between the links to improve the system performances by amplifying the signal prior to re-transmission to the next hope. However, EDFA also introduces its own noise, which also contributes to the overall SNR of the link.
4.4 Extension to the case of longer link spans In order to demonstrate to the effect of turbulence in a practical outdoor network, we have extended our investigations by considering the worst-case scenario (i.e., Figs. 3(a)-(c))
and scaling the total link span to 1500 m with plots shown as (cR), (eR), and (hR) in Fig. 4. The link span of 1500 m is illustrate the last mile access networks in urban areas [2]. Note that, for typical outdoor conditions, we have assumed a weak-to-moderate turbulence regime m-2/3 [1]. Fig. 4 shows the simulated BER performance against the optical
with
SNR for uncoded NRZ-OOK FSO for three scenarios of (i) plot (cR) only for the link 3 with m-2/3; (ii) plot (eR) for links 2 and 3 with corresponds to the link 1 with
=
m-2/3; and (iii) and plot (hR)
m-2/3 and the links 2 and 3 with
m-2/3.
As expected, the BER plots follow the same trend as in Fig. 3, but in the case of outdoor link, with increased SNR penalties between the different cases. For example, at a bench mark BER of 10-4 we notice SNR penalties of about 4, 5.8, and 9.1 dB for scenarios (cR), (eR), and (hR), respectively compared to the case with no turbulence.
5. Conclusion This paper investigated a novel experimental all-optical triple-hop AF relay-assisted FSO network under different turbulence conditions using a dedicated laboratory turbulence chamber. We considered seven scenarios based on occurrence of turbulence along the link span, and showed that the network BER performance is affected by the resulting turbulence induced fading. We presented experimental BER results, which were validated by numerical simulations based on Gamma-Gamma turbulence model showing good agreement. We also showed that the relay-assisted FSO system performance was most affected when turbulence was closer to the receiving end. For a more practical situation with a total link span of 1500 m, we showed that for a target BER of 10-4 the SNR penalties can reach up to 9.1 dB compared to the case with no turbulence. These results elucidated the impact of nonhomogeneous turbulence along the entire link span in a multiple-hop relay-assisted FSO network.
Acknowledgment This joint research is supported by the EU COST ICT Action IC 1101 and by the Grant Agency of the CTU in Prague, grant no. SGS17/182/OHK3/3T/13. The first author N. A. M. Nor received Ph.D sponsorship from the Ministry of Education Malaysia and she is with the International Islamic University Malaysia, Malaysia. (email:[email protected]).
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Fig. 1 Block diagram of the experimental setup. CL:Convex lens; ODA: Optical digital attenuator, SMF: Single mode fiber
G2
Plastic walls
(a)
Link 3
Link 1
G2
Plastic walls
Rx 2
Tx 3 G2
Link 2
Plastic wall
(d)
Link 3
G2
(e)
Tx 2
Rx 1
Rx 3
Tx 2
Rx 1
Rx 3
G1
G1
Link 2 Link 3
Link 1 Link 2
(f)
Link 3
Tx 3
Plastic wall
Turb 2
Rx 2
Tx 3 G2
Tx 1
Turb 1
Rx 2
Turb
Tx 1
Link 1
Rx 3
Tx 2
Rx 1 Link 1
Tx 3
Link 3
Turb
No turb
Rx 2
Link 2
Tx 1
Link 1
G2
(c)
Tx 1
No turb
Turb
Link 3
G1
Rx 3
Tx 2
Rx 1
Plastic wall
Turb
Link 2
(b) G1
Rx 3
No turb
Tx 3
Link 2
Rx 2
Tx 1
Link 1
Tx 2
Rx 1
No turb
Tx 1
Rx 3
Turb
Tx 3
Tx 2
No turb
Tx 3
Plastic walls
Rx 2
Link 1 Link 2 Link 3
Rx 1
Rx 3
Tx 2
Rx 1
No turb
Turb
Tx 1
G1
G1
Rx 2
G1
G2
(g)
Fig. 2. Different test cases with turbulence (turb): (a), (b), (c) links 1, 2, and 3 with turb.; (d) links 1 and 2 with turb. and link 3 is with no turb., (e): links 2 and 3 with the same turb. level and link 1 with no turb.; (f) all links with the same turb. regime, and (g) link 1 with different turb. level and links 2 and 3 with the same turb. level.
(a)
(b)
(c)
Fig. 3: Experimental (exp) and simulated (sim) BER vs. the optical SNR for uncoded NRZ-OOK triple-hop FSO relay-assisted system for: (a) no turbulence (No-turb) and links shown in Figs. 2(a), (b), and (c), (b) Noturb and links shown in Figs. 2(d) and (e), and (c) No-turb and links shown in Figs. 2(f) and (g), and sim (h).
Fig. 4 Simulated BER vs. the optical SNR uncoded NRZ-OOK for all-optical triple-hop FSO relay assisted link with a total link span of 1500 m for no turbulence (No-turb), and turbulence cases of (cR), (eR), and (hR).
PII: DOI: Reference:
S1573-4277(17)30158-3 https://doi.org/10.1016/j.osn.2017.11.002 OSN462
To appear in: Optical Switching and Networking Received date: 19 July 2017 Revised date: 16 October 2017 Accepted date: 6 November 2017 Cite this article as: Norhanis Aida Mohd Nor, Zabih Ghassemlooy, Stanislav Zvanovec, Mohammad-Ali Khalighi, Manav R. Bhatnagar, Jan Bohata and Matej Komanec, Experimental Analysis of a Triple-Hop Relay-Assisted FSO System with Turbulence, Optical Switching and Networking, https://doi.org/10.1016/j.osn.2017.11.002 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Experimental Analysis of a Triple-Hop RelayAssisted FSO System with Turbulence Norhanis Aida Mohd Nora*, Zabih Ghassemlooya,e*, Stanislav Zvanovecb, Mohammad-Ali Khalighic, Manav R. Bhatnagard, Jan Bohatab, Matej Komanecb a
Optical Communications Research Group, NCRLab, Faculty of Engineering and Environment, Northumbria University, Newcastle Upon Tyne, UK b Department of Electromagnetic Fields, Faculty of Electrical Engineering, Czech Technical University in Prague, Prague, Czech Republic c Institut Fresnel, UMR CNRS 7249, Ecole Centrale Marseille, Marseille, France d Department of Electrical Engineering, Indian Institute of Technology Delhi, Hauz Khan, IN-110016 New Delhi, India e QIEM, Haixi Institutes, Chinese Academy of Sciences, China *Corresponding authors: Norhanis Aida Mohd Nor, and Zabih Ghassemlooy
Abstract This paper outlines experimental investigation of the performance of the wireless optical network based on an all-optical triple-hop free space optical (FSO) communications employing amplify-and-forward relaying under the influence of atmospheric turbulence. We present new results for the bit error rate (BER) performance for seven possible turbulence network scenarios for relay-assisted FSO link and validate them with numerical simulations based on Gamma-Gamma turbulence model showing a good agreement between them. We also show results, which elucidate the impact of non-homogeneous turbulence along the entire transmission link span for the multiple-hop relay-assisted FSO system. More specifically we show that the BER performance considerably deteriorates for the case where turbulence is near to the receiver end. We outline that for a target BER of 10-4 the signal-tonoise ratio penalty can be as high as 9 dB compared to the case with no turbulence.
Keywords: All-optical relaying, atmospheric turbulence, BER, free space optics, multi-hop FSO network, relay-assisted FSO.
1. Introduction The rapid increase in the number of internet and mobile users together with higher data-rates demanding applications have necessitated the quest for alternative complementary wireless network technologies in order to circumvent the spectrum congestion problem currently being experienced in radio frequency (RF) based systems [1]. One possible viable solution is the free space optical (FSO) communications technology, which offers several advantages such as higher bandwidth, no RF induced electromagnetic interference, license free, low cost of installation and maintenance, and inherently secure due to a highly directional and narrow beam profile [2]. FSO systems used in the last mile access networks offer high data rates, with typical commercial products supporting up to 10 Gbps over a link span of few kilometres [3], increasing to a data rate of 1.6 Tbps over a much short link span of less than a few hundred meters using the wavelength division multiplexing technique in research laboratories [4]. However, the FSO technologies face a number of challenges including (i) the link availability under all weather conditions i.e., fog, rain, smoke, smog in big cities, and turbulence [5], and (ii) longer transmission spans in urban areas. Within this context, the relay-assisted FSO networks can address these challenges even under the influence of the diverse atmospheric channel conditions by extending the link span, ensuring higher link availability (i.e., 99.999 %) and improving performance (small-scale fading statistics) using a number of relays in place of a single long range span. A number of researchers have reported the reliability of relay-assisted FSO systems to broaden the coverage areas as well as to reduce the turbulence induced fading effects. Performance analyses for amplify-and-forward (AF) and decode-and-forward (DF) parallel or
serial relay-assisted FSO schemes were reported in [6] and [7]. More specifically, in [8] it was shown that relay-assisted transmission allows an increase in the power margin by ~19 dB and ~25 dB when using one and two (serial) relays between the source and destination, respectively, assuming a refractive index structure parameter
m-2/3 and a total
link span of 5 km in weak turbulence. In [9], a practically interesting approach
was
introduced, which employed an all-optical relay, that resulted in reduced delay latency and with significant simplification of the opto-electonic devices and signal processing at the relays. The bit error rate (BER) analysis of all-optical AF and DF relaying systems over lognormal fading was investigated in [9] using Monte Carlo simulations. It was shown that for a dual-hop system at a target BER of 10-5,
m-2/3, and a link span of 1 km, the total
transmission span can be increased by 0.9 km and 1.9 km using AF and DF schemes, respectively. In [10], a comprehensive and accurate outage probability Pout analysis of practical and exact models for dual- and multi-hop systems based on all-optical dual-hop AF relaying employing the erbium-doped fiber amplifier (EDFA) was presented showing Pout of 10-6 for
m-2/3 over a link span of 5 km. This configuration was further
analyzed in [11] for a dual-hop AF relaying system considering the effect of amplified spontaneous emission (ASE) and optical degree of freedom (DoF) of EDFA. It was shown that for a target Pout of 10-6, the performance gain (with respect to the direct link) reduced from 14.4 to 10.3 dB when DoF was increased from 10 to 1000 for
m-2/3
compared to a direct transmission at a 1550 nm wavelength over a link span of 10 km with no turbulence. Note that, all the analyses in [9], [10], and [11] were carried out the weak turbulence regime.
In [12], the detailed outage probability analysis of the all-optical AF
relaying system with subcarrier intensity modulation (SIM) based all-optical FSO networks
over Gamma-Gamma fading channel was reported where both the variable AF gain and the pointing error effect were considered. Recently a hybrid RF-FSO scheme was considered to further enhance the system performance when adopted between source-to-relay and from relay-to-destination, respectively [6], [13]. However, almost all works reported on this topic are theoretical in nature with very little or no experimental verifications. In [14], we experimentally investigated the performance of a dual-hop 10 Gbps FSO with turbulence using all-optical switching in the relay terminal. More recently, in [15] we, for the first time, compared the BER performance of a triple-hop AF relaying FSO link with both direct and dual-hop links. Nevertheless, in these works we considered identical turbulence conditions over the entire hops. However in real scenarios, it is highly probable that each hop will experience different turbulence regimes. This is because turbulence can be localized within a small area due to the different thermal flows in close vicinity of buildings, in open and semi-open urban and suburban areas, heating from air conditioning units etc. Therefore, in this paper we considerably extend our previous investigations by considering each hop as part of the longer transmission span in an optical wireless based network, which experiences different turbulence regimes, and consequently evaluate the system performance by means of experimental investigation and numerical simulations. To the best of our knowledge, such an investigation has not been reported in the literature before. The remainder of the paper is organized as follows: Section 2 describes the system and turbulence channel model, Section 3 enlightens the experimental campaign, then experimental results are explained in Section 4 and finally Section 5 concludes the paper.
2. System and Channel Models 2.1 System Model
The proposed all-optical AF relay-based system includes at least one intermediate terminal (the relaying node) between the source S and destination D, which amplifies and retransmits the optical signal. Let us denote the number of relay terminals between S and D by ( ),
M (i.e., R1, R2, …., RM) and the transmitted signal from the mth relay by {
}. The mth hop represents the channel between the mth and the (m+1)th relays. For ,
( ) denotes the transmitted signal from S, whereas the received signal at
can be
expressed as [9]: ( ) where
( )
( ) represents the received noise at
background radiations, and
( ),
(1)
, which is assumed to be mainly due to the
is the channel loss from (m-1)th to the mth terminal, which is
defined in terms of both the atmospheric attenuation
and turbulence
and is given by
[8]: (2)
.
Following amplification at the mth relay node within network, the re-transmitted signal is given as [9]: ( ) where and
( )
( ),
(3)
( ) is the ASE noise of the EDFA (modeled as an additive Gaussian white noise), is the gain of the mth optical amplifier. In most previous works reported on the relay-
assisted FSO networks, Gm was assumed to be fixed [9]. However, in this work we define it for the mth optical amplifier relay (m = 1, 2, 3) as [10]: √
(4) where
and
are the transmit and received powers at S and relay/destination, respectively.
Note that, we assume that the relay transmits the signal under a maximum total power constraint and Gm is self-adjusted in order to ensure a fixed output power with varying input
powers. Note that, this is typical for real optical networks, which is comparable to theoretical works based on assumptions made from the RF domain. The received signal at the jth receiving terminal j=1,2,…, M+1 where j=M+1 denotes the destination, can hence be formulated as follows [9]: ( ) ( )
(∏ ∑
)
( )
∏
( )
∏
(∑
( ))
(
( ).
5)
Note that, since in this work we consider a triple-hop network, therefore
.
2.2 Turbulence Model The atmospheric turbulence is mainly characterized by turbulence strength [2], [16].
, which is a measure of the
defined in terms of both the atmospheric pressure P and the
absolute temperature T, given as [16]: ( where
and
,
(6)
is related to the universal 2/3 power law of temperature variations by [16]: 〈(
where
)
and and
)〉
{
(7)
are the temperatures at two points separated by the propagation distance
,
stand for the inner and outer scales of the small-temperature fluctuations,
respectively. Another important parameter that is commonly used to determine the turbulence regime is Rytov variance
, which is defined as [16]: (
, 8)
where
is the wave number (or the propagation constant), and
propagation distance between the ( Gamma (
)
and the
is the
relays. Here, we use Gamma-
) distribution, which applies to all turbulence regimes, to describe the intensity
fluctuations where the probability density function is given by [16]: ( ) Here, ( ) and
( ) ( ) ( )
(
)
( √
)
(9)
( ) are the Gamma function and the modified Bessel function of the 2nd
kind of order (α-β), respectively. The parameters α and β are related to the turbulence conditions, for details see [16].
3. Experimental Setup Figure 1 shows the schematic block diagram of the experimental setup for the proposed network. It consists of three independent FSO hops each having a length of 2.2 m forming a total multi-hop link span of 6.6 m. As shown in Fig.1, we have used a total of three EDFAs with the last one acting as a booster amplifier. At the source, a BER tester (VeEX VePAL TX300 model) was used to generate a 10 Gbps non-to-return-to-zero on-off-keying (NRZ-OOK) data for intensity modulation (IM) of a laser source. The intensity modulated light beam is launched into the free space channel via a single mode fibre (SMF), a gradient index (GRIN) lens (Thorlabs 50-1550A-APC) and a plano-convex lens (N-BK7). Plano-convex lenses were used to ensure collimated optical beam transmission within the atmospheric chambers. Optical digital attenuators were used to control the level of the transmit power. At the other end of the channel, the incoming optical beam is launched into a SMF via a combination of plano-convex and GRIN lenses. At the receiving end of each chamber (i.e., hop) an EDFA is used to boost the signal level prior to transmission to the next hop. At the destination node, we used a BER tester and an optical
spectrum analyser (OSA) to assess the link performance in terms of the BER and signal-tonoise ratio (SNR), respectively. (Please note that, BER and SNR defined in this paper refer to the mean measured values). In order to generate different turbulence scenarios, external heating fans were used to blow hot air perpendicular to the propagating optical beam within the chamber. 20 remotely controlled temperature sensors were equally spaced along the propagation path at intervals Lp of ~0.28 cm in order to continuously monitor the temperature profile. We considered seven different turbulent scenarios within the optical wireless network, as illustrated in Fig. 2, and used the no turbulence case as a benchmark. Note that, with no turbulence the temperature along the propagation path was kept constant at the laboratory room temperature. We organized our investigation into three main categories: (i) a single hop with turbulence, see Figs. 2(a-c) with
=
m-2/3; (ii) two hops with turbulence, see Figs. 2(d) and m-2/3 for the links 1 and 2 (link 3 with no turbulence) and
(e), with
m-2/3 for the links 2 and 3 (link 1 with no turbulence); and (iii) three hops with turbulence with for the link 1 and
m-2/3, see Fig. 2(f), and with
m-2/3
m-2/3 for the links 2 and 3, see Fig. 2(g). Note that, we
used plastic dividers n order to isolate each section within a network and ensure a specific temperature gradient within the chambers. 4. Results and Discussions This section presents the measured BER results for the triple-hop all-optical relayassisted FSO network for the seven aforementioned turbulence conditions. The experimental results are also validated by means of numerical simulations. We furthermore evaluated the worst case scenario for each hop of 500 m long, as a typical practical case study e.g., as in the last mile network access. Table 1 shows all the key parameters adopted in the experiment work.
Table 1 The experimental setup parameters Parameter Data rate (NRZ-OOK) Laser source Wavelength Transmit power Aperture Gradient index lens Focal length f Convex lenses Diameter Max. output power EDFA Noise figure Sensitivity PIN photodiode Responsivity Total link distance
Value 10 Gbps 1550 nm 0.91 dBm 0.18 cm 10 cm 2.54 cm 21 dBm <7.5 dBm -16 to -1 dBm 0.6 A/W 6.6 m
4.1 One Link with Turbulence We begin with the case where only a single link within network is experiencing turbulence, see Figs. 2(a)-(c). Fig. 3(a) depicts the measured and simulated BER performance against the optical SNR for uncoded NRZ-OOK triple-hop FSO relay-assisted system with turbulence. Also shown for reference is the measured BER for the case with no turbulence. As we can see, the plot (c) displays the worst performance compared to the plots (a) and (b). For example, at a target BER of 10-4, which is lower then the forward error correction limit of 10-3, the SNR penalties are ~ 2.3, 3.5, and 4.5 dB for the plots (a), (b), and (c), respectively compared with the case with no turbulence. Note that, there is a good match between the measured and predicted plots, and the slight mismatch between them could be due to optical components losses and the ASE noise of EDFAs. These results show that in multi-hop FSO links the location of turbulence along the propagation path have a major impact on the entire link performance. The link performance is significantly affected when turbulence occurs within the vicinity of the receiver (Rx). 4.2 Two Links with Turbulence Now let us consider the case where two of the links (i.e., hops) within the network are experiencing the same turbulence levels. Fig. 3(b) depicts the BER against the optical SNR
for the uncoded NRZ-OOK FSO system and for the configurations shown in Figs. 2(d) and (e), where there is a good match between the experimental and simulated results. Here, we notice that the plot (e) display the worst case scnario, where turbulence is introduced close to the Rx end. In line with with the results shown in Fig. 3(a), higher SNR penalties are incurred with turbulence taking place closer to the Rx. For example, for a target BER of 10-4 the SNR penalties are ~3.7 and 5.3 dB for the plots (d) and (e), respectively compared with the case with no turbulence. Note that, the SNR penalties are also higher a single link span with turbulence, see Fig. 3(a). 4.3 All Links with Turbulence Finally, we consider the most common case where all hops of the optical wireless network are experiencing some level of turbulence, see Figs. 2(f) and (g), where the BER results are shown in Fig. 3(c). For fair comparison, we have also included a simulated plot (denoted as h), which is the inverse turbulence condition of (g) with for the link 1 and
m-2/3
m-2/3 for the links 2 and 3. As expected, the plot (h) with
the most significant turbulence effect around the Rx end has the worst performance. Compared to the no turbulence case, at a BER of 10-4 we have SNR penalties of about 3.5, 6.3, and 7.4 dB for plots (f), (g), and (h), respectively. Higher SNR penalties are due to reduced captured light at end of each hope because of turbulence induced beam scattering. Introduced EDFA between the links to improve the system performances by amplifying the signal prior to re-transmission to the next hope. However, EDFA also introduces its own noise, which also contributes to the overall SNR of the link.
4.4 Extension to the case of longer link spans In order to demonstrate to the effect of turbulence in a practical outdoor network, we have extended our investigations by considering the worst-case scenario (i.e., Figs. 3(a)-(c))
and scaling the total link span to 1500 m with plots shown as (cR), (eR), and (hR) in Fig. 4. The link span of 1500 m is illustrate the last mile access networks in urban areas [2]. Note that, for typical outdoor conditions, we have assumed a weak-to-moderate turbulence regime m-2/3 [1]. Fig. 4 shows the simulated BER performance against the optical
with
SNR for uncoded NRZ-OOK FSO for three scenarios of (i) plot (cR) only for the link 3 with m-2/3; (ii) plot (eR) for links 2 and 3 with corresponds to the link 1 with
=
m-2/3; and (iii) and plot (hR)
m-2/3 and the links 2 and 3 with
m-2/3.
As expected, the BER plots follow the same trend as in Fig. 3, but in the case of outdoor link, with increased SNR penalties between the different cases. For example, at a bench mark BER of 10-4 we notice SNR penalties of about 4, 5.8, and 9.1 dB for scenarios (cR), (eR), and (hR), respectively compared to the case with no turbulence.
5. Conclusion This paper investigated a novel experimental all-optical triple-hop AF relay-assisted FSO network under different turbulence conditions using a dedicated laboratory turbulence chamber. We considered seven scenarios based on occurrence of turbulence along the link span, and showed that the network BER performance is affected by the resulting turbulence induced fading. We presented experimental BER results, which were validated by numerical simulations based on Gamma-Gamma turbulence model showing good agreement. We also showed that the relay-assisted FSO system performance was most affected when turbulence was closer to the receiving end. For a more practical situation with a total link span of 1500 m, we showed that for a target BER of 10-4 the SNR penalties can reach up to 9.1 dB compared to the case with no turbulence. These results elucidated the impact of nonhomogeneous turbulence along the entire link span in a multiple-hop relay-assisted FSO network.
Acknowledgment This joint research is supported by the EU COST ICT Action IC 1101 and by the Grant Agency of the CTU in Prague, grant no. SGS17/182/OHK3/3T/13. The first author N. A. M. Nor received Ph.D sponsorship from the Ministry of Education Malaysia and she is with the International Islamic University Malaysia, Malaysia. (email:[email protected]).
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Fig. 1 Block diagram of the experimental setup. CL:Convex lens; ODA: Optical digital attenuator, SMF: Single mode fiber
G2
Plastic walls
(a)
Link 3
Link 1
G2
Plastic walls
Rx 2
Tx 3 G2
Link 2
Plastic wall
(d)
Link 3
G2
(e)
Tx 2
Rx 1
Rx 3
Tx 2
Rx 1
Rx 3
G1
G1
Link 2 Link 3
Link 1 Link 2
(f)
Link 3
Tx 3
Plastic wall
Turb 2
Rx 2
Tx 3 G2
Tx 1
Turb 1
Rx 2
Turb
Tx 1
Link 1
Rx 3
Tx 2
Rx 1 Link 1
Tx 3
Link 3
Turb
No turb
Rx 2
Link 2
Tx 1
Link 1
G2
(c)
Tx 1
No turb
Turb
Link 3
G1
Rx 3
Tx 2
Rx 1
Plastic wall
Turb
Link 2
(b) G1
Rx 3
No turb
Tx 3
Link 2
Rx 2
Tx 1
Link 1
Tx 2
Rx 1
No turb
Tx 1
Rx 3
Turb
Tx 3
Tx 2
No turb
Tx 3
Plastic walls
Rx 2
Link 1 Link 2 Link 3
Rx 1
Rx 3
Tx 2
Rx 1
No turb
Turb
Tx 1
G1
G1
Rx 2
G1
G2
(g)
Fig. 2. Different test cases with turbulence (turb): (a), (b), (c) links 1, 2, and 3 with turb.; (d) links 1 and 2 with turb. and link 3 is with no turb., (e): links 2 and 3 with the same turb. level and link 1 with no turb.; (f) all links with the same turb. regime, and (g) link 1 with different turb. level and links 2 and 3 with the same turb. level.
(a)
(b)
(c)
Fig. 3: Experimental (exp) and simulated (sim) BER vs. the optical SNR for uncoded NRZ-OOK triple-hop FSO relay-assisted system for: (a) no turbulence (No-turb) and links shown in Figs. 2(a), (b), and (c), (b) Noturb and links shown in Figs. 2(d) and (e), and (c) No-turb and links shown in Figs. 2(f) and (g), and sim (h).
Fig. 4 Simulated BER vs. the optical SNR uncoded NRZ-OOK for all-optical triple-hop FSO relay assisted link with a total link span of 1500 m for no turbulence (No-turb), and turbulence cases of (cR), (eR), and (hR).