Fiber-distributed Indoor High Bitrate Optical Wireless ...

Springer-Verlag journal manuscript No. (will be inserted by the editor)

Fiber-distributed Indoor High Bitrate Optical Wireless System Hani Al Hajjar · Bruno Fracasso · Dominique Leroux

Received: date / Accepted: date

Abstract In this paper, we describe a wideband Indoor Optical Wireless distribution system based on an infrared communication channel using a dedicated architecture. The idea is to provide narrow line-of-sight indoor free-space optical cells at very high rates through an optical fiber network. The used wavelength is 1550 nm for an eye safety and optical power budget reasons. To validate the system performance using standard On-Off Keying modulation, we calculate the power budget and simulate the overall link, showing that an implementation with commercially available components may lead to 2.5 Gbps operational optical wireless links. Keywords Indoor optical communications · Infrared · Free-space · Polymer optical fiber · Photo-receiver. 1 Introduction In the last decade, the number of lap-top computers, personal digital assistants (PDAs) and other mobiles terminals have massively increased. This evolution has led to a huge demand of wireless communications, in the purpose of avoiding wires and connectors to supply some kind of mobility in various places such as airports, rail stations, offices or homes [1]. To date, this mobility is Hani Al Hajjar Optics department, Telecom Bretagne, CS 83818, 29238 BREST CEDEX 3, FRANCE Tel.: +33-2-29001031 Fax: +33-2-29001025 E-mail: [email protected] Bruno Fracasso Optics department, Telecom Bretagne, CS 83818, 29238 BREST CEDEX 3, FRANCE Dominique Leroux Signal & Communications department, UMR CNRS 3192 Lab-STICC, Telecom Bretagne, CS 83818, 29238 BREST CEDEX 3, FRANCE

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Hani Al Hajjar et al.

mainly offered by Radio-Frequency (RF) communications using Wi-Fi channels, a widespread technology offering a maximum bitrate of 300 Mbps with the 802.11n standard. However, new indoor applications such as non-compressed HD video transfer or remote hard-disk backup require much higher bandwidths. Therefore, new wideband RF technologies have been proposed, such as the IEEE 802.15.3c standard, operating in the 57− 64 GHz unlicensed band [2]. This solution offers mm-wave wireless channels over a few meters range (femto-cells), at data rates in excess of 500 Mbps, with potential application to real-time multiple HD-TV video stream transfer or wireless data bus for cable replacement. At the in-building network level, the most efficient technique to distribute high-bitrate wireless picocells over larger distances (a few hundred meters to several kilometers) is the promising Radio-over-Fiber (RoF) technology [3], for which compact and cost-effective interfaces are still to be developed and spread commercially. In this framework, we present the fiber-distributed architecture of an optical wireless indoor system working at bitrates that exceed 2 Gbps. In section II, the proposed optical architecture is presented and the wavelength and fiber link constraints are discussed. The different stages of the downlink are considered in section III, including an output signal-to-noise ratio estimation to calculate the link power budget. The last two sections provide a system modeling with some simulation results, followed by the conclusion.

2 Optical cells over fiber distribution 2.1 Proposed architecture The proposed system to implement an indoor wireless optical transmission is depicted in figure 1. The overall principle takes back the basic architecture of RoF systems [4], except that the analog RF subcarrier is here removed and replaced by the optical digital high bitrate data stream. The system aims at transporting the optical wireless distribution from the access network (FTTH) to mobile terminals located in the rooms of a building or a home. It can also distribute very high bitrate streams (HD video) generated by a central server throughout the in-building space. Unlike RoF systems which transport RF cells through optical fibers, the wireless cells transmitted here are optical links with digital modulation schemes, hence reducing the system implementation complexity while providing the potential huge bitrates (several Gbps) of optical transmission links. Both the access traffic and locally generated home data streams are centralized using an optical control station (OCS), which can be viewed as an optical multi-port switch, with possible optical amplification at that stage. At the outputs of the OCS, the optical signals are distributed to optical access points (OAPs) on every room, using dedicated optical fibers. The downstream signals are then transferred from the OAP to the mobile terminals within a room using a wireless optical link signal. Compared to many standard optical wireless schemes using diffused links in

Fiber-distributed Indoor High Bitrate Optical Wireless System

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Fig. 1 Hybrid indoor optical fiber distribution system

a single room [5], the present fiber-distributed scheme offers Narrow Line-ofSight (NLOS) links, which enables the power link budget to be consistent with Gbps communications. The OAP is an all-optical beam steering system whose role is to track mobile receivers and to ensure the connection through a maximization of the signal-to-noise ratio (SNR). It operates in two modes to ensure the downlink setting and the optical signal transfer to the receiver: 1. Medium LOS (MLOS) mode: a signal with medium divergence is transmitted in free-space (10 − 30◦ ) to detect the presence of wireless terminals (the description of the procedure is out of scope). 2. Narrow NLOS (NLOS) mode: when a receiver is detected, the deflector switches to the NLOS geometry with a reduced beam divergence (2 − 5◦ ) to transmit the information to the mobile receiver, supposed to be fixed during the connection. The OAP is made up with a dynamic lens/mirror combination associated to a diffusing holographic optical element (HOE) [6], placed at the output of the fiber. The covered surface and the optical power at a given distance can be adjusted using the lens/HOE combination, whose role is twofold: (i) to ensure eye safety conditions and (ii) to shape the spatial configuration of the optical cell. The position and number of optical beams can possibly be dynamically adjusted using the lens-hologram configuration to provide reconfigurable all-optical hotspots. The covered area diameter ranges from 0.1 to 1 meter depending on the required mobility. If necessary, the hotspot area can be easily outlined using a visible light mark. This solution is viewed as a real

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high-bitrate home network solution providing data and health safety conditions, as well as reduced complexity/cost and lower energy consumption. The OAP generates an attenuation of about 3 dB, including both vignetting and absorption at the lens stage, as well as the limited diffraction efficiency of the holographic beam shaper. The OAP structure is currently being designed, but the details are out of the scope of this paper. In the present application, the upstream link is not supposed to contain very high bitrate data, and hence can be implemented using a RF channel (e.g. 802.11).

2.2 Constraints: eye safety, ambient light In the present system, the optical signal is transmitted in free-space, and the signal power should respect the international standard value of eye safety conditions (class 1 laser) limiting the acceptable emitted power in free-space at 1550 nm to 10 dBm, in the case of point sources. This upper limit is higher than the corresponding values at 850 or 1300 nm and 50 times more than the safe power-level in the the visible range [7]. In contrast to fiber communications, the optical free-space receiver is exposed to ambient light, introducing such additional noise sources as daylight, fluorescent or tungsten lamps, whose relative spectral irradiances are shown in figure 2 [8]. The ambient noise is clearly less harmful at 1550 nm, in comparison with its level at 850 nm or in the visible range. These two arguments, as well as the wide availability of optical components at this wavelength lead us to consider the 1550 nm wavelength in the proposed system.

Fig. 2 Spectral power densities (irradiances) of the three dominant sources of indoor background illumination


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2.3 Optical fiber link The fiber used to establish the interconnections between the rooms of a building (or house) should have suitable bandwidth characteristics while providing ease of deployment at a moderate cost. Although single-mode glass optical fibers (GOFs) have the potential of achieving tens of Gbps, they still suffer from high cabling and maintenance costs in the home network framework. Most solutions deployed at the user’s premises (including LANs) involve multimode GOF with large core diameters and more tolerant connectorization issues. Nevertheless, multimode GOF are still brittle and fragile to smaller bending radii. Mainly deployed in local area networks, Polymer Optical Fibers (POFs) emerge as an ideal medium for transmitting high speed and short distance (< 200 m) data links for in-building networks. Beyond local networks, POF is employed in boats, planes and cars to reduce the metal cable weight while providing high-speed data links. The ease of installation and maintenance of POF is its strong point, giving it a significant advantage over silica fibers. POF can be installed on any existing cable tray, without suffering any accidental bends and twist and do not fear nor generate electromagnetic radiation [9]. In addition, POF connectors are inexpensive, requiring no precise alignment of the cores. The Japanese company Asahi Glass has designed and fabricated the first gradient-index perfluorinated POF (PF-GI-POF) conveying its transmission performance similar to that of a silica fiber. In the first Telecom window (1300 nm wavelength), these fibers have an attenuation lower than 1 dB over 100 m while 10 Gbps bitrates have been reported over this distance. The average fiber loss at 1550 nm is 15 dB/100m, which corresponds to less than 3 dB in case of a standard indoor 20 m distribution length, and makes PF-GI-POF based systems compatible with 10GEthernet or FibreChannel4 standards. In laboratory trials, multi-level modulation schemes have even allowed some researchers to reach 40 Gbps over 200 m [10]. First simulations on this fiber using the VPI Transmission Maker simulator confirmed the good transmission performance in terms of impulse response and -3 dB bandwidth [11]. The overall downlink simulation is presented in section 4. This fiber was physically tested in the lab for several input launching conditions and different fiber core diameters. These tests confirm that a large (connectorization) offset exceeding 10 µm is permitted with less than 1 dB excess loss, while a uniform spatial distribution according to 2 orthogonal axes in free-space can be obtained using 50 µm core diameter PF-GI-POF [12].

3 NLOS free-space optical link design The all-optical downlink considered here is schematically depicted in figure 3. It is made up with the cascading of (i) a fiber section, (ii) a free-space section and (iii) a receiving module. The next step is to design the optimal link parameters in terms of optical budget and electrical SNR at the receiver.

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While the fiber section is characterized by a moderate loss (lower than 3 dB in the present case), the free-space link loss strongly depends on the wireless link geometry (see section 3.2).

Fig. 3 Downlink transmission channel between the OCS and the mobile receiver

3.1 Optical receiver Basically, the opto-electronic receiver in the present framework must have: (i) a wide bandwidth to allow for the transmission of Gbps data rates, (ii) a good sensitivity to maximise the system power budget. The receiver field of view is not really an issue here as we consider NLOS links. An optical filter is also used to reduce the noise generated by ambient light to a smaller level than that generated by the preamplifier electronics (transimpedance type). An optical concentrator is also used to increase the detection area without increasing the capacitance of the photodiode, which would result in decreasing the detection bandwidth. The concentrator compensates for a part of the high attenuation in free-space by transforming light rays, incident over a large area on its entrance aperture, into a set of light rays that emerge at the output from a smaller area. The maximum theoretical gain of an optical concentrator is given by the following relation (1) [13]: G=

n′ sin θ′ n sin θ

2

(1)

where n′ and n are the refractive indices of the concentrator, and the environment that surrounds the concentrator, respectively, and θ and θ′ are the


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maximum angle of rays impinging onto the concentrator and outgoing from the concentrator, respectively. The exit aperture of the concentrator is generally placed in front of the photodiode, which has a maximum acceptance angle of 60◦ . Assuming air at the entrance aperture, equation (1) reduces to G = ηc n′2 / sin2 θ, where ηc ≃ 0.7 [14]. 3.2 Free-space loss and signal-to-noise ratio In an optical wireless link, the geometrical characteristics of the received optical signal depend on the link geometry, i.e. the source radiation diagram, propagation path and relative orientation and position of the source and receiver. The NLOS link considered here is schematically depicted in figure 4. Path loss and multipath distorsion are minimized using a directional transmitter. The following assumptions are made. The signal emitted by the OAP is distributed over a small semi-angle α, yielding a coverage area (circular shape) Ac in the detection plane. The source and the receiver are supposed to be perfectly aligned (no tilt in both directions with respect to each other). Uniform ambient light noise originating from background light (sun, lighting) is distributed uniformly over a large semi-angle θa (θa = π/2, in practice). The corresponding spectral radiance is denoted by Lbk (W/m2 /sr/nm). The

ambient optical noise

coverage area

θa

α

2θi θc photodetector area

optical emitter optical concentrator

Fig. 4 Free-space link geometry. Both the coverage area and the detector surface are supposed to be circular, with areas Ac and Ad , respectively

receiver associates an optical concentrator and an optical band-pass filter, placed before a photoreceiver, whose first stage is a photodetector with area Ad . The signal source is a single frequency laser emitting at wavelength λ0 , whose linewidth is much smaller than the optical filter -3 dB bandwidth ∆λ. Under these assumptions, the optical signal power Ps captured by the detector is (2) Ps = Es GT Ad

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where ES is the irradiance (W/m2 ) of the optical signal impinging at the concentrator input, G = ηc n2 / sin2 θc is the concentrator gain for this particular signal and T is the average filter transmittance of the signal, depending on both the incidence angle range [−θi , +θi ] and the center wavelength λ0 . For the present NLOS link, we have T ≃ 1. If P0 is optical power emitted and d is the distance between the source and the receiver, then the received irradiance is Es = P0 /Ac , yielding, using the small angle approximation: Ps =

P0 n2 Ad π(αd sin θc )2

(3)

The corresponding free-space attenuation is given by F Sa = 10 log(Ad /πα2 d2 ). Typical numerical values are here Ad = 2 · 10−7 m2 , α = 2◦ , θc = 10 degrees, yielding F Sa ≃ 49 dB. The optical concentrator is supposed to be non-imaging and ideal with a constant gain within its field-of-view θc , so that the received background optical power is given by Pbk = Lbk Ad Gbk T0 ∆λ, where Gbk = n2 is the optical gain of the concentrator for the ambient radiation [15], T0 is the transmittance of the optical band-pass filter at λ0 , and ∆λ is the -3 dB bandwidth of the optical filter. Under these assumptions, we can estimate the signal-to-noise ratio (SNR) of the electrical signal detected by the photodiode in a direct detection scheme by: SNR1 =

RP02 n2 Ad (RPS )2 = 2eRPbk Be 2eπ 2 Lbk (αd sin θc )4 ∆λBe

(4)

where R denotes the responsivity of the photodiode and Be is the noise equivalent bandwidth. With the configuration listed in Table 1, we obtain SNR1 = 47.3 dB. One can note that this value is much larger than the corresponding SNR1 to be obtained with a diffuse link configuration, where the background noise is integrated over a large field-of-view. Other noise sources are negligible here, including the shot noise due to the signal and dark current, and the thermal noise arising from the shunt resistance. In the present configuration, the dominant noise factor will be generated at the preamplifier stage, and it can be expressed by the total input-referred noise current (rms) itot or by the overall noise equivalent power (NEP) of the photoreceiver. Typical √ for commercial devices (PINNEP values of 1 − 10 pW/ Hz can be noticed √ or APD-based), yielding itot = R · N EP Be = 4 · 10−7 A in the PIN case (R = 0.9 A/W), with Be = 2 GHz, and the corresponding output signal-tonoise ratio (i.e. after the preamplifier) SNR2 is then obtained by replacing the denominator in (4) by i2tot yielding SNR2 = 7.9 dB. The photoreceiver sensitivity σ depends on both the bitrate and the BER to be achieved. Considering a commercial PIN/TIA based device with σ = -26 dBm (at 2.5 Gbps and BER=10−7 ), the free-space link power budget is 10-(26)+16 = 52 dB, yielding a 3 dB power margin with respect to the free-space attenuation value considered above. If required by the application, the power budget could be improved by increasing the power emitted at the OAP, using an extended light beam distribution [18].


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Table 1 Free-space link parameters Photodiode responsivity (PIN) Emitted power (at the OAP) Concentrator refractive index Detector surface Ambient background spectal irradiance (1550 nm) Emitter-receiver distance Divergence angle Concentrator field of view Optical band-pass filter bandwidth Photodiode electrical bandwidth

R P0 n Ad Lbk d α θc ∆λ Be

0.9 A/W 10 mW 1.48 π(0.25 mm)2 0.012 W/m2 /nm 2.5 m 2◦ 10◦ 10 nm 2 GHz

4 Downink modelling and simulation The overall downlink can be modelled as the concatenation of a PF-GI-POF link and an optical free-space transmission. This optical hybrid channel is described and implemented using the VPI transmission Maker simulator [16]. This software is a powerful reference tool within the optical fibre transmission community, but cannot process the optical signal propagating outside the fibre. To this aim, Matlab co-simulation routines are generated to represent both free-space propagation and the optical concentration stage. We first evaluate the PF-GI-POF performance in terms of bandwidth (BW), impulse response, and detected eye diagram as a function of the launching conditions in the fiber, the geometric characteristics and the length of the fiber. The VPI software provides the dynamic characteristics as a function of the variations of the physical parameters of the system: index profile, length and conditions of use of POF, source type and used modulation scheme. The virtual component MultimodeFiber.vtms used in the VPI library introduces a radial profile of truncated parabolic index n(r) and a development of Sellmeier n(λ) defined by:  2  n1 1 − 2∆(r/a)β , if 0 ≤ r ≤ a (5) n2 (r) =  2 n1 (1 − 2∆) , if r > a and

n2 (λ) = 1 +

3 X Ai λ2 λ2 − li2 i=1

(6)

where a and β are respectively the fiber core radius and the profile coefficient, and the parameters li and Ai are the wavelengths and coefficients of the resonant material. The PF-GI-POF considered here is the GigaPOF model manufactured by Chromis Fiberoptics [17], whose cladding and core diameters are respectively 490 µm and 2a = 50 or 120 microns. The fiber numerical aperture is 0.19. If these data are known for the most common polymer fiber types (PMMA), obtaining them for the PF-GI-POF has required repeated contacts with the laboratory designing the polymer. The silica, PMMA and

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PF-GI-POF multimode fibers have been simulated in terms of BW and impulse response in the same conditions, using a Gaussian input signal of 20 ps (FWHM) modulating a DFB laser diode at 1550 nm, with a spectral width of 12 GHz and a spatial mode diameter of 10 µm [11]. This comparison proves the correct modeling of PF-GI-POF and its advantages for high bitrate indoor applications. For a system simulation including a free-space link, we use a 20 m- and 50 µm core PF-GI-POF span, which represents the average fiber link length in an indoor architecture. Free-space propagation is modeled as an attenuation depending on the divergence angle at the output of the fiber and the freespace distance. Optical noise with Gaussian statistics and the required power spectral density of 2.22 · 10−17 W/Hz (see (4)) is added to the optical signal to take the ambient light effect at 1550 nm into account. We use a 10 nm bandpass optical (dielectric) filter to reduce the background noise effect. The concentrator is also modeled in Matlab as a gain stage depending on the acceptance input angle and the photodiode area. The photoreceiver used in the simulation is composed of a PIN photodiode with a dark current of 3·10−10 A, a thermal noise of 10−11 A/(Hz)1/2 , a transimpedance amplificator (TIA) with a 5000 V/A transimpedance and an input-referred equivalent noise of 5.8 · 10−7 A. The last stage is a 4th -order Bessel bandpass electrical filter in the [10kHz - 2GHz] range. The system is simulated at 2.5 Gbps using On-Off Keying NRZ modulation at the transmitting stage.

Fig. 5 Simulated eye-diagram at the receiving stage of the optical downlink transmission, using a PIN-based photoreceiver (2.5 eGbps NRZ-OOK modulation)

Figure 5 shows the simulated output electrical eye-diagram with a 2 m freespace NLOS link propagating with a 2 degrees divergence. The optical power


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Fig. 6 Simulated eye-diagram at the receiving stage of the optical downlink transmission, using an APD-based photoreceiver (2.5 Gbps NRZ-OOK modulation)

launched from the OAP is 13 dBm. The simulation is performed on 2048 bits and the estimated BER (using a Gaussian estimation method) is 9 · 10−7 . As a comparison, an APD module is also tested as receiving module (multiplication factor M = 10), yielding a slight improvement in the eye-diagram (shown in figure 6), with an estimated BER of 1.4 · 10−7 .

5 Conclusion In this paper, we propose an all-optical fiber feeded indoor system distributing wireless cells at very high bitrate. The distribution scheme makes use of a polymer optical fiber network that is fully compatible with an indoor deployment. After describing the architecture and the different system modules, we validate the downlink budget to check the feasibility of the links. Then, we simulate the overall system using a VPI-Matlab cosimulation, using NRZ-OOK modulation at 2.5 Gbpss. The resulting eye-diagrams show an almost error-free transmission at the receiving stage. These results have to be confirmed by the physical implementation of an experimental test-bed, which is currently being built. Test results will be reported in a forthcoming publication. Acknowledgements The authors would like to thank Institut Telecom, France, for partly funding this project, the Coll` ege Doctoral International/Universit´ e Europ´ eenne de Bretagne (CDI/UEB) and ”Conseil R´ egional de Bretagne” for their financial help, and Marie-Laure Moulinard for fruitful discussions.

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