A Visible Light Localization Aided Optical Wireless System

Transcription

1 2nd IEEE Worksop on Optical Wireless Communications A Visible Ligt Localization Aided Optical Wireless System Giulio Cossu, Marco Presi, Raffaele Corsini, Pallab Coudury, Amir Masood Kalid and Ernesto Ciaramella Scuola Superiore Sant Anna University - TeCIP Via G. Moruzzi, 1, Pisa (ITALY) giulio.cossu@sssup.it Abstract We report about a Line of Sigt Optical Wireless Communication (LoS OWC) system aided by a simplified Visible Ligt localization algoritm for tracking purposes. We sow experimentally tat by combining te LoS OWC system wit te localization algoritm it is possible to provide a gross datarate exceeding 3 Mb/s over a 9 illumination angle by using a RGB visible-ligt LED at 9 cm distance. Index Terms Visible Ligt Communications, LED, Adaptive OFDM, Localization I. INTRODUCTION Optical Wireless Communication (OWC) systems ave been recently investigated for different applications, ranging from sensing [1] to data-transmission [2]. Among OWC systems, tose based on Visible Ligt are of particular interest for indoor environments since tey can combine illumination and communication functionalities. A great effort as been put to demonstrate different applications based on wite LEDs (eiter pospor-based and RGB). For communication purposes, Visible Ligt Communication (VLC) record as been demonstrated at data rates up to 83 Mb/s by combining advanced Digital Signal Processing and Wavelengt Division Multiplexing in RGB LEDs [3]. Yet, tis requires a line-of-sigt (LoS) system wit a focalization lens. VLC systems based on wite LEDs and fluorescent ligts ave been also demonstrated to be effective for localization purposes, as an aid for visually impaired people or to provide localization information in indoor environment, were GPS is not available [4], [5], [6]. Tese systems can provide up to sub-centimeter accuracy [7]. Recently, an interesting proposal suggested to exploit localization tecniques based on common radio infrastructures (eg. WiFi) as an aid to provide tracking information in widearea LoS optical wireless systems [8]. In tis case, tanks to a transceiver design tat relaxes te fiber-to-fiber alignment [9], a roug localization information (about alf-a-meter) is enoug, and te radio-based protocol can easily fulfill tese requirements [1]. However suc a resolution doesn t meet te requirements of ig bit-rate VLC systems. Indeed te very limited modulation bandwidt of visible ligt LEDs (usually less ten 2 MHz [11], [12]) and te relatively low detector responsivity in te visible region, make tis class of systems very sensitive to Signal to Noise Ratio (SNR) degradation: tis poses more stringent requirements on te transmitter-receiver alignment. Tose requirements are even more strict wen spectrally efficient modulation formats, suc as adaptive Ortogonal Frequency Division Multiplexing (OFDM), are implemented in to increase te net data-rate [13]. In tis case to meet te SNR requirements, te receiver needs a focalization lens wic drastically decreases te alignment tolerance angle down to few degrees. Fortunately, te relatively low modulation bandwidt of visible ligt LEDs allow to use large area potodiodes, tus avoiding furter SNR and alignment tolerance constraints. We terefore propose to use a localization algoritm based on visible ligt. We demonstrate experimentally tat te localization accuracy obtained by our solution is fully compatible wit te angle tolerance required by our focalized LoS VLC system based on adaptive OFDM. A proof of principle of tis tecnique is demonstrated in a 1-D system. By means of tis approac we acieve a localization error tat is low enoug to guarantee a data-rate between 3 and 41 Mb/s using only one transmitting RGB LED, covering an illumination angle of 9 and at 9 cm link lengt. II. OPERATING PRINCIPLE We consider a room were LEDs installed on te ceiling provide simultaneously for illumination, localization and data transmission. A minimum of tree LEDs is required for te localization, wile at least one LED lamp is required for te data transmission. A general sceme of te proposed metod is reported in Fig. 1. In tis case a system wit 4 auxiliary transmitters is sown. is te LED used for te data transmission, Data is te data receiver wit focalization lens, T x i, (i = 1,..., 4) and are respectively te auxiliary transmitters and receiver used for localization purpose. Data Tx and are as close as possible. As said, te lens in increases te SNR and improves te system capacity; on te oter and, it reduces drastically te acceptance angle for te incoming signal so it must be carefully aligned wit te Data Tx (LoS). In order to obtain te required LoS between Data Tx and Data, te two coordinates (d x, d y ) must be known wit an error well below te tolerance of te system. Te does not use any focalization system, in order to ave a wide acceptance angle /11/$ IEEE 828

2 Tx4 Tx1 given by te expression: Tx2 2 cosm (θ) cosm (ψ) Ts (ψ) g(ψ) (1) L2 were θ is te angle of irradiance, ψ is te angle of incidence at te, Ts (ψ) is te gain of an optical filter and g(ψ) is te gain of an optical concentrator placed in front of te detector [14]. P (θ =, ψ = ) is te received power at distance. In tis paper, we assume tat bot LED cip and detector ave a Lambertian radiation pattern wit different order, m and M respectively; moreover Ts (ψ) = g(ψ) = 1 because no filter and lens are being used at te. Wen bot T xi and are perpendicular to te floor we ave ψ = θ. In order to account for multiple reflections, we consider a pat-loss exponent correction factor γ [14]. Ten te Eq. 1 becomes, for te two T xi : P (θ, ψ) = P (, ) y Tx3 dx Data x dy Figure 1. Sceme of te proposed 2-D localization metod wit 4 auxiliary transmitters. Let us explain in detail our tecnique considering a 1-D system. Tis is reported in Fig. 2 were Li (i = 1, 2) are te distances between T xi (i = 1, 2) and ; is te distance between te planes of te transmitters and te receiver and Ω is te angle at wic te Data sould point in order to be in LoS wit. Te receivers and Data are placed at coordinate d tat sould be determined by te proposed localization algoritm. Tx1 l1 Tx2 l2 L2 θ1 θ2 Ω L1 d Figure 2. Description of 1-D localization algoritm. Te LEDs used for te localization sould also transmit a topology information, in order to provide te position of te T xi. For localization purposes, eac reference LED transmits a unique identifier. In our approac te unique ID is represented by a single tone. Eac tone is located at frequencies lower tan 7 khz, in a frequency region were tere is no overlap wit te ig data-rate signal. Te data transmission is acieved by using an adaptive OFDM signal, also known as Discrete Multi-Tone (DMT), over a bandwidt of 9 MHz. In order to establis te VLC link, te following steps must be followed. Te receiver first detects te number of reference LEDs, by counting te number of modulated tones. Ten te receiver computes its position witin te room by measuring te relative power emitted by eac localization LED using te propagation model described in in te following, and correlating it wit te location map of te localization LED, i.e. te coordinates li (see Fig. 2). To tis aim we recall tat, in te case of a single source, te power at te receiver is Pi (θi ) = Pi () γ cosm (θi ) cosm (θi ) i = 1, 2 Lγi (2) We used a value of γ = 4 as it fits well te typical range considered for indoor scenarios [15]. By using te relation Li = cos(θ, Eq. 2 can be rewritten as: i) Pi (θi ) = Pi () cosm+m +γ (θi ) i = 1, 2 (3) Using two auxiliary transmitters, as in Fig 2, we can remove te ambiguity in respect to te symmetry of te sources. Eq. 3 allows tus to individuate univocally te angles θi, provided tat we ave θi < π2. Measuring Pi (θi ) and knowing Pi () and te parameters m and M it is possible to obtain θi inverting Eq. 3. Once te couple θi as been determined, we obtain bot te displacement position d and te angle Ω using te known values of li and a simple trigonometric relationsip. For a 2-D system, te procedure must be modified, altoug it retains te same working principle of te 1-D case. In Fig. 3 we report a top-view of te system: te (wite box) is placed at coordinates (, ). Te T xi are placed along X and Y axes. Te is indicated by a gray box and placed at coordinates (dx, dy ). At tese coordinates, eac T xi delivers a fixed optical power as defined in Eq. 3: owever, tis information defines a circle of constant power for eac one of te T xi. Te coordinates (dx, dy ) can be ten evaluated by calculating te intersection point of tose circles. Once (dx, dy ) are known, te data receiver can cange its pointing angle toward te transmitter, establising a LoS link. A detailed derivation of te 2-D localization algoritm is currently under development. Of course, te localization accuracy increases by increasing te number of reference LEDs. A minimum of 3 reference LEDs is required for 2 D localization. In order to ensure LoS operation, te receiver sould be able to cange its orientation. Tis requires some adaptive optics, e.g. a steering mirror, or an electrical actuator able to rotate te lens-receiver pair over two axis. III. E XPERIMENT Te proposed metod as been experimentally investigated as described in tis section. Due to te lack of a large number 829

3 Data Rate (Mb/s) 42 y Tx3 φ = φ= +1 φ= Tx1 dx Tx2 Ω (deg) 5 x Figure 5. Experimental values of link data-rate vs displacement angle Ω. For optimal pointing angle (φ =, continous line). and at φ ± 1 (dots). dy 1 1 Data Tx4 FEC limit 2 BER Figure 3. Top View of te 2-D system: by using te power information from eac auxiliary LEDs (yellow), it is possible to find te coordinates of te (gray-box) in respect to te (wite box) placed at coordinates (,). 1.5 Bias Figure 6. AWG.5 φ (deg) BER as a function of te error on te pointing angle φ. EA f Data 1 d Real Time Oscilloscope Figure 4. Experimental Setup used to determine te angular tolerance to LoS mismatc of te VLC link. AWG: Arbitrary Waveform Generator; EA: Electrical Amplifier. of LEDs, we restrict our proof-of-concept demonstration to a 1-Dlocalization and a manually adapted receiver. As a preliminary step, we measured te performances of te data system, as te gross capacity vs. d and vs. angle Ω at a Bit Error Ratio (BER) as defined in Fig. 2. To tis aim, we used te setup presented in Fig. 4. Tis measurement allowed to evaluate te receiver (Data ) tolerance to te deviation of te pointing angle φ in order to provide te specification to te proposed localization metod. Te transmitter () was a commercially available wite composite RGB LED luminary (generating a luminous flux of 15 lm at typical driving currents of 35 ma, wit a radiation angle of 12, 12 MHz modulation bandwidt), consisting of tree cips radiating in te wavelengt regions of 625 nm (red), 525 nm (green) and 46 nm (blue). In order to drive all 3 LED cips simultaneously, tey ave been connected in series. Te DMT signal from te Arbitrary Waveform Generator (AWG) output was ten used to modulate te 3 LED cips under test. Before te modulation of optical sources, te AWG output signal was amplified (Mini Circuits ZHL 32A) and via a Bias-T combined wit te bias current (19 ma). Te bias value was cosen in order to optimize te link performance. After transmission over te visible-ligt cannel a commercially available silicon APD (Hamamatsu model C , 1 mm active area diameter, 1 MHz bandwidt) integrated wit a trans-impedance amplifier was used for detection of te optical signal. In addition we used a glass lens (25 mm focal lengt and 25 mm diameter) in front of te APD in order to increase te SNR of te received signal. Te electrical signal was acquired by a real time sampling oscilloscope for off-line post processing (including equalization and BER evaluation). Te DMT signal consisted of N = 128 sub-carriers witin a bandwidt B = 9 MHz (.73 MHz sub-carrier spacing). Bit and power loading was applied on N 1 sub-carriers (excluding te DC component). In all te following measurements, te distance was fixed 83

4 at 9 cm. At tis distance, te brigtness level was measured by a ligt-meter placed in front of te receiver and was found to be 3 lx (background ligt subtracted) for d =. Tis conditions is well below te value recommended by te standard for te working environments (5 lux) [16]. Te maximum gross capacity (bit rate) at a BER of (FEC limit), was measured as a function of te position d (i.e. angle Ω) for a pointing angle φ = (i.e. exact pointing toward te transmitter). Te same data-rate measurement vs. pointing angle φ was carried-out at tree representative displacements d: te zero-displacement position (Ω = ) and te two edge positions (Ω = ± 45, d = ± 9 cm). Te results are summarized in Fig. 5 were te continuous line sows te data-rate obtained for a BER of as a function of te displacement angle Ω, wit φ = (best pointing condition). Te diamonds and circles indicate te data-rate at Ω =, ± 45, for φ =, ± 1 respectively; we noted tat wen te pointing error exceeds ± 1 te received signal drastically drops and no furter measurement can be performed. However, as we see, a minimum capacity of 3 Mb/s was always guaranteed across te wole displacement range, wit a maximum value of 41 Mb/s. Te DMT adaptation algoritm was run at eac Ω value, tus te bit and power loading configuration was different for eac point. In Fig 6 we report a detailed analysis of te tolerance to te pointing angle φ. In tis case te data rate was fixed, te was at Ω =. In Fig. 6 we report te measured BER vs φ; from tis we see tat for φ = ± 1 te BER variation is below alf a decade. Furter increasing te error of te pointing angle results into a drastic performance decrease. We tus conclude tat te tolerance of te pointing angle is around ± 1. Te asymmetry in te curve is due to a non perfect alignment between te lens and APD receiver. AWG Bias a b ESA Tx Pre-Amp Figure 7. Setup for te caracterization of te radiation lobe and te acceptance angle of te auxiliary transmitter and receiver. We ten measured te radiation pattern of te auxiliary transmitters and te acceptance angle of te auxiliary receiver (indicated in te following as T x i, i = 1, 2 and respectively). Te auxiliary transmitter was a posporescent wite LED (generating a luminous flux of 22 lm at driving current of 7 ma). Te LED was incorporated in a polymeric lens tat narrowed its radiation pattern. Te auxiliary receiver was a Si potodiode (wit average.3 A/W responsivity across te LED emission spectrum) followed by a variable gain ig speed amplifier. For tis measurements bot te LED and te potodiode were mounted on steering plates and placed eac in front of te oter (see Fig. 7 tat illustrates te setup). In tis way, bot te LED and te potodiode could be rotated independently around teir axes. We define α and β as te LED and potodiode rotation angles. Te LED radiation angle was measured by rotating it around its axis, i.e. varying α, wile keeping te potodiode at β = ; similarly, te receiver acceptance angle was measured by rotating te PD respect by te angle β at α =. Te signal from a function generator at a frequency outside te DMT signal spectrum was used to drive te LED. Te DC driving current was 3 ma corresponding to te best LED working point. Te signal was acquired by means of an electrical spectrum analyzer (ESA) and te power of te modulating carrier was measured. Te collected data were fitted by a cos k ( ) function (we used te Lambertian approximation for bot te LED and PD [14]). We found tat tis is a good approximation in te range of ±55 (see Fig. 8). Te measured Lambertian emission order m for bot LEDs is found to be 2.99, wile an order M = 2 fits well te Si potodetector acceptance angle. Te effects of te variations of radiation pattern for different LED models will be te subject of future investigations. LED Radiation Power (a.u.) Angle α (deg) Receiver Acceptance (a.u.) Angle β (deg) Figure 8. Experimental measurement (stars) of te Lambertian radiation pattern of te reference LEDs (left) and angular acceptance of te receiver (rigt). Bot curves are fitted wit te a Lambertian function (solid line). Finally te setup described in Fig. 9, was realized in order to evaluate te effectiveness of te proposed pointing system. Te two T x i were positioned symmetrically, at a distance of 45 cm from te used for ig-capacity transmission. Te receiver was set at a distance = 9 cm from te line of te sources. Te receiver was mounted on a guide and can be moved at arbitrary x positions along X axis. For eac x position, it is possible to define te following 831

5 f 1 ~ AWG EA Bias Tx 1 Tx 2 L 2 f 2 ~ Ω (deg) 2 1 Maximum allowed pointing angle error q 1 L 1 q 2 d x Ω (deg) EA Figure 1. Experimental measurement of te pointing angle estimation error. ESA Figure 9. Complete Setup of te localization aided ig-capacity VLC link. Si PD: Si Potodiode; ESA: Electrical Spectrum Analyzer. parameters: te angle Ω, i.e te angle formed by te RGB LED and te receiver; te distances L 1 and L 2 between te auxiliary receiver and te two auxiliary transmitters; te angles θ 1 and θ 2, i.e. te angles formed between te auxiliary receiver and te auxiliary transmitters (T x 1 and T x 2 ). Te two T x i were modulated by a single frequency tone at f 1 = 5 and f 2 = 52 khz respectively. A first calibration was required, so te was positioned in front of te two T x i and te power P i (θ i = ), i = 1, 2, was measured. By moving te along te X axis and measuring P i (θ i ), te two angles θ i can be obtained inverting numerically te two equations 3. Te indices M and m were determined by te previous measurement. Once te couple θ i as been determined troug Eq. 3, it is possible to obtain bot te displacement position d and te angle Ω by using a simple trigonometric relationsip. We define Ω = Ω Ω m as te difference between te angle Ω evaluated by te proposed metod and te angle Ω m geometrically measured on te setup. In Fig. 1, we report an experimental measure of Ω as a function of Ω. As can be seen te corresponding pointing error was always below 1 as required by te data link specification (see Fig. 1). We note tat te estimation error is maximum wen te receiver is in front one of te T x i. As can be seen from Eq. 3, in tese positions θ = so tat te transmission function cos m+m+γ (θ) is maximum and its derivative against te displacement is close to zero. Tis results clearly in a greater position estimation error. As can be seen from Fig. 1, te estimation error is not exactly symmetrical. Tis can be due to several reasons. For example te two reference LEDs can ave a sligtly different radiation pattern. Notwitstanding, te overall error is witin te tolerances as reported in Fig. 4. IV. CONCLUSIONS We presented te experimental proof-of-principle of a VLC system based on RGB LEDs capable of providing a data-rate between 3 and 4 Mb/s witin an illumination angle of 9 at a distance of 9 cm. As te data link requires a strict LoS operation, we introduced a visible-ligt localization tecnique to provide tracking information to te receiver. Te angular resolution provided by te localization algoritm ( Ω < 1 ) allows to track te transmitter position witin te angular tolerance of te LoS VLC link. Te localization algoritm uses low frequency tones tat do not overlap wit te spectrum of te DMT data signal. REFERENCES [1] J. Llorca, A. Desai, U. Viskin, C. Davis, and S. Milner, Reconfigurable optical wireless sensor networks, in Proceedings of SPIE, vol. 5237, 24, p [2] K.-D. Langer, J. Grubor, O. Boucet, M. El Tabac, J. 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