An Aeronautical Visible Light Communication System to ... - IEEE Xplore

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Dhouha Krichene, Maha Sliti, Walid Abdallah, and Noureddine Boudriga. Communication Networks and Security Research Lab., University of Carthage, Tunisia.
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An Aeronautical Visible Light Communication System to Enable In-Flight Connectivity Dhouha Krichene, Maha Sliti, Walid Abdallah, and Noureddine Boudriga Communication Networks and Security Research Lab., University of Carthage, Tunisia ABSTRACT This paper proposes an aeronautical network architecture based on visible light communication (VLC) technology which targets the distribution of in-flight entertainment services. To this purpose, we investigate the deployment of LEDs within an aircraft cabin using two different wavelength assignment methods in the VLC cells. The first method combines both WDM and Direct sequence OCDMA techniques to reduce intra-cell and inter-cell interferences. In the second one, a two-dimensional OCDMA scheme is used to enable efficient sharing of resources between users. Moreover, an FSO-based inter-VLC-cells communication scheme is described to enable connectivity distribution among LEDs and inter-cells handover. This scheme is based on all-optical switching using code-words that uniquely identify the cells. Finally, a simulation work is conducted to evaluate the bit error rate of the proposed access control schemes for different configurations of the VLC system. Keywords: in-flight connectivity, VLC, OCDMA, FSO. 1. INTRODUCTION Offering an in-flight data communication service is becoming a trend that is adopted by many plane constructors towards the next generation aircrafts. Some companies are providing Wi-Fi based solutions to offer access to internet within the aircraft. However, the Wi-Fi spectrum band may interfere with the coexisting flight systems. Cognitive spectrum allocation methods were proposed to mitigate the interference between RF-based systems. However, promoting the primary flight systems in the spectrum allocation may degrade the quality of the perceived services, especially the in-flight entertainment multimedia applications. Optical wireless connectivity provides many advantages for this kind of applications because it uses optical wavelengths instead of radio frequencies, avoiding therefore the interference with radio navigation systems. Recently, visible light is increasingly investigated as a nature-friendly, low cost, high data rate and secure communication solution for indoor wireless applications. This technology uses either coloured LED chips or white LEDs mixing different monochromatic wavelengths to ensure ubiquitous lighting and communication without interfering with RF systems. This property can be beneficial for in-flight entertainment applications by providing higher transmission data rates, while not interfering with other navigation systems. Several research groups are addressing the usage of VLC technology to implement an in-flight networking infrastructure. In [1], authors introduce a VLC prototype for indoor applications, where the transmitter is equipped with a certain number of white LEDs to provide enough brightness for both illumination and high speed communication. In their prototype, they apply a blue filtering method to eliminate the yellow phosphor layer that radiates from the LED and which causes the degradation of the modulation bandwidth due to its long response time. Authors proved that blue filtering technique can provide higher speed communication with acceptable SNR and BER when combined with the adequate modulation and coding scheme. In [2], authors propose a full optical scheme for passenger connectivity within the aircraft. Their system uses the VLC as a downlink communication system while the infra-red mode is used as an uplink channel. However, they did not address access control in the network. Furthermore, they use Ethernet connection between the LEDs that are mounted on the aircraft ceiling. In [3], the authors propose a combined wavelength division and code division multiple access scheme for cells and users differentiation. For this purpose, two types of orthogonal codes are proposed. The Walsh-Hadamard (WD) code is used to differentiate users within the same cell, while the phaseshifted maximum-length PN sequences are assigned to neighbouring cells using the same wavelength channel in order to mitigate the inter-cell interference. In addition, the authors in this work consider an indoor environment that consists of an office with a rectangular geometry. This paper investigates the design of an in-flight communication infrastructure using an indoor VLC system, where several LEDs are deployed along the aircraft cabin to provide communication as well as illumination capabilities. Particularly, we propose multiple access schemes based on the WDM and OCDMA techniques using an appropriate optical code generation function. The proposed schemes reduce the inter and intra-cell interference. In addition, we define a new optical switching technique based on optical and orthogonal codes to ensure packet forwarding between different VLC cells. The reminder of the paper is organized as follows. Section 2, describes the global structure of the in-flight VLC network; Section 3 presents the two proposed access control schemes and investigates the codeword-based optical switching which is used to relay packets within an acceptable delay between the different LEDs composing the network. In Section 4, the system performance is analysed using simulation work. Finally, a conclusion is drawn in Section 5.

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2. Global System Architecture In this section, we describe the architecture of the VLC-based in-flight entertainment network. This architecture is depicted by Figure 1. It encompasses three communication sub-systems; namely: − The ground to aircraft link: this link provides a connection between the airplane and the related Internet Service Provider. It can be a Free Space Optic link via a satellite working as an intermediate node, − The network backbone: The aircraft data network is an all-optical network where the links connecting the LEDs on the ceiling of the cabin are FSO-based links. The FSO technology is selected to ensure a fast and high data communication rates. In addition, we assume that each LED has an integrated optical switch that enables packets forwarding from cell to cell. This can address passenger’s mobility within the aircraft. − The access network link: It is the user link within a cell, which is a line-of-sight VLC link from a LED lamp towards the user terminal. However, in the uplink direction, the link can be based on a direct FSO beam from the user terminal to the ceiling. We also note that the user terminal could be a computer or a smart phone equipped with an adapter that is able to perform an electro-optical conversion.

Figure 1. The proposed System architecture. In the sequel, we will focus on the design and the implementation of the two later parts of the network. One main problem that must be addressed is LEDs deployment on the ceiling of an aircraft cabin. Two methods are studied. The first one uses white LEDs with 4 coloured wavelengths that are red, green, blue and cyan. Each LED forms a VLC cell where a single wavelength is selected for communication and the other colours are only used for illumination. In the second method all the wavelengths are used for both communication and illumination. A two-dimensional OCDMA code is used to allow user access control to optical resources. These two methods will be detailed in the next Section. An issue that can be mentioned in our system, is the case where some passengers want to turn-off the lighting system while their neighbours need to remain connected. Two solutions can be envisioned to fix this problem. The first one is to deploy communication capability in the reading lighting lamp of every passenger such as the system developed in [2]. Nevertheless, this approach will increase the needed number of the communicating LEDs and the inter-VLC cells interference. The second solution will be to switch to infrared mode in the downlink instead of VLC mode if one of the neighbouring passengers would like to sleep. Therefore, the visible light could be turned off to guarantee the comfort of neighbours. However, the infrared light should be in this case tightly directive to the video terminal of the target user to ensure an acceptable in-flight connectivity. 3. User Access control and inter-cell communication In this section, we propose two methods to enable multiple-access in the VLC system. These two methods are based on the use of orthogonal optical codes (OOC) to discriminate between users. It is noteworthy that these codes are dedicated to users within the same cell to reduce the intra-cell interference. We also adopt a generation process based on the lattice point theory to comply with the orthogonality requirement [4]. 3.1 User access control based on WDM and OCDMA In the first channel assignment option we propose to use white LEDs combining four different wavelengths; namely, the red, green, blue and cyan wavelengths. In this type of LEDs, all the wavelengths are used for illumination, as studied in [5]. Nonetheless, only one wavelength is selected to be used for communication in each optical cell to enable the wavelength reutilization within the cabin. In order to reduce the inter-cell interference, we propose to assign the monochromatic wavelengths to the different cells using graph colouring concept, as depicted by Figure 2.

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It is worthy to note that each white LED covers 5 passengers’ seats and a monochromatic wavelength is selected to communicate optical data in the downlink direction within the cell in question. Each user will be assigned optical code-word using time domain coding on the wavelength selected for communication.

Figure 2. Monochromatic wavelengths assignment for communication using graph colouring. 3.2 User access control using two dimensional code-words The second option is based on 2D optical encoding scheme for the generation of OOC users’ code-words. This 2D coding scheme uses the 3 coloured wavelengths (red, blue, green) to construct a code-word for each user independently on his position in the aircraft cabin. Specifically, each data packet is switched to the target user having the appropriate code-word independently to which optical cell he is attached to. In this case, we assume that an embedded central unit will affect and distribute the orthogonal code-words for users. An example of 2D coding using 3 coloured wavelengths is illustrated by Figure 3.

Figure 3. 2D coding. In this example the code-word is written as a sequence (100000100000100) which is physical represented by a sequence of pulses that are transmitted during specific time interval called a chip using a given wavelength. In this case, the 2D code can be written as (λ000000λ100000λ200), where λ0,λ1, and λ2 are the wavelengths of the blue, red, and green colours respectively. 3.3 Inter-cells communication using codeword-based optical switching In addition to the user code-words, we propose to use the same code generation function to allocate orthogonal optical codes to different cells. Thus, each codeword uniquely identifies the considered cell. Whenever a user has data to communicate to a destination user belonging to a distant cell, or if a mobile user hands over from a cell to another, each source cell can switch a multicast traffic to the destination cell using those optical code-words. Recall also that multicast traffics are switched using FSO-based backbone which links the different LEDs. In this multicast switching, each cell considers four code-words indicating its direct neighbours: the left neighbouring cell, the right neighbouring cell, the front-end neighbouring cell and the backward neighbouring cell [6]. Moreover, a shortest path is defined in terms of number of hops by promoting the nearest cell with the lightest intra load traffic and which does not present inter-cell interference with the source cell. 4. PERFORMANCE EVALUATION In this section, we assess the performances of the proposed access control schemes using simulation work. We first present the adopted channel model and indoor environment parameters. Then we give numerical results that compare between the two wavelengths allocation methods in term of bit error rate. 4.1 Optical Channel Model The optical channel is characterised by two kinds of links that are the line-of-sight (LOS) link and the non-lineof-sight (NLOS) link. For the proposed indoor VLC applications, the contribution of the NLOS link is negligible compared with the LOS link. Moreover, we assume that the transmitting LED follows a Lambertian radiation pattern [7,8]. Therefore, the path loss can be expressed as:

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= α

(m + 1) Ar × cos m ( β ) 2π D 2

(1)

Where m is the order of the Lambertian radiation, Ar is the active receiving area, D is the distance between the transmitter and the receiver known also as the “vertical separation”, and β is the emission angle from the transmitter (the LED). It is worthy to notice that the Lambertian order m depends on the semi-angle φ1/2 at half luminance of the LED as follows: m=

− log(2) log(cos(ϕ1/ 2 ))

(2)

4.1 Simulation set-up

Figure 4.Cabin model and indoor aeronautical VLC environment. We assume that the cabin is a half-cylinder with radius r. To set up the VLC network inside the aircraft cabin, we propose to fix the LEDs on the ceiling according to a certain angle θ, as shown in Figure 4. As it had been previously indicated, the seats are organized into three rows. One in the middle and the two other rows are placed on the two sides of the cabin. We propose to fix three LEDs over each 5 seats belonging to the three parallel rows respectively. Because our system should support an acceptable illumination, the LEDs should be bright enough with wide radiation angle. In addition, each LED lamp consists of arrays of white LEDs to ensure the illumination requirement. For every simulation we evaluate the bit error rate (BER) for the two access control approaches by varying the passengers’ positions. The main objective is to achieve an appropriate dimensioning of the VLC cells that minimises interferences and reduces bit error rate. This is performed, by firstly, determining the signal to noise and interference ratio (SNIR) using the following formula inspired from [1]. SNIR =

N (εα Pt ) 2 Rb N 0 + I

(3)

Where Rb is the bit rate, ε is the responsivity, α is the path loss, Pt is the transmitting power, N0 is the noise power spectral density, N is the length in bits of the transmitted signal, and I is the power of interference resulting from adjacent cells that use the same wavelengths. By considering and on-off keying (OOK) scheme the BER can be expressed as follows: (4)

BER = Q( SNIR )

It is worthy to note that the adopted communication model does not take into consideration the effect of the reflection of optical rays on the cabin walls and seats. Table 1 summarizes the parameters used in the simulation work. Table 1. Simulation parameters. Simulation parameters Number of LEDs in each array group Optical Transmit Power (Pt) Half radiation angle Field of view Photodiode active receiving area

Value 3600 (60×60) 0.18 mW φ1/2 = 70 [deg] 60 [deg] 1 cm2

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Photodiode responsivity PSD of shot noise (N0) Bit rate Rb Radius of the cylinder (the cabin) Cabin length Angular position of LEDs in the ceiling of the cabin Number of users per cell

0.21 A/W 2×10-22 A2/Hz 1 Mbps 4m 48 m θ = [π/4, π/2, 3π/4] rad 5

4.2 Simulation results In this subsection, we compare between the two wavelength assignment methods in terms of BER in order to figure out how both methods deal with inter-cell interference. Figure 5, shows the variation of the bit error rate (BER) with the deviation between the position of the passenger and the angle θ that reflects the angular location of the LEDs in the ceiling of the cabin for both wavelength assignment methods. As it is shown by the curves, the BER is an increasing function of the deviation angle and varies from 10-6 to 10-2 for both methods. In fact, the smaller the angle is, the closer the passenger to the LED position in the ceiling of the cabin. Therefore, the interference coming from cells deployed on the other rows will be smaller. On the other hand, when the deviation angle is bigger, the passenger become nearer to the other LEDs lamps in the middle row, which increases the interference with this LEDs and hence decreases the Signal to Noise and Interference ratio (SNIR). This will degrade the BER as shown above. Besides, we notice that the BER of the two methods are close for small values of the deviation angle. Recall that in the first allocation method, a 1-D coding is applied to generate the OOC codes, while the second method uses a 2-D coding to generate the optical code words. However, when the angle is equal to π/3 = 1.0472 rad (the LEDs are closer to each other), the BER computed with the first method is less than the one obtained with the second method. This is because in the second method, all the wavelengths are used at the same time for communication, which will increase the interference between the adjacent users. Indeed, when a single wavelength is used for data transmission such that monochromatic wavelengths are assigned according to the graph colouring method (which is the case of the first method), the interference between users in the downlink will be reduced.

Figure 5. Variation of the BER with respect to the polar angle θ.

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Figure 6. Variation of BER in function of the position of a user along y-axis. Figure 6 shows the variation of the BER in function with the mobility of a user along the y-axis. We notice that as the distance separating the user from his cell becomes bigger, the BER increases. This is because when a passenger moves away from his cell, the vertical separation between him and the transmitter lamp will increase. Therefore, the quality of the received signal from his serving cell will decrease as well. Moreover, we observe that the first wavelength allocation method ensures better BER than the second method, especially when the vertical separation is important. This is because the interference obtained by the adjacent cells using the same wavelengths at the same time couple with the effect of the important distance separation. We can determine radius of a VLC cell, which cannot exceed 3 m to limit the inter-cell interference. In addition, although the BER values obtained by both methods are comparable, the 2-D coding ensures better data rate within a single cell than in the 1-D case. This is because, in the 2D encoding scheme, the three wavelengths are available for communication, which allows the support of an increasing number of users within the same VLC cell than in the 1-D case. Ultimately, a 2-D coding can be more appropriate for our system since it allows a higher data rates while reducing the BER caused by the inter-cell interference. 5. CONCLUSION In this paper, we proposed indoor VLC-based network architecture to provide In-flight entertainment applications. Moreover, we describe two methods to enable multiple-access in the VLC system. Both schemes are based on the combination of WDM and OCDMA techniques. In addition, we present an all-optical switching scheme based on FSO technology which uses optical code-words to enable communication between different cells while reducing the inter-cell interference. Finally, the performance evaluation of the proposed VLC network is performed using two multiple access schemes and some results related to VLC-cells dimensioning had been elaborated. REFERENCES [1] Z. Wu, J. Chau: Modelling and designing of a new indoor free space visible light communication system, in Proc. European Conference on Networks and Optical Communications (NOC 2011), Newcastle-UponTyne, Jul. 2011. [2] C. Quintana, V. Guerra, J. Rufo, J. Rabadan, R. Perez-Jimenez: Reading lamp-based visible light communication system for in-flight entertainment, IEEE Transactions on Consumer Electronics, vol. 59, no. 1, pp. 31-37, Feb. 2013. [3] K. Cui, J. Quan, Z. Xu: Performance of indoor optical femtocell by visible light communication, Optics Communications Journal., vol. 298-299, pp. 59-66, Jul. 2013. [4] B. D. Ivan and V Bane: Novel combinatorial construction of optical orthogonal codes for incoherent optical CDMA system, Journal of Lightwave Technology, vol. 21, no. 9, pp.1869-1875, Sep. 2003. [5] E.F Schubert: Light-Emitting Diodes, Second Edition, Cambridge University Press, 2006. [6] M. Sliti, W. Abdallah, N. Boudriga: An optical multicast protocol for LEO satellites based on optical codewords, International Journal on Advances in Telecommunications, vol. 7, no. 3&4, pp. 56-68, 2014. [7] J.M. Kahn, J.R. Barry: Wireless infrared communications, in Proc. of the IEEE, Feb. 1997. [8] T. Komine, M. Nakagawa: Fundamental analysis for visible light communication system using LED lighting, in Proc. of Ninth IEEE Symposium on Computers and Communications, 2004.

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