In the proposed system topology, each passenger is considered to be inside a microcell. However, it could be possible that nearby users or cabin illumination lamps generate harmful interference due to a wide lamp emission pattern or a high photodiode FOV. In addition, reflections on different objects could dramatically reduce the system performance. Here we opt for the use of optical lenses for collimating the light beam in the passenger’s table. A simulation based on a Monte Carlo-Ray tracing algorithm has been performed in order to calculate the Signal Noise Ratio (SNR) at different points of the user’s table. We have considered 3 emitters, corresponding to the three reading lights, and 740 receivers distributed over the passenger’s table.
The following table shows the parameters used in all the performed simulations.
Photodiode Area 66 mm2
Photodiode FOV 120º
Photodiode responsivity 0.63 A/W
Photodiode darkness current 2 nA
Transimpedance BW 5 MHz
Amplifier NEP 750 W/?Hz
Number of rays 100.000
Number of reflections 10
Objects reflection coefficients 0.6-0.7
Fig. 2 shows the cabin model developed, which has been used by the simulator to determine the environment under study.
Fig. 2. Aircraft environment model.
Different simulations varying the lambertian coefficient (n) of the lamp have been performed, getting reflectance patterns, and then SNR and SIR distribution patterns from a top view.
Fig. 3. SIR distribution for user 2.
The rectangle represents the passenger’s table, where the device should be placed, although a good system performance can be offered in all the area of the passenger’s seat. Fig. 3 shows SIR values at different points of the user’s table when a LED lamp with n=50 is used. Under this situation, the worst ratio is up to 16.85 dB. Fig. 4 shows the CR- DPPM (Constant Rate DPPM) bit error rate evolution when is interfered by the adjacent user. In spite of this is the codification scheme used to implement the downlink channel in this work, it can be changed without loss of generality by other PPM schemes such as the VPPM models currently under consideration by the IEEE 802.15.7 committee.
100 DPPM interfered by the adjacent user
Bit error 10- 4
10 – 6 SIR=15dB
10 – 8
8 9 10 11 12 13 14 15
Fig. 4. Simulated BER evolution of CR-DPPM codification scheme when is interfered by the adjacent user.
Based on these simulation results, we can affirm that the system performance reduction is negligible so as there is a BER deterioration of 0.25 dB. In spite of that, the effect could be mitigated increasing the lambertian coefficient of the lamp or even employing WDM (Wavelength Division Multiplexing) techniques over RGB LEDs. Fig. 5 shows the Signal Noise Ratio distribution over the user’s table, which shows values up to 70 dB. These ratios do not affect the system performance.
Fig. 5. SNR distribution for n=50 for user 2.
IV. VLC DOWNLINK IMPLEMENTATION
In order to use an illumination lamp as a VLC device, a codification scheme that allows a constant illumination level at the lamp and data transmission is needed. In this work, CR-DPPM (Constant Rate DPPM) is considered because of its simplicity and low cost of implementation, which is only based on an edge detector and a counter. This scheme does not only offer the advantages of DPPM (which encodes the data by modifying the distance between the pulses) , but also allows a constant bit rate, which makes the implementation of multimedia applications easier. In addition, it is suitable to be used simultaneously in communications and illumination systems due to the absence of light flickering.
Different codification schemes have been studied as well. OFDM techniques offer both good multipath and narrowband interference rejection response, but imply quite expensive hardware requirements . VOOK and VPPM schemes have been proposed in the under development VLC standard (802.15.7) as modifications of the OOK and PPM codifications respectively, with the aim of adding dimming capabilities. However, OOK does not guarantee the absence of flickering and both of them (OOK and PPM) have lower spectral efficiency than DPPM. Regarding the PPM scheme, it is characterized by having a better system performance against AWGN, but it is not a real necessity in VLC applications as the SNR values are commonly really high. The synchronization task, on its part, turns out to be much more difficult to accomplish than in CR-DPPM. If VPPM or VOOK are eventually defined as the codifications for the standard, many of these results would be also applied without significant modifications.
The communications behavior of LED lamps is limited by rise and fall times (100 ns for white phosphor LEDs), so the pulse width should be at least 200 ns, which is the upper bound of the lamp switching rate. In addition, the LED switching should be fast enough to avoid light flickering, fixing the lower frequency limit at several hundreds of Hz. The ratio between the ON and OFF periods determines the illumination level of the lamp.
The proposed topology might work with data coming from PLC, twisted pair cables or optical fiber. The first solution avoids the necessity of performing any changes in the cabin topology as it uses the power lines of the aircraft itself to transmit the data. However, it could be noisy and generate EM interference with the aircraft instrumental systems. We shall consider, as an alternative, Ethernet as a distribution network inside de aircraft and the use of a Power over Ethernet (PoE) system to feed them up, and so the same shielded twisted pair cable will be used to transmit the data and to power the lamps. This fact generates a weight reduction in the aircraft installation needed to offer this kind of services. As commented on above, there are two different adapters: the lamp adapter and the passenger adapter. Both of them have a similar functionality, but they differ in the power supply needed and the final optical interface they use to make the electro-optical conversion. The lamp adapter is fed with a PoE device and uses a visible light LED to do the transmission, whereas the passenger adapter uses the USB port as power supply, which offers up to 500mA at 5V.
A. Ethernet-wireless optical system adapter
The interface between the Ethernet data and the optical access point is based on a bridge between the network and the optical downlink. This bridge detects the packets, modifies them at the MAC layer and finally codifies the frames in CR-DPPM to be transmitted through the VLC downlink. Multiple access strategies are not to be considered a major concern for this application as the VLC emitter can be optically limited by
lenses to illuminate only the passenger’s area, and the uplink is pointing upwards. If needed, contention protocols can be used, but this is beyond the scope of this paper.
The designed adapter is responsible for both managing the Ethernet Controller, which means to control the packet read and write processes, and making the CR-DPPM codification/decodification. In order to implement the first task, a microcontroller with an embedded Ethernet module has been chosen. This property does not only offer a total system cost reduction, but also makes the communication task between both devices easier. This microcontroller is characterized by having five 8-bits-width ports, 4 external sw