J. Rabadan

Experimental evaluation of video transmission through LED illumination devices

In this paper we describe the implementation of a prototype of an optical wireless system based on visible white LED lamps, which allows a video broadcasting to reach a bit rate of 2 Mbps. This technology is usually called VLC or Visible Light Communications and presents several advantages as the robustness against EM interference, safety for human eye and security against undesired network access. These conditions make this system suitable for co-existing with commercial RF networks -WiFi, Bluetooth, etc.-, especially for in-house applications. For the uplink channel we have also included in the prototype an infrared 115 kbps transceiver. This VLC system could be used for supporting data transmission applied to low-speed sensor network connections as well. The electronic structure of a low-cost VLC transceiver, based on commercial off-the-shelf components and LED lamps is presented too. The modulation process and the Ethernet interface implemented in each access point are also described. Finally, some conclusions and application scenarios are drawn.

Reading lamp-based visible light communication system for in-flight entertainment

This paper explores the use of a reading lamp as an access point for a Visible Light Communications (VLC) downlink channel. We have established an infrared uplink channel based on a network adapter, supporting both a VLC receiver and an infrared emitter. The optical signal power distribution over the passenger area has been also studied using a Monte Carlo Ray-Tracing algorithm. The hardware implementation and testing results are also presented.

Design and Implementation of an Ethernet-VLC Interface for Broadcast Transmissions

In this work, a complete interface between Ethernet and Visible Light Communications (VLC) networks is presented. It requires the use of previously proposed DPPM schemes for this kind of applications, but with some modifications so as to keep constant duty cycle in order to assure its use in illumination facilities. The prototype has been tested using a 2Mbps VLC Link, obtaining distances of, at least, 3 meters in an interfering environment. The PPM characteristics allow transmission without severe signal degradation on this environment. This Ethernet-VLC interface is intended to demonstrate the capability of IP broadcast applications of this kind of devices. This interface performs packing functions and flow control.

Time-hopping spread-spectrum system for wireless optical communications

This paper describes an optical wireless system based on time-hopping spread-spectrum techniques, as a first approach, at infrared wavelengths. It can be used either for low-speed sensor interconnections or as a return channel in a visible light communications system. The main advantage of this scheme is an improvement in the narrowband interference rejection capability and the number of simultaneous users supported, compared with other wireless optical standards. Moreover, a wide analysis is carried out about the mutual interference between time-hopping spread-spectrum systems and other infrared ones. Finally, some conclusions are drawn about some design parameters of the time-hopping system in order to allow its compatibility with preexisting optical devices.

Experimental characterization of a direct sequence spread spectrum system for in-house wireless infrared communications

The design and experimental characterization of a wireless infrared direct-sequence spread spectrum (WIR-DSSS) system is presented This prototype support low and medium data rates (up to 64 kbps) enough for most consumer appliances, but can be easily increased to rates of about 1-2 Mbps if required. This system is designed to be used in the control and interconnection of heterogeneous appliances in the in-house environment, such as microwave ovens, washing machines, refrigerators, audio equipment, etc. The application of this technology for these kinds of channels allows the possibility of interchanging events without the need for a line-of-sight path between emitter and receiver. This eliminates the effect of ISI induced by multipath propagation and also permits the sharing of the channel avoiding the use of complex protocols (such as CSMA) using code-division multiple-access (CDMA) strategies.

Experimental characterization of a direct-sequence spread-spectrum optical wireless system based on pulse-conformation techniques for in-house communications

In this paper, the design and experimental testing of a direct-sequence spread spectrum (DSSS) system for wireless infrared LAN is presented. This method improves the wireless IR communications performances against multipath propagation and narrowband interference. We have implemented a IR-DSSS prototype with 100 MHz code frequency and 6 Mbps bit rate. We have also studied the effects of both Gaussian and sinusoidal signal conformation over wireless optical DSSS systems. These results show that these techniques reduce the adjacent-channel interference without loss of process gain or substantial reductions of BER vs. S/N performances. We also present some results about a simplified synchronization structure to be used on reduced-cost domestic equipment. These systems are suitable for different applications at low and medium rates, especially for in-house.

Considerations on modulations and protocols suitable for visible light communications (VLC) channels: Low and medium baud rate indoor visible ligth communications links

This paper describes some applications under development for optical wireless systems based on visible light communications (VLC). The indoor scenario defines services which can be provided to users inside a room such as video broadcasting or home automation. For this purpose a 2Mbps visible downlink and a 125 kbps infrared uplink were used. In this work, we present the Ethernet interface that has been implemented in each AP (access point), the light modulation used and the sensor network developed for demonstration. The main advantages of VLC technology are the robustness against electromagnetic (EM) interference, safety for human eye, and security against undesired network access.

Filtered modulation schemes for short distance infrared wireless communications

This paper describes two modem prototypes based on spectrally efficient filtered modulations for infrared wireless links. They are fully compatible with commercial IrDA and IEEE 802.11 standards. These links allow more channels, giving a more accurate use of the infrared spectrum. We compare the performances of the two main alternatives: phase (using FQPSK-2 systems) and frequency modulations (GMSK schemes). We have designed and tested 2 Mb/s IR modems with each modulation scheme. Extensive experimental results and BER measurements of these prototypes are also detailed.

Pulse width modulated optical OFDM

In this paper, the PWM encoding of an optical OFDM transmission is studied and simulated. Traditional optical OFDM schemes make use of linear drivers, but the inherent nonlinear behavior of the optical emitters produce a harmful effect on the demodulated signal. PWM allows the use of nonlinear drivers, avoiding this harmful effects and increasing the efficiency of the transmission. Furthermore, synchronization is easier and can be edge-directed, taking advantage of the pulsed nature of the PWM. As OFDM samples may be considered normally distributed, nonlinear predistortions can be used to increase the BER performance. The proposed modulation is studied in terms of spectral efficiency, energy efficiency and thoroughly simulated for different parameters.

Hybrid Visible Light and Ultrasound-Based Sensor for Distance Estimation

Distance estimation plays an important role in location-based services, which has become very popular in recent years. In this paper, a new short range cricket sensor-based approach is proposed for indoor location applications. This solution uses Time Difference of Arrival (TDoA) between an optical and an ultrasound signal which are transmitted simultaneously, to estimate the distance from the base station to the mobile receiver. The measurement of the TDoA at the mobile receiver endpoint is proportional to the distance. The use of optical and ultrasound signals instead of the conventional radio wave signal makes the proposed approach suitable for environments with high levels of electromagnetic interference or where the propagation of radio frequencies is entirely restricted. Furthermore, unlike classical cricket systems, a double-way measurement procedure is introduced, allowing both the base station and mobile node to perform distance estimation simultaneously.