F.J. Lopez-Hernandez

A VLC-based beacon location system for mobile applications

This paper proposes a low cost and complexity indoor location and navigation system using visible light communications and a mobile device. LED lamps work as beacons transmitting an identifier code so a mobile device can know its location. Experimental designs for transmitter and receiver interfaces are presented and potential applications are discussed.

Wireless optical communications for intra-spacecraft networks based on OCDMA with random optical codes

In recent years, spacial agencies have shown a growing interest in optical wireless as an alternative to wired and radio-frequency communications [1,2]. The use of these techniques for intra-spacecraft communications reduces the effect of take-off acceleration and vibrations on the systems by avoiding the need for rugged connectors and provides a significant mass reduction. Diffuse transmission also eases the design process as terminals can be placed almost anywhere without a tight planification to ensure the proper system behaviour. Previous studies [3,4] have compared the performance of radio-frequency and infrared optical communications. In an intra-satellite environment optical techniques help reduce EMI related problems, and their main disadvantages - multipath dispersion and the need for line-of-sight - can be neglected due to the reduced cavity size. Channel studies [5] demonstrate that the effect of the channel can be neglected in small environments if data bandwidth is lower than some hundreds of MHz.

LED jitter-induced limitation effects in the baud rate of VLC systems

In this paper we show that the effect of jitter due to driver and LED is the limiting factor in the baud rate in L-PPM formats for VLC systems.

VLC-based light-weight portable user interface for in-house applications

Advances in solid-state lighting have overcome common limitations on optical wireless such as power needs due to light dispersion. Its been recently proposed the modification of lamps drivers to take advantages of its switching behaviour to include data links maintaining the illumination control they provide. In this paper, a remote access application using visible light communications is presented that provides wireless access to a remote computer using a touchscreen as user interface.

Optical orthogonal codes for DS-CDMA in OWLS

We present a search-algorithm for the construction of optical orthogonal codes (OOCs) to be used in OWLS (Optical Wireless Links for intra-Spacecraft communications) systems. It is based on trial-and-error search, but a special effort is done in reducing the size of the initial set of sequences that are candidates to be part of the desired code. The approach for identifying that initial set of candidates is done through the mathematical concept of partitions. Constraints related to the desired auto- and cross-correlation properties are applied at different steps before the trial-and-error search commences, with the aim of keeping to a minimum the size of the candidates set.

Modulation schemes effect on the driver efficiency and the global VLC transmitter energy consumption

Visible Light Communications (VLC) make the most of the fast switching times of LEDs to transmit information. High transmission rates have been achieved with different modulation schemes as CSK or OFDM. However, the main asset of LED lamps, which is energy efficiency, has been overlooked. In this paper we analye how the modulation schemes affect the energy efficiency of LED lamps. We compare the effect of switched waveform (OOK, VPPM) and continuous waveform (OFDM) modulation schemes. Switched waveform signals present energy efficiency results between 15% and 30% better than continuous waveform signals.

Multi-wavelength modelling for VLC indoor channels using Montecarlo simulation

In this paper we present a compact and efficient method for the calculation of the channel response over indoor wireless optical channel for multi-wavelength signals. It is based on the Modified Montecarlo Algorithm but allows not only processing composed signals as used in VLC transmission with white or RGB LED emitters, but also modeling reflections where wavelength components of the incoming signals are modified, as could be the case of phosphor materials or wavelength-dependent reflecting surfaces. It also allows modeling not only lambertian reflections but also non-lambertian, quasi-specular surface, as could be plastic or metallic furniture. The algorithm uses a wavelength transformation matrix that can be defined for each reflecting material, and even for each incidence angle.

V-shape liquid crystal-based retromodulator air to ground optical communications

This paper describes the use of a 2D liquid crystal retro-modulator as a free space, wireless, optical link. The retro-modulator is made up of a retro-reflecting corner-cube onto which 2 cascaded V-shape smectics liquid crystal modulators are mounted. The communication link differs with respect to more conventional optical links in not using amplitude (nor frequency) modulation, but instead state-of-polarisation (SOP) modulation known as Polarisation Shift Keying (PolSK). PolSK has the advantage over amplitude modulation, that it is less sensitive to changes in the visibility of the atmosphere, and increases inherently the bandwidth of the link. The implementation of PolSK both in liquid crystal based and in retro-modulated communication are novelties.