Enrique Poves

Experimental optical code-division multiple access system for visible light communications

An optical code-division multiple access system (OCDMA) is presented for visible light communications (VLC). The OCDMA scheme is based on the recently proposed random optical codes (ROC), which do not present optimal correlation properties but are characterized by their ease of generation when large number of simultaneous users are considered. The synchronism mechanism when using ROCs is an important issue which is adequately addressed and solved by the designed system. An experimental prototype has been developed which allows us to transmit medium-quality audio data from several emitting sources simultaneously through different channels (using different codes), obtaining a practically error-free reception (imperceptible by the human ear) which perfectly agrees with the theoretical analysis.

Random optical codes used in optical networks

An analysis of a simple way to design generalised optical orthogonal codes to be used in optical code-division multiple-access systems is presented. They are random optical codes (ROCs). This novel family is especially suitable for optical networks with a large number of channels, low bit rate requirements, energy limitations and packed data. For example, sensor networks need these requirements. An application in an intra-spacecraft telecommand and telemeasurement (TC/TM) optical wireless network is also described. ROCs are selected randomly from all possible rearrangements. However, its probability of error is adequate in many useful parameter ranges. Moreover, ROCs present features not found in other families of codes such as huge cardinality and parameter adaptation to the system necessities. Here, the main contribution is to present a method to optimise the values of the parameters. Furthermore, the optimal weight of the codes is found, given two out of the other set of parameters, that is, the length, the number of users and the probability of error. In addition, a comparison between the length and the power consumption of ROCs and prime codes is also developed.

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.

Random optical codes in an intra-satellite optical wireless network

We present a novel family of optical codes to be used in intra-satellite optical wireless networks. They are random optical codes (ROCs). This family is specially adapted for optical networks where the number of channels varies, ever large, low bit rate requirements, energy limitations and packed data are present. For example sensor networks have these requirements. The implementation in an intra-spacecraft telecommand and telemeasurement (TC/TM) optical wireless network is also described. Besides we propose a method for the comparison between ROCs and generalized optical orthogonal codes (OOCs). A decision between both family based on the performance for a particular set of parameters is given by the method. An application of the method is also provided. We have compared the families in the intra-spacecraft optical wireless network.

Use of optical orthogonal codes for intra-spacecraft communications

This work presents the application of optical orthogonal codes (OOC) for intra-spacecraft communications within a sensor network with a large number of terminals. In particular, a novel family of optical codes named random optical codes is introduced for the target application. These codes allow a simple optimization of the length and weight of the code while maintaining a controlled probability of error and a low duty cycle. Also, due to a straightforward generation process they can be adjusted almost in real-time to adapt to new conditions like including new channels. This flexibility makes this family a good candidate for systems with strict restrictions over this parameters.

Comparison between the Performance of Algorithmic Optical Codes and Orthogonal Optical Codes in OCDMA Systems

A novel method to be used in OCDMA systems is exhaustively described in this paper. It uses algorithmic optical codes (AOCs). They are based on the signature sequence idea. However patterns are constantly changing. A common seed for pseudo-random sequence is the signature element. In this paper a derivation of the probability of error due to the multiple-access interference is provided. An other contribution of this work is also the comparison between the AOCs and the orthogonal optical codes (OOCs) with (auto- and cross-) correlation equal to one. Attending to the cardinality limitation problem observed in OOCs, the comparison is focused on the number of users allowed by the two different systems aforementioned. As result we state that a system using AOCs allows more users than one using OOCs when the codes are long enough.

Algorithm Optical Codes: An alternative to Random Optical Codes in an intra-satellite optical wireless network

We propose a novel family of optical codes to be used in intra-satellite optical wireless networks. They are algorithm optical codes (ADCs). This family is related with the recently introduced family random optical codes (ROCs). Both families of codes are designed to be used in optical code-division multiple-access (OCDMA) systems. Similarly than their predecessor, i.e. ROCs, ADCs are specially suitable for optical networks where the number of channels varies, ever large, low bit rate requirements, energy limitations and packed data are presented. For example sensor networks demand these requirements. A detailed description of AOCs is shown in this paper. Furthermore a comparison between them and ROCs is presented. We find that AOCs provide less probability of error than ROCs, for a given length of the code and a number of users. Moreover a system using AOCs can change the length of the code easier than a system using ROCs. This fact permits a more efficient accommodation to the actual number of users. Finally the implementation in an intra-spacecraft telecommand and telemeasurement (TC/TM) optical wireless network is also described. We compare the families in the intra-spacecraft optical wireless network.

Code acquisition of random optical codes in optical code-division multiple-access

In this study, the authors present a technique for code acquisition in optical code-division multiple-access systems using the recently introduced random optical codes. In these systems, the communication is divided in two parts: a synchronisation process and the data transmission. The main target of the study is to describe and analyse the new algorithm of synchronisation using Markov processes. Once described the code acquisition technique and established an estimation for the probability of false alarm (FA) of the proposed scheme, two different proposals for the optimisation of the code parameters are described. They are based on two different estimators for the probability of FA. The first one considers the expected value for the probability of FA. The second one is very much restrictive. It guarantees that the probability of FA is in a confidence interval. It takes the upper bound of the confidence interval as estimator for the probability of FA. Although the second one presents worse acquisition time, the authors consider that the gain provided by the first method is not sufficient enough to assume its loss in the reliability. Thus, the authors propose to guarantee the probability of FA in a confidence interval. A compromise between reliability and acquisition time is obtained with this second method.