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- DLL architecture for OFDM based VLC transceivers in FPGAPublication . Duarte, Luis; Rodrigues, Luis; Alves, Luis N.; Ribeiro, Carlos; Figueiredo, MónicaThis paper addresses the problem of achieving high bandwidth in a DLL design for OFDM based VLC broadcast systems. It describes the implementation of efficient Data Link Layer (DLL) and Forward Error Correction (FEC) modules in a Xilinx FPGA. The proposed DLL aims at furnishing the adequate means to fragment and route both high data-rate (HDR) and moderate data-rate (MDR) service requests while maintaining a continuous transmission flow. The FEC modules aims at providing sufficient error correction capabilities with reasonable computation overheads. Another goal was to develop these modules under a globally asynchronous locally synchronous paradigm, ensuring high modularity and performance.
- Lighting the Wireless World: The Promise and Challenges of Visible Light CommunicationPublication . Figueiredo, Mónica; Alves, Luis Nero; Ribeiro, CarlosLight-emitting diodes (LEDs) are becoming increasingly ubiquitous. They can be found in illumination appliances, phones, TVs, advertising panels, dashboards, and traffic signals, among others. Most illumination applications are becoming LED based, mainly due to their long operational lifetime and high energy efficiency, which is nowadays higher than 100 lm/W [1]. Other benefits include enhanced sustainability, a compact form factor, easier maintenance, and lower cost. For these reasons, LED lighting is expected to have a market share of 84% in the general illumination market by 2030 [2]. However, there is another characteristic that is not being fully exploited: LEDs are capable of switching their light intensity at a rate that is imperceptible to the human eye. This property has been used for dimming purposes but can also be utilized in the opportunistic deployment of value-added services based on visible light communication (VLC). Since 2011, VLC technology has gained momentum, supported by the release of the IEEE 802.15.7 draft standard [3] that defines the physical and medium-access control layers. This norm specifies data rates of up to 96 megabits/s for indoor and outdoor applications [3]. Since then, several research demonstrators have shown that VLC is capable of achieving gigabits-per-second transmission with commercial off-the-shelf red-green-blue (RGB) and phosphorescent white LEDs [4]. This is the result of the increasing attention that this technology has attracted in both the research community and global society in recent years. Figure 1 illustrates this trend over the last five years by depicting the number of results for the search term visible light communication conducted using the IEEE Xplore online library and Google search engine. A key VLC requirement is that it must be based on illumination-grade LEDs and comply with the illumination requirements and safety recommendations [5]. Also, data transmission should have minimal impact on LED performance, such as color temperature, color rendering index, and lifetime [6]. Most state-of-the-art VLC demonstrators have already proved they can achieve data rates compatible with the envisioned applications, but they do not address lighting quality issues. Thus, further investigation is still necessary to guarantee the seamless integration of lighting and communication services, which is crucial for the general deployment of this technology [7]. A pertinent question that one might ask is, “Why use light signals when we can use radio-frequency (RF) signals to communicate?” The visible light spectrum can be used synergistically with common radio technology. First, as the available RF bandwidth is limited, highly regulated, and increasingly congested, it may be helpful to use a portion of the spectrum that is unlicensed, currently largely unused, and amenable to spatial reuse. This is especially relevant in the realm of technologies beyond fifth generation (5G), where the density of users and devices with communication needs is predicted to scale up exponentially. Second, there are many application scenarios where the use of radio signals raises concerns related to e-smog, privacy, and security. Third, in scenarios where line of sight (LOS) and locality are important and the illumination infrastructure is already deployed (e.g., offices, stores, or vehicles), VLC can be a complement to current RF communications. Finally, light can be a good medium for low-cost and/or low-latency short-range links for near-field communications or high-bandwidth download links. Table 1 shows the most relevant visible light and RF signal characteristics, highlighting their complementarity. Broadcasting content to end users is the most natural service envisioned for directional technologies such as VLC. Luminaires are supposed to transmit a low- [8] or high-data-rate service [9] for end users located in their illumination area. However, VLC can also be used for full duplex communication, as long as the uplink and downlink can be separated [10]. This can be done by division in wavelength, in time, or in code or by resorting to spatial isolation. Alternatively, VLC can be combined with RF in heterogeneous networks [11]; VLC provides a high-capacity, uncongested, and unregulated downlink path, while RF is used in the uplink, where congestion is less likely. In the past couple of years, several interesting surveys have been published on VLC. Some are focused on physical-layer techniques [12], while others cover medium-access protocols [13], networking techniques and sensing [14], and lighting requirements [15]. A survey on more general optical wireless communications can be found in [16]. In this article, we provide a brief state-of-the-art overview of the technology and the main upcoming challenges.
- Live demonstration: 150Mbps+ DCO-OFDM VLCPublication . Figueiredo, Monica; Ribeiro, Carlos; Alves, Luis Nero; Figueiredo, MónicaThis demonstration showcases the potential of OFDM-based Visible Light Communication (VLC) systems for indoor high-speed communications using off-the-shelf LEDs, suitable for illumination. The physical layer was implemented in a Xilinx Virtex-6 FPGA using System Generator. It was designed with a modular architecture, enabling collaboration with other groups with interest in this field. This demonstration explores the optical front-end out-of-band bandwidth to transmit, showing the usefulness of OFDM-based schemes. Using 64QAM and 25MHz modulation bandwidth, it is possible to transmit over 50cm at 150Mbit/s with a BER smaller than the usual 3.8x10-3 limit.
- Time precision comparison of digitally controlled delay elementsPublication . Figueiredo, Mónica; Aguiar, Rui L.This paper compares the time precision of different digitally controlled delay cells. Other design metrics as delay, signal integrity, power and area are also considered. The precision is evaluated by the cell's uncertainty, given as the ratio between jitter and time delay, considering both thermal and power supply noise. The comparison is based on circuit simulation, in a 180nm technology, using the SPECTRE tool from Cadence. The comparison results will help the designer to choose the most appropriate delay cell for low uncertainty design.
- A Study on CMOS Time Uncertainty with Technology ScalingPublication . Figueiredo, Mónica; Aguiar, Rui L.This paper evaluates the clock generation quality of different digital circuits associated with clock generation and distribution. Circuit’s noise response, jitter, and uncertainty are evaluated for different noise sources and loading conditions. We present performance simulations for inverters and inverter chains implemented in different technologies from AMS and UMC foundries. We show that the device size-scaling trend is increasing the uncertainty associated with this circuits, decreasing their precision. The correlation between circuit’s parameters and selected performance metrics is also highlighted.
- Performance Analysis of 8-bit ODACs for VLC ApplicationsPublication . Dobesch, A.; Figueiredo, Mónica; Alves, L. N.; Wilfert, O.A discrete optical power level stepping technique in visible light communication (VLC), also known, as an optical digital to analog conversion (ODAC) has been proposed. This is an alternative concept for VLC front-end design, able to mitigate the LED intrinsic non-linearity. Additionally, it removes the need of an electrical digital to analog conversion (EDAC) in the driver stage. This paper provides an experimental evaluation of two different ODAC front-ends. The results investigate the spatial relation between the optical front-end and the optical receiver. In addition, the performance evaluation employs dynamic test metrics rather than conventional static metrics previously reported in the literature.
- A real-time platform for collaborative research on Visible Light CommunicationPublication . Ribeiro, Carlos; Figueiredo, Mónica; Alves, Luis NeroThis paper reports the implementation and performance results of a real-time Visible Light Communication platform. The platform uses a high-level software abstraction architecture where the algorithms are developed and tested in MATLAB, using a modular architecture. With this modular approach it is very easy to add/remove blocks, making it amenable to collaboration with other groups with interest in this field, offering a real-time test bed to evaluate the performance of different modules, algorithms and optical front-ends, which is currently not available. The physical layer is based on DCO-OFDM and is implemented in a Xilinx Virtex-6 FPGA. We explore the optical front-end out-of-band bandwidth to transmit, showing the usefulness of adopting OFDM-based modulations schemes for VLC. Our system transmits at 2m over an indoor free space channel with 12MHz bandwidth, with more than 23dB gain difference, using QPSK, thus achieving 24Mbit/s with a BER smaller than the usual 3.8×10-3 limit.