Browsing by Author "Ribeiro, Carlos"
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- A Software-Defined Radio for Future Wireless Communication Systems at 60 GHzPublication . Gomes, Rodolfo; Duarte, Luis; Ribeiro, Carlos; Caldeirinha, RafaelThis paper reports on a complete end-to-end 5G mmWave testbed fully reconfigurable based on a FPGA architecture. The proposed system is composed of a baseband/low-IF processing unit, and a mmWave RF front-end at both TX/RX ends. In particular, the baseband unit design is based on a typical agile digital IF architecture, enabling on-the-fly modulations up to 256-QAM. The real-time 5G mmWave testbed, herein presented, adopts OFDM as the transmission scheme waveform, which was assessed OTA by considering the key performance indicators, namely EVM and BER. A detailed overview of system architecture is addressed together with the hardware considerations taken into account for the mmWave testbed development. Following this, it is demonstrated that the proposed testbed enables real-time multi-stream transmissions of UHD video content captured by nine individual cameras, which is in fact one of the killing applications for 5G.
- All-digital reconfigurable STDCC radar baseband implementation in FPGAPublication . Duarte, Luís; Ribeiro, Carlos; Alves, Luís N.; Caldeirinha, Rafael F. S.This paper reports the architecture of an all-digital Swept Time-Delay Cross-Correlator (STDCC) baseband. Until recently, the sliding correlator technique has been mainly em-ployed for sounding the radio propagation channel. However, recent benchmarks have shown promising results in target detection context when compared to commercially available solutions. STDCC takes advantage of the sliding correlation properties of Pseudo-Noise (PN) sequences. Therefore, this paper presents the baseband generation for this new radar technique with on-the-fly sequence tuning using a Field-Programmable Gate Array (FPGA). The reconfigurable STDCC radar baseband generates both PN sequences digitally and requires a low-cost ADC to acquire the time dilated result. At the end, the proposed architecture is evaluated regarding resource usage efficiency and then the radar performance will be discussed in terms of the all-digital PN sequence spectrum and the real-time slide correlation. Our analysis confirmed a strong correlation between both sequence length and sampling frequency with radar detectable distance.
- Channel Estimation Schemes for OFDM Relay-Assisted SystemsPublication . Neves, Darlene; Ribeiro, Carlos; Silva, Adao; Gameiro, AtilioThe aim of this paper is to adapt and evaluate the performance of two classical pilot-aided channel estimation schemes designed for OFDM systems: least square and minimum mean square error, in a relay-assisted scenario. We consider the Amplify-and-Forward, which is the simplest relay protocol for cooperative systems. Also, we consider the use of a single antenna at all the terminals: base station, user terminal and relay node. In our scenario, it is assumed that some of the user terminals deployed in a certain area could act as relaying-able terminals for the communications of other users. In this communication we show that the classical channel estimation techniques designed for conventional wireless communications can be extended for relay-assisted based systems under some modifications.
- Consumer LED lamp with ODAC technology for high-speed visible light communicationsPublication . Figueiredo, Mónica; Ribeiro, Carlos; Dobesch, Ales; Alves, Luís N.; Wilfert, OtakarOptical digital-to-analog converters (ODACs) were recently proposed to mitigate the non-linear characteristic of light emitting diodes (LEDs) and to simplify the transceiver in optical wireless communication systems. This paper describes the implementation and performance of an ODAC lamp built with commercial phosphor coated LEDs. It is shown to have sufficient performance to cope with data-rates compatible with those recommended for Ultra HD quality internet download speed (25Mbps), despite the very low bandwidth of these LEDs. Also, the structure of a low-cost and high-speed OFDM-based Visible Light Communication transceiver, based on commercial LEDs and off-the-shelf components, is presented. This solution does not require D/A conversion or complex LED driving circuitry in the transmitter, which are usually seen as limiting factors to the deployment of high-speed VLC systems in the consumer LED lighting market.
- Disruptive Future of Radar Based on All-Digital PN Signal ProcessingPublication . Ribeiro, Carlos; Ferreira Gil, João; Caldeirinha, Rafael; Reis, Joao R.; Sardo, Andre; Duarte, Luis; Leonor, NunoThis paper presents the first results on the design and implementation of a real-time and high resolution monostatic radar at 24 GHz, based on the sliding correlation of pseudonoise (PN) sequences. The real-time radar, with a high time resolution better than 4ns, is used for moving target identification (MTI) in the presence of highly dense clutter, under harsh environments and severe weather conditions (fog, snow and fire smoke or plume). A radar signal processing based on alldigital PN sequences is proposed, which represents a quantum leap in radar future front-end architecture. Results obtained in a controlled environment, inside an anechoic chamber, are presented and a benchmark with a commercially-of-the-shelf solution is presented. © 2019 IEEE.
- 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.
- Estimation of CFO and Channels in Phase-Shift Orthogonal Pilot-Aided OFDM Systems with Transmitter DiversityPublication . Ribeiro, Carlos; Gameiro, Atílio; Ribeiro, CarlosWe present a CFO estimation algorithm and an associated channel estimation method for broadband OFDM systems with transmitter diversity. The CFO estimation algorithm explores the TD structure of the transmitted symbols carrying pilots and data, relying solely on the data component present on the symbols to estimate the CFO, thus avoiding additional overhead like training symbols or null subcarriers. An intermediate output of the CFO algorithm provides an easy-to-get initial CIR estimate that will be improved with the utilization of a TD LMMSE filter. The feasibility of the investigated methods is substantiated by system simulation using indoor and outdoor broadband wireless channel models. Simulation results show that the joint algorithms provide a near optimal system’s 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.
- A mm Wave solution to provide wireless Augmented Reality in classroomsPublication . Gomes, Rodolfo; Sismeiro, Luis; Ribeiro, Carlos; Sanchez, Manuel G.; Hammoudeh, Akram; Caldeirinha, Rafael F. S.This paper presents a very high throughput wireless data transmission solution for Augmented Reality (AR) Head Mounted Devices (HMDs) in the 60 GHz band. Its feasibility is assessed in an Orthogonal Frequency Division Multiplexing (OFDM) downlink communication towards five User Equipment (UE) devices, placed at different locations inside an indoor classroom environment scenario. Key Performance Indicators (KPIs) as Error Vector Magnitude (EVM), Signal-to-Noise Ratio (SNR), and path loss are considered for Quality of Service (QoS) assessment. Experimental results demonstrate the reliability of the proposed mmWave system, where a data rate of 312.5 Mbps is achieved over distances of 1, 2.4 and 4.1 m, regardless the UE position.