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- 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.
- 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.
- 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.
- Single carrier optical FDM in visible light communicationPublication . Saied, Osama; Ghassemlooy, Zabih; Kizilirmak, Refik Caglar; Dai, Xuewu; Ribeiro, Carlos; Zhang, Min.; Rajbhandari, SujanLight emitting diodes (LEDs) used in visible light communications (VLC) have a limited optical power-current linear range, which could be a problem in multi-level and multicarrier frequency modulation schemes. Orthogonal frequency division multiplexing (OFDM) as a popular scheme in VLC suffers from nonlinear distortion due to the high peak to average power ratio (PAPR). In this paper, we present a novel single carrier optical frequency division multiplexing signaling scheme for VLC, where the symmetrical characteristics of single carrier frequency division multiple access is investigated in order to reduce PAPR. Simulation results show that PAPR reduced by 10 dB compared to the traditional asymmetrically clipped optical OFDM. We show that the bit-error-rate performance of the proposed scheme is enhanced compared to ACO-OFDM when the limited dynamic range of digital to analog converter and LED is considered.
- 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.
- 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.
- 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.
- 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.
- A software-defined radio FPGA implementation of OFDM-based PHY transceiver for 5GPublication . Ribeiro, Carlos; Gameiro, AtílioThis paper introduces a software-defined radio implementation of an OFDM-based transceiver for the prototyping and testing of 5G physical layer algorithms. The implementation uses high level abstraction tools to develop and test the algorithms, significantly reducing the time and effort needed to test new features. The proposed architecture adopts interconnecting FIFOs between each functional block, reducing the critical paths and enabling complex designs to be implemented at higher clock rates. The proposed LTE-like transceiver is implemented using COTS FPGA and RF development boards. The real-time over-the-air demonstrator has an on-the-fly scalable bandwidth from 20 to 61.44 MHz, attaining close to 500 Mb/s when using 256-QAM modulation.