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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.
- Multi-Gigabit/s OFDM real-time based transceiver engine for emerging 5G MIMO systemsPublication . Ribeiro, Carlos; Gomes, Rodolfo; Duarte, Luís; Hammoudeh, Akram; Caldeirinha, Rafael F. S.This paper presents a highly scalable multi-Gigabit/s Real-Time (RT) Wideband (WB) Orthogonal Frequency Division Multiplexing (OFDM) processing chain for 5G applications. It is aimed to significantly reduce the Hardware (HW) footprint of Multiple Input Multiple Output (MIMO) systems. In this context, implementation complexity results for MIMO configurations of 2 × 2, 4 × 4 and 8 × 8, enabled at sampling rates of 245.76, 122.88, 61.44 and 30.72 MHz, respectively, are compared with the ones from conventional MIMO architectures. For example, for a 8 × 8 MIMO–OFDM configuration implementation, savings of up to 87% are achieved when compared with conventional design methods, in terms of DSP48E1 slices. Moreover, in a Xilinx Virtex 7 XC7VX485T, it is shown that a second processing branch might be implemented, leading to a 5 Gbps (using 1024 Quadrature Amplitude Modulation (QAM)) Single-Input Single-Output (SISO) link or a 16 × 16 MIMO configuration. Finally, in order to validate the proposed architecture, the impact of its inclusion on a complete Field Programmable Gate Array (FPGA) 8 × 8 OFDM system, at LTE's highest sampling rate of 30.72 MHz, is evaluated. Subsequently, it is demonstrated that this does not affect the performance of OFDM, even in the presence of Carrier Frequency Offset (CFO).
- An iterative pilot-data-aided estimator for SFBC relay-assisted OFDM-based systemsPublication . Neves, Darlene; Ribeiro, Carlos; Silva, Adão; Gameiro, AtílioIn this article, we propose and assess an iterative pilot-data-aided channel estimation scheme for space frequency block coding relay-assisted OFDM-based systems. The relay node (RN) employs the equalise-and-forward protocol, and both the base station (BS) and the RN are equipped with antenna arrays, whereas the user terminal (UT) is a single-antenna device. The channel estimation method uses the information carried by pilots and data to improve the estimate of the equivalent channels for the path BS-RN-UT. The mean minimum square error criterion is used in the design of the estimator for both the pilot-based and data-aided iterations. In different scenarios, with only one data iteration, the results show that the proposed scheme requires only half of the pilot density to achieve the same performance of non-data-aided schemes.
- Effects of Channel Estimation on Multiuser Virtual MIMO-OFDMA Relay-Based NetworksPublication . Jiménez, Víctor P. Gil; Ribeiro, Carlos; Gameiro, Atilio; Armada, Ana GarcíaA practical multiuser cooperative transmission scheme denoted as Virtual Maximum Ratio Transmission (VMRT) for multiple-input multiple-output-orthogonal frequency division multiple access (MIMO-OFDMA) relay-based networks is proposed and evaluated in the presence of a realistic channel estimation algorithm and using low-density parity-check (LDPC) codes. It is shown that this scheme is robust against channel estimation errors. It offers diversity and array gain, keeping the complexity low with a multiuser and multiantenna channel estimation algorithm that is simple and efficient. In addition, the combination with LDPC codes provides improved gains; diversity gains larger than 6dB can be easily obtained with a reduced number of relays. Thus, this scheme can be used to extend coverage or increase system throughput by using simple cooperative OFDMA-based relays.
- An iterative pilot-data aided estimator for OFDM-based relay-assisted systemsPublication . Neves, D.; Ribeiro, Carlos; Silva, A.; Gameiro, A.We propose and assess a pilot-data aided channel estimation scheme for realistic relay-assisted scenarios designed for the downlink SFBC OFDM-based systems. We consider that both the base station and the relay node are equipped with an antenna array whereas the user terminal is a single antenna device. The relay protocol considered is equalise-and-forward. The channel estimation method iteratively uses the information carried by the pilots and data to improve the estimate of the equivalent channels for the path Base Station - Relay Node - User Terminal. The MMSE criterion is used in the design of the estimator for both the pilot-based and data-aided iterations. The results show the benefits of cooperation in terms of MSE and also show that the proposed scheme allows, with a single data iteration, a reduction of the number of pilots when compared with non-data-aided schemes.
- A Time Domain Channel Estimation Scheme for Equalize-and-Forward Relay-Assisted SystemsPublication . Neves, D.; Ribeiro, C.; Silva, A.; Gameiro, A.In this paper we propose a scheme with a dedicated relay node for an OFDM-based system. We consider an antenna array at the base station and a single antenna at both the user terminal and the relay node. The relay-assisted protocol considered is equalize-and-forward. The proposed scheme requires the computation of parameters which depend on unknown variables at the destination and we show that under some assumptions the scheme is suitable to provide all those requirements. Also, we show that since the pilots at the relay node are properly designed the TD-MMSE estimator can still provide the required channel estimate. The system performance is evaluated by considering a typical pedestrian scenario based on WiMAX specification. The results show that the degradation in the estimator performance is negligible.
- 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.
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