Percorrer por autor "Torres, Paulo"
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- Adaptive error control coding for a mobile broadband systemPublication . Torres, Paulo; Caseiro, Carlos; Gusmão, AntónioThe Mobile Broadband System (MBS) under research in Europoe aims at offering to mobile users an ATM-based radio access to the future integrated broadband communications network. It is well known that a high-quality transmission channel can be assumed within the fixed broadband network; however, this is far from being expected in the radio links of the future mobile system. This paper is concerned with the design of an error control coding scheme for a first stage of MBS implementation. The proposed scheme is a very flexible, hybrid one, which is based on the concatenation of a convolutional inner code, and a Reed-Solomon outer code, and combines ARQ and FEC features in an adaptive manner, under an implicit evaluation of the channel state. It is powerful enough to cope with poor channel conditions and is compatible with both ATM characteristics and those of a wide range of services (including required BER below lo-' with a delay of a few ms). A set of simulation results illustrates the advantages of the proposed error control coding scheme.
- A class of iterative FDE techniques for reduced-CP SC-based block transmissionPublication . Gusmão, António; Torres, Paulo; Dinis, Rui; Esteves, NelsonFor conventional CP-assisted (Cyclic Prefix) block transmission systems, the CP length is selected according to the expected maximum delay spread. In this paper, an SDDC-aided (Soft Decision Directed Correction), iterative FDE (Frequency Domain Equalization) technique is presented for reduced-CP, SC-based (Single Carrier) block transmission systems using conventional frame structures. A more sophisticated Turbo SDDC-FDE technique is then proposed for improved performances in such systems, through a moderately increased complexity. The relations with some already known iterative FOE techniques are established, and a set of performance results is used to show the advantages of the SDDC-aided approach, namely the following: the fact that it can operate in an acceptable way even when, for the sake of implementation simplicity, no decoding operations are used to improve the iterative FDE process; the possibility of achieving the maximum power efficiency gain that a strong CP reduction allows.
- Code-assisted SC/FDE block transmission with reduced cyclic prefix overheadPublication . Gusmão, António; Torres, Paulo; Dinis, Rui; Esteves, NelsonFor conventional CP-assisted (Cyclic Prefix) SC/FDE implementations (Single- Carrier/Frequency Domain Equalisation), as well as for OFDM implementations (Orthogonal Frequency Division Multiplexing), the CP length is known to be selected on the basis of the expected maximum delay spread. Next, the data block size can be chosen to be large enough to minimise the CP overhead, yet small enough to make the channel variation over the block negligible. This paper considers the possibility of reducing the overall CP assistance, when transmitting sequences of SC blocks, while avoiding an excessively long FFT block for FDE purposes and keeping good performances through a moderate increase of the FDE receiver complexity. Firstly, we present an algorithm for a Decision-Directed Correction (DDC) of the FDE inputs when the CP is not long enough to cope with the time-dispersive channel effects. Next, we present and evaluate a novel class of reduced CP SC/FDE schemes, which takes advantage of the DDC algorithm for replacing ”useless” CP redundancy by fully useful channel coding redundancy. A very efficient block transmission, especially for both strongly time-dispersive and time-varying channel conditions, is then achieved.
- Códigos de controlo de erros para o sistema MBSPublication . Torres, PauloO objectivo do sistema MBS (Mobile Broadband System), actualmente objecto de pesquisa na Europa, no âmbito do projecto RACE R2067/MBS, é disponibilizar aos utilizadores móveis um acesso rádio de base ATM à futura rede de banda larga (IBCN). A implementação de um interface rádio para o sistema MBS não é tarefa simples, uma vez que as condições de propagação tornam os canais disponíveis bastante adversos à radiotransmissão (ritmos altos de transmissão sobre portadoras na banda das ondas milimétricas), e os objectivos dos serviços são ambiciosos (por exemplo: probabilidades de erro abaixo de 10-6 com atrasos de alguns milisegundos). Esta tese dedica atenção especial ao contributo de códigos de controlo de erros para bons compromissos entre qualidade do serviço e requisitos de potência de emissão. No início da tese, é descrito a interface rádio do sistema MBS (características e parâmetros principais), fazendo-se também referência às limitações e requisitos de transmissão. É apresentado em seguida o esquema de controlo de erros escolhido, baseado na concatenação de um código interno convolucional com rendimento 3/4 e de um código externo construído sobre um campo finito GF(26), que é um código RS(60,36) encurtado. Trata-se de um esquema híbrido (ARQ/FEC), muito flexível, desenhado para combinar capacidades de detecção e correcção de erros de um modo adaptativo; quando solicitado, é possível enviar um burst médio, com redundância adicional (para aumentar a capacidade de controlo de erros), em alternativa à retransmissão convencional. A operação do esquema de controlo de erros proposto é dependente de um limiar que é estabelecido de uma forma flexível, de acordo com os requisitos e as características do serviço, e, possivelmente, o estado do canal. Para finalizar a tese, são discutidos os resultados de desempenho num determinado cenário de simulação. Estes resultados demonstram claramente que o esquema de controlo de erros proposto é poderoso e flexível, podendo ser adaptado a uma variada gama de serviços e características do canal, e é uma contribuição importante para o desenvolvimento de um sistema eficiente.
- Detection issues with many BS antennas available for bandwidth efficient uplink transmission in a MU-MIMO systemPublication . Torres, Paulo; Gusmão, AntónioThis paper is concerned with Single Carrier (SC)/Frequency Domain Equalization (FDE) for bandwidth efficient uplink block transmission, with Quadrature Amplitude Modulation (QAM) schemes, in a Multi-User (MU) Multi-Input Multi-Output (MIMO) system. The number of Base Station (BS) receiver antennas is assumed to be large,but not necessarily much larger than the overall number of transmitter antennas jointly using the same time/frequency resource at Mobile Terminals (MT). In this context, we consider several detection techniques and evaluate, in detail, the corresponding detection performances (discussed with the help of selected performance bounds), for a range of values regarding the number of available BS receiver antennas. From our performance results, we conclude that simple linear detection techniques,designed to avoid the need of complex matrix inversions, can lead to unacceptably high error floor levels; however, by combining the use of such simple linear detectors with an appropriate interference cancellation procedure - within an iterative Decision-Feedback (DF) technique, a close approximation to the Single-Input Multi-Output (SIMO) Matched-Filter Bound (MFB) performance can be achieved, even for 64-QAM schemes, after a few iterations.
- Detection performance with many antennas available for bandwidth efficient uplink transmission in MU-MIMO systemsPublication . Torres, Paulo; Gusmão, AntónioThis paper is concerned with Single Carrier (SC)/Frequency Domain Equalization (FDE) for bandwidthefficient uplink block transmission, with Quadrature Amplitude Modulation (QAM) schemes, in a Multi-User (MU) Multi-Input Multi-Output (MIMO) system. The number of Base Station (BS) receiver antennas is assumed to be large, but not necessarily much larger than the overall number of transmitter antennas jointly using the same time/frequency resource at Mobile Terminals (MT). In this context, we consider several detection techniques and evaluate, in detail, the corresponding detection performances (discussed with the help of selected performance bounds), for a range of values regarding the number of available BS receiver antennas. From our performance results, we conclude that simple linear detection techniques, designed to avoid the need of complex matrix inversions, can lead to unacceptably high error floor levels. However, by combining the use of such simple linear detectors with an appropriate interference cancellation procedure - within an iterative Decision-Feedback (DF) technique -, a close approximation to the Single-Input Multi-Output (SIMO) Matched-Filter Bound (MFB) performance can be achieved after a few iterations, even for 64-QAM schemes, when the number of BS antennas is, at least, five times higher than the number of antennas which are jointly used at the user terminals.
- Error Correction Repetition Codes with Arduino and Raspberry PIPublication . Torres, Paulo; Malhão, SérgioThis work consists in the development of software with the main objective of increasing the quality of the communication system with Repetition Codes and GFSK modulation. To evaluate the probability of error we used simulation in software Python and implemented with hardware (Raspberry Pi, Arduino and transceiver NRF24L01).
- Evaluation of clipped-OFDM and SC/FDE alternatives for block transmission using iterative receiver techniquesPublication . Torres, Paulo; Gusmão, AntónioA CP-assisted (Cyclic Prefix) block transmission is widely accepted as a good choice for future mobile systems, taking advantage of low-cost, flexible, FFT-based (Fast Fourier Transform) signal processing technology with both OFDM (Orthogonal Frequency Division Multiplexing) and SC (Single Carrier) alternatives. The OFDM alternative provides a simple receiver implementation which allows good performances, by exploiting the channel coding effort, but is known to suffer from a high PMEPR (Peak-to-Mean Envelope Power Ratio), leading to power amplification difficulties. The SC/FDE (Frequency-Domain Equalization) alternative is easily compatible with low PMEPR values, but the FDE (MMSE) receiver performance is known to be far from the optimum. In this paper, we consider a reduced-PMEPR, ClippedOFDM transmission, where an advanced receiver performs the iterative cancellation of the deliberate nonlinear distortion effects. In parallel, we consider an advanced Turbo FDE receiver, as well as simplified iterative FDE receiver implementations, within the SC alternative. The "SC vs OFDM" comparisons of this paper, under the use of iterative receiver techniques in both cases, show a very significant overall advantage for the SC side.
- Iterative receiver technique for reduced-CP, reduced-PMEPR OFDM transmissionPublication . Torres, Paulo; Gusmão, AntónioWhen a low-PMEPR (Peak-to-Mean Envelope Power Ratio) is intended for OFDM (Orthogonal Frequency Division Multiplexing) transmission, a well known solution is to resort to signal processing schemes that involve nonlinear operations (e.g., an appropriate clipping) in the time domain. In this paper, where we consider an OFDM-based trans- mission under a deliberate nonlinear distortion and a com- plementary frequency-domain filtering, we assume that the length of the cyclic prefix (CP) is below the channel memory order. In this context, a new iterative receiver technique is proposed, which can cancel both the nonlinear distortion effects and the interblock interference effects, through a mod- ified iterative decision-feedback procedure, simultaneously providing the appropriate CP reconstruction effects. A set of performance results is used to show the practical interest of the proposed approach for power/bandwidth efficient OFDM-based block transmission.
- Iterative receiver techniques for SC-FDMA uplink block transmission: design and performance evaluationPublication . Charrua, Luís; Torres, Paulo; Gonçalves, Victor; Gusmão, António
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