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- Técnicas de codificação de canal e descodificação/detecção iterativa para comunicações móveis de alto débitoPublication . Torres, PauloEsta tese resultou duma pesquisa sobre a contribuição da codificação de canal e dos métodos de descodificação/detecção para a melhoria do desempenho geral do sistema, num contexto de transmissão por blocos com prefixo cíclico. O trabalho principal incidiu em técnicas avançadas de recepção iterativa, envolvendo descodificação SISO (Soft-In, Soft-Out) para cancelamento brando de interferências: interferência intersimbólica residual no caso SC (Single Carrier); interferência não-linear, devido a um ‘’clipping’’ no emissor, no caso OFDM (Orthogonal Frequency Division Multiplexing). Tanto para a opção SC como para a opção OFDM com “clipping”, são introduzidos nesta tese aperfeiçoamentos face a outras técnicas de recepção iterativa anteriormente propostas, incluído extensões para receptores com diversidade espacial, no que diz respeito à transmissão por blocos com prefixo cíclico completo, de acordo com a memória do canal. Uma das principais inovações desta tese é o algoritmo DDC (Decision Directed Correction) e suas aplicações numa transmissão em que o peso do prefixo cíclico é reduzido, preservando implementações simples baseadas na FFT (Fast Fourier Transform), nomeadamente para extensões das técnicas iterativas atrás mencionadas. Para concluir esta tese, são fornecidas comparações relevantes ''SC vs OFDM'', no contexto de recepção iterativa avançada, e é enfatizada a vantagem global da opção SC.
- A reduced-CP approach to SC/FDE block transmission for broadband wireless communicationsPublication . Gusmão, António; Torres, Paulo; Dinis, Rui; Esteves, NelsonFor conventional cyclic prefix (CP)-assisted single-carrier/frequency-domain equalization (SC/FDE) implementations, as well as for orthogonal frequency-division multiplexing (OFDM) implementations, 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 minimize 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 fast Fourier transform window for FDE purposes and keeping good FDE performances through low-complexity, noniterative receiver techniques. These techniques, which take advantage of specially designed frame structures, rely on a basic algorithm for decision-directed correction (DDC) of the FDE inputs when the CP is not long enough to cope with the time-dispersive channel effects. More specifically, we present and evaluate a novel class of reduced-CP SC/FDE schemes, which takes advantage of a special frame structure for replacing "useless" CP redundancy by fully useful channel coding redundancy, with the help of the DDC algorithm. When using the DDC-FDE technique with these especially designed frame structures, the impact of previous decisions, which are not error-free, is shown to be rather small, thereby allowing a power-efficiency advantage (in addition to the obvious bandwidth-efficiency advantage) over conventional block transmission implementations under full-length CP. Additionally, the DDC algorithm is also shown to be useful to improve the power efficiency of these conventional implementations.
- Practical implementation of repetition codesPublication . 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).
- 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.
- On the SCFDE uplink alternative to OFDM in a massive MU-MIMO contextPublication . Torres, Paulo; Charrua, Luís; Gusmão, AntónioThis paper deals with Single Carrier (SC)/Frequency Domain Equalization (FDE) as an uplink alternative to Orthogonal Frequency Division Multiplexing (OFDM) for a Multi User (MU)-Multi-Input Multi-Output (MIMO) system where a ”massive MIMO” approach is adopted. In this context, either an optimum Minimum Mean-Squared Error (MMSE) linear detector or appropriate reduced-complexity linear detection techniques are considered. Regarding performance evaluation by simulation, two semi-analytical methods are proposed - one method in the optimum (MMSE) case and the other one in the reduced-complexity cases. This paper includes performance results for uncoded 4 Quadrature Amplitude Modulation (QAM) SC/FDE transmission and a MU-MIMO channel with uncorrelated Rayleigh fading, under the assumptions of perfect power control and perfect channel estimation.The accuracy of performance results obtained through the semi-analytical simulation methods is assessed by means of parallel conventional Monte Carlo simulations. The performance results are discussed in detail and we also emphasize the achievable ”massive MIMO” effects, even for the reduced complexity detection techniques, provided that the number of BS antennas is much higher than the number of antennas which are jointly employed in the terminals of the multiple autonomous users. Appropriate ”SC/FDE vs OFDM” comparisons are also included in this discussion of performance results.
- On the uplink detection performance in MU-MIMO broadband systems with a large number of BS antennasPublication . Torres, Paulo; Charrua, Luís; Gusmão, AntónioThis paper deals with Cyclic-Prefix (CP)-assisted uplink transmission - Orthogonal Frequency Division Multiplexing (OFDM)-based or Single Carrier (SC)/Frequency Domain Equalization (FDE)-based - within a Multi-User (MU)-Multi-Input Multi-Output (MIMO) system where a large number of Base Station (BS) antennas is adopted. In this context, either a Minimum Mean-Squared Error (MMSE) optimum linear detection or reducedcomplexity linear detection techniques are considered. Regarding performance evaluation by simulation, appropriate semi-analytical methods are proposed, and their accuracy is assessed under the assumptions of perfect power control and perfect channel estimation. This paper includes a detailed evaluation of Bit Error Rate (BER) performances for uncoded 4-Quadrature Amplitude Modulation (QAM) schemes - with both OFDM and SC/FDE - and a MU-MIMO channel with uncorrelated Rayleigh fading. The accuracy of performance results obtained through the semi-analytical simulation methods is assessed by means of parallel conventional Monte Carlo simulations. The performance results are discussed in detail, with the help of selected performance bounds, and we also emphasize the achievable ”massive MIMO” effects, even for the reduced-complexity detection techniques, provided that the number of BS antennas is much higher than the number of antennas which are jointly employed in the terminals of the multiple autonomous users. Appropriate ”SC/FDE vs OFDM” comparisons are also included in this discussion of performance results.
- Mobile communications with STBC and turbo codesPublication . Torres, Paulo; Costa, Ludovic Mamede; Gusmão, AntónioUsers are increasingly demanding about the quality and consumer batteries of their mobile terminals to gain access to services imposed by operators. In order to satisfy the customers, operators must provide a good quality, high transmission rates to allow videoconferencing communications and low power consumption in mobile terminals, so that battery last longer. In mobile communications, data are transmitted over a channel that is affected by noise, thus affecting the quality of data received, and thus can degrade the received information. In traditional communications, SISO (Single-Input Single-Output systems) with one transmitting and one receiving antenna, are not efficient to minimize noise caused by the channel. Solutions such as increased bandwidth and increased power transmission would solve the problem, however, are not reliable. Although these solutions are valid in theory, none of them is put into practice, because increasing the transmission of mobile terminals, there would cause an increase in battery size, price and increasing the size of mobile terminals and the fact that it could be detrimental to our health. Increasing the bandwidth would be an easy way out and solve the problems of errors and low transmission rates, however there is a price to pay, the spectrum allocation is expensive, so this technique is not feasible. This paper aims to provide efficient solutions to improve the efficiency of power, namely to achieve a good quality with a low power consumption on the handset. The spectral efficiency is improved through the implementation of MIMO (Multi-Input Multi-Output) systems, i.e., multiple transmit antennas and multiple receiving antennas, using error correction codes. Through the union of these two techniques is possible to obtain a low error probability with low power consumption. This paper presents how MIMO solution the STBC (Space Time Block Codes) encoding. The Alamouti code [1] is used in this coding, which consists of data transmission with two transmit antennas and one or more receiving antennas. Other STBC codes for multiple antennas and multiple receiving antennas are presented in the paper, these codes developed by Tarokh et al [2]. The novelty of this study, presented here, is the use of MIMO systems using error correction codes (Turbo-codes[3]). In the turbo code decoding algorithms are used with soft outputs, as the MAP (Maximum a Posteriori) algorithm, Log-MAP and max-log-MAP. The error probability performance results, presented in this paper were obtained by Matlab Simulations with 4-QAM modulation in Rayleigh channel. The SISO systems are compared with MIMO systems with and without error correction. The simulation results show that there is a significant improvement when the MIMO systems are used compared to the SISO systems, and that with error correction lower error probability is achieved with lower energy consumption.
- Performance evaluation of SC-FDMA uplink block transmission schemes using iterative receiver techniquesPublication . Charrua, Luís; Torres, Paulo; Gonçalves, Victor; Gusmão, AntónioThis paper deals with CP-assisted block transmissionsolutionsforfuturemobilebroadbandsystems,inthecontext of a Single Carrier (SC)-Frequency Division Multiple Access (FDMA)uplink.Twoalternativechoicesareconsideredregarding the subcarrier mapping rule: a ”localized” subcarrier mapping where user’s data occupy a set of consecutive sub carriers (Rule 1); a ”distributed” subcarrier mapping where user’s data occupy a set of uniformly spaced subcarriers (Rule 2). Detailed performance evaluations, in this paper, involve the consideration of two iterative receiver techniques which can be regarded as extensions of iterative receiver techniques proposed previously within a single user context. A selected class of multipath radio channels, providing a range of channel time dispersion levels, is assumed for performance evaluation purposes,andasetofmatchedfilterboundsonreceiverperformance plays a relevant role in ”achievable performance” comparisons. Both the impact of the mapping rules and that of the iterative receiver techniques considered here are evaluated in detail. The performance advantages under ”Rule 2” are emphasized, for practically the entire range of the channel time dispersion levels assumed in the paper and both specific iterative receiver techniques.
- Space time block codes with turbo codesPublication . Torres, Paulo; Costa, Ludovic Mamede; Gusmão, AntónioThis paper aims to provide efficient solutions to improve the power efficiency, like implementation of schemes using MIMO (Multiple-Input Multiple-Output), with error correction codes. Thanks to the union of these two techniques is possible to obtain a low error probability with low power consumption. This paper shows STBC (Space Time Block Codes) with codes and diversity in the reception using a MRC (Maximum Ratio Combining). In this solution are studied Alamouti codes [1] and those proposed by Tarokh et al [2], with four and eight transmit antennas. Turbo Codes are used in the decoding algorithm with soft output, the log-MAP. The simulation results of the performance of error probability (BER) are performed in MATLAB with 4-QAM modulation in a Rayleigh channel.
- Uplink performance evaluation of broadband systems which adopt a massive MU-MIMO approachPublication . Torres, Paulo; Charrua, Luís; Gusmão, AntónioThis paper deals with an Orthogonal Frequency Division Multiplexing (OFDM)-based uplink within a Multi User (MU)-Multi-Input Multi-Output (MIMO) system where a ”massive MIMO” approach” is adopted. In this context, either an optimum Minimum Mean-Squared Error (MMSE) linear detection or a reduced-complexity Matched Filter (MF) linear detection are considered. Regarding performance evaluation by simulation, several semi-analytical methods are proposed: one performance evaluation method in the optimum (MMSE) case; two performance evaluation methods in the MF case. This paper includes performance results for uncoded 4-Quadrature Amplitude Modulation (QAM)/OFDM transmission and a MUMIMO channel with uncorrelated Rayleigh fading, under the assumptions of perfect power control and perfect channel estimation. The accuracy of performance results obtained through the semi-analytical simulation methods is assessed by means of parallel conventional Monte Carlo simulations [10]. The performance resultsare discussed indetail andwe also emphasize the achievable ”massive MIMO” effects, even for the reducedcomplexity detection techniques, provided that the number of BS antennas is much higher than the number of antennas which are jointly employed in the terminals of the multiple autonomous users.
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