Enhanced carrier frequency offset estimation for OFDM using channel side information

Carrier frequency offset (CFO) in OFDM systems, which induces the loss of orthogonality among OFDM subcarriers, can result in significant performance degradation. As a result, it needs to be estimated and compensated for. In this paper, we present a general CFO estimator based on the maximum likelihood (ML) estimation criterion, with which CFO can be obtained using training OFDM symbols, pilot tones, null subcarriers, or a combination of them. Using the proposed CFO estimator, the performance of CFO estimation can be significantly improved by taking advantage of the channel side information. In particular, using the channel statistics information, such performance improvement can be achieved for low SNR values and all SNR values over Rayleigh fading channels and Ricean fading channels, respectively. When the complete channel impulse response (CIR) information is available, simulation results will show that the performance improvement can be more than 6dB. To further demonstrate the capability of the proposed CFO estimator, we will consider an OFDM system using the signal structure of the IEEE WLAN standard 802.11a. Compared with previous work using null sub-carriers alone, we will show that by taking advantage of the pilot tones, null. sub-carriers, and channel statistics, the performance of CFO estimation can be improved by about 2dB

Channel and frequency offset estimation schemes for multicarrier systems

Doutoramento em Engenharia ElectrotécnicaO presente trabalho aborda o problema da estimação de canal e da estimação de desvio de frequência em sistemas OFDM com múltiplas configurações de antenas no transmissor e no receptor. Nesta tese é apresentado o estudo teórico sobre o impacto da densidade de pilotos no desempenho da estimação de canal em sistemas OFDM e são propostos diversos algoritmos para estimação de canal e estimação de desvio de frequência em sistemas OFDM com antenas únicas no transmissor e receptor, com diversidade de transmissão e MIMO. O estudo teórico culmina com a formulação analítica do erro quadrático médio de um estimador de canal genérico num sistema OFDM que utilize pilotos dedicados, distribuidos no quadro transmitido em padrões bi-dimensionais. A formulação genérica é concretizada para o estimador bi-dimensional LS-DFT, permitindo aferir da exactidão da formulação analítica quando comparada com os valores obtidos por simulação do sistema abordado. Os algoritmos de estimação investigados tiram partido da presença de pilotos dedicados presentes nos quadros transmitidos para estimar com precisão os parâmetros pretendidos. Pela sua baixa complexidade, estes algoritmos revelam-se especialmente adequados para implementação em terminais móveis com capacidade computacional e consumo limitados. O desempenho dos algoritmos propostos foi avaliado por meio de simulação do sistema utilizado, recorrendo a modelos aceites de caracterização do canal móvel multipercurso. A comparação do seu desempenho com algoritmos de referência permitir aferir da sua validade. ABSTRACT: The present work focus on the problem of channel estimation and frequency offset estimation in OFDM systems, with different antenna configurations at both the transmitter and the receiver. This thesis presents the theoretical study of the impact of the pilot density in the performance of the channel estimation in OFDM systems and proposes several channel and frequency offset algorithms for OFDM systems with single antenna at both transmitter and receiver, with transmitter diversity and MIMO. The theoretical study results in the analytical formulation of the mean square error of a generic channel estimator for an OFDM system using dedicated pilots, distributed in the transmitted frame in two-dimensional patterns. The generic formulation is implemented for the two-dimensional LS-DFT estimator to verify the accuracy of the analytical formulation when compared with the values obtained by simulation of the discussed system. The investigated estimation algorithms take advantage of the presence of dedicated pilots present in the transmitted frames to accurately estimate the required parameters. Due to its low complexity, these algorithms are especially suited for implementation in mobile terminals with limited processing power and consumption. The performance of the proposed algorithms was evaluated by simulation of the used system, using accepted multipath mobile channel models. The comparison of its performance with the one of reference algorithms measures its validity