This paper considers two simple procedures to improve the channel frequency response estimate for uplink Zero Padded MC-CDMA systems. The first approach uses a precomputed matrix that projects the original discrete frequency response onto the subspace where the true frequency response lies. The second uses a constrained MMSE optimization that forces the estimate to belong to this subspace. Analytical expressions for the mean square error and its upper and lower bounds are presented for the conventional and the two improved channel estimators. Numerical results show significant reduction in both the multiple access interference and the noise terms of the estimation error when using the improved estimators. Simulation results indicate that MMSE receivers using each of the improved procedures achieve gain in bit error rate performance.

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Resumo-Este trabalho investiga o impacto de estimativa de canal melhorada no desempenho dos sistemas MC-CDMA e MC-DS-CDMA, com intevalo de guarda ZP (Zero Padding), no enlace direto. Nós apresentamos modificac ¸ões nas estimativas de canal MMSE assistidos para os sistemas. Três versões de estimadores são apresentadas. A primeira estratégia consiste em aplicar uma matriz de projec ¸ão na estimativa inicial da resposta em freq üência do equivalente discreto do canal com o intuito de eliminar componentes indesejáveis do ruído. A segunda estratégia aplica uma restric ¸ão inicial na soluc ¸ão MMSE, para obter a estimativa da resposta ao impulso do canal e, a seguir, convertê-la para o domínio da freq üência. Os resultados obtidos, por simulac ¸ão em computador, são apresentados em termos de erro médio quadrático (MSE) e taxa de erro de bits (BER). O desempenho dos sistemas utilizando a estimativa com restric ¸ão inicial mostra-se superior para os dois sitemas considerados.

This paper addresses pilot-assisted estimation of frequency-selective time-invariant channels in multicarrier Code Division Multiple Access communications systems (MC CDMA and MC DS-CDMA). Performance in terms of normalised mean square error (NMSE) is derived for two discrete channel frequency response estimators: a conventional estimator based on the minimum mean square error criterion, and an improved estimator exploiting subspace relationships between the frequency and impulse responses of the discrete channel. For MC DS-CDMA, NMSE performances of both estimators result in closedform solutions. For MC CDMA, a closed-form solution is derived for the NMSE of the conventional estimator; upper and lower bounds are provided for the NMSE of the improved estimator. Furthermore, for the particular case of identically distributed discrete channels with common arbitrary power delay profile and uncorrelated weights, a closed-form expression for the NMSE of the improved estimator is also obtained. Numerical results illustrate the accuracy of the proposed NMSE expressions.