New approaches for channel estimation in wireless communications

This thesis first proposes a computationally efficient, pilot-aided linear minimum mean square error (MMSE) batch channel estimation algorithm for the orthogonal frequency division multiplexing OFDM systems in unknown wireless fading channels. The batch linear MMSE will be converted to the sequential linear MMSE estimator due to fast convergence property and the simple structure. In addition to OFDM systems, focusing on transmit diversity (OFDM) transmission through frequency selective channels, this thesis pursues a channel estimation approach in time-domain for both space-frequency OFDM (SF-OFDM) and space-time OFDM (ST-OFDM) systems. This thesis also proposes a computationally efficient, pilot-aided linear minimum mean square error (MMSE) time domain batch channel estimation algorithm for OFDM systems with transmit diversity in unknown wireless fading channels. The proposed batch approaches (with or without transmit diversity) employ a convenient representation of the discrete multipath fading channel based on the Karhunen Loeve (KL) orthogonal expansion and finds MMSE estimates of the uncorrelated KL series expansion coefficients. Based on such an expansion, no matrix inversion is required in the proposed MMSE estimators. Moreover, optimal rank reduction is achieved by exploiting the optimal truncation property of the KL expansion resulting in a smaller computational load on the estimation algorithm. The performance of the proposed approaches is studied through analytical and experimental results. Then, in order to explore the performances, the stochastic Cramer-Rao lower bounds are considered for the proposed approaches. The effect of a modeling mismatch on the performances of the estimator is also analyzed. In order to explore the performance of the transmit diversity OFDM systems, the closed-form expression for the average symbol error rate (SER) probability is also derived for the maximum ratio receive combiner (MRRC) in these systems. Simulation results confirm our theoretical analysis, and illustrate that the proposed channel estimation algorithms for OFDM systems with and without transmit diversity are capable of tracking fast fading and frequency selective fading, respectively and improving overall performance.