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“Design and Analysis of OFDMA and SC-SDMA For Broadband Multiuser Communications”
by Man-On Pun
May 2006
This thesis addresses two challenging issues in the design of orthogonal frequency division multiple access (OFDMA) and single-carrier space-division multiple-access (SC-SDMA) systems, both of which are among the most promising multi-access technologies for the next-generation broadband multiuser communications. In the first part, a maximum likelihood (ML) estimator of the carrier frequency offset (CFO), timing error and channel response of each active user in an asynchronous OFDMA uplink transmission is proposed by exploiting a training sequence. The Cramer-Rao bound (CRB) of the frequency estimate is derived. Furthermore, it is shown that the accuracy of the frequency estimate obtained by the proposed ML estimator asymptotically achieves the CRB. To further reduce the computational complexity of the proposed ML estimator, we develop a sub-optimal estimator using the von Neumann series to approximate a matrix inverse required by the ML estimator. Simulation results indicate that the performance of the sub-optimal estimator is similar to that of the ML estimator even when the approximation order is low. We also propose two iterative receivers for uncoded and coded OFDMA uplink transmissions based on expectation maximization (EM)-type algorithms so as to decouple the frequency and channel estimation process from the data detection process. In particular, the receiver proposed for the coded systems exploits the soft-decision feedback derived from a maximum a posterior (MAP) decoder to provide more reliable synchronization, channel estimation and interference suppression. The proposed receivers perform data detection directly without the need of returning CFO estimates to active users to achieve frequency synchronization. This significantly reduces the system overhead and solves the problem of outdated information that occurs in rapidly-varying channels due to feedback delay. Simulation results indicate that the proposed receivers can provide accurate data detection for unsynchronized OFDMA uplink transmissions over doubly-selective fading channels. In the second part, we consider an SC-SDMA downlink system in which both the base station (BS) and mobile terminals (MTs) are equipped with multiple antennas. We propose to perform frequency-domain linear combining at each MT and frequency-domain pre-equalization at the BS. Furthermore, a novel scheme is developed to jointly optimize the design of frequency-domain pre-equalization and combining based on the minimum mean squared error (MMSE) criterion, assuming that the BS has perfect channel state information (CSI). The performance of the proposed scheme is analyzed in terms of mean squared errors (MSE). Simulation results demonstrate the effectiveness of the proposed scheme.
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