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Multiple antenna techniques (MAT) are expected to be used in next generation cellular systems to provide higher capacity, higher data rates and improved signal quality. In this paper, the spectral efficiency of a broadband mobile OFDM system using ideal adaptive modulation/coding is evaluated for various multiple antenna techniques. A generic model to evaluate the system performance for such a complex system is presented. Cellular system-level results are given for a single transmit antenna, dual transmit diversity with Alamouti encoding, and various MIMO techniques (idealized and practical). The impact of spatial correlation of the fading on the system performance is studied and quantified. Results indicate that the theoretical MIMO capacity is only moderately sensitive to correlated fading at the receive array with uncorrelated fading at the transmit array. Moreover, a practical scheme using a separate stream on each transmit antenna with a linear MMSE receiver is found to be much more sensitive to correlated fading. When the TX and RX arrays consist of only two antennas, Alamouti encoding with receive diversity is a simple technique that performs quite well.