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Closed-form expressions for the ergodic capacity for downlink transmission of a distributed antenna system are derived, based on different cooperative transmission schemes including single-antenna transmission (SAT) and dual-antenna transmissions (DAT). For DAT, we consider space-time block coding (STBC) for the open-loop operation, and maximum ratio transmission (MRT) for the closed-loop operation. The probability density function (PDF) of the signal-to-interference ratio (SIR) for each scheme is presented, followed by the capacity expressions in terms of the received power components for the significant signal sources and the interference sources. The switching criteria among the transmission schemes are proposed and used for evaluating the capacity gain over the conventional SAT. The derived results are verified via Monte-Carlo simulations in a Rayleigh fading environment, for a given geographic location of the mobile station (MS). It is also demonstrated that an instantaneous adaptation of the transmission schemes, if applicable, can provide further capacity improvement over individual transmission schemes.