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Coordinated multipoint (CoMP) transmission has been widely recognized as a spectrally efficient technique in future cellular systems. To exploit the abundant spatial resources provided by the cooperating base stations (BSs), however, considerable training overhead is required to acquire the channel information. To avoid the extra overhead outweighing the cooperative gain, we propose a method that allows each user to select transmission mode between coherent CoMP and non-CoMP. We first analyze the average throughput of each user under CoMP and non-CoMP transmission after taking into account the downlink training overhead. A closed-form mode selection rule is then developed, which depends on the user location and system settings, i.e., the number of cooperating BSs and transmit antennas, training overhead, and cell-edge signal-to-noise ratio (SNR). Simulation results show that the proposed downlink transmission mode selection method achieves higher throughput than CoMP for cell-center users and than non-CoMP for cell-edge users after accounting for the overhead. As a by-product, the backhaul load is also significantly reduced.