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In multiple-input-single-output (MISO) broadcast channels, the achievable diversity gain is limited by the number of source transmitter antennas. Cooperative transmission is able to provide another dimension of diversity gain improvement. In this paper, we introduce a spectrally efficient cooperative transmission protocol in MISO broadcast channels using linear precoding and nonorthogonal relaying. The proposed protocol can achieve maximum diversity gain expressed as the sum of the number of source transmitter antennas and the number of available relays. The diversity and multiplexing tradeoff of the proposed protocol outperforms the noncooperative scheme, even when only a single relay is used. For a large number of relay candidates, the diversity and multiplexing tradeoff of the proposed protocol completely surpasses the existing noncooperative scheme.