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A novel energy-efficient collaborative Alamouti space-time block code (STBC) design is proposed for a wireless communication system having two transmitter antennas and a single receiver antenna. This new design allows us to have more options for variable-rate modulation for fading channels with enhanced performance for signal transmission than the conventional Alamouti coding scheme while having complex-symbol-decodable complexity for the maximum likelihood (ML) detector. Subject to the constraints on a fixed transmission bit rate and unity average energy per each transmitter antenna, the optimal linear constellation combinations with the optimal energy-scales as the coefficients are attained by maximizing the coding gain. It is shown that the optimal coding gain of the newly-designed code is larger than that of the conventional Alamouti code if the transmission bit rate is greater than two bits per channel use.