In this paper, bit error performance of the cooperative diversity system based on quadrature signaling with 16 quadrature amplitude modulation (QAM) is analyzed. An adaptive cooperative diversity system based on quadrature signaling is proposed to increase bit error performance. The level of square envelope of fading coefficients of interuser channels and main channels between users and base station with respect to a threshold value is taken as the cooperation criterion for the proposed system. This threshold value is optimized by using computer simulations to minimize the bit error probability. An analytical upper bound expression is derived for the bit error probability of the proposed system. It is shown by computer simulations that the adaptive cooperation based on quadrature signaling significantly improves the system performance.