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By combining adaptive modulation (AM) and automatic repeat request (ARQ), a cross-layer design (CLD) scheme for MIMO system with space-time block coding (STBC) and imperfect channel state information (CSI) is presented in this paper, and the effect of the imperfect CSI on the system performance is investigated. Based on the imperfect CSI, the probability density function of the effective signal to noise ratio (SNR) is derived, and the fading gain switching thresholds subject to a target packet loss rate and fixed power constraint are also derived. According to these results, closed-form expressions of the average spectrum efficiency (SE) and packet error rate (PER) of the system are further derived, respectively. These derived expressions include some existing expressions with perfect CSI as special cases, and can provide good performance evaluation for the CLD system with imperfect CSI. Numerical results verify the validity of the theoretical analysis. The results show that the SE and PER of the system with imperfect CSI are worse than those with perfect CSI due to the estimation error, but their performance difference will be reduced as estimation accuracy increases.