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In this paper, a space-time coded adaptive MQAM scheme with variable power is developed for MIMO system. Under the target BER and average power constraint, the optimum SNR switching thresholds for the developed scheme are generated by using the Lagrange method, which can make the scheme maximize the spectrum efficiency (SE). Besides, the closed-form expressions for the SE and average BER of the scheme are derived. Simulation results show that our scheme is superior to the corresponding constant-power space-time coded adaptive MQAM scheme. Moreover, the SE and average BER from theory computation accords with those from the simulation results.