We present an adaptive space time block code (STBC) for 2×2 MIMO systems designed to be used with powerful forward error correcting (FEC) codes and having a low ML-detection complexity. STBCs are commonly designed according to the rank-determinant criteria, suitable for high signal to noise ratio (SNR) values. However, powerful FEC codes like turbo codes achieve iterative convergence at low to moderate SNR. Thus, we first propose a non-asymptotic STBC design criterion based on the bitwise mutual information (BMI) maximization at a specific target SNR. According to this criterion, an adaptive matrix D based STBC is defined for a wide range of SNRs. Resulting code outperforms the original matrix D and closes the gap to the more complex Golden code in the context of a WiMAX system.