Space time block codes (STBCs) are commonly designed according to the rank-determinant criteria, suitable for high signal to noise ratio (SNR) values. However, capacity approaching forward error correcting codes, used in practical communication systems, achieve iterative convergence at low to moderate SNR. In this paper we first present a non-asymptotic STBC design criterion based on the bitwise mutual information (BMI) maximization at a specific target SNR. According to this BMI criterion, we then optimize a trace-orthonormal-based STBC structure. Therefore, designed STBC becomes adaptive with respect to the SNR. Proposed adaptive trace-orthonormal STBC shows identical or better performance than 2×2 STBCs of a turbo-coded WiMAX system.