Singular-value decomposition (SVD)-based multiple- input multiple-output (MIMO) systems have attracted a lot of attention in the wireless community where the whole MIMO channel is decomposed into a number of unequally weighted inde- pendent single-input single-output (SISO) channels. The unequal weighting of the SISO channels has led to intensive research on bit- and power allocation even in MIMO channel situations with poor scattering conditions identified as the antenna correlation effect. In this situation, the unequal weighting of the SISO channels becomes even much stronger. In comparison to the SVD- assisted MIMO transmission, geometric mean decomposition (GMD)-based MIMO systems are able to compensate for the drawback of weighted SISO channels when using SVD at the cost of remaining interferences which can be easily removed by using dirty paper precoding. Together with different QAM constellation sizes per layer, bit loading and power allocation can be helpful to balance the bit-error probabilities in the activated number of MIMO layers. The novel contribution of this paper is that optimal and suboptimal power allocation solutions are investigated under the assumption of unequal SISO channels as well as different QAM constellation sizes per MIMO layer. Our results show that GMD-based MIMO transmission has the potential to significantly simplify the process of bit and power loading and outperforms the SVD-based MIMO transmission as long as the same QAM-constellation size is used on all equally- weighted SISO channels.