Power allocation and bit loading for spatial multiplexing in MIMO systems

The power assignment problem is important for multiple-input-multiple-output (MIMO) systems to achieve high capacity. Although this problem is solved by the well-known water filling (WF) algorithms, this does not provide an optimal solution if the system is constrained to a fixed raw bit error rate threshold and to discrete modulation orders. An approximate approach, called QoS based WF, is proposed to solve the power assignment problem with such constraints. It is shown to outperform quantization of the conventional water filling solution and a well known bit loading algorithm (Chow´s algorithm) used in digital subscriber lines (DSL).