Power allocation based on fast Water-Filling for energy efficient OFDM and MIMO transmissions

We consider power allocation in energy efficient OFDM (Orthogonal Frequency Division Multiplexing) or MIMO (Multiple-Input Multiple-Output) transmissions. An energy-per-goodbit (EPG) metric is used to gauge the average energy consumed for transmitting each bit. Existing works minimize EPG by searching for a set of dual variables which are used to compute the optimal power allocation in a Water-Filling (WF) expression. The searching process is complex and requires a long running time. In this paper, we propose a new algorithm to compute the optimal power allocation without searching any dual variables. Our algorithm is based on an iterative calculation of the total transmission power and it converges when the minimum EPG is reached. It takes WF as the basic building block. Unlike the conventional WF approach which needs to search for a Lagrange multiplier (i.e., the water level), we additionally propose a novel WF algorithm without searching the Lagarange multiplier. Numerical results show that our power allocation algorithm for EPG minimization, together with the embedded fast WF algorithm, can run multiple times faster than the existing ones.