Optimal co-phasing power allocation for coordinated OFDM transmission

Abstract

This paper considers an orthogonal frequency division multiplexing (OFDM) coordinated transmission system in which K coordinated transmission points (CTPs) coherently transmit and allocate power across N subchannels under individual power constraints. In maximizing the system capacity, we derive an optimal co-phasing power allocation solution with the following property: For any subchannel, if the power allocation of one CTP is zero, then the power allocation of all the other CTPs must be zero. Otherwise, the non-zero power allocation of all CTPs follows a proportional rule establishing a relationship between the power allocation for all the subchannels and CTPs. This highlights that the optimal power allocation is different from classical waterfilling and maximum ratio transmission (MRT), as more power is not necessarily allocated to the subchannels with better channel conditions. Based on this property and our derived solution, we successfully reduce the constrained optimization problem with NK variables into an unconstrained one with only K variables, which simplifies computation significantly. Numerical results are presented to verify our theoretical findings.