Optimal Power Allocation for DAS-OFDM under Joint Total and Individual Power Constraints

We derive new optimal power allocation solutions for a distributed antenna system with orthogonal frequency division multiplexing (DAS-OFDM), in which K remote radio heads (RRHs) allocate power over N subchannels with both individual RRH and total system power constraints. The individual RRH constraints may be necessary due to hardware limitations, while the total power constraint allows the system to limit the overall energy consumption for cost and/or green environmental factors. In order to obtain some insight on the optimal power allocation, we focus on a simple two-RRH case in this paper. The resulting optimal power allocation solution has an interesting sparse feature: Among N subchannels, at most 1 subchannel can be allocated power for joint transmission by the two RRHs, and the rest of the subchannels must be served by a single RRH. A novel subchannel rearrangement scheme is presented to identify the possible joint-transmission subchannel, as well as which RRH the remaining subchannels are associated with. Numerical results are presented to verify our theoretical results.