Precoding Design for Massive MIMO Systems with Sub-connected Architecture and Per-antenna Power Constraints

This paper provides the necessary conditions to design precoding matrices for massive MIMO systems with a subconnected architecture, RF power constraints and per-antenna power constraints. The system is configured such that each RF chain serves a group of antennas. The necessary condition to design the digital precoder is established based on a generalized water-filling and joint sum and per-antenna optimal power allocation solution, while the analog precoder is based on a perantenna power allocation solution only. We study the analytically most interesting case where the power constraint on the RF chain is smaller than the sum of the corresponding per-antenna power constraints. For this, the optimal power is allocated based on two properties: Each RF chain uses full power and if the optimal power allocation of the unconstraint problem violates a per-antenna power constraint then it is optimal to allocate the maximal power for that antenna.