Radio resource allocation is critical in order to manage available bandwidth and optimize network behavior. In Centralized-Radio Access Networks (C-RAN), simple Radio Units (RUs) are deployed and can be controlled remotely to allow cooperation. In this paper, we consider micro-cell systems with low-to-medium load regimes. We study the resource allocation problem with minimization of the energy consumption due to radio transmission and signal processing. We propose to mix two transmission modes over a reference period allowing resource reuse: one with all RUs transmitting the same signal, the other with each RU transmitting a specific signal. We impose the same target rate for each user equipment (UE), the same transmit power for the RUs, and a maximum resource block (RB) number, which defines the constraints. The resulting optimization problem is formulated so as to be mixed integer linear programming (MILP). Simulations are run using CPLEX Python API and show the efficiency of the proposed scheme to increase the capacity while consuming the least energy. We also observe that the mode with all RUs simultaneously serving the same UE is more efficient (fewer RB and lower consumed energy) than the mode with a single RU activated at a time.