## 2 Architecture of the Microgrid-Based Base Transceiver Station (MG-Based BTS)
### Proposed Architecture for the MG-Based BTS  
Figure 2 depicts the block diagram of the proposed architecture for a grid connected BTS with solar generation based on a microgrid architecture.  
The MG-based BTS consists of the aforementioned BTS along with a PV system with Maximum Power Point Tracke (MPPT) control and a DC bus connected to the BTS rectifier. This DC bus acts as the microgrid main bus to which the different elements are connected. The BESS and non-critical loads are directly connected to the DC bus without DC/DC converters. This is the approach adopted by most companies with the aim of maximizing short-term profitability, since a more technologically sophisticated design would increase the costs. In the BTS architecture proposed in this paper, all loads (critical and non-critical) and connections of the different parts of the MG-based BTS become controllable elements through a set of switches (Figure 2). Furthermore, several variables, such as currents and voltages in loads and the BESS are monitored. Thus, different operating modes and energy flows within the BTS can be managed by designing a local controller.  
This local controller is implemented on an Single-Board omputer (SBC) Raspberry Pi 4 model B and some electronics associated. The reason for choosing this SBC is three-fold: firstly, a reduction of power consumption is required; secondly, because of its high processing power; finally, the Raspberry Pi platform has been successfully used in similar works [33; 34; 35]. The associated electronics consists of a main board and a series of latching relays electrically connected to the main board. In the main board, several sensors and conditioning electronics have been included to collect electrical parameters of the installation, such as BESS and load currents, BESS voltage and some ambient parameters (temperature, humidity, etc.). These parameters are sent to the SBC through I2C protocol. The local controller, which also receives the DC bus voltage and PV modules parameters from the MPPT solar charger controller through Modbus protocol, processes all this information to calculate other variables, such as the State of Charge (SoC) of the BESS, which allows the MG-based BTS to be managed. To implement the control of the MG-based BTS, there are  
Figure 2: Base Transceiver Stations transformed into DC Microgrids.  
also three latching relays which connect or disconnect the BESS and the non-critical loads to the DC bus, and the rectifier to the grid. Henceforth, these associated electronics will be referred as the SBC driver, since it acts as the bridge between the local controller in the SBC and the hardware of the installation.  
Likewise, to provide the basis for the smart sensing strategy, a server based on IEC61850 standard has been implemented. This allows the MG-based BTS to be remotely controlled as a VPP node using an external IEC61850 client. Previously, an IEC61850 plant model of the MG-based BTS is proposed to store data in a standard way.  
The MG-based BTS has been designed for the worst-case scenario for a macro BTS with a rated power of 3 kW. The experimental setup has been developed using the same RF equipment and power amplifiers as those installed in 3-kW BTSs. The rated power is equally split among the three controllable loads: two critical loads corresponding to the always-on transceivers and a non-critical load for transceivers that can be switched off, and auxiliary equipment, such as the cooling system, when necessary, and lighting. This power configuration can be easily scaled to meet the requirements of more power-demanding BTSs.