## 2 Methodology and Experimental Setup
### Empirical Experimental Setup  
The TI evaluation RF kit TRXEBK is the choice platform for the experimental wired DC PLC at high frequencies, since it provides a common software and hardware environment for both interest communication frequencies of 868 MHz and 2.4 GHz, ensuring consistency at the empirical investigation stage. Two PCs were used to control and manage the transmitter and receiver via a USB cable trough the TI's SmartRF Studio 7 dedicated interface. The SMA output of each transceiver was connected through a 50 \(\Omega\) coaxial cable, i.e., the same was previously used to connect the R&S oscilloscope to a custom prototyping PCB board designed for the DC power network coupling. The coupling to a network for transmitting and receiving data through and from a physical signal is mainly based on impedance matching principles, such as for optimal power transfer to a load or for minimum noise [60]. Neither of the methods come without criticism [61; 62]. Additionally, regardless of the impedance circuitry used for coupling and recently applied communication standards [63; 64], the signal injected for PLC encounters multiple reflections, attenuations and additive noises, resulting from unplanned parameters, such as the cable branches' unknown connection state, the random coupling of loads and various impulsive interferences [65; 66]. In the case of the DC power bus bound by Lithium-ion energy storage, such as in this study, simplifications of coupling circuits can be achieved for complex network measurements [12; 67] and communication signal injection [25; 26; 68; 69] through capacitive connections. This simplification exploits the capacitive property of buffering the DC voltages, while presenting transparency to the AC signals; however, due to the various number of unknown factors that may play a role during communication, an empirical test for different capacitive values may provide a more realistic picture of possible communication measures and adjustments. Following the above reasoning, three identical boards were made, each presenting four different coupling capacitors, 1 \(\mu\)F, 1 nF and 1 pF, similar to  
Figure 4: MATLAB Simscape test setup for 868 MHz and 2.4 GHz communication over the DC bus composed of four Li-ion cells organized in parallel (**A**) and series (**B**). those used for the MATLAB simulation. The capacitive coupling PCB is subsequently connected via banana-ended cables to the four 18650 Li-ion cells arranged in parallel and then in series. Each cell presents 100% SOC and it presents a nominal voltage of 4.2 V and 3500 mAh capacity. Figure 5 illustrates the setup and the general arrangement and device interconnections. The empirical measurements include two more additional setups than in the MATLAB simulation, to complete the picture of using high-frequency RF transceivers with DC PLCs. Since both CC1200 and CC2520 transceivers are used in wireless applications, a setup testing the possible wireless connection between the Tx and Rx in a straight line separated by 2 m cables and 0.1 m distance (i.e., the PCB size) between the cables' ends was investigated to, first, reference the worst communication case scenario, i.e., if the PCB and the batteries were removed while the cables and transceivers were still in place and running. A second setup, connecting the end cables to the interfacing PCBs without adding the Li-ion cells and holders to the scenario was designed to test the best communication case scenario, when the voltages, currents and additional circuit parasitic elements are missing. The two additional setup roles present potential reference frames for the tested DC PLC, the disconnection, and only the PCB connection, providing the overall operating margins that, in theory, may be described by RSSI, BER and PER. Across all four empirical setups, the Tx-Rx data link was tested with three different payload sizes of 10, 60, and 120 bytes.