# A Comparative Study of On-Body Radio-Frequency Links in the 420 MHz-2.4 GHz Range
## 2 Materials and Methods
### RF Technologies
#### 2.2.2 450 MHz Body-Coupled Communication and On-Body Monopoles  
Body Coupled Communication (BCC) should only be tested with isolated power supplies such as battery-powered instruments to prevent coupling. Additionally, the size of the instrument's ground plane should be minimized to limit over-the-air coupling via the instrument. This is because a bigger reference plane will increase the coupling of the system to its environment. Consequently, it was chosen to use the small form factor evaluation kit STEVAL-FKI433V2 [41] (STMicroelectronics, Geneva, Switzerland). This kit consists of an S2-LP radio daughterboard plugged onto a Nucleo Board STM32L053R8 (Figure 3a,b). The latter embeds a Cortex-M0+ that controls the radio and allows data download to a computer for processing. Both boards are powered by a single 3.6 V battery (Figure 3c). For our experiments, two of these modules are used: one for transmission and one for reception. The BCC electrodes are composed of two distinct parts. On one side is a 2 cm by 2 cm backplane that consists of a single copper layer PCB. On the insulating side, a snap is glued. Both are wired to an SMA connector, as depicted in Figure 4a. The second part is a pre-gelled standard medical electrode from Covidien (Kendall disposable electrode, Arbo H124 SH [42]) which can be snapped in the BCC electrode described above (Figure 4b). A 15 cm long SMA cable directly connects the electrode to the radio, with no matching network added. For comparison purposes, a pair ofwire antennas in a monopole type package (see Figure 4c) were also used to perform the same tests. The antennas were included in the evaluation kit STEVAL-FKI433V2 with the sub-GHz radio and the Nucleo board and exhibit an S\({}_{11}\) value of \(-\)8 dB at 450 MHz. The antennas were directly connected (no matching) to the same transceivers used for BCC for the sake of comparability between both measurement setups. via a 90\({}^{\circ}\) SMA connector (Figure 4c), positioning them perpendicular and 2 cm above the surface of the arm. In order to measure the attenuation of the channel under different conditions, the transmitter radio (TX) was set up to feed a continuous wave (CW) at the frequency range of interest: 420 MHz to 510 MHz with steps of 10 MHz. The receiver radio (RX) was set up to store the received signal strength indicator (RSSI) values in the flash memory of the microcontroller on the motherboard. Because the TX and RX are asynchronous both boards follow a 5 s cycle for a given frequency. Once the cycle begins, the TX waits 750 ms before transmitting a continuous wave, lasting 3 s. At the end of the 5 s cycle, the TX switched to the following pre-programmed frequency and waits for the start signal of the following cycle. On the RX side, as soon as the program is triggered, the board begins  
Figure 3: (a) Mother board: Nucleo STM32L053R8, (b) daughter board with ST S2-LP sub-GHz radio, (c) 3.6 V battery. saving the value of the RSSI register in the Coretex-M0+ RAM every 40 ms. A total of 116 samples are collected per frequency step. This phase lasts for 4.64 s. At the end of the cycle, the data saved in the RAM are transferred to the flash memory of the microcontroller. The radio switches to the following pre-programmed frequency and awaits the start signal.The timing of start signals is kept fixed by interruptions from the on-board timer, and the measurement is triggered manually one-time by an operator.. The flexibility of this approach and the accuracy of the timer offer a certain flexibility, relaxing the synchronization constraint on the operator triggering the initial start signal on both TX and RX at the same time. Prior to the measurement, a relative power calibration was performed. It consists of setting the output power of the TX radio to 0 dBm (an inaccurate value when measured with a precision instrument) and recording the RSSI on the RX radio for all the frequencies of interest. The radio offers a function that converts the RSSI to dBm.