## 2 Design and Experimental Setup  
### Wireless ECG Module  
SCA is characterized by abnormal ECG signals that occur without prior indication, often when patients are outside hospitals. Ideally, low-power (long battery life), low-cost, and miniaturized compact ECG recording systems enable chronic disease management and preventive healthcare with a bit error rate of \(<\)10% [17]. An ECG monitoring system ideally begins with collecting ECG data from a human subject. Zeolite and polydimethylsiloxane (PDMS) composite dry electrodes are used for acquiring ECG signals. The primary advantage of the zeolite-based dry electrode is user comfort and reliable long-term ECG  
Figure 1: Overview of the ECG wireless SCA transmission system. acquisition. Dry electrodes using 13X Zeolite-PDMS with a diameter of 2 cm and a filler concentration of 12% \(w/w\) are proposed and are illustrated in Figure 2a. Placing the electrodes in the precordial position, ECG was acquired from a subject while strolling (limited to 20 steps) [15]. The weak ECG signal needs to be amplified and filtered for noise components before transmitting the signal. In our prior work, we designed and built a compact ECG processing unit on an FR4 substrate with three main components. An AD623 amplifier with 60 dB gain, an RC-based notch filter to remove power line interference (60 Hz), and a third-order low-pass filter using Texas Instruments's TL081 operational amplifier to dismiss higher-order frequencies (\(>\)100 Hz) is implemented on an FR4 substrate for processing the ECG signal and is shown in Figure 2b. The specific values of the resistances and capacitances can be found in our prior work [14]. The target of the wireless ECG system is to transmit the ECG data with low power consumption \(\leq\)28 mW and low bit error rate \(\leq\)10%. This work of a wireless ECG module extends our prior ECG acquisition and ECG processing system [14; 15]. For transmission, the ECG signal is digitized, and an ADC of 1.8 \(\times\) 4.3 cm\({}^{2}\) size from an Arduino nano microcontroller with a resolution of 10 bits, a maximum sampling rate of 1M samples per second is incorporated, as shown in Figure 2c. The advantage of Arduino is that it is low-cost, compact, and has an option to choose from 10-16 bits of resolution. For a transmitter, a low-power nRF24L01 transceiver module, of size 2.3 \(\times\) 1 cm \({}^{2}\), as shown in Figure 2d, is used. The core printed circuit board (PCB) circuit diagram is shown in Figure 3. The transceiver transmits data using Gaussian frequency-shift keying (GFSK) modulation. The Gaussian type of FSK modulation uses a Gaussian filter to shape the pulses before they are modulated. It is well suited for narrow bandwidth biosignals such as ECG data transmission. The transceiver operates at 2.4 GHz, with a data rate of 250 kbps. The pin configuration for nRF24L01 and connections to an Arduino are shown in Table 1. For our design, the transmitter output power is programmed to \(-\)18 dBm. The distance between the transmitter and receiver is 5 m. An nRF24L01+ can transmit up to 100 m in an open field at 250 kbps [8]. The nRF24L01 transceiver module communicates over a 4-pin serial peripheral interface (SPI) with a data rate of 250 kbps. National Instruments's myDAQ ((Model number: 78132901) acts as a transducer to input the acquired ECG signal into the ADC of the Arduino microcontroller. The selection of input ports, and after the digitization, the flow of bits is directed to the output ports in MATLAB. Transmitting at 2.4 GHz is achieved using a single-chip radio transceiver, nRF24L01, which is compatible with Arduino. The transceiver has an option of a higher data rate (250 kbps to 2 Mbps) and transmission distance and hence is a better choice for transmitting large volumes of ECG data. The experimental setup for wireless transmission and reception of ECG bits is shown in Figure 4. The role of myDAQ is to apply the analog ECG signal to nRF24L01. A single-direction communication system is implemented in this study.