# Flexible Wearable Heart Rate Monitoring System and Low-Power Design: A Review
## 6 Design of the Flexible Wearable Heart Rate Signals Transmission and Output Module
### Low-Power Hardware Design for Flexible Wearable Heart Rate Signal Transmission and Output Module  
In terms of hardware transmission for flexible wearable systems, both wired and wireless methods are commonly employed. Wired transmission methods include USB interface transmission, serial port transmission (UART) [121], or Ethernet port transmission via Ethernet. Wired transmission has advantages in stability, anti-interference performance, and data transmission rate, but it is limited by the wired connection mode, resulting in lower flexibility. Wireless transmission methods adopt modules such as BLE, Wi-Fi, and Zigbee and so on. The advantages of wireless transmission lie in high flexibility, convenient deployment, and adaptability to miniaturized designs; however, its drawbacks include transmission stability being affected by the environment and transmission rate sometimes being slower than that of wired transmission. Given that wireless transmission can enhance the flexibility of flexible wearable heart rate monitoring systems, this section primarily takes wireless transmission as an example to introduce the low-power design of the transmission and output module in flexible wearable systems. For example, [PERSON] et al. [122] developed a Bluetooth communication and output module for flexible wearable heart rate signal transmission with the low-power CC2541 chip as the core, whose structure is shown in Figure 6a. As the CC2541 chip itself consumes extremely low power, the module further reduces energy consumption by adopting a working-standby mode: it switches between working and standby states every 10 s to avoid prolonged high-power operation. Eventually, the overall power consumption of the Bluetooth communication module based on this chip can be controlled within 3 mW, fully meeting the low-power requirements for the long-term operation of flexible wearable systems. In addition to the BLE module, for scenarios requiring longer transmission distances, Wi-Fi modules [124; 125] can leverage their advantages of a high data rate and long-distance  
Figure 6: (**a**) Low-power Bluetooth module based on CC2541 chip [122]; this image is licensed under CC BY 4.0, copyright 2019, IEICE TRANSACTIONS on Information and Systems. (**b**) Program diagram of the DCT compression algorithm. Reproduced with permission [123], copyright 2011, IEEE. (**c**) Program diagram of the DCT-IV compression algorithm. Reproduced with permission [71], copyright 2016, IEEE. transmission for data relay, ensuring stable transmission of heart rate data from the collection end to remote servers or receiving devices. For example, [PERSON] et al. [126] developed a Wi-Fi-based flexible wearable heart rate cloud monitoring module through hardware selection and peripheral circuit design. The module reduces energy consumption at the core level by adopting low-power chips, and furthermore, the voltage regulators in peripheral circuits stabilize voltage and optimize current transmission, thus avoiding non-ideal operating states caused by voltage anomalies and directly reducing both dynamic and static power consumption. To further minimize module power consumption, the adopted MQTT protocol streamlines the protocol header and reduces communication overhead, making it suitable for networks with limited bandwidth and consequently lowering transmission energy consumption. It consumes more than 25% less power than traditional wired transmission and output. Furthermore, the Zigbee module [127; 128] demonstrates unique value in constructing complex heart rate monitoring networks by virtue of the dual advantages of its self-organizing network capability and low-power characteristics. The module adopts the IEEE 802.15.4 low-power protocol, which consumes only microampere-level current in sleep mode. Combined with a dynamic wake-up mechanism, it can activate the node communication function on demand.