shown in Figure 21. Experimental results demonstrated the limitation of using a single sensor in real situations and the ability of BWRSN in detecting emergency situations while monitoring breathing. The research in [95] described the design of a monitoring system for the purpose of baby monitoring. This system consists of a monitoring unit and a receiver unit. The monitoring unit is to be hanged on the side of the infant's crib to detect his/her breathing and heart rate. Collected signals are then communicated wirelessly to the receiver unit. Alarm sound and red lights will flash on both units if the child respiration and heart rate is detected to be too weak. The monitoring unit consists of an RF circuit to send and receive the radio signal, a microcontroller for processing, an Xbee chip for communication with the receiver, and several buttons and switches. The generated signal for transmission is a single tone at 5.8 GHz. The Xbee wireless transceiver communicates with the receiver in the monitoring unit and keeps it updated with the current alarm status. The choice of 5.8 GHz is due to its suitability in detecting vital signs, besides being an unlicensed ISM band, with wide availability of low-cost components. This prototype is capable in detecting vital signs up to 1.15 m, while the receiver unit can be as away as 50 m to be able to receive data from the monitoring unit. The block diagrams of the monitoring and receiving units are shown in Figure 22. In situations where a large network of radars and communication devices are co-located, their co-existence in the same spectrum will increase interference Sensor 1 Sensor 2 Base Station Sensor 3 Sensor 4 Figure 21. Sensor nodes and base station setup in [93] © 2016 IEEE. VCO REG Power Amp RF Out Baseband Circuit Tx Antenna MUX RF In Mixer REG Amps LNA Rx Antenna REG Speaker µC LEDs XBee Low Battery Detector Battery Charger REG Speaker Buzzer Power Battery Charger Monitor XBee µC Low Battery Detector LEDs Receiver Figure 22. Block diagram of monitoring system described in [95] © 2009 IEEE. FIRST QUARTER 2021 IEEE CIRCUITS AND SYSTEMS MAGAZINE 67