Wireless Bio- medical Sensor Network for Heartbeat and Respiration Detection

A wireless bio-medicalr sensor Network was designed to detect a human heartbeat and respiration signals without direct skin contact. In order to design a wireless bio-medical sensor quantitatively, the signal to-noise ratio (SNR) in the baseband output of a sensor should be calculated. Therefore, we analyzed the SNR of the wireless biomedical sensor, considering the signal power attenuation in a human body and all kinds of noise sources. Especially, we measured a residual phase noise of a typical free-running oscillator and used its value for the SNR analysis. Based on these analysis and the measurement results, a compact, low-cost 2.4 GHz direct conversion bio-medical sensor was designed and implemented in a printed circuit board. The demonstrated sensor consists of two printed antennas, a voltage-controlled oscillator, an I/Q demodulator, and analog circuits. The heartbeat and respiration signals acquired from the I/Q channel of the sensor are applied to the digital signal processing circuit using MATLAB. ECG (electrocardiogram), and reference respiration signals are measured simultaneously to evaluate the performance of the sensor. With an output power of 0dBm and a free running oscillator without a phase locked loop circuits, a detection range of 50 cm was measured. Measurement results show that the heart rate and respiration accuracy was very high.