An ultra-wideband (UWB) pulse radar is proposed to detect human respiration behind non-metallic and nontransparent objects. The proposed radar transmits impulse-radio UWB (IR-UWB) signals with a reconfigurable center frequency in the range of 3.55 to 4GHz. Then, the received signal from the environment is transferred to the baseband in the analog domain for further processing. In this system, to reduce complexities and problems related to frequency domain analysis, time domain analysis of the base-band signal is utilized, to extract the target respiration. Since the target can be located at an environmental frequency null, the proposed system utilizes a frequency-tuning mechanism that maximizes the down-converted baseband signal, thus preventing signal loss due to the nulls. Moreover, here, only in-phase downconversion is utilized, since the proposed frequencysearching mechanism also finds the proper frequency that the received RF signal and the local-oscillator (LO) signal are nearly in-phase. The proposed system is implemented using off-the-shelf components at a low cost. The implemented system is tested in a real environment and successfully detects human respiration, behind a 30cm thick brick wall as well as a wooden door, up to a distance of 2.5m, with less than 5% error in the respiration frequency detection. It also shows superior performance, as compared to conventional I/Q techniques.

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