Indoor near-field target detection characteristics under radio and radar joint operation at 2.4 GHz ISM band

This paper presents an experimental research work on detecting an indoor near-field target at 2.4-GHz industrial, scientific, and medical (ISM) band. The research is based on hypotheses that if the detection target is in proximity to antennae, then the dominant electromagnetic (EM) wave couplings would happen between the target and antennas. Any other EM wave interaction between the antennas and indoor background objects would be secondary to the target-to-antennas EM-wave interaction. This unique coupling may allow us to distinguish and measure the EM wave energy scattered by the target to the receive antenna in the form of signal power using a spectrum analyzer. The detection power is a strong function of the distance between the target and receiving antenna, which could be characterized by radar equations. In such scenario, the hardware module configured to test the hypotheses plays the role of a radar sensor that detects indoor targets at near-field region under the coexistence and cooperation of WiFi radio link at the ISM band. In the experiment, at the presence of a flat round metal target with radius 5 cm, the measured detection signal power ranged from 10 nW to 1 mW (-30 dBm) when the sensing distance (r) decreased from the edge of a radiative near-field (r = λ = 12.5 cm) to the reactive near-field region (r = λ/2π = 2 cm). These were well-agreeable with theoretical estimations by radar equations, validating the hypotheses.