An Ambiguity-Free Depth Detection Method for Wireless Capsule Endoscopy by Combining Frequency Locking and Signal Strength Tracking of Self-Injection-Locked Radars

This paper presents a novel depth detection method without range ambiguity for localizing wireless capsule endoscopy (WCE). The WCE may locate in a several-wavelength depth from the skin during its entire traveling path of the gastrointestinal (GI) tract. This work introduces a range ambiguity problem when the phase of the RF signal is utilized to estimate the target distance in conventional continuous-wave Doppler radars or self-injection-locked (SIL) radars. To solve this problem, we propose a new SIL-based radar with both functions of frequency locking and signal-strength tracking of the received signal. The measurement results show that the average depth error is 1.4 mm, and the maximal error is 3.3 mm for the target in a depth range of 10-50 mm. This demonstrates that the phase wrapping can be effectively unwrapped so that the implant depth can be unambiguously determined over a long depth range.