Analysis of M-ary PSK Bio-degradable Tag

Previous work published by the authors has considered a passive bio-degradable implant as a novel method to monitor the healing of soft tissue trauma. By implanting a passive tag inside the wound area during the final stages of surgery can allow early stage diagnosis of infection. The tag is passive so an off-body receiver uses backscattering waves to communicate with the implanted tag. As the tag degrades, its electrical length changes. This change can be monitored from outside the body by measuring its forward transmission coefficients. Infection results in an increase in the rate of degradation due to local changes in temperature and acidity. Therefore, monitoring the rate of change in degradation allows us to infer the onset of infection. In this paper we introduce a more efficient method for detecting an implanted passive biodegradable tag by measuring both in-phase and quadrature components of the signal. As the tag's electrical length changes, the complex RF response of the tag produces an angular shift on the I-Q plane. This feature can be exploited to create a degradation pattern which can give a response similar to M-ary PSK. The number of degradation stages of the tag specify the number of points on the constellation diagram.