Near field radio link propagation model for wireless capsule endoscopy

Abstract

Wireless Capsule Endoscopy (WCE) is used widely as an implantable medical device to diagnose any irregularity inside the gastrointestinal tract in the human body. The distance between transmitter and receiver antenna, operating at 402 MHz, is considered to be in the near field region due to the distance being shorter than the wavelength. The previous radio link estimation by using the Friis equation is debatable in the near field situation. The radio link calculation method in the near field region is investigated in this research. Normal Mode Helical Antenna (NMHA) is proposed as the antenna used for the application in WCE due to its ability to be designed in small size and high efficiency. The designed transmitter NMHA is placed within a human body phantom with a dielectric constant of 11.6 F/m that represents fat tissues. The receiver NMHA is placed outside the phantom at a distance of 0.1 to 1.0 m from the transmitter. To establish a link budget for this condition, important parameters such as received power (PR), transmitted power (PT), the efficiency of the transmitter antenna ( T) and receiver antenna ( R), reflection loss at the boundary which includes transmitted signal from vacuum to phantom (T1) and transmitted signal from phantom to free space (T2), dielectric loss (L ), spherical spreading of power, power density (W), and effective antenna aperture (Ae) are investigated. Based on the electromagnetic simulation results, the distance dependency of W is examined and the result of W 1/(distance)2 is obtained. As for the simulation results for Ae, the relation of Ae of the receiver antenna is examined at different distance points and results of Ae = -30 dB is obtained. The simple equation of Ae is derived. The radio link equation is formed by employing W, Ae and other propagation loss factors of a medium that is presented in this thesis. The proposed equation is then compared with the power received, PR and S21 obtained from simulation. The calculated and simulated S21 obtained at distance 0.1 m for fat phantom is -53.6 and -53.3 dB respectively. To validate the link design equation, different human body cases such as skin and stomach are studied. The calculated and simulated S21 results for skin phantom are -57.3 and -56.6 dB while for stomach phantom are -50.1 and -50.2 dB. The agreement between the calculated and simulated results shows that this equation can be used in the near field region. The proposed equation also agrees well for all three tissues.