What is the in-body propagation loss model for communicating with a swallowed wireless capsule?

The in-body propagation loss model for communicating with a swallowed wireless capsule describes the attenuation of RF signals as they propagate from an ingestible capsule endoscope (located in the gastrointestinal tract) through body tissues to an external receiver on the body surface. The propagation loss is much higher than free-space loss due to the high permittivity and conductivity of human tissue. The loss components are: tissue absorption (the dominant loss mechanism; body tissue absorbs RF energy and converts it to heat; the absorption coefficient increases with frequency; at the MICS band (402-405 MHz): tissue loss approximately 1-3 dB/cm in muscle and approximately 0.3-1 dB/cm in fat; at 2.4 GHz: tissue loss approximately 3-10 dB/cm in muscle; the total tissue path from the GI tract to the body surface is approximately 5-15 cm, depending on the capsule's location), dielectric boundary reflections (each tissue boundary (muscle-fat, fat-skin) reflects some power due to the impedance mismatch; the reflection loss at each boundary is approximately 1-5 dB), and free-space divergence (the electromagnetic wave also diverges as it propagates, adding a 1/r^2 component). The total path loss: PL_total = PL_free-space + tissue_absorption + reflection_losses. For a capsule at 10 cm depth, operating at 402 MHz: PL approximately 40-60 dB. At 2.4 GHz: PL approximately 60-90 dB. Models: the IEEE 802.15.6 standard provides a statistical path loss model for BAN channels including in-body to on-body links: PL(d) = PL(d_0) + 10n × log10(d/d_0) + S, where n approximately 4-8 (much higher than free-space n=2), d is the depth in the body, and S is the log-normal shadowing.