Wireless Communication for Custom Hearing Instruments

Wireless communication for custom hearing instruments has been studied. The main focus has been propagation and antennas for ear-to-ear communication at 2.45 GHz. Custom hearing instruments are sp ecially made for the individual user’s ear canal. Thus the devices are placed in the ear at various depths and orientations. A new geometrical theory of diffraction formulation for the on-body creepingwave propagation has been developed. It is the first attempt of a general formulation for all opaque, convex, and lossy dielectric objects. The formulation has been validated against an eigenfunction solution on a cylindrical geometry. It is valid for geometries down to a size κ κ2+τ2 > λ/2 and up to a torsion limit of τ /κ < 2 as long as the geometries are opaque and convex. The ear-to-ear on-body channel between hearing instruments on opposite sides of the head has been characterized by a new ear-to-ear propagation model. The model is based on the new developed geometrical theory of diffraction formulation. It is the first to find the geodesic lines that connect the ears. The model shows a clear path behind the head and a clear path over the top of the head. In front of the head multiple geodesic paths exist. The main one is the one over the chin. For an omni-directional antenna, the front paths would provide the main contribution to the ear-to-ear path gain. The model has been validated against simulations. It has been shown that the model predicts the interference between the different paths correctly. This has been shown with frequency sweeps and by a rotation of the antenna in the ear. Multiple new antenna designs has been presented. The antennas are the first in a realistic in-the-ear position to achieve a path gain better than –80 dB. The antennas are matched for the entire 2.45 GHz industrial, scientific, and medical band. The antenna designs confirmed that a polarization perpendicular to the surface of the head is best to optimize the radiation efficiency. For two similarly sized and placed antennas, the efficiency improved by 5 dB. The magnitude and phase of the radiation pattern can change the contributing geodesic paths and thus their interference. The effect from the radiation pattern is seen to be more than 10 dB for the ear-to-ear path gain. Rotation and change of the placement in depth is shown to have significant effect. The effect from rotation is not intuitively predicted, but can be more than 10 dB. If the device is moved further into the ear canal the radiation efficiency drops with one to two decibels per millimeter. In summary, the work gives a theoretical explanation to the creeping waves observed as well as a propagation model to estimate and analyze the ear-to-ear path gain. Finally, suitable antenna designs with guidelines for future designs for custom hearing instruments are given.