Wireless Communication for Hearing Aid Systems

Baqer Nour, PhD student
Olav Breinbjerg, Professor
Ole Sigmund, Professor
Niels Asger Mortensen, Associate professor

1 December, 2008 – 30 November 2011. 

Technical University of Denmark. 

Hearing aids already constitute an advance technology and wireless coupling to hearing aids will open of a range of completely new functionalities. This constitutes the motivation for the project “Wireless Communication for Hearing Aid Systems” that was proposed by a consortium including Teknologisk Institute, Widex, and Technical University of Denmark in April 2007 and approved for funding by Højteknologifonden in July 2007.

Recent years’ tremendous development of wireless communication involving ever smaller terminals mobile phones, GPS receivers, PDAs, headsets, etc. – has caused a significant research interest in antenna miniaturization. The challenge is not just to develop geometrically small antennas but electrically small antennas that are less than half a wavelength of the RF signal – the traditional minimum length of resonant antennas. Additionally, the propagation to and from hearing aids, in particular between the hearing aids on opposite sides of the human head, constitute a major challenge since such propagation channels have not been employed previously and thus not been the subject of technical research.

This Ph.D. project foresees 3 major areas of investigation:

1. Ear-to-ear communication
The wireless coupling between hearing aids on opposite sides of the human head constitute two major challenges. First, the propagation around the head, and scattering from nearby structures (ground, walls, etc.).  The relative significance of these mechanisms, that may depend on the frequency of the signal, is not understood and must be studied through theoretical and numerical analysis as well as experiments. Second, depending upon the frequency of the signal, the antennas may be in each other’s near-fields and the spatial structure of the propagation wave is thus far more complicated than in almost other radio communication systems where the transmitting and receiving antennas operate in the far-fields with the propagation wave thus being a plane wave. This may have a strong influence on the antennas that can be used for this type of communication. 

2. Propagation measurements
In order to carry out experimental measurements of the ear-to-ear communication, which is not possible with standard measurement set-ups, it is necessary to develop a dedicated measurement facility. This will include, in addition to the implementation of RF hardware and head phantoms, proper schemes for taking in account the disturbing influence of connecting cables, support structures, and nearby-objects which becomes significant with the small size of the hearing aid antennas. Furthermore, numerical simulations and experimental tests must be carried out to assess the accuracy of this new type of propagation measurements and develop an accuracy budget. 

3. Magneto-dielectric antennas
Antenna miniaturization is essential for wireless communication in hearing aid systems since the mechanical as well as electrical size of the antennas must be small there are several techniques for antenna miniaturization and recently there has emerged a significant interest in magneto-dielectric antennas. The magneto-dielectric materials facilitate miniaturization through a combination of magnetization and polarization, expressed in terms of the permeability and permittivity, yielding a smaller wavelength inside the material and thus allowing for a mechanically smaller antenna structure compared to antennas without such materials. In contrast to purely dielectrically loaded antennas, the magneto-dielectric materials can maintain an intrinsic impedances close to that of free space and may thus provide a better matching of the antenna at its input and output terminals as well as allowing larger bandwidth.


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