Spatial Diversity for Short Range Communication in Home Care Systems Using One Antenna Element

It is becoming increasingly important for countries like Japan, Finland and Italy to enable as wide as possible home care for their elderly citizens because of the rapidly changing age structure of the population and high hospital costs. Wireless sensors hidden into furnishings ( as a Reference a Finnish on going project ) can provide necessary information for high quality planning of home care visits. The relative positions of the radios are now fixed because of their relations to the furnishings. When conductive or metallic walls, floors or objects are found in the apartment, an antenna can hit a radio fade caused by shadowing or summing up of waves. The Link Quality Indicator of a ZigBeereg receiver can vary up to 50 units within short distances because of a floor, compromising the radio range. This paper describes principles of a radio construction which is able to move the antenna element inside the enclosure of the radio unit along an X-shaped path along the major mechanical plane of the radio unit. Two electrical motors dynamically optimise the location of the internal antenna element to fit the current radio propagation environment. The paper also describes the feasibility and cost factors associated with the new structure. The achieved effect is a useful countermeasure against shadowing and multipath in the 2.44 GHz band but provides only a partial solution to the problem in the 868 MHz band, due to space limitations in a typical radio unit.