Minimisation Techniques of Multiple Antennas in Mobile Phones and Effect of the Capacity of Multielement Antenna System

Abstract

With the recent progress and rapid size decrease of mobile phones, the design of antennas for small mobile phones is acquiring great importance. In view of this situation, the design concept of antenna systems for small mobile phones are discussed, referring to the trends in modern mobile communications and the demands for future antenna systems such as MultipleInput Multi-output (MIMO) systems. MIMO systems use multiple antenna elements at both the transmitter and receiver to improve the capacity over single antenna topologies in multipath environment. In such systems, the antenna properties as well as the multipath channel characteristics play key roles in determining communication performance. Despite the overwhelming amount of papers published in the area of MIMO systems during the past years, the mechanisms for successful implementation of multiple antennas in a mobile phone are largely unstudied, and how their performance can be optimised is not fully explored. Therefore, there is a need for new methods from which the antenna designer can evaluate the MIMO performance of mobile phone antennas and use the results to determine how they are performing. Issues considered include channel capacity, capacity versus signal to noise ratio, and capacity for MIMO systems with unequal numbers of antennas in link ends, and the impact of antenna element properties on the MIMO system performance. A simple closed formed equation to calculate the envelope correlation between any two antennas in a MIMO system of an arbitrary number of antenna elements is derived. The equation uses the scattering parameters obtained at the antenna feed point to calculate the envelope correlation coefficient. This approach has the advantage that it does not require knowledge of the antenna radiation pattern. Numerical data are shown to validate the approach. It is found that choosing configurations that maximise the distances between the open ends of the Planar Inverted-F Antennas (PIFAs) yields the lowest mutual coupling as well as the lowest envelope correlations.When a small antenna is attached to a small metal object, like the metal chassis of a mobile phone, the size and shape of the object and the position of the antenna on it can have a strong effect on the antenna performance. It is shown that the optimal location of a camera or a loudspeaker could be determined directly from the raw unprocessed electric near-field distribution. The result is that metallic objects should be located in areas below local minima in the electric field amplitude of the total field. Finally, the thesis demonstrates a 30% size reduction by inductive or capacitive loading of the PIFA