Calculation of the spatial envelope correlation between two antennas in terms of the system scattering parameters including conducting losses

The envelope correlation for a two-element antenna array may be calculated using the antenna radiation fields, or more simply from the scattering parameters of the system. The use of scattering parameters provides a major simplification over the direct use of field data. In this paper we propose a modification of the scattering parameter method which also includes the antenna losses. This approach has the advantage of simplifying the antenna design process, especially when low envelope correlations are needed. It also offers a better prediction of the spatial envelope correlation, and a good framework for understanding the effects of the mutual coupling. The accuracy of this proposed method is illustrated by two examples.

I. Introduction

Mobile communication systems where there is only one antenna at both the transmitter and the receiver are known as Single Input Single Output (SISO) systems. SISO system capacity is limited by the Shannon-Nyquist criterion [1]. In order to increase the capacity of the SISO systems to meet the high bit rate transmission demanded by the modern mobile communications the bandwidth and/or the power have to increase significantly. Fortunately using MIMO systems (Multiple Input, Multiple Output) has the potential to increase the capacity of the wireless system without the need to increase the transmission power or the bandwidth. On the other hand, mutual coupling between the antennas degrades the antenna system diversity performance; therefore designers try to minimize the mutual coupling of the antenna system while maintaining the matching requirements. MIMO systems are required to deliver maximum capacity with minimum bit error rate (BER). To achieve that, the antenna arrays should have high gain, narrow lobe patterns, and reasonable separation between the elements. In mobile communication the antenna spacing is usually small, thus the impact of the mutual coupling will be not negligible. Mutual coupling increases the spatial correlation between the array elements. Also, it deforms the radiation pattern of each array element, which affects the diversity gain.