Optimal noise matching condition for mutually-coupled antenna arrays

It has long been appreciated that mutual coupling affects the signal correlation at the outputs of a receiving array. For the signal response, a decoupled array has maximum discrimination between different signal multipath arrival angles, leading to optimal performance from an antenna diversity point of view in a multipath environment with uniformly distributed arrival angles. Matching networks have been designed for specific array configurations to achieve this decoupling. More recently, it has been demonstrated that an array must be decoupled in order to obtain optimal system noise performance. Using the network models developed, it can be proved that system noise for an array attached to low noise amplifiers (LNAs) is minimized over all possible signal arrival angles only if the array appears effectively uncoupled to the amplifiers. If an array is mutually coupled, to minimize amplifier noise a multiport matching network must be inserted between the array output ports and the amplifier input ports that decouples the array and diagonalizes the effective mutual impedance matrix seen by the amplifiers looking into the matching network. In this paper, we discuss some of the physical implications of this optimal multiport noise matching condition for a mutually coupled array.