I. Introduction
With the advances in cyber-physical technology, the physical systems facilitating modern world have become more and more connected, while the growth in scales and dimensionalities in these systems have made the control and monitoring more difficult. Conventional control mechanisms utilize all available inputs and outputs from system specifications. The problem is that as the number of inputs or outputs scales up with system size, implementing the control would necessitate a large volume of sensors and actuators for measuring the outputs and feeding the control inputs, as well as demand an expensive bandwidth for all-to-all communication between sensors and actuators. Moreover, large-scale network systems such as power systems often exhibit a low-rank coherent dynamics. In this sense, employment of full outputs or inputs may not be necessary for the control objective. This has led to the problem of sensor and actuator placement, that is to employ only a limited number or preferably a small number of sensors or actuators to achieve a near optimal performance of a controller as of full feedback.