Continuous-time directed information and its role in communication

The notion of directed information was recently introduced for stochastic processes in continuous time. The key idea of the definition is to consider all possible time partitions of a given interval. Unlike the definition of mutual information of discrete-time random variables with continuous alphabets where the supremum over all possible partitions of the alphabets plays an important role, here the infimum over all possible time-partition plays an important role. We show that the fundamental limit on reliable communication for a wide class of continuous-time channels with feedback are characterized using the notion of continuous-time directed information.