Wikipedia
User-in-the-Loop (UIL) refers to the notion that a technology (e.g., network) can improve a performance objective by engaging its human users ( Layer 8). The idea can be applied in various technological fields. UIL assumes that human users of a network are among the smartest but also most unpredictable units of that network. Furthermore, human users often have a certain set of (input) values that they sense (more or less observe, but also acoustic or haptic feedback is imaginable: imagine a gas pedal in a car giving some resistance, like for a speedomat). Both elements of smart decision-making and observed values can help towards improving the bigger objective.
The input values are meant to encourage/discourage human users to behave in certain ways that improve the overall performance of the system. One example of a historic implementation related to UIL has appeared in electric power networks where a price chart is introduced to users of electrical power. This price chart differentiates the values of electricity based on off-peak, mid-peak and on-peak periods, for instance. Faced with a non-homogenous pattern of pricing, human users respond by changing their power consumption accordingly that eventually leads to the overall improvement of access to electrical power (reduce peak hour consumption). Recently, UIL has been also introduced for wireless telecommunications (cellular networks).
Wireless resources including the bandwidth (frequency) are an increasingly scarce resource and the while current demand on wireless network is below the supply in most of the times (potentials capacity of the wireless links based on technology limitations), the rapid and exponential increase in demand will render wireless access an increasingly expensive resource in a matter of few years. While usual technological responses to this perspective such as innovative new generations of cellular systems, more efficient resource allocations, cognitive radio and machine learning are certainly necessary, it seems that they ignore a major resource in the system, namely the users. Wireless users can be encouraged to change their "wireless behavior" by introducing incentives, e.g., differentiated pricing. In addition, the increasing concern for the environment and the considerable yet invisible environmental effects of wireless use can be tapped into in order to convince "greener" user to change their wireless behavior in order to reduce their carbon footprint.
UIL used in wireless communications is referred to as the Smart Grid of Communications. It aims for avoiding a location of bad link adaptation or excess use during the busy hour.