Fig. 5

Illustration of dummy location (user) selection. It depicts a scenario that users send requests to the LBS server in a sparsely populated region. The solid red dot A represents the real user, the solid black dots represent the selected dummy locations (users) by the K-SDCA algorithm, and the numbers indicate the order in which they are selected. The hollow dots represent the unselected dummy locations. The solid lines between two locations represent the connection between dummy locations selected in the odd round, and the dotted lines are vertical lines representing the distances from the location point to the straight line