You are correct. The blank space is caused by the tab bar. Override the y position in the view by adding the additional space when you receive keyboard should appear and not in your viewDidLoad or any place else because it will then be hidden behind the tab bar. Then you need to set it back to normal by removing that added value when the user is finished using the keyboard. eg.
// Declaration
var tabBarHeight : CGFloat = 0.0
// viewDidLoad
tabBarHeight = tabBarController?.tabBar.frame.size.height
// When the user begins using the keyboard move the view containing the textfield
yourView.frame.origin.y = (tabBarController?.tabBar.frame.origin.y + tabBarHeight )
// When the user finishes using the keyboard
yourView.frame.origin.y = (tabBarController?.tabBar.frame.origin.y - tabBarHeight )
If you have constraints, you will need to adjust them. Good answer here:
https://stackoverflow.com/a/27870216/4008175
This answer will assist you in listening to when the keyboard has been activated and deactivated:
https://stackoverflow.com/a/27087733/4008175
Good luck!