Halliday &
Matthiessen (2014: 423):
It is important to make a distinction between a preposition group, such as right behind or immediately in front of, which is a Modifier-Head structure expanded from and functionally equivalent to a preposition, and a prepositional phrase, which is not an expansion of anything but a clause-like structure in which the Process/Predicator function is performed by a preposition and not by a verb.