In chapter 2 section "Multiplexing for 4G" it is stated that "The minimal schedulable unit, called a Resource Element (RE)...", but REs cannot be scheduled individually; the minimal schedulable unit is whole PRB pair (12 subcarriers and 2 slots) assuming legacy LTE, not the IoT "addons".
This is even later indicated as a constraint on scheduler (note that 7x12 is PRB for normal CP). The colors on figure 4 suggest that you can schedule blocks smaller than PRB which is not possible.