Hi,
As per my understanding you are configuring the LTE simulation with the predefined Reference Measurment Channel "R.12". By default, the configuration is designed for four antenna ports (CellRefP=4) and you want to simulate it with single antenna.
Referring to the following MathWorks documentation:
The "CellRefP" parameter controls the number of cell-specific reference signal (CRS) antenna ports and accepts the value of "1", "2" and "4". Therefore you can change the value of CellRefP to "1" to ensure that the eNodeB transmits using a single antenna.
I hope this resolves your query!