If you have an older Raspbian image then these instructions may work for you.
The Raspberry Pi uses the serial port as a monitor, by default, so you will need to disable this so that you can use the serial port for the base station receiver.
Run the Raspberry Pi configuration tool to disable the serial port:
Select the Interfacing Options menu option:
Followed by Serial menu option,
then select NO to the question "Would you like a login shell to be accessible over serial?".
Then "Yes" to "Would you like the serial port hardware to be enabled?".
Exit save and reboot.
Special instructions for Raspberry Pi 3 only:
You need to disable Bluetooth so that the serial port will work.
Page down to the bottom of the file and add the following line:
Add the following line:
Exit, save and reboot.