Which parameter and Data Type should i set for spatial reference in new tool in ArcGIS 10.1?

I developed a python script for user defined grid. I want to make a tool from this script. The problem Im facing is to set up parameter for spatial reference. I used GetParameterAsText function for spatial reference and put 'Spatial reference' in data type while setting the tool in ArcGIS 10.1. It seems that function is not working. I don't want to ask user to input the Factory code for coordinate rather want to ask them to choose coordinate system as they want to set up for their output feature class. How can I setup the spatial reference using GetParameterAsText function and ask user to choose their reference system?