It is just some kind of emulated single mode wave from infinity. How big and what is the phase of the signal? It really does not matter. We use some assumed signals to find the network s-parameters. Then, the properties of your network are already known. You can use the s-parameters to post processing the I/O. For example, you can put some specified signal into the input and see what the output is. It can be done in F-domain on MODUA and MDSPICE. You can also do it on T-domain on MDSPICE.