Noting that you are preparing a User Requirements document, in the System Architecture section you might want to be identifying the system architecture as part of user requirements, e.g., what are the existing architecture? are you expected to keep them, replace them, or have your new system working with them? etc.
In other words, you might not want to elaborate the architecture of your future system for too much in your user requirements, unless there are something specific from the user that need to be captured.
Just my 2 cents.