Photo ID checks are performed during the event days each time you enter the venue. Make sure you carry valid ID with you at all times. Identification documents MUST show your name in a Latin alphabet (no Chinese, Japanese, Korean or Arabic characters) and match the name on your pass.
If you are using your Digital Badge there is no need to produce ID once you have uploaded and had your photo validated at badge collection. If your photo was validated at Mobile World Congress 2015, it may be reused without having to revalidate, unless you change the photo.