Using mobile phones in Japan could be a struggle sometimes. Although you can buy a prepaid sim at airports, it usually either lacks the capacity or necessary services to communicate.
Since the Japanese government is strict for the usage of SMS and phone calls, it is unlikely to get a sim-card including these services.
If you can even use a mobile , it is possible , such as in reference to online guide book , http://www.intheluggage.com/ will travel becomes greater ease
Also, Wi-Fi spots are not so in common even in Tokyo so that you could struggle a little bit to communicate in Japan.
However, there is a service called Line to be a solution.
This social tool is very famous among Asian countries including Japan.
I strongly recommend you to download this application and communicate for free with Wi-Fi.
Also, Japanese government provides a free Wi-Fi for a few weeks designed for travelers.
In order not to be upset after arriving in Japan, it is important to know what plan or what tool you should use in order to communicate with others.