Spotify is planning to introduce a free mobile version of its streaming music service, according to The Wall Street Journal. The free version is said to be ad-supported and focused around playing personalized radio like Pandora or Rdios Stations feature, though users will also be able to play a limited number of songs on demand — its otherwise unclear how the free tier will differ from the existing Radio. Spotifys current on-demand mobile service is only available to premium subscribers, with its free version restricted to PCs.