Konami recently announced that the previously announced Metal Gear Solid V: Ground Zeroes is indeed a prequel to Phantom Pain and will be releasing on current and next gen consoles on March 20th, 2014!
Metal Gear Solid V: Ground Zeroes acts as a prequel to events in the forthcoming Metal Gear Solid V: The Phantom Pain. The game introduces a new open-world environment to the series, alongside changing day and night environments, changing weather elements, and extensive AI advances.
See screenshots below! (Clicking on an image triggers the gallery for easier viewing).