Vico VR is racing to be one of the first to make mobile VR positional tracking available to consumers. Vico VR uses an external sensor but doesn't need a PC to control it. Instead the sensor has its own processor that sends the 3D depth data wirelessly via Bluetooth to your smartphone.
Here are some demos from Vico VR:
Although the demos above show the VicoVR being used with a Samsung Gear VR (reviewed here), VicoVR is also compatible with Cardboard, as well as any Bluetooth-enabled gaming platform such as AndroidTV or Apple TV.
Vico VR is moving to market fairly quickly. Theyv'e successfully completed a crowdfunding campaign recently. The dev kit as available now, and thus far they are still showing a scheduled release date of December 2016 for the consumer version. Hopefully, they are indeed able to deliver positional tracking for mobile VR.
Thanks to David Grass for bringing this to my attention!