How can I determine if a cell phone or laptop is in inside or outside of an office using passive RFID tags?
The answer would depend on whether you wanted to know where the smartphone or laptop was located within the building when it was inside, or just whether it was anywhere within the facility.
If you just wanted to know if a device was inside or outside of the building, you could set up portal readers at the entrances and exits. You would need to set up your antennas so that at least two read the tag. If antenna A read the tag first, and then antenna B, that would indicate that a person was traveling out of the building, whereas if antenna B interrogated the tag first, and then antenna A, that would indicate that he or she was entering. Cognizant Technology Solutions did this back in 2008, when the technology was not nearly as good as it is now (see Eating Your Own Dog Food: Cognizant Uses RFID to Track Laptops).
If you needed to know where in the building a device was located, you would need to either install portals at every doorway or use a passive real-time location system, which employs phased-array antennas to locate tags in three-dimensional space.
I should point out that tracking cell phones would not be easy. First, you would likely need a special tag that could work on or near metal components. Second, the human body is mostly composed of water, which absorbs RF energy, so if a person were to walk through a portal with a smartphone in his or her pocket and the tag close to that individual's leg, it would be very difficult to read that tag. That being said, Guard RFID Solutions offers an active tag for tracking cell phones (see Aerospace Company Tests RFID Solution for Tracking Cell Phones).
—Mark Roberti, Founder and Editor, RFID Journal