Dennis:

I have experienced weak cell phone signal in a brand new 4-story 180-bed long term care facility. But I'm not sure if it is due to the low-e glazing. There are so many other factors that contribute to bad cell phone signal. The building is in a rural area, and it's quite possible that the distance to the nearest cell tower is too far. It may also be due to all the metal in the building, such as the metal deck holding the concrete floor slabs, the rebar in the concrete, the steel framing, the metal studs in the walls, or the sheet metal in the ductwork. As you probably already know, metal reflects radio waves.

To answer your question about the low-e glazing, I don't know of a viable alternative at this point in time. Low-e in itself is metallic. Window films that go on the inside face of the glass are also metallic based. (Window films by themselves without Low-E is not recommended, by the way). If you are that concerned about cell-phone signal, it may be worth looking into a signal repeating system, involving an external antenna and internal antennas spread throughout the building.