I have an interesting dilemma. I use my Sprint Touchpro2 as a modem to provide my netbook and ipod touch with internet access at the office. I work in an office that has large windows, almost floor to ceiling approximately 9 feet high. My office is separated from these windows by a drywall partition about 6 or 8 feet away and my desk is about 12 to 14 feet away from that wall. While downloading file to my computer with the phone at my desk my bandwidth is about 4 to 8 Kb/s if I go by the wall it increases dramatically to 40 to 50 Kb/s if I go by the window bandwidth increases to 90 to 110 Kb/s. I have never seen such a large drop in bandwidth over such a short distance. I am assuming it's some kind of interference but what. Wifi operates at 2.4 and 5.0 Ghz. Cordless phones usually 900 Mhz and 2.4/5 Ghz. But my office doesn't have any of those. What else could produce interference in the 1.9 Ghz spectrum.
My guess is that it's the building it self.. As you are having more success as you get closer to the open window. As you move away from that window the signal from the tower needs to pass through more of the building (steel and other heavier materials), not just the glass window.
I suspect attenuation, not interference. Does the displayed signal strength drop? Almost certainly, the cell site is not directly in front of your floor to ceiling window. Instead the signal is coming in diagonally to the glass, so the deeper into the building you are, the more "shaded" you are from the cell site. COtech
If you are in an area with borderline reception, your connection quality/speed will fluctuate like that. Could be signal loss as COtech said, but may also be fading dips. Fading (or multi-path propogation) is when the signal transmitted by the tower bounces off buildings, walls, etc and arrives at your phone at different times (ie: out of phase) and cancells each other. Think when your car is stopped at a traffic light and your FM radio is suddenly all static, but then you roll forward 2 feet and suddenly you get the music back. Same thing.