Okay, so i'm sat at my work desk on Three with a Nexus 5. I generally have 3 Bars of signal. The second I run a speed it shoots to full bars.

If I leave it stationary and just watch it, it will move from 1 bar to full.

are you in an area where the indoor signal for Three is marginal?

is this not a power saving feature? i've heard people mention it before. in that when it's not transferring data it only needs to just be aware of it's base station.

or are you moving it when you do the test. as in is it in your pocket, flat on your desk or whatever then you pick it up and do the speed test?

I'm indoor in an area with fantastic signal. On my speedtests I break 15mb/s. The speedtest is just when laid on my desk.

At home, it's full all the time upstairs and does the same behavior downstairs.

I was thinking the opposite. i.e when it's dropping to 1 bar it's putting it in a higher power state to keep the base station at a higher signal.

The phones measuring the pilot (CPICH) signal for the bar display, and this is a fixed power signal so is not subject to any power changes, as such it should be pretty constant in a fixed position. The phone doesn't have a low power cellular state when in idle. It does measure more often when active, but this doesn't change what it's measuring.

It may be that as the phone becomes active it moves to a different cell/carrier with a different power setting, or simply that you've moved it slightly, a few cm can make a difference.

Okay, I only notice this effect indoors so it would seem. For small movements could it be as simple as the building supports aren't in the way 2cm one side but are the other?

Thanks.

The wavelength of 3G at 2100 MHz is 14cm, statistically radio fades in size are around a quarter of this, so 3-4 cm can move you from a deep radio fade (10dB or more) into a better signal, this could easily be 2 bars, depending on your handset. Even if you're stationary, everything else is moving around you which affects where these fades occur even when the handset is stationary (this is fast fading). Indoors, there are more obstructions around, which can cause more slow fading, and is due to there being something physical in the way. The first few slides of the link below with some pretty pictures may help. The maths does get pretty complicated pretty quickly!

http://staff.neu.edu.tr/~fahri/wireless_chp2.pdf

Cellular systems are designed to cope with these signal fades, but most of the clever stuff only happens when you are active.

Looking at the link now.

That last sentence is interesting, I'm guessing it only happens when you're active due to the hit in power needed to try and reform the original signal?