What do cell phone reception bars mean?
They don’t mean much of anything, it turns out. I don’t know what they’re displaying for GSM, but probably what they’re displaying is the signal strength. For CDMA (which is what I know about) that’s what they display, but in CDMA the signal strength is highly deceptive because it doesn’t inform you of what the noise floor is. The technical term is “EC/I0” (pronounced “ee-see-over-eye-naught”) and it refers to the amount of the signal which is usable. In CDMA you can have strong signal (4 bars) and lousy EC/I0 and not be able to carry a call, and you can have low signal (zero bars) and excellent EC/I0 and carry a call fine. But they can’t display EC/I0 because it fluctuates wildly (it could go from zero to four bars and back to zero again in just a few seconds) and would terrify users, so they display the signal strength, which at least has the virtue of being stable, though it doesn’t really mean much. Even worse… there is no industry standard for what “one bar” or “two bars” means.
The biggest reason why the signal strength isn’t very helpful for GSM is what’s known as “multipath”. The signal reaching you from the cell can arrive by different routes which are slightly different lengths, meaning that they’re offset in time. If you’ve ever seen “ghosts” on a TV when using rabbit-ear antennas, you’re seeing the same effect. The “ghost” is a multipath component. In GSM (or any other cellular system using TDMA) multipath is a form of interference. Strong multipath components can render the primary signal useless. The “signal strength” tells you how strong the primary signal is, but doesn’t tell you anything about how much multipath interference you’re getting. Multipath by its nature is highly variable from one location to another. In some places moving just 2 meters can radically change how much of it you receive, thus drastically altering the ability of the phone to carry a call, without changing the signal strength in any way.
This is my understanding based on working for a telco mobile data system several years ago. Please note that I was a programmer, not an RF engineer, so this may not be accurate. The GSM standard does not specify the meaning of the signal bars on your handset (correctly known as the “signal quality estimate”). Each manufacturer uses their own formula to work out how many bars you see. This varies not only between phone makers, but also between models, and between firmware versions of the same model. In short, you can’t compare phones using signal bars. You *can* – to a limited extent – compare the signal strength in different locations using the same phone, but even that isn’t reliable. What I’m about to describe is the typical behaviour for the Motorola devices I’m familiar with. Let me repeat, this may be out of date, and isn’t necessarily typical of any other devices, but having said that, it’s probably not far off. When your phone is idle, the signal quality estimate (SQE) is calcul
