Cellphone "Bar Display" Indicators

Apple's initial reaction to the iPhone 4 signal strength issue was an admission that they miscalculated the number of signal strength bars that they show. Steve Jobs mentioned in today's press conference that there is no standardized algorithm for calculating "bars" so the industry has been on it's own. But there must be some general consensus out there because a new version of iOS was released "fixing" this issue. Clearly a favorable "bars" indication is somewhat of a selling point for both a phone and a carrier, so there is some grey area here.

But my question is, what do the bars actually stand for? If you ask someone just walking down the street, chances are they might say that it indicates the strength of the cell signal. I would imagine that in the middle of a city were there are many cell stations and lots of radio frequency noise, you would see "five bars" pretty much all the time. But out in rural areas where there is comparably far less radio energy around, you might expect to see very few bars. Of course this doesn't seem to jive with the realities of cell phone usage today.

Regardless of how much radio energy is around you, the only thing you really care about is how much "signal" is there that you can actually use. So, many cellphone signal strength indicators display the far more useful "signal to noise ratio", or the amount of signal you have to use that is above the noise floor. In the middle of a city where there is lots of radio frequency noise around, there may be less useful signal even if the overall power of that signal is greater than that in a rural area. Background noise, as it turns out, is a very significant factor.

But wether or not to show the signal to noise ratio isn't the only decision to be made. When you "death grip" a cellphone, the signal strength understandably drops. Obviously the instant you grip the phone, the signal has dropped. But for marketing reasons, it makes sense to add some delay to the bar display because the drop might only be temporary. Some phones take upwards of one minute to accurately display the new level. (to it's credit, the iPhone is fairly quick to respond - which has been somewhat of it's undoing in the recent days) Obviously what is going on here is some sort of average over time with the time being a big variable manufacturer to manufacturer.

Almost comically, an increase in signal strength is usually indicated many times faster than a decrease. Release your grip on a cellphone near you or check YouTube to see how quickly some manufacturers spring back. Obviously there is some discrepancy from device to device and marketing definitely plays a pivotal role.

Regardless, there isn't much clarity around what signal bars indicate and there needs to be a standard. It is already hard enough to evaluate problems with the various carriers around the world without "bar discrepancy" getting in the way. Does anyone know of an effort underway?

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Cellphone Bar Display Signal Strength iPhone 4

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Comments (2)

Mike Demler from San Jose, CA

Hello Anders,

I ran across your blog post through Twitter. You make an interesting point regarding SNR & the bar display, but I'm afraid that even there it is not possible to standardize in a meaningful way. Perhaps if there was only one radio band and one modulation scheme in the U.S. we could get closer to a standard measurement, but that is just not the case.

Another problem is that we are not talking about analog demodulation schemes. The 3G signals are received through an analog/RF front-end (RF/AFE) and demodulated digitally. When designers test performance of their chips, they need to run through a number of configurations for interference products, etc. There is no one type of "noise".

I think that all one can say about the bars on a given phone is that they are an indicator of the relative quality of the signal being received. No absolute metric can be established.

-Mike Demler

Anders from Cambridge, MA

Thanks Mike, good points. Check out Mike Demler's "The World is Analog" blog as well:

http://www.the-world-is-analog.blogspot.com/