Meaning of Signal Strength Bars

Apple's antennagate controversy is now growing old and a bit stale. The company has come out with a half-hearted response to dealing with the problem, but it appears increasingly likely that this will prove sufficiently satisfying to their customers, especially since most have not experienced the problem of fringe-area performance. There has even been some talk about the antenna being the reason for delaying release of the white iPhone 4, or then again maybe not.

Although I've already touched on this issue, there is one aspect that I think is worthy of another blog post. It comes about from a few subsequent discussions I've had with people, including iPhone 4 users with the product in question. This has to do with a fundamental misunderstanding of how cell phones are internally engineered. I often take it for granted that in this modern age when it seems everyone is conversant with the high-tech products and services that they use every day, that there is also some basic understanding of that technology. This is frequently untrue. Rather than seeing the cell phone for what it is, internally -- a collection of modular building blocks that are essentially independent and communicate over interfaces -- some (most?) people see the cell phone as a monolithic device. That is the impression I get as to why there is still so much confusion between the dependence of the iPhone 4's antenna performance on how it's held and the number of bars on the signal strength display.

But touching the hot spot doesn’t always ruin the call, even if it lowers the number of bars. In several cases, when I was already on a call with three or four bars showing, I deliberately covered the hot spot with my hand, and the call continued normally, strong and clear, even though the bars dropped to one or two.

On that basis I decided to break down the problem into its component parts so that the relationship is more clearly illustrated, and in particular why the number of bars of signal that is displayed can be so distinct from fringe-area performance. Hopefully someone will benefit from this. To those who do understand the technology well, please forgive me for any errors due to my coarse description of the technology details.

First however, let's look more closely at weak-signal performance, where the received signal strength is close to the minimum for a usable signal. This is determined by the signal's field strength -- which is independent of the phone -- and the phone's antenna and radio module. As can be seen in the diagram, the gray area where performance is marginal is wider for the now-obsolete analogue technology (such as AMPS) than for digital. Degradation as an analogue signal weakens is more gradual, and is manifested with increasing noise and distortion until the other person's voice becomes completely unintelligible, and finally descends into silence and call termination.

With digital, degradation may not be noticed until the bit error rate is high enough to cause drop-outs. From there it takes only a little less signal to reach the condition where the bit stream cannot be decoded, resulting in silence and then call termination. In other words, the gray area of received signal strength where the signal is distorted or subject to drop-outs is narrower for modern digital phone transmission. As mentioned in my earlier article, this is one reason why the number of bars can be uncorrelated with reception quality: at any level above the minimum signal strength to achieve a low bit error rate, digital reception is pretty much perfect.

With that out of the way, let's return to the above diagram. The radio module is a self-contained unit, with reception dependent on no other component other than than the antenna system. If the antenna efficiency is impaired (or the signal from the carrier's base station is reduced by terrain or obstructions, or there is interference from another source) the signal that the receiver has to work with is reduced. There are differences among phone in their antenna designs and placement, and some variation in the performance of radio modules from component manufacturers, but for any one phone there are no other factors we need to be concerned about. The antenna plugs in one end, and audio in and out plugs in the other end, plus various control lines and power.

One thing the radio module produces is an indication of signal level (using an internal signal sampler) that can be read and used by other modules. This is typically to be interpreted on a relative scale where zero is approximately the level where communication with the network base station (or tower, if you like) is marginal or lost.  On the Nexus One phone, the relative scale goes from 0 to 31, where 0 is referenced to -113 dBm and 31 is -51 dBm. Below and above these signal strengths will produce 0 and 31, respectively. On other phones the granularity may be less (less than 32 values) and the interval between values may be non-constant (it's a constant 2 dB on Nexus One). However, these are at best nominal values and could vary, perhaps even quite a lot, since there is no real technical need to ensure accuracy.

Whatever the granularity and range, these values are mapped by software to something that is displayed for the convenience of the phone's user. The mapping can be linear (as shown) or any function at all that maps from the signal strength value to the indicator icon. The important part here is that the mapping is arbitrary. I've shown it as mapping the signal strength to one of 5 different displays -- 0 to 4 bars on the screen icon -- because that is a common format.

When Apple talked of making a software change, it was this mapping function they were discussing; they proposed changing one arbitrary mapping to another arbitrary mapping, one where the number of bars would be higher for lower signal strength values. If by chance this wasn't clear before, you should now be able to see that this mapping function has nothing whatsoever to do with the performance of the antenna system and radio module; the mapping function does not impact reception quality so modifying it does not solve the iPhone 4 antenna problem.

Of course changing the mapping function so that it shows more bars at lower signal strengths doesn't hurt. It may even be reassuring to those who worry that a call will be interrupted, or never begun, when there is just one bar displayed. This really isn't a terrible idea since, as described earlier and in the previous article, performance is probably just fine at that signal strength on a digital network so why not remove the source of worry. The thing Apple should not do is claim that this change solves the problem, since it clearly does no such thing.

There is also the matter of the phone's transmission performance since it, too, is equally dependent on the antenna; antennas are in general reciprocal in receive and transmit usage, so a reduction of antenna performance affects transmission (from the phone to the base station/tower) equally. Since that process is a story by itself, I'll stop here, having covered the reception issue which is the larger problem.