Apple Claim Signal Strength Display Causes iPhone 4 Issue

Apple are claiming that the problem with the iPhone 4 is only down to the signal strength display. But, many users have demonstrated that the drop in signal strength affects both data throughput and can cause dropped calls. As I mentioned yesterday Anandtech have measured the signal strength drop and found that it’s very large.

Apple have said:

Upon investigation, we were stunned to find that the formula we use to calculate how many bars of signal strength to display is totally wrong.

Our formula, in many instances, mistakenly displays two more bars than it should for a given signal strength. For example, we sometimes display four bars when we should be displaying as few as two bars.

This does make some sense. The users affected may be in poor signal areas that are being reported as good signal areas. So, the degradation is exaggerated. However, that doesn’t show that Anandtech or the other testers are wrong. Anandtech’s test didn’t depend on the bar display, they used a piece of software that extracts the signal strength in dB from the radio.

Also, the problem is reported to affect both WCDMA(3G) and GSM EDGE. The signal levels involved in these two protocols are quite different. The algorithm or formula that takes signal strength information (and sometimes other info) and produces the display of signal strength bars is generally different. That is, WCDMA has an algorithm to produce the bar display and EDGE has a different algorithm.

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3 Responses to “Apple Claim Signal Strength Display Causes iPhone 4 Issue”

  1. Gravatar of Norman Yarvin Norman Yarvin
    2. July 2010 at 22:34

    The nonlinear mapping of dB received to signal bars that Anandtech noted seems to me like it’s probably what Apple is talking about. In that mapping, about half of the range of signal strengths mapped to five bars; then at the bottom, the number of bars changes rapidly. So if Apple adopted a more linear mapping, the number of bars would change more slowly in places where the signal is close to giving out.

    That doesn’t make 20dB of loss any more acceptable, of course; it would just lessen the “OMG I lost four whole bars” factor… at the price of making it harder to optimize your phone’s performance when the signal is weak, which is when optimizing performance is most critical.

  2. Gravatar of rt rt
    2. July 2010 at 22:49

    Non-linear mappings are quite normal for this.

    I’ll explain why… Let’s say your just below a cell tower. In that case you have no problem with signal strength at all, what determines the data throughput you’ll get is the limitations of the protocol and the network’s optical fibre backbone. The protocol provides a maximum modulation rate that provides the highest theoretically possible data rate. As you walk away from the cell tower that rate can be provided until the signal strength drops below some threshold. Then after that the error rate increase and throughput drops. The system responds by changing to a different modulation rate, this is “adaptive modulation”. Then as you walk still further away you pass into a lower modulation rate.

    This system has coarse granularity at high signal-strength levels and fine granularity at low signal-strength levels. That’s why the signal strength indicators work as they do. Between 51 and 91dBm you won’t see much difference in performance, but after that the performance change comes quickly. So, having irregular sized steps is the right thing to do.

  3. Gravatar of Norman Yarvin Norman Yarvin
    3. July 2010 at 04:24

    Oh, I agree that it’s the right thing to do; but I’m guessing that Steve Jobs doesn’t agree, and instead thinks of it as the bug that he was “stunned to find”. (It certainly is causing him more PR problems than a linear mapping would… and he’s a computer guy, not an antenna math guy, so to him, a linear mapping is, I’m guessing, “obviously” the right thing to use, so anything else can just get labeled a “bug”, even if it’s really a useful feature like this is.)