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Comment by userbinator | Hacker News Reader

A friend recently got a (carrier-supplied) phone and has been complaining about how it would often have no reception despite showing a good signal; taking mine to the same areas on the same carrier and doing a comparison, mine was indeed showing no bars on the signal indicator. The difference is, mine predates this stupidity, and I can also see the details in the MTK Engineer Mode app, which shows the actual signal strength --- it was around -140dBm when it was showing 0 bars.

The signal strength measurement is actually standardised: https://en.wikipedia.org/wiki/Mobile_phone_signal#ASU

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> taking mine to the same areas on the same carrier and doing a comparison

Unfortunately I don't think it's that simple. I've seen one phone simultaneously show significantly different numbers of bars for two SIMs installed in it for the same exact network and operator. After a while they become similar... then differ again... etc.

I have no clue how to explain it yet, but what I do know is that it literally makes no sense with a naive model of how these work, whether you try to explain it as reception or deception.

The phone selects a RAT (radio access technology) and frequency for each SIM slot.

After selecting, each SIM slot is subject to inter freq / inter RAT reselection / handover.

Both are controlled by messages received from the tower (e.g. on 4GLTE, for reselection, System Information messages), though there is an additional constraint: what's supported by/enabled in the phone.

Perhaps one SIM slot was in the connected state and the other was in the idle state at one point. So the reselection logic applied for one and the handover logic applied for the other. There is for example a problem called ping pong handover. Once a phone is switched to a different frequency or RAT, the tower may have the phone be sort of stuck in the new frequency, until the conditions of the previous RAT or frequency improve substantially, in order to prevent the phone being like a ping pong ball between the two. This frees resources that would otherwise be spent on repeated handover-related messages.

Each frequency has its own signal strength (free space path loss, transmit power, one frequency might be on one tower and another might be on another, etc).

This is usually for a good reason - dual sim phones are almost always “DSDS”, or “Dual SIM Dual Standby”. The secondary SIM, because it doesn’t need to make a data connection, parks itself on the lowest-frequency (and therefore usually lowest-bandwidth) connection it can find. Meanwhile, your data-connected SIM is busy trying to stream a video or upload your photos, so it’s using a higher-frequency + higher bandwidth connection, resulting in a lower signal strength.

> Meanwhile, your data-connected SIM is busy trying to stream a video or upload your photos

You're making huge and incorrect assumptions here, no? This also happens when your phone is entirely idle... and it randomly changes if you sit still for some time...

Your phone is never idle. Open your adb logs sometime and you'll see that it's doing a ton of stuff all the time. Much of which involves a network connection.

So your phone is basically always doing something, and frequently sending and receiving data when you assume it's doing nothing. By design, radios hop around between channels as conditions change. Another device somewhere outside your house kicked off a big transfer and your device hopper channels to avoid interference. Random atmospheric conditions introduced new noise, or another channel cleared up. This is standard, normal behavior for WiFi, Bluetooth, cellular, and essentially every other type of modern digital radio.

What you're seeing is normal and expected behavior. Modern radios and operating systems are vastly more complex than you're assuming.

How do you know that your phone is entirely idle?

I also have multiple SIMs set to the same network, with sometimes a different amount of bars shown.

I guess the bars aren’t realtime but updates every x seconds? I summed no malice.

> I have no clue how to explain it yet

Android is quiet lazy searching for towers.

I think it has more to do with the cellular modem itself, or precisely the firmware it's running; of which there is much more diversity on the Android side.

As I read this comment on my iPhone 15, I have 1 bar of 5G signal on one esim and 3 bars of signal on the other.

This suggests that the issue is not related to Android.

When we visit downtown of our city, I get great data coverage. My wife, on a different model but same gen iPhone and same plan, gets nothing. Her phone shows three or four bars but her apps won't load anything.

No idea why, especially since I'm the one who installs ad blockers and such. Her phone is essentially stock.

> My wife, on a different model but same gen iPhone and same plan, gets nothing.

Some generations, different Apple models have pretty different radios. Is there a difference in bands or ?

is it perhaps iPhone 16e?

15 Pro Max

I highly recommend Network Cell Info Lite app for the network diag. It shows signal strength with all details for each of the SIM modules, shows on a map in real time where are the base station you are currently connected to, and other interesting statistics.

There’s no way you earnestly recommend the hot steaming pile of ad-ridden caca I just installed on my phone.

That might be the worst app I’ve used on my iPhone in a year. Better off vibe coding an app to give you signal strength.

Well, there is a paid version (which I didn't try), presumably without ads. As for the Lite version, I don't know exactly why, but ads in this particular app never load on my phone. I see only a black space at the bottom where they supposedly should be visible, but they never load for some reason, without any action from my side. And I think the same happened on my previous phone too, need to check later.

Sorry, I came off too strong since you were just making a recommendation. But man is it bad on iOS.

Just tried the Android version and it was ok.

Does iOS not have built in network signal details?

I don't need to install an app on my Android phone to see my network signal strength. It's kinda hidden though.

Settings->About Phone->Click the sim slot you want to see info for

-140 dBm is far beyond no coverage, yeah. -120 dBm is pretty much when LTE stops working (sometimes it can painfully stretch to -123 to -125 but usually not because of noise etc)

Yes, even thermal background noise (the noise level that exists even in complete absence of RF) would be expected to be above -140dBm. It scales with channel size and temperature.

As near as I can tell, the smallest subcarrier 5G can use is 15kHz, the thermal noise floor for a 15kHz channel at room temp (300K) would be -132 dBm.

My guess is whatever chip doing the measurement simply couldn't measure that low accurately, or it reports "nothing detected" as -140 dBm.

GPS receivers can go down to around -160dBm, but that's a very low bitrate signal. Nonetheless it is possible to receive such signals via CDMA techniques.

Reminds me of "The Hunt For Red October" and the sonar consoles "magma displacement" reportings...

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