Hacker News Reader

Comment by BatteryMountain | Hacker News Reader

Here's another one for you: today I found out that my dual-sim android phone has a single radio that is shared between the two sims. So when you are on a call, the 4G/5G doesn't work on the other sim, likewise, sometimes the the voice sim is unavailable while the other sim engaged with 4G/5G things... which is just stupid. Can't they just add two radios/chips (like many wifi AP's....)? So the whole setup on a consumer phone is actually quite unreliable because of reusing a single radio. I thought it was my buggy code, but nope, just the radio being shared and the mobile operator seeing the sim as offline. Crazy.

View on HN

I can't speak to your specific setup, but if the two SIMs use different carriers then they will often need to use different frequencies. A phone could in theory use twice as many radios, but the emitted power limits would be the same, so you'd have reduced range or throughout for each (plus additional size, weight and power for the second radio). Access points have different SWaP budgets than phones.

When I travel, I try to use an eSIM that uses a network my home provider partners with. This seems to improve the chance that voice and data over LTE or 5G NR work simultaneously.

> the emitted power limits would be the same

Dumb question: If the limit is L, why do regulators care if I have one device with two radios each outputting L or if I have two devices each with one radio outputting L. Either way, total output is 2L.

What is the objective?

* To limit range? Then regulators shouldn't care about 2 radios in one device outputting L, as long as one radio doesn't output 2L.

* To avoid cooking nearby electronics and people? How do they account for the unpredictable, theoretically limitless number of devices in an area? What about someone working in a cell phone store or passengers on a Tokyo train at rush hour? Do regulators just set L low enough that any realstic number of devices together is safe?

Usually, the exposure limit is based on an active phone next to an ear. RF field strength falls off with the square of distance, so a phone on each side of your head is most dangerous to the respective ear and bits of your brain close to your skull; each phone is considered acceptably safe to the middle of your brain and the opposite ear.

Common dual SIM configuration eTLA(extended three-letter abbreviations) are:

  - DSDA: Dual SIM Dual Active, theoretically can call and do everything on both SIMs simultaneously  
  - DSDV: Dual SIM Dual VoLTE, can standby on both at all time but can't "use" at the same time     
  - DSDS: Dual SIM Dual Standby, can standby on both on so long radios don't conflict(3G+4G|4G+5G)
  - DSSS: Dual SIM Single Standby, can physically mount two SIMs to just electronically switch between

It depends on the phone. The rule of thumb in the past for typical dual-SIM phones was nG on the first SIM, n-1G on the second. Then, after you encounter that for the first time, you make sure that your next phone has a baseband that supports nG on both SIMs, although with 5G it's become a bit blurred, typically 5+4 or 4+4, not that that makes any difference (5 vs 4 I mean).

Voice is normally done using VoLTE now.

Yep, although (at least in my experience) it's not enabled by default. I also learnt about VoWiFi (also called "WiFi Calling") that allows you using WiFi for calling, including when you're abroad or you don't even have phone signal.

Support for VoWiFi is a bit weird. Some operators don't allow it outside of your region (for example, the EU), some others will allow it and even bill you as if it were a local call made from your country.

Also, although it's in theory made only for working through WiFi, seems like if you have a second SIM or eSIM with a data plan (for example, a travel SIM local for the country you're on vacations) and some combination of phone and/or operators, it can use the second SIM as the data provider for WiFi Calling.

For the major US carriers, since they shut down 3G, VoLTE is the only way calls work. Which meant that a non-VoLTE-but-4G-capable device can no longer make 911 calls.

I was using a Samsung S5 running LineageOS for a while with Google voice for calls, but the no 911 thing is one of the main reasons I finally moved to a different phone. The S5 supports VoLTE but only with stock firmware.

Auch, wasn't familiar with that, haven't been in the US for a while. At least around here (EU) is not enabled by default on phones you buy off the shelf. I'm not sure about phones bought from the carrier.

What part of EU doesn't have VoLTE enabled? It is definitely widely used here in Sweden, though they simply forbid phones unable to do emergency calls over WiFi (including existing phones, which forced me to buy a new one. That was very irritating).

To explain myself better: it's readily available on the carriers, but if you just buy an off the shelf phone and put a SIM on it, it won't be enabled automatically (at least in a couple countries I tested it), and you'll need to go to your Android SIM/carrier settings menu and enable it yourself.

LTE was originally designed as a data-only upgrade to 3G. It became the full on successor to 3G, but launched with voice spec left as TBD. It eventually got defined, but ended up being a cursed nonstandard SIP-VoIP software thing, and so a lot of carriers and networks implement whitelists for VoLTE phones to manage anxieties around it.

VoLTE was an advertising feature for a long time. Major carriers here were advertising it as active and available for at least a year before it was actually available.

Not for everyone in many places. Many carriers still only allow VoLTE for a limited alow-list of phone models. Any customers of those with less popular models still gets dropped to 3G calls.

In the US all 2G and 3G has been shut down and VoLTE is the only way voice calls are done on 4G and Voice over New Radio on 5G

Or you can only use 2 sim cards at a time even tho both can connect over wifi calling.

Radio desense is a thing, though you'd probably still come out ahead.

An additional radio would raise the price of your phone for at least another 200 or 300 dollars. Try to enable VoLTE if available for your operator. That makes the phone use the LTE connection for voice and don't drop the Internet connection while you talk.

Dual SIM Dual Active phones are under £100 in the UK, I imagine under $100 in the US.

Dual SIM Dual Standby (two or more SIMs, one radio) is cheaper, less battery, less space, and usually good enough. It's literally a few dollars, not 200-300.

Seems like I forgot to specify "5G" (those radios tend to be expensive) as the original poster was asking for an additional 4G/5G radio.

DSDA in 5G is rare, and I can't reliably find a phone that can do it in EU or US (I can find higher end chipsets that can do it, but phone vendors don't seem to follow through with it). Usually it's 4G+5G or 4G+4G.

You can get an additional phone with a screen, 5g radio, ram and everything for $150. An additional radio should not raise the price for another 300.

I assume for something like a phone these things are non-linear. going from 2 PCIe lanes (nand, radios) to 3 (another radio) may cause an SOC to jump from entry-level to mid-level, larger PCB and increased layer count, more antenna for simultaneous use, etc. so its not just the cost of the chip but all the secondary effects.

In many phones, some of the radio hardware is integrated into the SoC. Adding a second, independent radio is a larger engineering task.

A non-linear cost for a small fraction of the phone. A slightly larger PCB with two more layers is about twice the price for the PCB, more antennas is a couple dollars, a one lane PCIe switch is a few dollars if that's cheaper than upgrading your SOC. And if it shrinks your battery 10% you save money on that part.

I blame niche-ness far more than the actual design and production costs.

Edit: fixed the pcb wording

> PCIe switch is a few dollars if that's cheaper than upgrading your SOC

perhaps you're more familiar with these options than I am, but I see 4 lane PCIe switches are 10x10 mm and maybe they are a few dollars but take more board space, new/larger power supplies, etc so it seems that the costs are more than just a few dollar PCIe switch chip. not sure how many phone PCBs can handle additional 100 mm2 but definitely looking forward to learning more.

[1] https://www.diodes.com/products/connectivity/pcie-packet-swi...

add to the antennas are likely based on best/cheapest efficiency for the available area, then additional antenna would reduce the area for each and require more effort/cost.

> not sure how many phone PCBs can handle additional 100 mm2

You are clearly out of your league. Get some 20 y/o flip Motorola, disassemble it and think, think hard.

No, 'PCIe switches' has nothing to do with the smartphones at all - so their PCB real estate do not indicate anything.

It won't, but the pre-made platform solution you pick and the prompt you send to Chinese ODM who do all the hard work must specify it has to be (DSDA|DSDV|DSDS|DSSS), or otherwise it'll default to DSDS.

The chips cost under $100 even for top tier modems from Qualcomm.

> Can't they just add two radios/chips

no. Too expensive.

Ironically, the cheapest phones available all tend to have Dual SIM Dual Active, usually because (in Europe at least) many have a second SIM card specifically for international calls.

So yes, it's "too expensive" because it's higher than $0, but it's not "too expensive" as in it would cost a lot.

Reply on news.ycombinator.com